Distinguishing Characteristics of Spring and Summer Onset El Niño Events

Abstract

AbstractEl Niño events can be classified into two categories according to the onset time: the spring (SP) El Niño with onset time from April to June and the summer (SU) El Niño with onset time from July to October. The SP El Niño is a basin-scale phenomenon and is closer to the conventional ENSO. It goes through the earlier and stronger heat build-up process, and the earlier occurrence of westerlies in the equatorial Pacific, which can partly explain its earlier onset time. For SU El Niño, in contrast, the anomalous signals, such as SSTAs, zonal wind anomalies, and subsurface variations, are much weaker, which can be attributed to the weaker accumulation of warm water and shorter duration of positive Bjerknes feedback. During its peak phase, anomalous southeasterlies over the eastern Pacific enhance the wind–evaporation–SST (WES) feedback and impede the development of positive SSTAs there, and then lead to a west shift of SSTA center. Recharge/discharge processes exist in both types of events but are weaker in the SU type, which may be caused by the lack of meridional Sverdrup transports as a result of weak zonal wind anomalies. A heat budget analysis demonstrates that the relative importance of thermocline (TH) and zonal advective (ZA) feedbacks in SP and SU El Niño is different. In SP El Niño, the TH feedback is dominant compared to ZA feedback in both the GODAS and SODA datasets. In SU El Niño, however, these two terms are equally important in GODAS, but not in the SODA dataset.