Influences of Indian Ocean interannual variability on different stages of El Niño: A FOAM1.5 model approach

Abstract

Both the tropical Indian and tropical Pacific Oceans are active atmosphere-ocean interactive regions with robust interannual variability, which also constitutes a linkage between the two basins in the mode of variability. Using a global atmosphere-ocean coupled model, we conducted two experiments (CTRL and PC) to explore the contributions of Indian Ocean interannual sea surface temperature (SST) modes to the occurrence of El Nio events. The results show that interannual variability of the SST in the Indian Ocean induces a rapid growth of El Nio events during the boreal autumn in an El Nio developing year. However, it weakens El Nio events or even promotes cold phase conversions in an El Nio decaying year. Therefore, the entire period of the El Nio is shortened by the interannual variations of the Indian Ocean SST. Specifically, during the El Nio developing years, the positive Indian Ocean Dipole (IOD) events force an anomalous Walker circulation, which then enhances the existing westerly wind anomalies over the west Pacific. This will cause a warmer El Nio event, with some modulations by ocean advection and oceanic Rossby and Kelvin waves. However, with the onset of the South Asian monsoon, the Indian Ocean Basin (IOB) warming SST anomalies excite low level easterly wind anomalies over the west tropical Pacific during the El Nio decaying years. As a result, the El Nio event is prompted to change from a warm phase to a cold phase. At the same time, an associated atmospheric anticyclone anomaly appears and leads to a decreasing precipitation anomaly over the northwest Pacific. In summary, with remote forcing in the atmospheric circulation, the IOD mode usually affects the El Nio during the developing years, whereas the IOB mode affects the El Nio during the decaying years.