On the low western Pacific sea levels observed prior to strong East Pacific El Niños

Abstract

Abstract. Research, based on both observations and ocean models, has indicated that strong East Pacific El Niños are preceded by the development of unusually low sea levels at the western end of the North Equatorial Trough early in the year. This results in an increase in the strength of the North Equatorial Counter Current which, aided by low sea levels due to the annual Rossby wave, then transports West Pacific Warm Pool water to the central and eastern Pacific. Here, an ocean model is used to investigate the factors affecting sea level prior to the 1982–1983 East Pacific El Niño. The results indicate that during this period the reduction in sea level was caused by Ekman pumping, due to the local winds, acting over a period of many months. The north–south distances involved mean that such upwelling can be connected to the westerly wind phase of Madden–Julian Oscillations. Since the 1980s, the quality and quantity of global wind measurement has steadily improved. So, if the Ekman pumping hypothesis is correct, the better-quality wind data available prior to the 1997–1998 and 2015–2016 East Pacific El Niños should also show unusually large Ekman pumping in the same region, over a similar long period. This is tested and found to be correct. However, a study of the integrated Ekman pumping for the period 1976 to 2015 indicates that in some years similar events did not develop despite a comparable amount of pumping during the first part of the year. The results imply that significant Ekman pumping early in the year is a necessary but not sufficient requirement for the development of a strong East Pacific El Niño.