Modulation of ENSO on the annual cycle in the equatorial eastern Pacific

Abstract

Abstract The annual cycle of sea surface temperature (SST) in the equatorial eastern Pacific (EEP) shows a distinctive change between the warm and cold phases of the El Niño-Southern Oscillation (ENSO), with a much-enhanced cold tongue in boreal fall during La Niña than during El Niño. The specific dynamic and thermodynamic processes through which ENSO modulates the annual cycle during the various phase of the annual cycle were investigated through an analytical heat budget model. It is found that the modulation of ENSO is primarily through ocean dynamic heating processes, including anomalous temperature advection by mean vertical and meridional currents and mean temperature advection by anomalous zonal and vertical currents. While El Niño induced increase of column water vapor causes the increase of downward longwave radiation and thus the weakening of the cold tongue, an enhanced surface latent heat flux due to the increase of surface wind speed tends to strengthen the cold tongue and thus oppose the ocean dynamic effect. Such ENSO impacts persist from boreal fall to boreal winter and weaken substantially during the subsequent spring.