Kelvin-Wave-Induced Anomalous Advection and the Onset of Surface Warming in El Niño Events

Abstract

The initial surface warming of the 1982 El Niño event was of quite different timing and pattern from that associated with most El Niño events; strong anomalous warming occurred first in July along the equator and subsequently along the South American coast. We show here that a simple advective model for tropical ocean surface warming can produce anomalous sea-surface temperature (SST) fields like those found in the first few months of the 1982 El Niño. The model physics assumes that the existing SST field is advected by anomalous currents to produce the anomalous warming, and that the anomalous currents are those induced subsequent to the passage of downwelling Kelvin wave front(s). With the initial SST field taken to be that of July 1982, the anomalous eastward currents of the model lead to a satisfactory prediction of the evolution of anomalous SST for several months. Numerical experiments with a fully nonlinear and thermally active ocean model support the physical relevance of the more idealized study. The anomalous horizontal advection model can also account for the initial SST evolution during the more common type of El Niño event. The reason that a similar anomalous current field can produce two such different warming patterns is that the gradients of SST along the equator have strong seasonal variation. If anomalous eastward currents are generated along the equator between February and April, when the climatological zonal SST gradient is small, little equatorial warming will occur and so coastal warming is observed first; this is the case in most El Niño events. But if the same anomalous currents occur later in the year, when there is typically a strong zonal temperature gradient, strong equatorial surface warming will occur prior to coastal warming, as happened in 1982. The pattern of SST changes resulting from remote westerly wind changes in the tropical Pacific thus is very strongly linked to the annual cycle of SST.