Two key parameters for the El Nio continuum: zonal wind anomalies and Western Pacific subsurface potential temperature

摘要

Different types of El Nio (EN) events have recently been discussed. Based on NCEP-NOAA reanalysis data this analysis explores a number of key parameters that cause a range of EN types over the period 1980-2013. EN events are divided into three types depending on the spatial and temporal evolution of the sea surface temperature anomalies (SSTA): Central Pacific (CPEN), Eastern Pacific (EPEN), and Hybrid (HBEN). We find that EN is a continuous spectrum of events with CPEN and EPEN as the end members. This spectrum mainly depends on two key parameters: the 130A degrees E-160A degrees E Western Pacific 5-250 m subsurface oceanic potential temperature anomaly about 1 year before the EN peak (typically January and February), and the 140A degrees E-160A degrees W cumulative zonal wind anomaly (ZWA) between onset and peak of the EN event. Using these two parameters, about 70 % of the total variance of the maximum SSTA realised in different Nio regions can already be explained up to 6 months before the maximum SSTA occurs. This offers a rather simple potential for ENSO prediction. A necessary condition for the evolution of an EPEN, the Western Pacific is in the recharged state. Strong and sustained westerly wind anomalies in Western Pacific can then trigger a Kelvin wave propagating to the eastern Pacific. Both parameters, potential temperature and zonal wind anomaly, constructively interfere. For a CPEN, these parameters are much less important. Kelvin wave propagation is not involved in the evolution of the event. Instead, the Central Pacific warming is caused locally by a zonal advection feedback and local air-sea interaction as already demonstrated in previous studies. The HBEN occurs when both parameters interfere in different ways: (1) Western Pacific is weakly charged, but strong westerly ZWA are observed that reduce the equatorial upwelling in the Central Pacific while the triggered Kelvin wave is too weak to have a significant effect; (2) Western Pacific is strongly charged but only weak westerly ZWA develop, so that the resulting Kelvin wave cannot fully extend into the eastern-most Pacific.