Sensitivity of the Asian summer monsoon to aspects of sea-surface-temperature anomalies in the tropical Pacific Ocean

The response of both the time of onset, and the strength of the Asian summer monsoon, to regional aspects of the sea surface temperature (SST) anomalies in the tropical Pacific Ocean associated with El Nino/Soufhem Oscillation (ENSO) has been investigated through a series of sensitivity experiments with the Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). This paper builds on the work of Ju and Slingo (1995) and on their hypothesis that the relationship between the Asian summer monsoon and ENSO involved the latitudinal position and strength of the tropical convective maximum (TCM) over Indonesia and the west Pacific in the preceding spring. The inference from their results was that the modulation of the TCM might be associated either with changes in the Walker circulation through the influence of the east Pacific SST anomalies, or with changes in the local Hadley circulation associated with the in situ SST anomalies in the west Pacific. The investigation has focused on the particular contrasting years of 1983 and 1984. The experiments described in this paper are designed to isolate the effects of the principal SST anomalies in the east and central Pacific, associated with El Nino/La Nina, from those of opposite sign which develop in the west Pacific as a complementary pattern during the mature phase of El Nino/La Nina. The results of the experimentation suggest that, at least for the test cases of 1983 and 1984, the modulation of the Walker circulation, with implied additional subsidence over the eastern hemisphere, is the dominant mechanism whereby the Asian summer monsoon is weakened during El Nino years. However, the late onset during El Nino years may also be associated with the complementary cord SST anomalies in the west Pacific which delay the northwards transition of the TCM. During La Nina, the modulation of the Walker circulation appears not to be the controlling factor which determines the stronger monsoons. The UGCM results suggest that the complementary warm SST anomalies in the west Pacific enhance the TCM, and it is this in situ response by the TCM that leads to an early onset and stronger monsoon. The importance of warm anomalies in the west Pacific in the development of a strong monsoon has been investigated further through a case-study of the 1994 season. The year 1994 was an El Nino year in which the monsoon was unexpectedly active, but which was also marked by warmer than normal SSTs in the west Pacific. The sensitivity experiments have also elucidated the role of El Nino in influencing the precursory signature of stronger subtropical westerlies over India and south-east Asia during the winter and spring preceding weak monsoons. The results suggested that the equatorwards shift of the subtropical jet is a remote response to the warm SST anomalies in the central and east Pacific associated with El Nino.