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Abstract

In this study, two types of El Nino events are classified based on spatial patterns of sea-surface temperature (SST) anomaly. One is cold tongue (CT) El Nino, which can be regarded as the conventional El Nino, and the other is Warm Pool (WP) El Nino. The CT El Nino is characterized by relatively large SST anomalies in the NINO3 region (5S-5N, 150-90W), while the WP El Nino is associated with SST anomalies mostly confined to the NINO4 region (5S-5N, 160E-150W). In addition, spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of El Nino events. Furthermore, difference in the transition mechanism between two types of El Nino is clearly identified. That is, the discharge process of the equatorial heat content associated with the WP El Nino is not efficient due to the spatial structure of SST anomaly; as a result, it cannot trigger a cold event. It is also demonstrated that the zonal advective feedback (i.e., zonal advection of mean SST by anomalous zonal current) plays a crucial role in the development of a decaying SST anomaly associated with the WP El Nino, while the thermocline feedback is a key process during the CT El Nino.