Influence of the Indian summer monsoon on ENSO

Historical records (approximately 100 years) of Indian summer monsoon rainfall and El Nino/Southern Oscillation (ENSO) indices show a strong negative correlation. This negative correlation is strongest for east Pacific sea surface temperature anomalies (SSTA) that occur during the months of December through to March, which is about three to six months immediately following the monsoon season (June to September). Based on this correlation, one is tempted to speculate that monsoon variability affects ENSO variability. However, it is well known that ENSO is phase locked to the annual cycle in that the largest SSTA occur at the end of the calendar year. In other words, an ENSO which originated well before the summer monsoon season will have its peak amplitude at the end of the calendar year. The purpose of this study is to explain the impact of the monsoon which has a strong seasonal preference on ENSO which has a life cycle of about 4 years. First, a 50-year atmospheric general-circulation model simulation with climatological SST is examined to determine the tropical Pacific wind-stress anomalies that are associated with a variable monsoon but that are also independent of SST variability in the tropical Pacific. Using simple statistical techniques, it is found that a weak (strong) monsoon results in a weakening (strengthening) of the trade winds over the tropical Pacific. To examine how these 'monsoon-forced wind-stress anomalies' in the tropical Pacific affect ENSO, simulations were made with a simple coupled model that does not include the effects of a variable monsoon. The effects of the monsoon are then added in the coupled model by either specifying the strength of the monsoon or by parametrizing the strength of the monsoon in terms of the coupled-model simulated SSTA in the east Pacific. Based on these coupled simulations, a variable monsoon enhances the ENSO variability, particularly three to six months after the monsoon ends, and can also serve as a trigger mechanism for ENSO. It is found that an ongoing warm (cold) ENSO event is made even warmer (colder) by a weak (strong) monsoon. Similarly, warm (cold) events are weakened by a strong (weak) monsoon. These results also reproduce the observed lag/lead ENSO-monsoon relation where the maximum negative correlation between the monsoon and the SSTA in the east Pacific occurs 3-6 months after the monsoon season.