The March 1997 Westerly Wind Event and the Onset of the 1997/98 El Niño: Understanding the Role of the Atmospheric Response

Abstract

In a previous study, the effect of the March 1997 Westerly Wind Event (WWE) on the evolution of the tropical Pacific Ocean was studied using an ocean general circulation model (GCM). The response was characterized by (i) a cooling of the far western Pacific (?0.8°C), (ii) a rapid eastward displacement of the warm pool (2000 km in a month), and (iii) a weak warming of the central eastern Pacific along the path of the oceanic Kelvin wave, excited by the WWE (?0.5°C). In this study, the atmospheric response to these aspects of the sea surface temperature (SST) response are investigated using an atmospheric GCM forced with the SST anomalies from the ocean-only experiments. The results have demonstrated that the three aspects of the SST anomaly field, generated by the WWE, themselves initiate two types of atmospheric response, both of which favor a rapid growth toward El Niño conditions. First, the eastward displacement of the warm pool, together with the reduction of the east?west SST gradient along the forced oceanic Kelvin wave path, results in a weakening of the trade winds in the central eastern Pacific. Second, the eastward displacement of the warmest water from the western to the central Pacific, giving rise to a cooling in the far western Pacific, induces an eastward shift of convection that consequently promotes the occurrence of further frequent and intense WWEs in the following months. The characteristics of these later WWEs are controlled both by the eastward extension of the warm pool and by the SST gradients established in the far western Pacific. The implications of these results for the onset of the 1997/98 El Niño have been considered, with the conclusion that the intense March WWE strongly contributed to the early onset and rapid growth rate of the 1997/98 El Niño, not only by its direct impact on the ocean, but also by the atmospheric variability induced by the oceanic changes that developed following this event.