A high-latitude North Pacific trigger for extreme El Niño events — Australian Meteorological and Oceanographic Society

A high-latitude North Pacific trigger for extreme El Niño events (#165)

Hyunsu Jo 1 , Wenju Cai 1 , Jae-Heung Park 2 , Yoo-Geun Ham 3
  1. Oceans and Atmosphere, CSIRO, Melbourne, VIC, Australia
  2. IPRC, Honolulu, Hawaii
  3. Chonnam National University, Gwangju, South Korea

Extreme El Niño events, as occurred in 1982/83, 1997/98 and 2015/16, cause massive socio-economic impacts worldwide. However, debates continue as to how extreme El Niño events are generated. Previous studies have focused on potential tropical causes such as westerly wind bursts, or triggers of a southern hemisphere origin, such as southerly jets that convert to westerly at the equator. Here we show that unusually strong cyclonic circulation anomalies develop in the high-latitude Northeastern Pacific 14 months prior to an extreme El Niño peak phase. The cyclonic circulation anomalies move equatorward through positive ocean-atmosphere feedback loops. The associated anomalous southwesterly winds to the south drive warm sea surface temperature (SST) anomalies through a wind-evaporation effect, which in turn force the southward moving cyclonic anomalies over the subtropical central North Pacific. As a result, SST and ocean heat content over the central tropical Pacific increase during the ensuing seasons, conducive to an extreme El Niño event in the following winter. The triggering mechanism is confirmed in a multi-century model experiment, providing a potential long-lead precursor of the extreme El Niño.

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