Interactive feedback between ENSO and the Indian Ocean

A feedback process of the Indian Ocean SST on ENSO is investigated by using observed data and atmospheric GCM. It is suggested that warming in the Indian Ocean produces an easterly wind stress anomaly over Indonesia and the western edge of the Pacific during the mature phase of El Nino. The anomalous easterly wind in the western Pacific during El Nino helps a rapid termination of El Nino and a fast transition to La Nina by generating upwelling Kelvin waves. Thus, warming in the Indian Ocean, which is a part of the El Nino signal, operates as a negative feedback mechanism to ENSO. This Indian Ocean feedback appears to operate mostly for relatively strong El Ninos and results in a La Nina one year after the mature phase of the El Nino. This 1-yr period of phase transition implies a possible role of Indian Ocean-ENSO coupling in the biennial tendency of the ENSO. Atmospheric GCM experiments show that Indian Ocean SST forcing is mostly responsible for the easterly wind anomalies in the western Pacific.