Do Sea Surface Winds and Currents in the Equatorial Pacific Ocean Derive the Extreme El Niños?

Abstract

El Niño is the largest natural climate variability event on an interannual time scale occurring in the equatorial Pacific Ocean, and is linked to global climate change. The north equatorial countercurrent (NECC) is considered a significant feature of the tropical Pacific current system due to its location and eastward flow direction. The NECC has been suggested as a current that transports the warmer western Pacific waters to the eastern Pacific to trigger the El Niño event in the equatorial Pacific. We investigated how the movements of zonal wind stress (ZWS) and ocean surface currents (OSC) contribute to the sea surface temperature (SST) changes during the El Niño period. During moderate to severe Central Pacific El Niño events, the continuous flow of the NECC extending from the El Niño monitoring region into the eastern Pacific is rarely observed. In addition, a significant increase in ocean heat content compared to the 27-year climatological normal has been identified at temperatures above 28 °C. Therefore, we propose a hypothesis that the additional heat is supplied from the subsurface source and the warm pool is expanded by the subsurface equatorial countercurrent, known as the Equatorial Undercurrent, rather than by the surface currents. The heated water is expected to contribute to the evolution of El Niño by upwelling to the surface along the equator in a north–south symmetric feature.