Interannual variations of North Equatorial Current transport in the Pacific Ocean during two types of El Niño

Abstract

Interannual variations of Pacific North Equatorial Current (NEC) transport during eastern-Pacific El Ninos (EP-El Ninos) and central-Pacific El Ninos (CP-El Ninos) are investigated by composite analysis with European Centre for Medium-Range Weather Forecast Ocean Analysis/Reanalysis System 3. During EP-El Nino, NEC transport shows significant positive anomalies from the developing to decay phases, with the largest anomalies around the mature phase. During CP-El Nino, however, the NEC transport only shows positive anomalies before the mature phase, with much weaker anomalies than those during EP-El Nino. The NEC transport variations are strongly associated with variations of the tropical gyre and wind forcing in the tropical North Pacific. During EP-El Nino, strong westerly wind anomalies and positive wind stress curl anomalies in the tropical North Pacific induce local upward Ekman pumping and westward-propagating upwelling Rossby waves in the ocean, lowering the sea surface height and generating a cyclonic gyre anomaly in the western tropical Pacific. During CP-El Nino, however, strength of the wind and associated Ekman pumping velocity are very weak. Negative sea surface height and cyclonic flow anomalies are slightly north of those during EP El Nino.