Importance of mean state in simulating different types of El Nino revealed by SNU coupled GCMs

Recent studies suggest that there are two types of El Nth events, which differ in terms of zonal distribution of sea-surface temperature (SST) anomalies. In this study, we investigate mechanisms in controlling simulation of two-types of El Nino using three different versions of the Seoul National University (SNU) air-sea coupled general circulation models. The occurrences of two types of El Nino are related to the simulated climatological SST over the eastern Pacific. It is found that a model with relatively less canonical (or frequent Warm Pool or Central Pacific) El Nino occurrence has colder cold tongue in the equatorial central Pacific. Due to the cold mean SST and associated dryness over the eastern Pacific, positive El Nino-related SST anomalies over the eastern Pacific cannot trigger local convection effectively. The eastern Pacific dryness leads to the confinement of anomalous convective activity in the western Pacific, which results in weak canonical El Nino and reduction in canonical El Nino occurrence. Instead, the confined convective activity in the western Pacific can lead to strong SST anomalies over western-central Pacific through local air-sea interaction, which can increase Warm Pool (WP) El Nino occurrence. In addition, we found that mean equatorial thermocline structure is also related to the occurrences of WP El Nino and canonical El Nino that is, a model with deep thermocline depth simulates less WP El Nino occurrence, consistent with Yeh et al. (2009). (C) 2013 Elsevier Ltd. All rights reserved.