|Session:||Large-scale ocean circulation and fluxes (02B)|
Agulhas Current Time-series (ACT): Towards a multi-decadal index
of Agulhas Current transport
Beal , Lisa1; Cipollini, Paolo2; Lutjeharms, Johann3
1University of Miami, UNITED STATES;
2National Oceanography Centre, UNITED KINGDOM;
3University of Cape Town, SOUTH AFRICA
ACT is a US-led, NSF-funded project with the goal of building a multi-decadal time series of Agulhas Current volume transport as a contribution to the Global Ocean Observing System. The Agulhas is an important component of the global thermohaline circulation and changes in its interocean flux have been linked to the end of the last ice age. Knowledge of the seasonal and interannual variability of Agulhas transport, its sensitivities and feedbacks, will help us understand its role in our current changing climate. The ﬁrst phase of ACT requires deployment of an array of instruments across the Agulhas Current and along an altimeter ground track, to obtain a three-year time series (2010-2013) of transports via in situ measurements. The second phase will be to correlate the along-track altimeter data with these measured transports to produce a proxy for Agulhas Current transport, which can be extended forwards and backwards in time. Ultimately,a twenty-year proxy of Agulhas Current transport will provide an important climate index for the Indian and global oceans, which can be compared to other climate indices, such as the Indian Ocean Dipole and the Atlantic Meridional Overturning, as well as to other western boundary currents, such as the Florida Current and Kuroshio time series.
Determination of Surface Wind Vector and Stress Fields Using METOP/ASCAT and QuikSCAT/SeaWinds Retrievals
Bentamy, Abderrahim / AB; Croizé-Fillon, Denis / DCF
Since March 2007 two scatterometers onboard MetOp-A and QuikSCAT satellites provide routinely and continuously 10-m surface wind speed and direction with high spatial resolution. Due to the scatterometer sampling scheme, the use of the retrievals for operational oceanic forcing is rather limited and especially at global scale. Indeed, most of the oceanic models require gridded wind fields. A number of efforts have been made to produce regular in space and time wind fields from scatterometer off-line wind observations. However, in order to minimize the “trackiness” effect related to the scatterometer sampling over a swath, the method, used to estimate the gridded wind fields, averages at each grid point observations occurring within specified space and time windows. The mapping methods tend to smooth some instrumental and/or geophysical events such as measurement errors, and rapid spatial or temporal wind variability. Another limitation of scatterometer wind observations is related to the nearshore use. Due to land contamination scatterometer wind retrievals are generally not available in these areas. To overcome these limitations especially associated with operational use, new surface wind fields, estimated from the remotely sensed wind data are calculated over global ocean with a spatial resolution of 0.25° in longitude and latitude, and temporal resolution of 12 hours. Their quality is investigated through various comparisons with surface wind vectors from buoys moored in various oceanic basins. The comparisons show that speeds and directions compare well to in-situ data. The root-mean-squared differences of the wind speed and direction are similar to those estimated for buoy hourly measurement and scatterometer retrieval comparisons..
TERRA NOVA BAY POLYNYA: A SMALL COASTAL AREA AFFECTING BASIN SCALE OCEANIC CONDITIONS
Budillon, Giorgio; Fusco, Giannetta; Rusciano, Emanuela; Spezie, Giancarlo
Dipartimento di Scienze per l'Ambiente - Universita' di Napoli 'Parthenope', ITALY
It is widely recognize the crucial role of the brine release during the sea ice formation processes for the formation of the Antarctic Bottom Water (AABW). Modification of thermohaline properties have been recently detected in the abyssal basins close to the Ross Sea pointing out its key role. Actually the Ross Sea is known to be an important area for the formation dense shelf water, the two most important shelf waters have peculiar characteristics in salinity (High Salinity Shelf Water - HSSW, S > 34.8 and temperature close to the freezing surface value) and in temperature (Ice Shelf Water - ISW, pot. temp. < 2.1 °C and S ≈ 34.6). Recent studies showed a relative short residence times of the Ross Sea shelf waters, the 3-4 years seems a reasonable period representative of the time interval between their formation and their ventilation of the deep ocean at the shelf break. The Italian CLIMA project started in 1995 a monitoring program of the polynya collecting both oceanographic and meteorological data in order to study at different time scales the atmospheric forcing (heat fluxes), HSSW formation rates, and the thermohaline characteristics of the water column. SSM/I remote sensed data of the last 15 years allowed us to discriminate two different behaviour of the polynya during the summer and winter conditions. The former is peculiar of the period between December-March and the polynya is often completely ice free, while the latter is between April-November when the polynya shows an average dimension of about 1500 km2 with a maximum extension of about 5000 km2. The TNB polynya is controlled by the katabatic wind regimes which is responsible for both the formation of the sea ice and for its continuous removal keeping the sea surface ice-free despite a continuous production. The analysis of the meteorological data collected by the Italian meteorological AWS network (www.climantartide.it) stressed the importance of the persistency of the katabatic regime (mainly the offshore component of the wind) which plays a major role for the maintaining the polynya opened. Analyzing the katabtic events we observed that they concentrated during the May-October with a significant interannual variability. The maximum in the offshore wind speed was regularly detected in June/July, few months later, in September/October, a salinity maximum appears in the surface layer (120 m depth approx.); while the former is related with a maximum in the sea-ice production in TNB polynya the latter marks the ending phase of the polynya. During this stage the polynya may still be open by the presence of relatively intense offshore winds but the production of sea-ice (and the associate release of brine) is not allowed by the shortness or by the infrequency of the katabatic wind regime. Along the water column the salinity freshen between March and June while it shows a sharp increase between July and October. A relatively surface warm layer 150 m deep appears in November producing a thermocline between 150 - 500 m which persists during the austral summer (December - February) and rapidly disappears at the beginning of the winter conditions (March) cause the increase of vertical turbulent mixing provided by the katabatic events. The estimated yearly averaged surface heat budget remarks a large variability in the last two decades showing higher values between 2001 to 2006 (maximum in 2003 with an average heat loss of -313 Wm2); the minimum heat loss (-58 Wm2) occurred in 1996 and the mean value for the entire period was -140 Wm2. The estimated HSSW mean yearly production span in the same period between 0.7 to 2.0 Sv with an estimated average value of 1.2 Sv. In the deeper layers of the water column a general freshening was detected from 1995 up today (ΔS = -0.06 at 900 m deep) with a short positive anomaly measured in correspondence of the period 2002-03 associated to the maximum of the heat loss at the surface (i.e. maxim in the HSSW estimated formation). A remarkable role in the controlling the thermohaline condition of the TNB water column may be played also by the heat and salt carried by the modified Circumpolar Deep Water which flows along the Ross Sea slope and periodically may reaches the polynya area. Recent works analyzed at the shelf break of the Ross Sea the variability of the shelf waters involved in the ventilation processes and in the AABW formation showing an significant correlation with the TNB thermohaline variability. Therefore the Terra Nova Bay polynya represents, not only for the logistic facilities due the proximity of the Italian scientific station, a unique polar environment to realize a relative cost-effective multi-disciplinary observing system that will provide the long-term measurements needed to improve understanding of climate change and variability, biogeochemical cycles and the coupling between climate and marine ecosystems in polar environments.
Dissolved Chlorofluorocarbons as Transient Tracers in the CLIVAR Repeat Hydrography Program
Bullister, John L.1; Sonnerup, Rolf E.2; Moore, Dennis W.1; Fine, Rana A.3; Min, Dong-Ha4; Smethie, William M.5; Warner, Mark J.2; Tanhua, Toste6; Keike, Dagmar7; Steinfeldt, Reiner7; Rhein, Monika7
As part of the CLIVAR Repeat Hydrography Program, a number of key hydrographic sections sampled in the 1990s as part of the World Ocean Circulation Experiment (WOCE) are being re-occupied at approximately decadal intervals. Measurements of a suite of physical and chemical properties are being made at full depth, closely spaced (nominally 30 nautical mile) CTD/rosette stations, with water samples collected at between 24 and 36 depths per station.
Key goals of the chlorofluorocarbon (CFC) studies in this program are to document the invasion of these compounds into the ocean interior and apply the data to:
Significant differences in CFC concentrations (and CFC-derived water mass ages) have been observed during CLIVAR reoccupations of WOCE sections. Transient tracer combinations including Sulfur Hexafluoride (SF6) measured on the same samples have been used to constrain transit time distributions in the ocean interior to help separate tracer age changes due to decadal changes in ventilation along the sections from artifacts due to the impact of mixing on tracer ages. The multiple tracer technique can also be used to correct mixing biases' effects on anthropogenic CO2 estimates that rely on CFC ages. The addition of SF6 to CLIVAR sampling plans also affords an unambiguous age tracer in the upper water column during this and coming decades.
Recent studies using CLIVAR repeat hydrography data have revealed wide-spread changes in the temperature and salinity of abyssal waters during the past several decades. In many cases these changes are strongly correlated with regions of significant CFC invasion along the sections. CFCs thus provide sensitive indicators of regions of the deep ocean where surface-derived climate change signals propagate into the ocean interior on decadal time scales.
Bias in the bathythermograph records and its impact on ocean climate analysis
Carton, J.A.1; Giese, B.S.2; Chepurin, G.A.1; Seidel, H.2
1University of Maryland, UNITED STATES;
2Texas A&M University, UNITED STATES
This paper examines the impact of correcting time-dependent temperature bias in bathythermograph observations on a global data assimilation–based reanalysis of ocean circulation. Three different estimates of temperature bias are considered: that of Hanawa et al. (1995), Levitus et al. (2009), and Wijffels et al. (2008). Examine of 36-year analysis experiments repeated with each show that the latter two bias correction algorithms reduce the observed warm bias in global heat content significantly, notably in the decade and a half beginning in the early 1970s, and again in the early 1990s. The results indicate that the bias correction of Wijffels et al. overcompensates after 1995, leading to a slight cool bias in the resulting analysis. Examination of the mean temperature structure from the experiments shows that generally the impact of the bias correction on the analysis is what one might expect – at the depths where the correction is largest the analysis is modified proportionately. Since the Wijffels et al. (2008) correction is larger than the Levitus et al. its impact on the analysis is larger. The exception to this result occurs in the equatorial thermocline where the cooling of the observations associated with the Wijffels et al. correction actually results in a warming of the analysis in excess of 0.3oC (the impact of the Levitus et al. correction is less than half of that). To understand the cause of this large and counterintuitive result the impact of the bias correction on the time-dependent circulation is examined. Interestingly, the Levitus et al. correction causes a larger impact on El Nino than on the mean state, including a reduction of the magnitude of the temperature anomalies associated with ENSO by 10-20% and a strengthening of the currents in the eastern equatorial Pacific by up to 50%.
Eddy-balanced buoyancy gradients on eastern boundaries and their
role in the Meridional Overturning Circulation
Cessi, P; Wolfe, CL
Scripps Institution of Oceanography - UCSD, UNITED STATES
It is demonstrated that eddy fluxes of buoyancy at the eastern and western boundaries maintain alongshore buoyancy gradients along the coast. Eddy-fluxes arise near the eastern and western boundaries because on both coasts buoyancy gradients normal to the boundary are strong. The eddy fluxes are accompanied by mean vertical flows that take place in narrow boundary layers next to the coast where the geostrophic constraint is broken. These ageostrophic cells have a velocity component normal to the coast that balances the geostrophic mean velocity. It is shown that the dynamics in these thin ageostrophic boundary layers can be replaced by effective boundary conditions for the interior flow, relating the eddy flux of buoyancy at the seaward edge of the boundary layers to the buoyancy gradient along the coast. These effective boundary conditions are applied to a model of the thermocline linearized around a mean stratification and a state of rest. The linear model parametrizes the eddy fluxes of buoyancy as isopycnal diffusion. The linear model produces horizontal gradients of buoyancy along the eastern coast on a vertical scale that depends on both the vertical diffusivity and the eddy diffusivity. The buoyancy field of the linear model agrees very well with the mean state of an eddy-resolving computation. Because the East-West difference in buoyancy is related to the zonally integrated meridional velocity, the linear model successfully predicts the meridional overturning circulation.
Water flux and Phosphorus transport in the mixed layer of the northern Red Sea and Gulf of Suez
El-Saharty, A.A.1; Said, M.A.1; Abbas, M.A.2
1National Institute of Oceanography & Fisheries, Alexandria, Egypt., EGYPT;
2National Institute of Oceanography & Fisheries, Alexandria, Egypt, EGYPT
The study presents an attempt to calculate water flux and phosphorus transportation in the upper 50m layer (mixed layer) of the northern Red Sea and Gulf of Suez using data collected during the joint Russian-Egyptian expedition onboard the Russian R/V "Professor Bogorov" which took place during March 1990. The hydrographic structure of the study area indicated the existence of an inflow of low salinity (40.10), warm (>22°C) and <28.3 surface water from the Red Sea into the Gulf of Suez and an outflow of a more saline (>40.40), colder (<22°C) and relatively high density ( >28.3) subsurface water in the opposite direction. This water is forming in the entrance area of the Gulf, sinking as indicated by the down-sloping of the isotherms, isohalines and isopleths and entering the Red Sea as a mid-deep water. Vertical profiles of temperature, salinity and at some stations indicated that, the water temperature in the upper layer down to about 200 m depth shows a great uniformity in temperature. Below that depth, the water temperature decreases with increasing depth to reach 21.60°C at 500 m depth. Salinity and values at these stations show also great variations in the upper 200 m layer. Below this layer, the salinity and increase with depth to reach 40.46 and 28.5 at 500 m depth respectively. The distribution of phosphate in the investigated area showed that all the surface waters are nearly depleted in the phosphate and lie near 0.1 mole PO4-P/l. An apparent first peak lies nearly between 50 and 100m depth. All the stations showed gradual increase of the phosphate with depth till 500m. Phosphorus transported to the area from the west accounts for 316.05 tons/day, while 1.34 tons/day are transported from the east. Phosphorus flux from the south plays the most important role; it reaches 1212.67 tons/day. From Gulf of Aqaba, 2.25 tons/day enters the area. Cumulatively about 1530 tons phosphorus/day enters the upper 50m layer, of which only 117.63 tons/day enters the Gulf of Suez. The rest may be exhausted in plant growth or through sinking to the lower layers.
Observing System Simulation Experiments for the Atlantic Meridional Overturning Circulation.
Halliwell, George R1; Thacker, Carlisle1; Yang, Haoping1; Garraffo, Zulema2
1NOAA/AOML, UNITED STATES;
2MPO/RSMAS, University of Miami, UNITED STATES
We report on initial efforts associated with a new project supported by NOAA and the National Oceanographic Partnership Program to quantitatively evaluate observing system strategies for the purpose of monitoring the Atlantic Meridional Overturning Circulation. The primary strategy is to use Observing System Simulation Experiments (OSSEs) to evaluate the impact of new and planned observing systems, and also to use Observing System Experiments (OSEs) to evaluate the impact of existing observing systems. The procedure for evaluating a proposed observing system using an OSSE is straightforward: (1) the proposed observations are sampled from an ocean model simulation known as the "nature run" that is assumed to represent the true ocean; (2) noise is added to make the observations more representative of what might actually be observed with actual instruments; (3) the observations are assimilated into a different ocean model, known as the operational model, to correct it with respect to the nature run; (4) an ocean forecast initialized with the corrected fields of the operational model is performed; and then (5) the quality of the forecast is evaluated against the nature run. We summarize our plans for developing and validating an ocean OSSE capability at NOAA/AOML and the University of Miami over the next 1-2 years. During this development period, our initial effort toward monitoring the AMOC is focused on evaluation of a multi-model ensemble to quantify the significant errors in the representation of the AMOC that exist in present-day ocean models, and also on identifying the best choice for use as a nature run. Error covariances determined from this analysis are then used to perform "virtual" OSSEs, which enable evaluation of the potential impact of observations without actually having to actually sample the observations from a nature run and assimilate these into the operational model.
Surface drifter measurements in the Mediterranean and Black Seas
Gerin, R.; Poulain, P.-M.
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), ITALY
Surface currents in Marginal seas, such as the Mediterranean and Black Seas, can be measured efficiently from the drifts of Lagrangian instruments tracked by, and transmitting data to, satellite systems (Argos, Iridium). These surface drifting buoys (called drifters) are low-cost, expandable systems that measure sea surface temperature (SST) and currents through their displacements between satellite fixes. Drifters are actually quasi-Lagrangian since they do not perfectly follow the surface water because of the effects of the winds and waves acting on them. However, over the past two decades, drifters, such as the CODE and SVP designs, have been developed to reduce these effects. In the Mediterranean and Black Seas, satellite-tracked drifters were operated starting in 1986 and 1999, respectively, as part of national and international scientific projects and in support of military operations. They have been deployed from research vessels and from ships of opportunity (ferries, small boats, etc.). The data of most drifters operated in the Mediterranean and Black Seas between 1986 and 2009 have been assembled in a common database in which all the data have been processed and quality controlled uniformly. This database and corresponding graphical products are available online whereas partial databases have also been released on CD-ROMs. The data of more than 1000 drifters (mostly corresponding to CODE and SVP designs) have been included in the database. Among all the studies in which drifter data can be utilized, it is important to mention their use in concert with satellite maps of SST or surface chlorophyll concentration. The combination of Lagrangian data and satellite maps provides a remarkable description of the often complex spatio-temporal variability, at sub-basin and meso-scales, of the surface circulation and its connection with the SST and chlorophyll fields. From a more statistical point of view, drifter data are also used to compute pseudo-Eulerian maps of mean circulation and sub-grid/temporal (also called eddy) variability. Kinetic energy levels of the surface currents can also be mapped. In addition, using the Lagrangian nature of the drifters, Lagrangian statistics (time-lagged auto-covariance, diffusivity, etc.) can be calculated to investigate the absolution and relative dispersion of water parcels. International collaboration is crucial to obtain a useful drifter dataset even at the scales of the Mediterranean and Black Seas. The combination of data from drifters with similar properties (same depth of drogue, same effects of wind and waves) in a marginal sea is needed to provide a better description of the complex spatial and temporal variations of the surface currents. Colleagues from the USA, Spain, France, Italy, Tunisia, Ukraine and Russia have contributed to the Mediterranean and Black Sea drifter database between 1986 and 2009. Starting in late 2006, the Mediterranean Surface Velocity Program (MedSVP) has been formed to coordinate the use of surface drifters in the Mediterranean and Black Seas and to make their data available in near-real time (daily) for several end-user applications, including their assimilation into operational numerical forecasting models. Hence, through MedSVP, drifter data are now readily available and represent an important component of the Mediterranean Operational Oceanography Network (MOON). Recent examples of Mediterranean and Black Sea drifter programs are presented, including the EGYPT-EGITTO experiment in the southeastern Mediterranean, and the TSS experiment with main focus on the Marmara Sea.
Surface Fluxes in High Latitude Regions
Gille, S.1; Bourassa, M.2; Bitz, C.3; Carlson, D.4; Cerovecki, I.1; Cronin, M.5; Drennan, W.6; Fairall, C.7; Hoffman, R.8; Magnusdottir, G.9; Pinker, R.10; Renfrew, I.11; Serreze, M.12; Speer, K.2; Talley, L.1; Wick, G.13
1University of California San Diego, UNITED STATES;
2Florida State University, UNITED STATES;
3University of Washington, UNITED STATES;
4British Antarctic Survey, UNITED KINGDOM;
5National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory, UNITED STATES;
6University of Miami, UNITED STATES;
7NOAA Earth System Research Laboratory, UNITED STATES;
8Atmospheric and Environmental Research, UNITED STATES;
9University of California Irvine, UNITED STATES;
10University of Maryland, UNITED STATES;
11University of East Anglia, UNITED KINGDOM;
12University of Colorado, Boulder, UNITED STATES;
13NOAA Environmental and Technology Laboratory, UNITED STATES
Improving knowledge of air-sea exchanges of heat, momentum, fresh water, and CO2 is critical to understanding climate, and this is particularly true in high latitude regions, where anthropogenic climate change is predicted to be exceptionally rapid. However, observations of these fluxes are extremely scarce in the Arctic, the Southern Ocean, and the Antarctic marginal seas. High winds, high sea state, extreme cold temperatures, seasonal sea ice, and the remoteness of the regions all conspire to make observations difficult to obtain. Existing gridded flux products can differ substantially, by 50 W m-2 or more in the case of heat fluxes, and in many cases there is no clear consensus about which flux products are most reliable. Gains would be achieved with improvements in the accuracy of scatterometer winds at high wind speed and with improvements in heat fluxes to achieve 0.01Nm-2 and 10 W m-2 accuracy (averaged over several days) with 25 km grid spacing. Progress in this regard will require a combination of efforts, including a concerted plan to make better use of ships of opportunity to collect meteorological data, targeted effort to deploy a few flux moorings in high wind regions, and improved satellite retrievals of flux related variables. A sustained flux observing system might eventually rely extensively on satellite data, but it will require in situ monitoring from ships of opportunity and buoys as ground truth and to support continuing algorithm improvements.
Observations of Atmosphere-Ocean Freshwater Input With In Situ and Satellite Measurements of Surface Salinity and Rain
Gommenginger, Christine P; Banks, Christopher; Srokosz, Meric A; Snaith, Helen
National Oceanography Centre, Southampton, UNITED KINGDOM
Two new satellite missions in 2009 and 2010 will attempt for the first time to measure ocean surface salinity remotely from space. The European Space Agency (ESA) mission for Soil Moisture and Ocean Salinity (SMOS) and the US/Argentina mission Aquarius both aim to deliver regularly updated global maps of sea surface salinity (SSS). These will eventually produce the first global view of the variability over oceanic basins and seasonal to inter-annual scales of this hitherto poorly known field. Together with sea surface temperature (SST), SSS determines surface water densities and has important implications for air-sea exchanges of freshwater, heat and carbon. In the North Atlantic, sinking of dense waters associated with the Meridional Overturning Circulation leads to the heat transport, which gives Northern Europe its relatively mild climate compared to other regions at the same latitude.
Given the technical and scientific innovation of flying an L-band interferometric radiometry in space, there are many challenging aspects linked with the calibration and validation of SMOS SSS. For example, while L-band (1.4 GHz ≈ 20cm wavelength) is the frequency with highest sensitivity to salinity, that sensitivity remains weak and is dominated instead by surface roughness effects. Those effects are poorly known, primarily because few observations sites provide surface water salinity and temperature measurements together with full descriptions of wind and sea conditions. In the case of SMOS, the absence of an ancillary surface roughness measuring payload only compounds the problem of retrieving salinity, by introducing uncertainties linked to possible biases and sampling errors in the operational wind and wave fields used for salinity retrieval.
One key aspect of measuring salinity from space is that significant spatio-temporal averaging over several satellite overpasses is necessary to achieve sufficient accuracy for meaningful comparisons with in situ data. Conventional validation methods, based on instantaneous comparisons with collocated in situ measurements, are thus of limited use. Both SMOS and Aquarius aim to produce composite salinity maps with 0.1psu accuracy averaged over 10-30 days and 1-2 degree resolution grid. To compare these against accurate but sparse in time and space in situ observations of near-surface salinity measurements will call for new methodologies to combine related information from different sources, each with different error and sampling characteristics.
In this paper, we begin by focusing on the North Atlantic area, examining the variability of key oceanic and atmospheric parameters like sea surface salinity, sea surface temperature and ocean roughness, using earlier studies in the literature and currently available observations and climatologies. Surface salinity observations are available from a multitude of sources (see e.g. Figure 1), from moored buoys and platforms (e.g. PIRATA, Station Mike), underway temperature and salinity (e.g. Voluntary Observing Ships, Ferrybox, research cruises), XBTs and of course Argo. Argo provides the most extensive and uniformly distributed dataset of salinity measurements but its shallowest measurements are rarely closer than 10 meters to the surface. In many places where the surface is well mixed, the salinity at 10 meters depth will differ little from the salinity measured by satellite in the top centimeter. But this will not be the case in areas of intense precipitation. Thus, precipitation satellites may also yield information relevant to assessing the validity of new satellite salinity data. In the end, combining satellite and in situ measurements may lead to important insight about atmospheric freshwater fluxes into the ocean as well as the degree of near-surface ocean mixing.
Long term ocean variability and effects on regional ocean dynamics
Goni, Gustavo1; Bringas, Francis2; DiNezio, Pedro N.2; Lumpkin, Rick3; Garzoli, Silvia L.3
1National Oceanic and Atmospheric Administration, UNITED STATES;
2Cooperative Institute for Marine and Atmospheric Studies, University of Miami, UNITED STATES;
3National Oceanic and Atmospheric Administration, Atlantic Oceanographic and Meteorological Laborator, UNITED STATES
Ocean variablity during the last fourteen years (1993-2007) is analyzed using different sets of satellite observations and products. Results show that trends in sea surface temperature are mostly positive in the northern hemisphere and negative in the southern hemisphere. Linear trends in the Southern Hemisphere show basin wide changes in the Pacific Ocean with positive values in the west and negative values in the east. In regional scales the variability of the sea height may be due to changes in the location and intensity of western boundary currents. Results are presented jointly with trends in wind field to investigate their possible link to an atmospheric response. Changes in the intensity and location of main currents are inferred from the variability in the eddy kinetic energy field. Results of the analysis indicate that several boundary currents show an increase in eddy and meandering activity. Among them, it is observed a change in the main location of the Gulf Stream and a southward shift of the Brazil/Malvinas Confluence in the South Atlantic. The variability of the Brazil-Malvinas frontal region is presented in terms of two parameters: the separation of the Brazil Current from the continental shelf break and the southernmost location of the Brazil Current Front. During the study period, these parameters exhibit a shift to the south of approximately 1.5 and 0.8 degrees, respectively. Simultaneously, the interior of the South Atlantic subtropical gyre exhibits an expansion with mean increase of 2 cm of dynamic height per decade. Statistically significant changes are not observed in the geostrophic transport of the Brazil and Malvinas currents, which theory and models have shown to govern the separation of the BC, thus suggesting that a different mechanism governing the low-frequency changes of the Brazil Current front. The shift of the Brazil Current front is hypothesized here to be associated to this concurrent expansion of the South Atlantic subtropical gyre. This shift is consistent with the trend in the latitude of the maximum basin-averaged wind stress curl, suggesting that the migration is driven by Sverdrup dynamics and that longer term estimates of the frontal location can be derived from wind fields. This proxy for the Confluence location is derived from the NCEP/NCAR reanalysis v.2 fields for the period 1979—2007, with results suggesting that the front was at an anomalously northward position in the early part of the modern altimetry period. Correlation of this decadal record with SST anomaly suggests that the wind changes may have been driven by warming along the Agulhas-Benguela pathway. Comprehensive numerical experiments will ultimately be needed to determine the origin of these changes.
Seven Years of measuring the Makassar Strait throughflow, the primary component of the Indonesian Throughflow
Gordon, A. L.1; Susanto, D.1; Huber, B1; Sulistyo, B2; Supangat, A2
1Lamont-Doherty Earth Observatory, UNITED STATES;
2Agency for Marine and Fisheries Research (BRKP), Jakarta, INDONESIA
The transfer of tropical Pacific water into the Indian Ocean through the Indonesian seas, the Indonesian Throughflow (ITF), is a significant part of the ocean system of interocean fluxes, ocean-scale heat and freshwater budgets and sea-air fluxes, providing an oceanic interactive link with the ENSO and Asian monsoon climate features. The ITF to a large extent governs the overall oceanographic stratification, circulation and ecosystems within the Indonesian Seas. Makassar Strait is primary inflow pathway of the ITF, carrying >80% of the total ITF. The Makassar throughflow has been observed within the 45 km wide Labani constriction near 3°S for a 1.5 year period in 1997/98 as part of the Arlindo program, and for three years, 2004-2006, as part of the INSTANT program. The observed transport in the Makassar Strait, from January 2004 through November 2006 is ~11.6 x106 m3/sec, 27% larger than observed during 1997 when a strong El Niño suppressed the flow. As the ITF transport varies with ENSO, and likely other climate indices (e.g. Indian Ocean Dipole) a multi-year record is needed to fully appreciate its characteristics and links to the regional and larger scale climate system. Directly after the recovery of the NSF funded INSTANT western Makassar mooring in November 2006, a NOAA funded mooring was deployed at the same site (2°51' S; 118°28' E; 2147 m) on 22 November 2006. The NOAA-Mak was recovered on 31 May 2009, and re-deployed for another 2 years to continue to build the time series. We now have a 5.5-year continuous time series of Makassar throughflow; with the Arlindo data we have a full 7 years of Makassar throughflow recorded. During the INSTANT periods ENSO was in a weak El Niño state, with a brief La Niña phase occurring in early 2006. The NOAA mooring period spans a time of an overall weak La Niña phase. Except for the Arlindo period, there is no clear correlation of the Makassar throughflow to ENSO, but it is noted that neither the INSTANT or NOAA time series recorded during strong ENSO episodes. The December 2006 through May 2009 record displays many of the same attributes as revealed by the INSTANT data: a clear seasonal behavior with maximum flow in August, with minimum flow in November. The particularly weak flow of November 2007 may be a consequence of a strong Kelvin Wave derived from the Indian Ocean. The mean flow within the thermocline and deeper as measured by the NOAA-MAK mooring is strikingly similar (less than 10% difference) to that measured during the INSTANT period. However the flow at 40 meters is notably weaker in the NOAA record, with an average southward speed of 0.3 m/sec versus 0.4 m/sec in the INSTANT 2004-2006 record.
The Solomon Sea observed by glider and altimetry
Gourdeau, L.1; Kessler, W.2; Davis, R.3
2NOAA/PMEL, UNITED STATES;
3SCRIPPS, UNITED STATES
The Solomon Sea with intense western boundary currents like the New Guinea Coastal Current is a key region for the tropical/subtropical connexion and for the feeding of the Equatorial Under current with possible effect on ENSO modulation. The sharp Papua-New Guinea coastline and the Solomon Sea with its narrow straits to the north impose strong topographic constraints on the flow that is little documented so far. Long-term observations in the region are sparse, and so far the Argo floats array does not sample this enclosed area well. Climatologies are hampered by the sparse data coverage. Gliders are autonomous underwater platforms that are moved over the water column by modifying their buoyancy and “glide” using wings that confer a horizontal velocity associated with their vertical displacements. Gliders are expected to be an important contribution to monitor boundary current, especially in regions of difficult accessibility. An experimental glider monitoring of the LLWBC within the Solomon Sea is currently tested to understand how the inflow distributes within the Solomon Sea. Five glider missions have been operated from August 2007 to January 2009 showing the huge variability of the transports in relation with ENSO conditions and eddy activities. Another information on this variability comes from altimetry where the Solomon Sea exhibits the highest levels of sea level variability of the whole South Equatorial Pacific Ocean. Surface geostrophic current, as eddy kinetic energy level, deduced from altimetry reflect most of the transport variability in the Solomon Sea. The satellite data are useful to replace the data along the glider track in a synoptic context whereas the glider data are useful to test how the surface information from altimetry is representative of the dynamics at depth. The complementary of both datasets: gliders and altimetry, motivates this study.
Minimization of the Impact of Sampling Errors in VOS-based Global Air-Sea Flux Fields
Gulev, Sergey K.
IORAS, RUSSIAN FEDERATION
Sampling uncertainties in the voluntary observing ship (VOS)-based global ocean-atmosphere flux fields were estimated by sub-sampling of reanalyses and operational forecasts. In poorly sampled regions sampling errors amount to 2.5°-3°C for air temperature, 3 m s-1 for the wind speed, 2-2.5 g kg-1 for specific humidity, and 15%-20% of the total cloud cover. The highest sampling errors in surface fluxes range from 30 to 80 Wm-2. In poorly sampled subpolar latitudes of the Northern Hemisphere and throughout much of the Southern Ocean the total sampling uncertainty in the net heat flux can amount to 80-100 W m-2. The largest uncertainties in linear trend estimates are found in relatively poorly sampled regions like the high-latitude North Atlantic and North Pacific as well as the Southern Ocean, where trends can locally show opposite signs when computed from the regularly sampled and undersampled data. Spatial patterns of shorter-period interannual variability can be also affected by sampling uncertainties, especially in the Labrador Sea and northwest Pacific as well as the Southern Ocean, where trends can locally change sign due to sampling uncertainty. In order to minimize sampling errors in surface air-sea flux fields we suggest an approach based on estimation of probability distributions for surface fluxes. We apply the modified Fisher-Tippett (MFT) distribution providing accurate estimation of all statistical moments and estimation of surface turbulent fluxes of rare occurrences. Application of MFT allows for abating sampling uncertainties by 3 to 10 times. We will also demonstrate the application of this approach for the reconstruction of surface ocean-atmosphere heat fluxes over the North Atlantic for the last 127 years (1880-2006). Reconstructed fluxes reveal long-term trends, implying, for example, about 4 W/m2 per decade growing sensible heat fluxes in the Labrador Sea and about 2 W/m2 per decade secular increase in the Central subpolar gyre in the Atlantic.
MORE: Five Years of Radiative Air-Sea Flux Measurements in the Atlantic Ocean
Sinitsyn, Alexey1; Kalisch, John2; Gulev, Sergey K.3; Macke, Andreas2
1IORAS, RUSSIAN FEDERATION;
3IOARS, RUSSIAN FEDERATION
The Meridional Oceanic Radiative Experiment (MORE) is a joint initiative of the P.P. Shirshov Institute of Oceanology (IORAS) and the Leibniz Institute of Marine Sciences at the University of Kiel (IFM-GEOMAR) MORE is set up to conduct long-term, high quality measurements of surface parameters and fl uxes in the Atlantic Ocean with a particular emphasis on short wave (SW) and long-wave (LW) radiation fluxes. These are needed for proper quantification of the global ocean heat balance. Since 2004 under MORE there has been carried 8 cruises with direct measurements of short-wave and long-wave radiation fluxes along with the observation of basic meteorological variables and cloud sky imaging taken with approximately similar sampling in different latitudes in the belt from 60N to 60S at the Atlantic meridional section. Thus, these data account for most potential cloud conditions at sea. In the poster we will demonstrate the pilot MORE results, first of all the potential of using MORE data for the development of new parameterizations of radiative fluxes at sea. For instance we will demonstrate that new parameterizations demonstrate statistically significant improvement of the accuracy compared to the other schemes based exclusively on the total cloud cover, being especially effective under high cloud cover and conditions close to complete overcast. Finally the poster will discuss the perspectives of the improvement of observations of clouds and radiative fluxes for achieving better accuracy of global and regional air-sea interaction estimates.
MEAN SEA SURFACE HEIGHT IN THE WORLD OCEAN USING ARGO FLOAT AND ALTIMETRY
Abe, H; Hanawa, K
Since satellite altimetry provides only anomalies of sea surface height (SSH) from mean SSH field temporally averaged for 1992-1999, it is needed to estimate mean SSH field to obtain absolute SSH field from some reference surface, such as 2000db. In the present study, we newly developed procedure to obtain mean SSH field using the Argo float data and satellite altimetry data. By subtracting satellite SSH anomalies from the SSH estimated using temperature and salinity profile, we can obtain mean SSH. However, we cannot expect the exact coincidence in place and time between satellite altimetry and Argo float observation. Therefore we first estimate the statistics of temporal and spatial behavior of SSH variation using satellite altimetry and determine the match-up condition between satellite altimetry and Argo observation. The match-up conditions are different in latitude and longitude. It was found that, although the obtained mean SSH field is similar to that obtained using climatological temperature and salinity profiles such as WOA05 as a gross, there are significant differences especially in the regions of the western boundary currents and the Antarctic Circumpolar Current.
Atlantic Meridional Overturning Circulation Simulated by NCEP GODAS
Huang, Boyin1; Xue, Yan1; Behringer, David W.2; Arun, Kumar1
1Climate Prediction Center/NCEP, UNITED STATES;
2Environmental Modeling Center/NCEP, UNITED STATES
The Atlantic meridional overturning circulation (AMOC) was estimated using the NCEP operational Global Ocean Data Assimilation System (GODAS), which assimilates observed temperature and synthetic salinity profiles down to 750m. The averaged (1982-2004) AMOC is 17 Sv at 37N (Fig. 1), consistent with other observed analyses and model simulations.
The AMOC was also estimated from two experimental GODAS runs, one that is identical to the GODAS except assimilating observations down to 2200m (GODAS_deep) and the other assimilating observed temperature and Argo salinity profiles down to 750m from 2001 to 2006 (GODAS_Argo). The GODAS_deep AMOC drifted upward for about 5 years from its initialization in 1979 provided by GODAS, and then became stationary until the Argo data became available in 2000 when the upward drift started again. The averaged (1982-2004) AMOC in the GODAS_deep is 26 Sv, about 9 Sv stronger than that of GODAS. However, the strength of the GODAS_Argo AMOC is similar to that of the GODAS. The results suggested that the strength of AMOC can be reasonably assimilated by a shallow data assimilation scheme, and the quality of AMOC simulation was not necessarily improved by using a deep data assimilation scheme. The impacts of Argo salinity on the AMOC simulation will also be analyzed.
An EOF analysis of the AMOC simulated by the operational GODAS indicates that the AMOC variation has a large spatial structure with maximum variability located at 45N and 1500m (Fig. 2a). The first EOF pattern represents 50% variance, and its principle component indicates an increasing trend from 1982 to 1994, and a decreasing trend from 1995 to present (Fig. 2b), in a good agreement with the AMOC variability estimated at 40N.
Since AMOC is density-driven, it is critical to analyze the features of temperature and salinity variability, and their contributions, to density variability. We analyzed the features of temperature and salinity variability in the GODAS and compared them with observations. It is found that the AMOC variability in GODAS is largely driven by temperature variability since salinity variability was severely underestimated by synthetic salinity. However, since upper ocean heat content can be used as the AMOC fingerprint, the AMOC variability in GODAS appears reasonable, and has a potential to be used in monitoring and assessing the current conditions of AMOC.
Objectively Derived In-Situ Turbulent Flux Climatology: Application to Tropical Atlantic Variability
Hughes, Paul1; Bourassa, Mark A.1; Rolph, Jeremy2; Smith, Shawn R.2
1Florida State University, UNITED STATES;
2Center for Ocean-Atmospheric Prediction Studies, UNITED STATES
In order to better understand both regional and global climate variability accurate estimates of the surface turbulent fluxes (momentum, latent and sensible heat) between the ocean and atmosphere on multiple spatiotemporal scales is essential. Observationally, the near ocean surface properties are primarily measured by ships, moored and drifting buoys, and satellites. Direct measurements of the fluxes between the ocean and atmosphere are sparse in both space and time and thus woefully inadequate to capture variability on basin-wide to global scales. Consequently, the turbulent fluxes need to be estimated using surface meteorological data obtained from said oceanic sources, numerical weather prediction (NWP) models, or a combination thereof. This study highlights the third generation Florida Statue University (FSU3) monthly mean 1°x1° gridded wind and surface flux product and its application to tropical Atlantic sea surface temperature (SST) variability. In particular, the consequences of formulating the turbulent fluxes via the classical time-averaging method is examined. The FSU3 product is constructed from in situ ship and buoy observations via a variational technique and includes: wind stress, latent heat flux, sensible heat flux, pseudostress, scalar wind speed, specific humidity, and air temperature. The resulting fields are available for the Atlantic, Pacific, and Indian oceans from 1978 through 2004.
Annual Signal Modulation of the Kuroshio Through-flow Volume Transport South of Japan Leading West Pacific Pattern
Ichikawa, H.1; Nagano, A.1; Ichikawa, K.2
2Kyushu Univ. and JAMSTEC, JAPAN
The present study shows the possible societal benefit of eMonitoring ocean-atmosphere interactions in western boundary current extensions in the North Pacific.
As the Western Boundary Current of the North Pacific Subtropical Gyre, the Kuroshio south of Japan is considered to play an important role in a global climate system through transporting a large amount of heat which is released to the atmosphere in the Kuroshio Extension region. However, the relation between the Kuroshio and the global climate has not been clarified yet mainly because the inter-annual variation of the Kuroshio transport south of Japan is not clear due to large fluctuations of volume transport caused by recirculation eddy to the south of Shikoku and meso-scale eddies from the east.
For getting the data necessary for estimating the net volume and heat transports of the Kuroshio through-flow south of Japan by reducing the dominant recirculation and eddy components, we conducted the moored array observation from July 2004 to October 2006 using 9 current meters at 9 sites and 11 pressure-gauge-equipped inverted echo sounders (PIES) along the ASUKA (Affiliated Surveys of the Kuroshio off Cape Ashizuri) -line north of 30°N, and the 30°N line from 135°E to 142°E.
By combined use of these moored observation data and satellite altimeter data, we have estimated a 16-year long time series from 1993 of the volume transport of the Kuroshio through-flow (KTVT) across the ASUKA-line as shown in Fig.1. It is obvious in this figure that annual signal is much more dominant than any other components in KTVT, and its amplitude changes year by year while it has the maximum every winter. Focusing on this dominant inter-annual amplitude modulation in the KTVT annual signal, we have examined the relation of KTVT with the climate indices such as Multivariate ENSO Index (MEI), Arctic Oscillation (AO) Index, Pacific Decadal Oscillation (PDO) Index, West Pacific Pattern (WP) Index and others, not by wavelet analysis but by correlation analysis due to lack of available data duration length of KTVT time series.
The cross correlation function, R, between annual signals in KTVT and WP is found to have statistically significant maxima of -0.73 at lag (L) = -19-month (WP leads KTVT) and -0.72 at L = 65-month (WP delays KTVT) while other indices have not so large correlation maxima as WP. This result indicates that, while KTVT is affected more directly by WP before 19 months, it has some feedback process affecting the annual signal of WP after 65 months, and that we can predict well the 65-month later temperature and precipitation over the North Pacific associated with WP using present KTVT.
For improving the prediction accuracy of temperature and precipitation over the North Pacific by better understanding of various ocean-atmosphere interaction processes connecting WP with KTVT, we must keep continuing the monitoring of KTVT and surface heat flux in the Kuroshio and Kuroshio Extension region.
Volume Transport Variability in the Northwestern Weddell Sea Seen in a Global Ocean Model (OCCAM)
Kerr, R.1; Heywood, K. J.2; Mata, M. M.1; Garcia, C. A. E.1
2UEA, UNITED KINGDOM
The Synoptic Antarctic Shelf-Slope Interactions Study (SASSI) project has conducted multidisciplinary studies on the continental shelf and slope at Antarctic margins during the International Polar Year (IPY-2007/09). In summary, during the IPY several countries contributed to SASSI project with short synoptic transects that were undertaken circumpolarly and radiated outwards across the Antarctic continental shelf and slope. One of those is the high sampled WOCE SR4 hydrographic section starting near the tip of the Antarctic Peninsula across the Weddell Sea, which is one of the main areas of Antarctic Bottom Water (AABW) export to the global oceans. As part of the SASSI project and because of the high spatial-temporal resolution available, we have chosen to analyze the 1/12° simulation obtained with the Ocean Circulation and Climate Advanced Modelling (OCCAM) model to investigate the temporal variability of AABW (i.e. Weddell Sea varieties) volume transport in the northwestern Weddell Sea. Here, we focus in the volume transport variability. The mean total full depth cumulative volume transport obtained was respectively 28.6 ± 8 Sv (1 Sv = 106 m3s-1) and 28.7 ± 10 for section 1 (i.e. the western part of the WOCE SR4 section) and section 2, this is somewhat lower than the transport (i.e. 46 ± 8 Sv) obtained during the summer 2007 cruise of the Antarctic Drift Experiment Link to Isobaths and Ecosystems (ADELIE) project [Thompson & Heywood 2008]. On the other hand, this is the mean volume transport considering all the simulated years (i.e. 1988-2004). It is not unexpected that the bottom layer volume transport is also underestimated by the model (i.e. 11.6 ± 4 Sv – section 1 and 10.7 ± 4 Sv – section 2. This could be probably associated with the weaker current velocity representation by OCCAM model in the Weddell Gyre [Renner et al. 2009]. The monthly variability of the total volume transport, considering both the entire section and only the neutral density layers >28.26 kg.m-3, shows the maximum (minimum) transport occurring in June (January). Comparing with the results from Fahrbach et al. , there is a delay of one month in the model. However, the monthly variability of the total volume transport in the model is in phase with sea ice fraction monthly average in the Antarctic Peninsula sector. The annual variability of the total volume transport of section 1 is not in phase with the sea ice parameters. In contrast, the annual average of the bottom volume transport is ~2 years lagged with both the sea ice fraction and the sea ice thickness variability. Other parameters (as the wind patterns) are under investigation to try to explain these findings.
Formation rates of Labrador Sea Water inferred from repeated tracer sections
Kieke, Dagmar; Rhein, Monika
University of Bremen, GERMANY
Since more than three decades oceanic measurements of anthropogenic tracers such as chlorofluorocarbons (CFCs) have served as valuable tools to investigate the spreading and the formation of deep water components in wide-spread regions of the Atlantic Ocean. Labrador Sea Water (LSW) is the shallowest components contributing to the cold and deep branch of the Meridional Overturing Circulation. Evidence from the past two decades have shown that the formation history of LSW in the Labrador Sea succumbs to substantial interannual and decadal variability. Repeated tracer sections on basin-wide scales have yielded valuable information on LSW formation changes. Since 1997 the spatial data coverage in the subpolar North Atlantic is sufficient to determine the CFC-12 inventories of Labrador Sea Water (LSW) on biennial time scales. Temporal changes in the CFC-inventories are therefore used to infer LSW formation rates and associated uncertainties. In contrast to oceanic convection peak times in the early 1990s producing a dense, deep mode of LSW, the convection activity of the late 1990s resulted in a shallow and lighter mode, called upper LSW. The results indicate a weakening of the total LSW formation in the years 1997-2005 which corresponds to a decrease of the baroclinic mass transport of the upper 2000m between Bermuda and the central Labrador Sea which is thought as an index of the strength of the subpolar North Atlantic gyre. The fact that the atmospheric signal of CFC-12 does not further increase, may hamper in future the detection of an oceanic CFC-12 increase due to water mass formation processes and will lead to greater uncertainties when considering changes in CFC-12 inventories. Therefore, we present first results from basin-wide measurements of sulphur hexafluoride (SF6) in the subpolar North Atlantic in conjunction with CFC-12 measurements and investigate the potential of using the transient signal of SF6 to estimate LSW formation rates.
Introduction of Japanese Ocean Flux data sets with Use of Remote sensing Observations (J-OFURO) Version 2
Hiroyuki, Tomita1; Kutsuwada, Kunio2; Akiyama, Masatoshi2; Iwasaki, Shinsuke2
2Tokai University, JAPAN
We have constructed ocean surface flux data sets mainly using mainly satellite data. The data set named Japanese Ocean Flux data sets with Use of Remote sensing Observations (J-OFURO) has been provided to scientists since 2002 (Kubota et al., 2002) and has been used in many research studies. Recently, new surface heat flux data sets have been constructed in J-OFURO, thereby upgrading it to version 2 (J-OFURO2). Version 2 has many improvements over version 1 for the estimation of momentum turbulent heat fluxes. For example, multisatellite data are used in J-OFURO2, while data from only one DMSP-SSM/I sensor has been used for the estimation of turbulent heat fluxes in J-OFURO1. We are going to introduce J-OFURO2 products with evaluation results.
U.S. AMOC Program
Duke University, UNITED STATES
The U.S. Atlantic Meridional Overturning Circulation (AMOC) program is a near-term priority of the Ocean Research Priorities Plan issued in January of 2007. A panel of U.S. scientists developed a five-year implementation strategy, released in October of 2007, that laid the groundwork for a program that will develop the initial components of an AMOC monitoring system and AMOC prediction capability. The overall objective of this research program is to investigate the physical variability of the AMOC such that researchers will be able to understand and predict the impact of AMOC variability on regional and global climate, ocean ecosystems, sea level, sea ice and the global carbon budget. Specifically, the three principal objectives of this program are: Objective 1: The design and implementation of an expanded AMOC monitoring system Objective 2: An assessment of AMOC’s role in the global climate Objective 3: An assessment of AMOC predictability An AMOC Science Team has been formed and is implementing the program. Given the scope of this program, on both the observational and modeling fronts, the US AMOC Science Team is interested in partnering with international collaborators to meet many of the stated goals. Progress and future plans for each of these US AMOC program objectives will be presented at Ocean Obs 09.
Multi-year Observations of the Brazil Current Baroclinic Transport Variability Near 22oS
Mata, Mauricio1; Caspel, Mathias1; Fonteles, Caio1; Goni, Gustavo2; Baringer, Molly2; Cirano, Mauro3
2NOAA, UNITED STATES;
Despite of the recognized importance of the western boundary currents (WBC) to the oceanic and climate systems from regional to basin-wide scales, the Brazil Current (BC) remains one of the least studied and understood of all WBCs, especially in terms of its associated variability. Several aspects of BC low-latitude variability remain unexplained mostly due to the lack of reliable observations and consistent time series. As the BC mean flow is relatively weak, eddy features can impose a large variability to the current, leading to uncertainties in the baroclinic transports estimates from hydrographic snapshots. In this sense, we have set up a partnership joining Brazilian institutions and NOAA to fund and run a long-term high-density XBT line in the southwestern Atlantic in order to improve our understanding about the region and particularly the BC variability. The project has been labeled MOVAR (Monitoring the upper ocean transport variability in the western South Atlantic) and the line has been designated in the NOAA/AOML high-density program as AX98. The line was set up using a ship of opportunity scheme between Rio de Janeiro and the Brazilian navy oceanographic post at Tridade Island (POIT, 30oW 20oS). The Brazilian navy visits the POIT regularly (≈ every 3 months) to take supplies and exchange personnel. Thus, since August 2004, the same transect has been repeated eighteen times using the same sampling scheme providing valuable novel data in the study area. For example, the zonally integrated baroclinic transport (relative to 700 dbar) has proven to be much variable both temporally (4 Sv ± 3 Sv) and spatially (zonal fluctuations of the BC axis of more than 150nm). Those fluctuations are further related to the presence/absence fo the Vitoria Eddy, a transient feature already described for the region.
Maximenko, Nikolai1; Niiler, Peter2; Schneider, Niklas1; Di Lorenzo, Emanuele3; Hafner, Jan1; Sasaki, Hideharu4; Melnichenko, Oleg1
1IPRC/SOEST, University of Hawaii, UNITED STATES;
2Scripps Institution Of Oceanography, UNITED STATES;
3Georgia Institute of Technology, UNITED STATES;
4Earth Simulator Center, JAMSTEC, JAPAN
Accumulation of large, high-quality satellite and in situ data led to significant improvements in the description of the mean dynamic ocean topography (MDOT) [Maximenko et al., 2009]. The new models of MDOT, with the resolution improved to 50-100 km, have not only revealed important details in the complex mesoscale structure of circulation systems such as the Gulf Stream, Kuroshio Extension, and Antarctic Circumpolar Current, but also led to the discovery of new anisotropic jet-like features in ocean circulation referred to as "striations" [Maximenko and Niiler, 2005; Maximenko et al., 2005; Maximenko et al., 2008].
While somewhat similar features -- alternating zonal jets, are predicted by a number of theories inspired by the banded cloud patterns in the atmospheres of Jupiter and Saturn [Galperin et al. 2004], preliminary analysis of satellite and high-resolution ocean models reveal that striations (at least at the sea surface) are inconsistent with the two-dimensional, geophysical turbulence, which produces jets through the combination of processes commonly known as the "Rhines mechanism" [Rhines, 1975]. Equally unlikely is the role of the PV staircases that could be formed by breaking Rossby waves [Baldwin, 2007]. The uniqueness of the ocean dynamics comes from the existence of the continents and culminates in the generation of large gyres associated with essentially meridional flows. To remain time-invariant in such a flow, striations behave as waves rather than as inertial jets. Once detected, the striations, both stationary and periodic in time, are found to be common throughout the ocean, although their properties varying to different degrees both geographically and interannually.
This paper outlines the main challenges of the striation study, both observational and theoretical. It discusses hypotheses of the forcing and dynamics of these features, interaction between striations and mesoscale eddies, and presents evidence that striations play an important role in regularizing the otherwise random eddy field. The striations are shown to be not just an artifact of misinterpreted moving eddies, but a structure retaining its coherence on spatiotemporal scales significantly exceeding the eddy scales (in some reported cases, up to thousands of kilometers and 15 years). Also discussed is the impact of striations on the climate system, both through the ocean dynamics and air-sea interaction, and possible differences between the circulation regimes in the upper and intermediate-depth ocean. It is also noted that techniques currently employed to map the sea level anomaly, derived from the along-track satellite altimetry, may tend to convert the signal from striations into the one from a train of eddies. We demonstrate the importance of the combined use of data of satellite and in situ observations, and realistic high-resolution global ocean general circulation model along with theoretical analysis and numerical experimenting with the regional ocean model system.
Mediterranean subsurface circulation and thermohaline properties from ARGO data
Notarstefano , Giulio; Menna, Milena; Poulain, Pierre Marie
In order to examine subsurface currents and thermohaline properties of the Mediterranean Sea, we used profiling floats deployed as a part of the International Argo program, since 2003. These floats are programmed to execute 5-day cycles drifting at a neutral parking depth of 350 m and CTD profiles from either 700 or 2000 m up to the surface. The Argos positions are used to estimate the circulation at the parking depth. This study involves a sophisticated determination of the surface and sub-surface displacements. From these, the subsurface velocities at the 350 m parking depth are estimated. Finally, the estimated subsurface velocities are used to compute pseudo-Eulerian circulation statistics, including maps of mean circulation and eddy variability in the Mediterranean Sea. We also used 5 years of Argo data (2004-2009) to study the spatial structures and the temporal variability of temperature and salinity in the Mediterranean Sea at surface, at 700 and 2000 m and at the depth of the salinity maximum. The dataset allows us to reconstruct the main spatial structures of salinity and temperature in the entire Mediterranean on a 2 degrees squared boxes. The analysis of the temporal variability in selected sub-basins of the Mediterranean reveals a positive trend of salinity at the depth of the salinity maximum in the Levantine basin.
THOR: long term observations of MOC variability
in the North Atlantic
Nunes, N.1; Østerhus, S.2
2Bjerknes Centre for Climate Research, NORWAY
The variability of the ocean circulation in the North Atlantic has direct implications for the European climate, and for the global climate through its effects on the meridional overturning circulation (MOC). The new EU-funded THOR project ("Thermohaline Overturning: at Risk?") aims to quantify the range and probability of changes associated with MOC variability using palaeoclimate studies, long term observations and numerical models of ocean circulation. We present the observation system currently available for THOR, as well as plans for developing and enabling near real-time data transfer capabilities from deep sea moorings. This observation system, consisting of arrays of self contained instruments as well as ship- and space-borne measurements, will provide accurate time series of mass, heat and salt fluxes at key locations, allowing for the first time to assess the strength of the Atlantic Overturning Circulation.
Seasonal and Interannual Variation of North Pacific Subtropical Mode Water in 2003−2006
The University of Tokyo / JAMSTEC, JAPAN
Temperature and salinity data from 2003 through 2006 from Argo profiling floats have been analyzed to examine the formation and circulation of the North Pacific Subtropical Mode Water (STMW) and the interannual variation of its properties over the entire distribution region. STMW is formed in late winter in the zonally-elongated recirculation gyre south of the Kuroshio and its extension, which extends north of ≈28°N, from 135°E to near the date line. The recirculation gyre consists of several anticyclonic circulations, in each of which thick STMW with a characteristic temperature is formed. After spring, the thick STMW tends to be continually trapped in the respective circulations, remaining in the formation region. From this stagnant pool of thick STMW, some portion seeps little by little into the southern region, where southwestward subsurface currents advect relatively thin STMW as far as 20°N to the south and just east of Taiwan to the west. The STMW formed in the recirculation gyre becomes colder, less saline, and denser to the east, with an abrupt change of properties across 140°E and a gradual change east of 140°E. The STMW formed east of 140°E exhibits coherent interannual variations, increasing its temperature by ≈1°C from 2003 through 2006 and also increasing its salinity by ≈0.05 from 2003 through 2005. These property changes are clearly detected in the southern region as far downstream as just east of Taiwan, with reasonable time lags.
Chile Ocean Observing System
Pizarro, O.1; Hormazabal, S.2; Shaffer, G.3; Ramos, M.4
1DEGEO & COPAS, University of Concepción, CHILE;
2DGEO, University of Concepcion, CHILE;
3DGEO, Univerity of Concepcion and Department of Geophysics, University of Copenhagen, DENMARK;
4CEAZA & University of Concepcion, CHILE
We describe the present state of the Chile Ocean Observing System and present main results from mooring and ship based time series. The first long- term observational program to study ocean circulation off Chile considered the deployment of two currentmeter moorings near 30ºS in November 1991. Those moorings have been maintained for several years and, except for relatively short periods, long time series for these sites are presently available. After this initial effort, further initiatives have been accomplished to extend observations to northern (~21°S in 1998) and southern (~37°S in 2003) locations. Presently, six subsurface moorings, conforms the base of a regional ocean observing system. The observing system includes 3 deep ocean moorings (deployed at about 150-200 km from the coast in a water depth of ~ 4400 m near 21°S, 30°S and 37°S), 2 slope moorings (deployed over a depth of ~900 m at 30°S and 21°S) and one continental shelf mooring (deployed near 37°S). The deep-ocean moorings are equipped with traditional current meter and sediment traps, and during some period we have included in all the line 300 kHz or 75 kHz ADCPs. Additionally, several monitoring cruises are maintained for different programs. Off northern Chile seasonal oceanographic cruises have been carried out since the beginning of the 80's supported by the Chilean government and the fishery industry, while near 37°S an oceanographic station over the continental shelf is monthly visited since 2002. There a complete set of biological and biogeochemical data is being collected by the University of Concepcion. These efforts, although modest to cover the large lack of information in the zone, have revealed novel dynamical features underlying low frequency variability. The alongshore current over the slope near Iquique (21ºS) and Coquimbo (30ºS) shows impressive intraseasonal fluctuations (oscillations with periods between 30 and 90 days). Such oscillations are forced by equatorial Kelvin waves that, after hitting the South American coast, generate coastally trapped waves. These oscillations decay rapidly offshore and are not present in the oceanic current records. In contrast, the oceanic time series near 30ºS show large variability at lower frequencies related to mesoscale eddies and meanders of the alongshore flow. On the other hand, the seasonal cycle of the current is related to both, the annual variation of the alongshore wind stress near the surface and annual and semiannual equatorial disturbances. At lower frequencies, interannual oscillations of the flow are closely related to the El Niño-La Niña cycles. Both seasonal and interannual disturbances can leave the coast and propagate offshore modulating the flow and thermocline depth several hundred of kilometers offshore in the region. Progress of the Chile ocean observing system has been possible thanks to various research projects, leaded by researcher from different international and national institutions. Particularly, the ocean moorings are maintained in a collaborative effort between the Catholic University of Valparaiso, the University of Copenhagen, and the University of Concepcion. Contributions from the Hydrographic and Oceanographic Service of the Chilean Navy (SHOA) and from the Fisheries Development Institute (IFOP) have been also essentials to maintain the moored lines.
The RAPID-MOC/MOCHA Mooring Array at 26ºN in the Atlantic
Rayner, D.1; Bryden, H. L.2; Wright, P. G.2; Collins, J.2; Kanzow, T3; Johns, W. E.4; Barringer, M. O.5; Meinen, C. S.5; Cunningham, S. A.2
1National Oceanography Centre, UNITED KINGDOM;
2National Oceanography Centre, Southampton, UNITED KINGDOM;
3IFM-GEOMAR, University of Kiel, GERMANY;
4Rosenstiel Scholl of Marine and Atmospheric Science, University of Miami, UNITED STATES;
5Atlantic Oceanographic and Meteorological Laboratory, National Oceanic and Atmospheric Administratio, UNITED STATES
The Atlantic Meridional Overturning Circulation (AMOC) at 26.5°N carries a northward heat flux of 1.3 PW. Northward of 26.5°N, over the Gulf Stream and its extension much of this heat is transferred to the atmosphere and is responsible for maintaining UK climate about 5°C warmer than the zonal average at this latitude. However, previous sparse observations did not resolve the temporal variability of the AMOC and so it is unknown whether it is slowing in response to global warming as suggested by recent model results. In 2004 NERC, NSF and NOAA funded a system of observations in the Atlantic at 26.5°N to observe on a daily basis the strength and structure of the AMOC. Two papers (Cunningham, et al., 2007 & Kanzow, et al., 2007) demonstrated that not only does the system of observations achieve a mass balance for the AMOC, it reveals dramatic and unexpected richness of variability (fig. 1): the AMOC mean strength and variability is 18.5±4.9 Sv. From estimates of the degrees-of-freedom the year-long mean AMOC is defined with a resolution of around 1.5 Sv so abrupt changes would be readily identified and long-term changes will be measured relative to the 2004-2005 average. The NERC contribution to the first four years of continuous AMOC observations was funded under the directed programme RAPID Climate Change. Following an international review of the system NERC will continue funding to 2014 under the programme RAPID-WATCH. The NSF and NOAA have also continued funding and commitments so that the system can continue operating at the same level of activity as during the period 2004-2008. The objectives of RAPID-WATCH are: To deliver a decade-long time series of calibrated and quality-controlled measurements of the Atlantic MOC from the RAPID-WATCH arrays and; To exploit the data from the RAPID-WATCH arrays and elsewhere to determine and interpret recent changes in the Atlantic MOC, assess the risk of rapid climate change, and investigate the potential for predictions of the MOC and its impacts on climate. The 26.5°N Atlantic section is separated into two regions: a western boundary region, where the Gulf Stream flows through the narrow (80km), shallow (800m) Florida Straits between Florida and the Bahamas, and a transatlantic mid-ocean region, extending from the Bahamas at about 77°W to Africa at about 15°W (fig. 2). Variability in Gulf Stream flow is derived from cable voltage measurements across the Florida Straits, and variability in wind-driven surface-layer Ekman transport across 26.5°N is derived from QuikScat satellite-based observations. To monitor the mid-ocean flow we deployed an array of moored instruments along the 26.5°N section. The basic principle of the array is to estimate the zonally integrated geostrophic profile of northward velocity on a daily basis from time-series measurements of temperature and salinity throughout the water column at the eastern and western boundaries. Inshore of the most westerly measurement of temperature and salinity, the transports of the Antilles current and deep western boundary current are monitored by direct velocity measurements. The array as deployed in 2008-2009 consists of a total of twenty-one moorings and twelve landers in three sub-arrays (fig. 2). The principal moorings measure the density profile at the eastern and western boundaries through use of CTDs. In the west the continental shelf forms a “wall” whereas in the east a series shorter moorings step up the slope reducing the influence of bottom triangles. Each sub-array also includes four bottom pressure landers that are serviced in alternate years so that each recovery provides a two-year record with a year’s overlap with the previous lander to remove instrument drift. The Mid-Atlantic Ridge moorings provide full depth density profiles either side of the ridge to allow separation of the eastern and western basin MOC contributions. The western boundary sub-array includes current meters to directly measure the currents in the western boundary wedge. The contribution of the Antarctic Bottom Water is captured through an offshore mooring in the western boundary combined with a mooring on the western flank of the mid-Atlantic ridge. In addition to the moorings listed above, the western boundary sub-array also contains three full depth moorings and four landers from the University of Miami that act as a backup to the density profile moorings and also provide the thermal-wind shear and measured velocities of the deep western boundary current (Johns, et al., 2008). Cunningham, S. A., et al. (2007), Temporal variability of the Atlantic Meridional Overturning Circulation at 26.5°N, Science, 317, 935-938. Johns, W. E., et al. (2008), Variability of shallow and deep western boundary currents off the Bahamas during 2004-2005: First results from the 26°N RAPID-MOC array, J. Phys. Oceanog., 38, 605-623. Kanzow, T., et al. (2007), Flow compensation associated with the MOC at 26.5°N in the Atlantic, Science, 317, 938-941.
Time series of transport variability and spreading paths of the North Atlantic Subpolar Gyre
Stroeh, A.; Mertens, C.; Kieke, D.; Rhein, M.
Bremen University, GERMANY
The Subpolar Gyre of the North Atlantic Ocean is one of the key regions for the earth´s climate system. Warm and saline waters of the North Atlantic Current (NAC) are transferred into the subpolar and polar regions, and subsequently returned as the deep and cold limb of the Atlantic Meridional Overturning Circulation (AMOC). Model simulations hint to a relation between deep water formation, the strength of the Subpolar Gyre and the intensity of the AMOC. To measure the variability of the NAC and thus the strength of the Subpolar Gyre, an array of 4 inverted echo sounders with bottom pressure sensors (PIES) was deployed along the Mid Atlantic Ridge between 47 and 52°N in August 2006. The location of the PIES allows the separation of the main NAC spreading paths through the fracture zones. The data were retrieved by acoustic telemetry in August 2008 and 2009, while the array remained at the seafloor to complete its scheduled 5-year deployment period. The travel time measurements of the PIES are combined with hydrographic data from profiling Argo floats, and ship measurements to calculate time series of hydrographic properties and the baroclinic transport variability. The horizontal bottom pressure differences are used to estimate the barotropic contribution.
Long term direct observations on currents and volume transport in the Mozambique Channel
Ridderinkhof, Herman1; Ullgren, Jenny1; van Aken, Hendrik1; van der Werf, Petra2; van Leeuwen, Peter Jan2; de Ruijter, Wilhelmus2
1Royal Netherlands Institute for Sea Research, NETHERLANDS;
2Institute for Marine and Atmospheric Research Utrecht University, NETHERLANDS
Since 2003, a team of Dutch physical oceanographers maintains a mooring array with current meters and T-S sensors across the narrowest part of the Mozambique Channel in the South West Indian Ocean. The goal is to study the time variability of the currents and volume transport in this branch of the global ocean circulation. The observations form part of OceanSites. An analysis of the volume transport through the channel, including the variability on interannual and seasonal time scales is presented for the period 2003-2008. The mean volume transport over the entire observational period is about 15 Sv poleward. Seasonal variations have a magnitude of some 5 Sv and can be explained from the variability in the wind field over the western part of the Indian Ocean. Interannual variability has a magnitude of about 10 Sv and is only slightly smaller than the mean. This time scale of variability is related to variability in the Indian Ocean Dipole, showing that it forms part of the variability in the ocean-climate system of the entire Indian Ocean. The relatively strong inter annual variability stresses the importance of these long term direct observations.
A New Mean Dynamic Topography Computed Over the Global Ocean Ffrom GRACE Data, Altimetry and In-situ Measurements
Rio, M.-H.1; Moreaux, G.2; Bourgogne, S.2; Lemoine, J.-M.3; Bronner, E.4; Schaeffer, P.1
Accurate knowledge of the ocean Mean Dynamic Topography at all spatial scales is mandatory for the full exploitation of altimetric data including their assimilation into operational ocean forecasting systems.
In the framework of the French SLOOP project, whose objective is to improve the processing of altimetric data for the open ocean, a new Mean Dynamic Topography has been computed, with improved data and methodology compared to the previous RIO05 MDT field.
The estimation is based on two steps. First, a large scale Mean Dynamic Topography was computed from the CLS01 altimetric Mean Sea Surface and the latest geoid model computed at GRGS from 5 years of GRACE data. For the first time, the geoid covariance error matrix is used for a more accurate estimation of the large scale MDT and associated error.
The second step of the estimation consists in combining altimetric sea level anomalies and in-situ measurements to compute synthetic estimates of the MDT and the corresponding mean currents. While the RIO05 MDT was based on 9 years of in-situ data (dynamic heights and drifting buoy velocities), the new field benefits from an enlarged dataset of in-situ measurements covering 15 years from 1993 to 2008 and including the latest ARGO reanalysis from the Coriolis center. Moreover the processing of the in-situ data has been improved: A new Ekman model was computed to extract the geostrophic velocity component from the drifting buoy measurements. The handling of hydrologic measurements has also been improved so as to allow for the inclusion of T,S profiles referenced to different pressure levels in the computation of the synthetic heights, resulting in a better sampling of the ocean, mainly in coastal areas.
The resulting new Mean Dynamic Topography is then validated comparing it to other existing solutions, based on observations or models.
Mapping the Ocean Interior's Currents From Altimetry, SST and In-Situ Measurements
Rio, M-H; Mignot, A.; Mulet, S.
Monthly maps of the ocean currents from the surface to 1500m have been computed for the 1993-2008 period combining altimetry to temperature and salinity data through the thermal wind equation. The 3D monthly current field compares well to the Mera-11 and Glorys reanalyses from the Mercator operational ocean forecasting system. Also, the regression coefficients computed between the new field and in-situ subsurface velocities measured by RAFOS and P-ALACE floats reach 0.66 and 0.65 for the zonal and meridional components respectively. The analysis of these new 3D observed currents shows that the NAO-related modifications of the North Atlantic subpolar gyre already observed at the surface extends down to 1500m. Finally, the estimated currents are used to monitor the strength of the North Atlantic Meridional Overturning Circulation (MOC) over the 1993-2008 period. Results are compared to previous studies as well as to estimates based on the RAPID-MOC array measurements.
Energetics From Drifting Buoys in the Southwestern Atlantic Ocean
Rota de Oliveira, Leopoldo1; Mata, M.2; Piola, A. R.3; Soares, I.2
1UNIVERSIDADE FEDERAL DO RIO GRANDE, BRAZIL;
2Universidade Federal do Rio Grande, BRAZIL;
3Universidad de Buenos Aires, ARGENTINA
The Southwestern Atlantic mean surface circulation, its associated variability and energetics are studied through the analysis of 16 years (1993-2008) of surface drifter data from the NOAA (SVP) velocity data binned onto a 0.5° x 0.5° grid. Special attention is given to three main regional features of the domain: the Brazil Current (BC), the Malvinas Current (MC) and the Brazil-Malvinas Confluence (BMC).
The overall result shows that the current NOAA drifter database is robust enough to significantly increase the resolution of the circulation features in the area, when compared with previous studies. Moreover, that fact led to a notable improvement in the estimates of several variables and parameters of the surface circulation and motivated us to pursue a robust analysis of the region’s energetics. Furthermore, from the analysis of the data it was possible to calculate the barotropic energy conversion term. The results unveil that the barotropic conversion fluxes are present in the Brazil Current (BC), Malvinas Current (MC) and the Brazil-Malvinas Confluence (BMC).
The highest Mean Kinetic Energy (MKE) values are found along the path of the Western Boundary Currents in the area (WBCs), particularly in the MC north of 45°S, where values approaching 2500 cm2 s-2 where computed. The Eddy Kinetic Energy (EKE) field is normally associated with mesoscale activity. The highest EKE was found in the vicinity of the BMC, where values approach 3000 cm2 s-2. High EKE in the BMC has been attributed to the frequent observation of mesoscale eddies and meanders. When compared to the BC, the MC current displays lower values of EKE (<265 cm2 s-2) indicating that the eddy activity is lower in the latter. Indeed, when comparing the EKE with the MKE, the MC displays a clear dominance of the mean flow over the eddy variability. The conversion of MKE to EKE is given by the BT term and can be used as an indicator for barotropic instability. Wherever this term is positive, MKE is being converted to EKE through the work of the Reynolds stresses on the mean shear. However, there is a dynamic distinction between those currents, which was identified by the barotropic conversio term (BT). On the Brazil Current, the BT conversion goes from MKE to eddy kinetic energy. Conversely, on the Malvinas Current the term BT in the most of the path is negative, i.e., conversion of the EKE to MKE.
The term Production of Mean Kinetic Energy (PKE) by the eddies indicates that they are producing mean kinetic energy in the most of the path of MC. However, with BC the scenario is the opposite. The net exchange between mean and eddy kinetic energy is characterized by the term BT – PKE, so the mean flow loses kinetic energy to the eddies in the Brazil Current, but the situation is again opposite with Malvinas Current. The anticyclonic branch of the BC includes the region of its recirculation cell. In the most of this region the eddies lose kinetic energy to the mean flow (e.g. Ivchenko et al. ) but the reverse energy flow is also possible. On the cyclonic branch of the MC the mean flow loses energy to the eddies. These results supporting old investigations [Webster, 1961; Schmitz e Niiler, 1969; Brooks e Niiler, 1977] which suggest that different energetic systems can exist in the cyclonic and anticyclonic branches of the western boundary currents.
Water flux and Phosphorus transport in the mixed layer of the northern Red Sea and Gulf of Suez
Saharty, Abeer; Said, Mohamed
National Institute of Oceanography&Fishers,Egypt, EGYPT
ABSTRACT The study presents an attempt to calculate water flux and phosphorus transportation in the upper 50m layer (mixed layer) of the northern Red Sea and Gulf of Suez using data collected during the joint Russian-Egyptian expedition onboard the Russian R/V "Professor Bogorov" which took place during March 1990. The hydrographic structure of the study area indicated the existence of an inflow of low salinity (40.10), warm (>22°C) and <28.3 surface water from the Red Sea into the Gulf of Suez and an outflow of a more saline (>40.40), colder (<22°C) and relatively high density ( >28.3) subsurface water in the opposite direction. This water is forming in the entrance area of the Gulf, sinking as indicated by the down-sloping of the isotherms, isohalines and isopleths and entering the Red Sea as a mid-deep water. The distribution of phosphate in the investigated area showed that all the surface waters are nearly depleted in the phosphate and lie near 0.1 mole PO4-P/l. An apparent first peak lies nearly between 50 and 100m depth. All the stations showed gradual increase of the phosphate with depth till 500m. Phosphorus transported to the area from the west accounts for 316.05 tons/day, while 1.34 tons/day are transported from the east. Phosphorus flux from the south plays the most important role, it reaches 1212.67 tons/day. From Gulf of Aqaba, 2.25 tons/day enters the area. Cumulatively about 1530 tons phosphorus/day enters the upper 50m layer, of which only 117.63 tons/day enters the Gulf of Suez. The rest may be exhausted in plant growth or through sinking to the lower layers. The calculated downward phosphorus transport amounts to 872.79 tons/day.
A revisit of the reason why the properties of the Central Mode Water in the North Pacific changed in regime shifts
Sato, Kanako1; Suga, Toshio2; Nonaka, Masami1; Hosoda, Shigeki1; Kobayashi, Taiyo1; Iwasaka, Naoto3; Oka, Eitaro4; Hiromichi, Ueno5
2JAMSTEC and Tohoku University, JAPAN;
3JAMSTEC and Tokyo University of Marine Science and Technology, JAPAN;
4JAMSTEC and the University of Tokyo, JAPAN;
5Hokkaido University, JAPAN
The Central Mode Water (CMW) is known as a water mass in the lower part of the ventilated pycnocline in the North Pacific subtropical gyre. It was reported that CMW got warmer, saltier and lighter remarkably just after the regime shift at the end of 1980s (Suga et al., 2003). This change was considered to result from the decrease of heat and freshwater losses at the sea surface in the CMW formation region accompanied by weaker westerly wind. However, it was recently found using atmospheric reanalysis data that the change of these factors was not large enough to explain the change of CMW properties. Rather, a high-resolution OGCM indicated that the increase of the sea surface temperature and salinity in the CMW formation region might be due to northward movement of the Kuroshio Extension and acceleration of the upstream Kuroshio Extension jet. Therefore, we hypothesize that the dynamical change of the Kuroshio Extension affects a northward supply of high temperature and salinity water to the northern margin of the subtropical gyre and makes the CMW properties change. To inspect this hypothesis, we analyzed Argo data from 2000. The Kuroshio Extension moved southward rapidly at the beginning of 2006, although the shift was small compared to that at the end of 1980s. Associated with this shift, sea surface temperature and salinity in the area north of the Kuroshio Extension decrease from January to March in 2006. Temperature and salinity at the core of CMW observed from May to September in 2006 also decreased compared to those in 2005 by 1°C and 0.1, respectively. Although heat and freshwater losses in the CMW formation region increased in winter of 2006, the change of these factors was not large enough to explain that of CMW properties. Thus, the change of CMW properties in 2006 supports our hypothesis.
Verification of Numerical Weather Prediction Marine Meteorology using Moorings: An OceanSITES Application
Schulz, Eric1; Weller, Robert A.2
1Bureau of Meteorology, AUSTRALIA;
2Woods Hole Oceanographic Institution, UNITED STATES
There has been an increased emphasis on coupled ocean-atmosphere modelling at the Australian Bureau of Meteorology with the development of routine seasonal forecasting and ocean modelling capabilities. The model forcing via fluxes at the air-sea interface is critical to the forecasting skill of the models. Verification of numerical weather prediction marine meteorology against moored buoy observations is undertaken routinely to establish the uncertainty in modelled fluxes. The OceanSITES program provides an overarching structure to moored arrays of long-term deepwater reference stations that span the global oceans. One of the benefits of OceanSITES is the provision of common data formats and metadata that enable ingestion of numerous data-streams into analysis activities. The real-time, daily mean marine meteorology observations from 17 high quality "flux reference stations", spanning all the oceans (excluding the Southern Ocean), from 66O North to 20O South are used to validate forecasts (0 out to 10 days) for two numerical prediction models (GASP and the newly implemented ACCESS-G). The current observational array is concentrated in a band along the tropics, and expansion into the extra-tropics is essential if the array is to be considered truly global.
A monitoring system for the South Atlantic as a component of the MOC
Speich, Sabrina1; Garzoli , Silvia2; Piola, Alberto3; Baringer, Molly4; Barreiro, Marcelo5; Biastoch, Arne6; Budillon, Giorgio7; Byrne, Deirdre8; Campos, Edmo9; Chereskin, Teresa10; den Tom, Matthis11; Dijkstra, Henk11; Donohue, Kathleen12; Gladyshev, Sergey13; Goni, Gustavo4; Gordon, Arnold14; Guerrero, Raul15; King, Brian16; Macrander, Andreas17; Mata, Mauricio M.18; Matano, Ricardo P.19; McDonagh, Elaine16; Meinem, Christopher S.4; Meredith, Michael Paul P.20; Nof, Doron21; Ollitrault, Michel22; Owens, Breck23; Reynaud, Thierry22; Rupolo, Volfango24; Shuckburgh, Emily F.20; Sokov, Alexey13; Troisi, Ariel H.3; Watts, Randy12; Winbush, Mark12; Zahn, Rainer25; Baehr, Johanna26
1Laboratoire de Physique des Océans UMR 6523 CNRS-IFREMER-IRD-UBO, FRANCE;
2NOAA-AOML, UNITED STATES;
4NOAA/AOML, UNITED STATES;
5Univ. De La Republica, URUGUAY;
7Univ. Parthenope, ITALY;
8Univ. of Maine, UNITED STATES;
10Scripps, UNITED STATES;
12URI, UNITED STATES;
13SIO, RUSSIAN FEDERATION;
14LDEO, UNITED STATES;
16NOCS, UNITED KINGDOM;
19OSU, UNITED STATES;
20BAS, UNITED KINGDOM;
21FSU, UNITED STATES;
22LPO UMR6523 CNRS-IFREMER-IRD-UBO, FRANCE;
23WHOI, UNITED STATES;
26Univ. Hamburg, GERMANY
We will report on the two SAMOC workshops (san Ceferino, Argentina in 2007 and Paris, France in 2009) that gathered scientists from Argentina, Brazil, France, Germany, Italy, Russia, Spain, South Africa, Uruguay, United Kingdom, and the United States to foster collaborations and to discuss the design and implementation of an observational system to monitor the South Atlantic’s branch of the Meridional Overturning Circulation (SAMOC). The workshops were financially supported by the NOAA Climate Program Office and US CLIVAR. After reviewing and discussing existing modeling and observational efforts in the South Atlantic Ocean we came to the conclusion they are yet inadequate to monitor the MOC. Discussions were focused on the design of an observational array that was adequate for this purpose and that has started to be implemented more consistently since 2008. Plans have been established to coordinate modeling efforts, and the implementation of such longterm observing network in the Drake Passage, in the region between South Africa and Antarctica, and on a zonal transect nominally across 30°S.
The Great Barrier Reef Ocean Observing System Moorings array: Monitoring Coral Sea Impacts on the Great Barrier Reef
Steinberg, C.; McAllister, F.; Brinkman, G.; Pitcher, C.; Luetchford, J.; Rigby, P.
Australian Institute of Marine Science, AUSTRALIA
Since 1987 Great Barrier Reef weather and water temperature observations have been transmitted in near real time using HF radio from pontoons or towers on coral reefs by AIMS. In contrast oceanographic measurements have however been restricted to loggers serviced at quarterly to half yearly downloads. The Great Barrier Reef Ocean Observing System (GBROOS) is a regional node of the Integrated Marine Observing System (IMOS). IMOS is an Australian Government initiative established under the National Collaborative Research Infrastructure Strategy and has been supported by Queensland Government since 2006. GBROOS comprises real time observations from weather stations, oceanographic moorings, underway ship observations, ocean surface radar, satellite image reception and reef based sensor networks. This paper focuses on an array of in-line moorings that have been deployed along the outer Great Barrier Reef in order to monitor the Western Boundary currents of the Coral Sea. The Westward flowing Southern Equatorial Current bifurcates into the poleward flowing East Australian Current and the equatorward North Queensland Current. The 4 mooring pairs consist of a continental slope mooring, nominally in 200m of water and one on the outer continental shelf within the GBR matrix in depths of 30 to 70m. The array is designed to detect any changes in circulation, temperature response, mixed layer depth and ocean-shelf interactions. A review of likely impacts of climate change on the physical oceanography of the GBR is providing a basis upon which to explore what processes may be affected by climate change. Sample data and results from the initial year of observations will be presented.
Formation and Export Rates of North Atlantic Deep Water
Steinfeldt, Reiner; Rhein, Monika
University of Bremen, GERMANY
The concept from Bolin and Rhode (1973) of transit time distributions (TTDs) for reservoirs is applied to North Atlantic Deep Water (NADW) in the subpolar North Atlantic. The 'reservoirs' are the different density classes of NADW, i.e. Upper Labrador Sea Water (ULSW), Labrador Sea Water (LSW), Gibbs Fracture Zone Water (GFZW) and Denmark Strait Overflow Water (DSOW). The TTDs for these reservoirs are computed as volume integral of pointwise TTDs, which are inferred from CFC data collected between 1997 and 2005. It will be discussed, in how far these TTDs and their temporal derivatives can be used to infer ventilation, formation, and export rates for NADW. These results will be compared with direct observational data, e.g. the export of NADW from the subpolar North Atlantic in the deep western boundary current as reported in Schott et al. (2006). Bolin, B., and H. Rohde, A note on the concepts of age distribution and transit time in natural reservoirs, Tellus XXV, 1, 1973. Schott, F. A., J. Fischer, M. Dengler, and R. Zantopp,Variability of the Deep Western Boundary Current east of the Grand Banks, Geophys. Res. Lett., 33, L21S07, doi:10.1029/2006GL026563, 2006.
Upper Layer Variability of Indonesian Throughflow
Susanto, R. Dwi1; Fang, Guohong2; Supangat, Agus3
1Lamont Doherty of Columbia University, UNITED STATES;
2First Institute of Ocenography, Qingdao, CHINA;
33. Agency for Marine and Fisheries Research, Jakarta, INDONESIA
Indonesian throughflow (ITF), the transfer water mass and heat flux of tropical/subtropical Pacific water into the Indian Ocean through the Indonesian seas plays significant part of the global ocean system of interocean fluxes, ocean-scale heat and freshwater budgets, sea-air fluxes and biogeochemical exchange. The ITF is believed to play interactive link with Asia-Australian monsoon, ENSO and Indian Ocean Dipole, and to the large extent governs the overall oceanographic stratification, circulation, and ecosystems within the Indonesian Seas. Although the ITF measurements have been conducted for more than two decades including a simultaneous measurement at various straits during INSTANT program in 2003-2006, and Makassar ITF in 2006-2009, they failed quantify upper layer variability and freshwater fluxes which is important for the mixing and sea-air interaction within the region. The ITF branches through the South China Sea-Karimata Strait, and Tores Strait have always been ignored and have received little observational attention. There have been no field measurements to quantify the total transport and its associate heat-freshwater fluxes, even though trajectories of sea surface drifters of the Global Drifter Program from August 1988 to June 2007 have indicated that the Karimata Strait is another important channel for the Throughflow from the SCS to the Indonesian Seas. Since December 2007, South China Sea - Indonesia Seas Transport/Exchange (SITE) has been measured using trawl resistant bottom mounted ADCP deployed in the Karimata Straits, an international collaboration between Lamont Doherty Earth Observatory (LDEO) of Columbia University-USA, Agency for Marine and Fisheries Research (BRKP)-Indonesia, and First Institute of Oceanography-China. Preliminary analysis indicated that the annual mean may be small ~1-1.5Sv, however, the seasonal volume transport associated with monsoon can reach as large as 4.4Sv. In addition, two bottom mounted ADCP have been deployed in the Sunda Strait in November 2008 to measure the water mass and fresh water fluxes between Java Sea and eastern Indian Ocean which is the center of Indian Ocean Dipole. For future observation, we should have an integrated observation of Indonesian throughflow and biogeochemical properties, to fill the gap of the map of global climatological mean of pCO2 and net sea-air flux of CO2.
Analysis of a 44 - Year Hindcast for the Mediterranean Sea :
Comparison with altimetry and in situ observations
Vidal-Vijande, E.1; Barnier, B.2; Molines, J.M.2; Tintoré, J.1; Pascual, A.1
1IMEDEA (Instituto Mediterraneo de Estudios Avanzados), SPAIN;
2LEGI (Laboratoire des Ecoulements Géophysiques et Industriels), FRANCE
Intermediate (150-600m) and deep (600m - bottom) layers show a clear positive temperature trend, with almost no variability in the deep layers, probably caused by the atmospheric forcing which has too low a resolution as to allow for deep water formation in the Mediterranean Sea.
Mean surface salinity for the entire Mediterranean basin is signiﬁcantly lower in ORCA R025-G70 than in MEDAR (~0.3 psu), and could mean that the surface salinity restoring might be too weak, without suﬃcient evaporation to compensate for a weak atmospheric forcing (ERA40) water loss ﬂux. The mean temperature of the Mediterranean Sea is also slightly higher than the observations (linked to the ERA40 underestimation of winter heat loss), which in combination to the lower salinities, leads to a less dense water mass.
Discrepancies between observed and OGCM-simulated anomalies in recent SSTs of the tropical Indian Ocean
Meteorological Research Institute, JAPAN
We investigated the discrepancy between observed and ocean general circulation model (OGCM)-simulated anomalies in recent sea surface temperatures (SSTs) of the tropical Indian Ocean. Observed SSTs show a warming beginning in the late 1990s, whereas simulated SSTs show a cooling over the same period. Examination of surface heat fluxes in the OGCM showed that the simulated SST cooling was caused primarily by a decreasing trend in the reanalyzed solar radiation used as the surface boundary condition. In the atmospheric reanalysis, the decrease in solar radiation was attributed to an increase in cloud cover, deduced from precipitation data, and in part responding to observed local warming of the Indian Ocean SSTs prescribed as the lower boundary condition. Observation-based estimates of precipitation, however, show no significant increasing trend, so no increase in cloud cover is indicated. Caution is needed when atmospheric reanalysis data are used for surface boundary conditions for OGCMs.
Direct velocity measurements of deep circulation southwest of the Shatsky Rise in the western North Pacific
Yanagimoto, D.1; Fujio, S2; Kawabe, M2
1The University of Tokyo, JAPAN;
2Ocean Research Institute, The University of Tokyo, JAPAN
The Lower Circumpolar Deep Water (LCDW) transported by the global deep circulation from the Southern Ocean is known to spread to the basins in the North Pacific after passing the Central Pacific Basin (e.g., Mantyla and Reid, 1983). Though the western North Pacific has so complex bottom topography that observational studies on the deep circulation has been made difficult, Johnson and Toole (1993) and Kawabe et al. (2003) found an eastern branch of the deep circulation flowing northward through Wake Island Passage and a western branch flowing through the East Mariana Basin at low latitudes (Fig. 1). Yanagimoto and Kawabe (2007) indicated the both branches have northwestward flows between the Shatsky Rise and the Ogasawara Plateau, which is an eastward bulge of the Izu-Ogasawara Ridge.
Continuous Observations From the Weather Ship Polarfront at Station Mike
Yelland, Margaret J.1; Holliday, N. P.1; Skjelvan, Ingunn2; Østerhus, S.3; Conway, T. J.4
1NOCS, UNITED KINGDOM;
3Bjerknes Centre for Climate Research, NORWAY;
4NOAA/Earth System Research Laboratory, UNITED STATES
This poster will describe the various observations made from the world's last remaining weather ship "Polarfront" which operates year-round at Station mike (66 N, 2 E). Station Mike has been occupied by a weather ship continually for 60 years. Data presented will include the high resolution, 60 year, full depth hydrographic observations, the 30 year atmospheric gas sampling data, and the 30 year wave measurements. Also presented will be the results of more recently installed systems, such as the continuous surface water sampling and direct measurements of the air-sea fluxes of momentum, heat and CO2.