Two Types of El Nino Events: Cold Tongue El Nino and Warm Pool El Nino

In this study, two types of El Nino events are classified based on spatial patterns of the sea surface temperature (SST) anomaly. One is the cold tongue (CT) El Nino, which can be regarded as the conventional El Nino, and the other the warm pool (WP) El Nino. The CT El Nino is characterized by relatively large SST anomalies in the Nino-3 region (5 degrees S-5 degrees N, 150 degrees-90 degrees W), while the WP El Nino is associated with SST anomalies mostly confined to the Nino-4 region (5 degrees S-5 degrees N, 160 degrees E-150 degrees W). In addition, spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of El Nino events. Furthermore, the difference in the transition mechanism between the 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 owing to the spatial structure of SST anomaly; as a result, it cannot trigger a cold event. It is also demonstrated that zonal advective feedback (i.e., zonal advection of mean SST by anomalous zonal currents) plays a crucial role in the development of a decaying SST anomaly associated with the WP El Nino, while thermocline feedback is a key process during the CT El Nino.