The Effect of the South Pacific Convergence Zone on the Termination of El Nino Events and the Meridional Asymmetry of ENSO

During large El Nino events the westerly wind response to the eastern equatorial Pacific sea surface temperature anomalies (SSTAs) shifts southward during boreal winter and early spring, reaching latitudes of 5 degrees-7 degrees S. The resulting meridional asymmetry, along with a related seasonal weakening of wind anomalies on the equator are key elements in the termination of strong El Nino events. Using an intermediate complexity atmosphere model it is demonstrated that these features result from a weakening of the climatological wind speeds south of the equator toward the end of the calendar year. The reduced climatological wind speeds, which are associated with the seasonal intensification of the South Pacific convergence zone (SPCZ), lead to anomalous boundary layer Ekman pumping and a reduced surface momentum damping of the combined boundary layer/lower-troposphere surface wind response to El Nino. This allows the associated zonal wind anomalies to shift south of the equator. Furthermore, using a linear shallow-water ocean model it is demonstrated that this southward wind shift plays a prominent role in changing zonal mean equatorial heat content and is solely responsible for establishing the meridional asymmetry of thermocline depth in the turnaround (recharge/discharge) phase of ENSO. This result calls into question the sole role of oceanic Rossby waves in the phase synchronized termination of El Nino events and suggests that the development of a realistic climatological SPCZ in December-February/March-May (DJF/MAM) is one of the key factors in the seasonal termination of strong El Nino events.