Tropical Impacts of SST Forcing. A Case Study for 1987 versus 1988

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  • 1 NASA/Goddard Institute for Space Studies and Columbia University, New York, New York
  • | 2 Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison. Wisconsin
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Abstract

The response of the NASA/Goddard Institute for Space Studies GCM to large tropical sea surface temperature (SST) anomalies is investigated by evaluating model simulations of the particularly contrasting summer monsoon seasons 1987 and 1988. These years are representative of the warm and cold phases, respectively, of a recent ENSO event. An ensemble averaging the results of three simulations was considered for each season, using monthly mean observed SST anomalies for June–August 1987 and 1988 as lower boundary forcing. Consistent with the ECMWF-analyzed winds, the simulators based on 1988 as compared to 1987 SST exhibit stronger upper-tropospheric irrational circulation between the monsoon regions and the Southern Hemispheric subtropical anticyclones, a stronger Pacific Walker cell and a weaker subtropical westerly jet over the South Pacific. In the same vein, the modeled precipitation, indicating a more northerly position of the Pacific ITCZ in 1988 compared with 1987, is supported by satellite observations of outgoing longwave radiation and highly reflective clouds.

Abstract

The response of the NASA/Goddard Institute for Space Studies GCM to large tropical sea surface temperature (SST) anomalies is investigated by evaluating model simulations of the particularly contrasting summer monsoon seasons 1987 and 1988. These years are representative of the warm and cold phases, respectively, of a recent ENSO event. An ensemble averaging the results of three simulations was considered for each season, using monthly mean observed SST anomalies for June–August 1987 and 1988 as lower boundary forcing. Consistent with the ECMWF-analyzed winds, the simulators based on 1988 as compared to 1987 SST exhibit stronger upper-tropospheric irrational circulation between the monsoon regions and the Southern Hemispheric subtropical anticyclones, a stronger Pacific Walker cell and a weaker subtropical westerly jet over the South Pacific. In the same vein, the modeled precipitation, indicating a more northerly position of the Pacific ITCZ in 1988 compared with 1987, is supported by satellite observations of outgoing longwave radiation and highly reflective clouds.

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