Long-Term Variations of Broad-Scale Asian Summer Monsoon Circulation and Possible Causes

The widely applied Webster-Yang index (WYI), which measures the broad-scale dynamical features of the Asian summer monsoon (ASM), has experienced robust interannual and interdecadal variations and a decreasing tendency, with apparent shifts in 1972. The WYI exhibits moderate variability and frequent positive phases before 1972, intensive interannual variability during 1972―98, and an obvious decreasing tendency and mainly negative phase afterward. The vertical shear easterly anomalies over the tropics/subtropics and the anomalous vertical shear anticyclonic circulation over Eurasia (Eu) are the background for the decreasing WYI, associated with reduced summer precipitation around the Bay of Bengal and Sumatra. On interdecadal time scales, the negative (positive) Atlantic multidecadal oscillation (AMO) is characterized by cooling (warming) in Eurasian tropospheric temperature (TT) via the North Atlantic Oscillation. Global warming manipulates the increasing tendency and the interannual variability of TT over the Indian Ocean (IO). The mutual effects of AMO on Eurasian TT and global warming on Indian Ocean TT correspond to the similar decreasing tendency and interdecadal shift of the difference in TT between Eurasia and the Indian Ocean (EuTT ― IOTT) with those of the ASM. Thus, the AMO and global warming seem to cause the interdecadal variability of ASM. Although the interannual relationship between Niño-3 SST and ASM weakens recently as a result of the weakening tendency of ASM, the Niño-3 SST still plays an important role in ASM variability via EuTT ― IOTT anomalies. In addition, the WYI in the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP―NCAR) reanalysis shows a larger decreasing tendency for 1999―2010 compared to other reanalysis products, a plausible reason for the inconsistent variations between land―sea thermal contrast and the NCEP―NCAR WYI during that period.