The Obduction of Equatorial 13°C Water in the Pacific Identified by a Simulated Passive Tracer

Abstract

The obduction of equatorial 13°C Water in the Pacific is investigated using a simulated passive tracer of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). The result shows that the 13°C Water initialized in the region 8°N?8°S, 130°?90°W enters the surface mixed layer in the eastern tropical Pacific, mainly through upwelling near the equator, in the Costa Rica Dome, and along the coast of Peru. Approximately two-thirds of this obduction occurs within 10 years after the 13°C Water being initialized, with the upper portion of the water mass reaching the surface mixed layer in only about a month. The obduction of the 13°C Water helps to maintain a cool sea surface temperature year-round, equivalent to a surface heat flux of about ?6.0 W m?2 averaged over the eastern tropical Pacific (15°S?15°N, 130°W?eastern boundary) for the period of integration (1993?2006). During El Niño years, when the thermocline deepens as a consequence of the easterly wind weakening, the obduction of the 13°C Water is suppressed, and the reduced vertical entrainment generates a warming anomaly of up to 10 W m?2 in the eastern tropical Pacific and in particular along the coast of Peru, providing explanations for the warming of sea surface temperature that cannot be accounted for by local winds alone. The situation is reversed during La Niña years.