Wind stress and ENSO dynamics

Abstract

Recent studies indicate that coupled general circulation models (GCM) display mixed success in simulating the properties of the El Ni\~{n}o/Southern Oscillation (ENSO). Specifically, aggregate runs of various GCMs typically show spatial and temporal disparities compared to observations. We turn to the Zebiak-Cane model of intermediate complexity for the tropical Pacific ocean to study the response of the sea surface temperature (SST) and inter-annual variability of ENSO under different wind stress conditions. Spatial variation is modeled after the meridional structure of the zonal wind stress based on TAO/TRITON measurements, and furthermore on reconstructed Pliocene conditions from the HadCM3 PlioMIP Exp 2 wind data. Temporal variations are introduced with a simple seasonal cycle. Among all conditions simulated, only very weak trade winds result in a "permanent" \EN with weak SST gradients and no variability. A cold tongue (CT) structure is always present when the wind is above a minimum critical value, although the oscillations vanish if the wind crosses a threshold maximum value. Apart from these extremes, periods of $3\sim4$ years are found with $\Delta SST$ amplitudes of 5\degrees $\sim$ 9\degrees.