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Understanding the sensitivity of North American drought to Tropical Pacific SSTs in present and past climates
Sang-Ik Shin, NOAA-CIRES Climate Diagnostics Center, Boulder, CO; and R. S. Webb, P. D. Sardeshmukh, R. J. Oglesby, and J. J. Barsugli
An atmospheric general circulation model coupled to a mixed-layer ocean outside the tropical Pacific basin is used to understand the sensitivity of North American drought to anomalous tropical Pacific SSTs in mid-Holocene (ca. 6,000 years BP) and current climates. In the mid-Holocene simulations, the orbital parameters and atmospheric trace gas concentrations are set to mid-Holocene values. Despite the "global change" resulting from this, the sensitivity of North American drought to tropical Pacific SSTs remains similar to that in the current climate. In both climates, cooler La Niņa-like conditions in the tropical Pacific result in enhanced wind-evaporation-SST (WES) feedbacks in the tropical Atlantic. Their net effect is to produce a northward shift of the North Atlantic ITCZ. This northward shift in turn blocks the moisture supply from the Gulf of Mexico into continental North America, causing drier conditions over North America. Local reduced soil wetness further amplifies the drying. Our simulated decrease of North American precipitation in the mid-Holocene is most pronounced during the growing season, and is more consistent than previous studies with observational paleoclimatic evidence derived from paleovegetation and sedimentary records.
Session 2, Climate Predictions on Seasonal and Interannual Time Scales: 1(parallel with Session 1)
Monday, 10 January 2005, 1:30 PM-5:30 PM
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