On the variety of coastal El Niño events

We examine the connection between interannual anomalies of sea surface temperature (SST) in the central and far-eastern equatorial Pacific associated with basin-scale and coastal El Niños. Variations of the SST anomalies in these two regions are largely coherent, meaning coastal El Niños mostly occur together with the commonly studied basin-scale El Niños. Of particular interest for this study though is the understanding of the coastal El Niños that are not accompanied by basin-scale El Niños or that follow basin-scale El Niños. Such coastal El Niños can have catastrophic societal consequences in western South America. We identify seven coastal El Niños during 1979–2017, namely 1983, 1987, 1998, 2008, 2014, 2015, and 2017. These coastal El Niños are driven by different mechanisms. The coastal El Niños in 1983, 1987 and 1998 occurred after basin-scale El Niños. A unique feature of such extreme basin-scale El Niños like in 1982–1983, 1986–1987, and 1997–1998 is an equatorially centered intertropical convergence zone during its decaying phase. As a result, positive SST anomalies persist, and sometimes even strengthen, in the eastern Pacific in the subsequent boreal spring/early-summer, leading to coastal El Niños. The coastal El Niños in 2014 and 2015 on the other hand resulted from westerly wind bursts in the western Pacific that forced downwelling Kelvin waves and a thermocline depression in the far eastern Pacific. The formation of coastal El Niños in 2008 and 2017 were associated with westerly surface wind anomalies in the eastern equatorial Pacific and largely driven by ocean surface heat flux anomalies. These two coastal El Niños occur during the warm phase of the seasonal cycle, so that warm SSTs are amplified and/or the warm season is extended along the west coast of South America. Thus, there is a wide variety of the coastal El Niños in terms of evolution, mechanism, and timing.