A Study On The Interdecadal Changes Of The South Asian Summer Monsoon Precipitation In The Late 1990s

The drying trend over the South Asian Summer Monsoon(SASM)area has been the focus of monsoon rainfall studies during the recent two decades.This paper analyzed the spatial and temporal distribution characteristics of summer monsoon precipitation in South Asia,thus found the inhomogeneous between its interdecadal evolution rules and spatial variations.The research emphasized their rationality of the regional average monsoon precipitation index,therefore evaluated the impact of different tropical oceans on the large-scale circulation around South Asia through simulation experiments.The main conclusions are as follows:(1)Significant interdecadal changes occurred in the SASM rainfall in 2000,characterized by increased rainfall over the northwestern India extending from the Arabian Sea to western Bay of the Bay of Bengal(BOB),and decreased rainfall over the southern South Asia and the western coast of the Indian Peninsula.(2)This paper quantified the dipole pattern of interdecadal precipitation change in South Asia by defining the PR index,and investigated the influence factors of PR through the regression analysis of PR,height field,wind field and OLR field,we got the conclusion that the South Asia High(SAH)had a positive correlation with the PR index during the period from 1979 to 2000,and had a relatively smaller impact on PR after 2000.The OLR of Marine continents was closely related to the PR index from 2001 to 2017,while the correlation was weak before 2000.(3)Numerical experiments CAM4 is used to diagnose the possible effect of global and regional SST changes in different tropical oceans on changes in the SASM.The global SST changes can capture the general features in the large-scale circulation over South Asia,especially the anomalous cyclone that is related to the westerly and easterly wind anomalies present in the southern and northern part in the upper level,respectively.The tropical SST forcing dominates the global SST effect,but with a somewhat weaker overall amplitude.(4)The tropical Pacific/Indian Ocean SST anomalies respectively dominated the Walker/regional Hadley circulation changes,while the sinking anomalies over the southern SASM further got enhanced by the warmer tropical Atlantic SST.Even though the global sea surface temperature(SST)changes can induce the general changes in the SASM circulation,the spatial inhomogeneity of this interdecadal change is beyond the AGCM's scope.