On Equatorial Waves and El Niño. I. Influence of Initial States on Wave-induced Currents and Warming

Abstract

We present results from three numerical model experiments designed to study the thermal and hydrodynamics changes associated with downwelling Kelvin wave passage and east coastal reflection along and near the equator. The model employs primitive equation dynamics in two active layers and a full thermodynamics equation, so that sea surface temperature, thermocline displacement and sea level are each independently predicted. Wind and thermal finding are used. The surface layer is a slab mixed layer using Kraus and Turner-style bulk physics. Kelvin waves are excited by introducing a westerly wind anomaly in the western part of the basin, and the temperature and current changes caused by the waves are studied as the wave fronts propagate through the circulation forced by three different mean wind fields: no mean winds, southerly men winds and easterly mean winds. The wave-induced changes depend strongly on the conditions that prevail when the waves are forced. Anomalous advection of the existing SST field is the primary SST change mechanism. The two internal Kelvin-wave modes allowed by the model sometimes induce comparable temperature changes near the east coast MA sometimes the effect of one mode substantially dominates that of the other. The shear mode wave does not always propagate to the east coast; it can be destroyed by nonlinear effects associated with the meridional circulation along the equator. Temperature changes near the east coast, similar in magnitude to those observed in the early stages of El Niño events, are caused in the mean southerly wind case, but no broad westward tongue appears latter on. The implications of these results on existing models of El Niño and for future model studies are examined.