Response of the Asian summer monsoon to changes in El Nino properties

Diagnostics from observed precipitation and National Centers for Environmental Prediction-National Center for Atmospheric Research re-analysis products reveal that after the 1976-77 climate shift in the Pacific there was a dramatic change in the response of the Indian summer monsoon (ISM) to El Nino, particularly during the months of July and August. Based on 1950-75 (PRE76) and 1977-2001 (POST76) El Nino composites: the western North Pacific monsoon (WNPM) was stronger than normal in both periods; the ISM was weaker than normal during the entire monsoon season in PRE76, but in POST76 was weaker only during the onset and withdrawal phases. In terms of observed sea surface temperature (SST) during July-August, the major differences between the two periods are the presence of cold SST anomalies over the Indo-Pacific warm pool and the intensity of warm SST anomalies in the central Pacific in POST76. The effect of these differences on the ISM is investigated in a suite of experiments with an Atmospheric General Circulation Model (AGCM) that has a realistic monsoon precipitation climatology. Separate ten-member ensemble simulations with the AGCM were conducted for PRE76 and POST76 El Nino events with SST anomalies inserted as follows: (i) tropical Indo-Pacific (TIP), (ii) tropical Pacific only (TPO), and (iii) tropical Indian Ocean only (TIO). Qualitatively, TPO solutions reproduce the observed differences in the monsoon response in both periods. Specifically, during July-August of POST76 the cold SST anomalies in conjunction with remote subsidence suppress precipitation (3-5 mm day(-1)) over the maritime continent and equatorial central Indian Ocean. Inclusion of Indian Ocean SST anomalies in the TIP runs further suppresses precipitation over the entire equatorial Indian Ocean. The low-level anticyclonic circulation anomalies that develop as a Rossby-wave response to these convective anomalies increase the south-westerlies over the northern Indian Ocean, and favour a stronger ISM and WNPM. During PRE76 the non-occurrence of cold SST anomalies over the Indo-Pacific warm pool reinforces El Nino's suppression on the ISM. In contrast, TIO solutions show a reduced ISM during July-August of POST76; the solutions, however, show a significant effect on the WNPM during both PRE76 and POST76 periods. It is argued that SSTs over the entire tropical Indo-Pacific region need to be considered to understand the El Nino Southern Oscillation-monsoon linkage, and to make predictions of rainfall over India and the western North Pacific.