Sensitivity of a GCM Simulation of two Contrasting Indian Monsoons to SST Anomaly Distributions

Summary

 One of the major forcings for the interannual variability of the Asian Summer Monsoon is the Sea Surface Temperature (SST) distribution in the tropical Pacific Ocean. El Niño years are characterized by a negative Southern Oscillation Index (SOI) and decreased monsoon rainfall over India leading to drought conditions. On the other hand, La Nina years are characterized by a positive SOI and generally good monsoon conditions over India.

The monsoon ENSO relation is not a consistent one. The monsoons of 1991 and 1994 are good examples. The spring SOI was the same (−1.3) during both years. However, the All India Summer Monsoon Rainfall (AISMR) was 91.4% of normal in 1991 and 110% in 1994. Though the SOI was same during the spring of both years, the spatial distribution of SSTs was different.

In the present study, the impacts of different SST distributions in the tropical Pacific Ocean, on the monsoons of 1991 and 1994 have been examined, to assess the UKMO-unified model’s sensitivity of SST. It is observed that the simulated monsoon was much stronger in 1994 than in 1991, in terms of precipitation and circulation. The wind and the Outgoing Long-wave Radiation (OLR) simulated by the model are compared with NCEP/NCAR reanalyses data, while precipitation is compared with Xie-Arkin merged rainfall data.