Oceans and Drought
Martin Hoerling(1), Siegfried Schubert(2)
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Evidence for the ocean's role in driving precipitation variability is presented. We begin by exploring the causes for seasonally recurring dry and wet seasons, known as the monsoons in tropical latitudes, and assess the role played by the seasonal cycle of the ocean's response to solar insolation. We next examine the ocean's role in the year-to-year and longer-term disruptions in normal rainfall patterns, conditions typically referred to as drought and pluvial. We focus on studies of sea surface temperature (SST) impacts on several historical droughts and draw assessments of ocean observing system requirements that could facilitate drought early warning. These must be viewed as preliminary and tentative given the nascent stage of our knowledge of the ocean stemming partly from the paucity of climate-quality ocean observations. A critical and quantitative evaluation of the importance of subsurface oceanic processes through which sea surface temperature conditions may develop and affect the risk of drought on decadal time scales, for instance, is not yet possible. Further modeling studies are also required to better identify those particular regional features of sea surface temperature anomalies that are most relevant for drought prediction.
We highlight three particular aspects of the relationship between oceans and drought. The first is the link between the seasonal pulse of monsoons and the seasonality of SSTs, and we show that the north-south migration of monsoon rains is strongly tied to the ocean's response to the annual march of the Sun. The second is the link between interannual-to-decadal droughts and anomalous ocean states. It is shown that SST variability enhances drought variability over most land areas within 40°N-40°S, indicating that the more extreme climatic states of precipitation arise partly as a consequence of sea surface temperature variability. We demonstrate this link for two widely studied historical events---- the mid-20th Century droughts over North America including the Dust Bowl and the great Sahelian drying in the later half of the century. A third feature concerns the link between the oceans and drought in a warming climate. Ocean temperatures have risen over the warmest portions of the tropical oceans, likely due to the effect of increasing greenhouse gases. The effect of such sea surface temperature trends on regional rainfall trends during 1977-2006 is discussed.
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This paper shall be cited as:
Hoerling, M. and Schubert, S., (2010). "Oceans and Drought" in Proceedings of OceanObs’09: Sustained Ocean Observations and Information for Society (Vol. 1), Venice, Italy, 21-25 September 2009, Hall, J., Harrison, D.E. & Stammer, D., Eds., ESA Publication WPP-306, doi:10.5270/OceanObs09.pp.22
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