Sensitivity of Asian summer monsoon and tropical circulations to 1987 and 1988 sea surface temperature anomalies



In this work, the impact of Sea Surface Temperatures (SSTs) during 1987 and 1988 on the Northern Hemisphere summer mean monsoon is studied using an Atmospheric General Circulation Model. Four model experiments were carried out to assess the relative impacts of observed SST fields of global(El), tropical(E2), tropical Pacific(E3) and tropical central and eastern Pacific(E4) oceans. It was seen that in El, the model captured a number of important observed circulation and precipitation features of 1987 and 1988 monsoons. The upper tropospheric outflow during 1988 was strong over India, Indian ocean and western Australia and the inflow was strong over central and eastern Pacific oceans. During 1987, the upper tropospheric velocity potential minimum over western Pacific shifted east by 20º of its 1988 position. The Tropical Easterly Jet (TEJ) over equatorial Africa was stronger in 1988 than in 1987. The precipitation maximum over western Pacific shifted to central and eastern Pacific during 1987. Over southern India, south west Indian ocean and parts of Sahel, there was increased rainfall during 1988 compared to 1987. However, it was less over northeast India in 1988 than in 1987. In experiments E2, E3 and E4, the large-scale structures of the upper tropospheric velocity potential difference between 1987 and 1988 were similar to those in El. However, some important changes were seen in E3 and E4. The divergent flow during 1988 was weaker over Indian ocean and monsoon region with the center of velocity potential differences shifted to the east in E3 and E4 than in El. Consistent changes were seen in the wind fields also. The TEJ was weaker during both the years 1987 and 1988 in E3 and E4 than in El.

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