FY 1991

Evolution of the western Pacific Ocean during the 1986–1987 El Niño

Toole, J., T. Delcroix, G. Eldin, E. Firing, M. Francis, C. Henin, S. Jiang, L. Mangum, R. Millard, J. Picaut, S. Pu, M. Radenac, Z. Wang, and E. Zou

In Air-Sea Interaction in Tropical Western Pacific, Proceedings, US-PRC International TOGA Symposium, China Ocean Press, Beijing, 1988, 79–90 (1990)

A series of 12 meridional transects along longitude 165°E in the western equatorial Pacific Ocean were made between mid-1986 and mid-1988: a time interval spanning an El Niño/Southern Oscillation (ENSO) event. Data collected on these cruises provide a detailed (albeit temporally sparse) view of the oceanic changes which occur in the western Pacific during an ENSO event. The present work focuses on the evolution of the upper ocean thermohaline and zonal velocity fields as revealed by high resolution hydrographic casts and direct near surface velocity measurements. Onset of the El Niño was characterized by anomalous eastward transport of warm surface waters equatorward of 10° latitude (where anomalies are relative to a mean state defined by historical hydrographic data and recent observations during non-ENSO periods). The thermocline in the western Pacific shoaled in response to this export of surface water; near-equator surface dynamic height relative to 10hPa at event peak was 20 dyn cm below historical non-El Niño levels. Vertical displacements of the water column which resulted in the dynamic height changes were confined to the upper 300-400 m of the ocean. Later in time, strong westward transport anomalies were observed and by mid-1988, sea level had recovered to its historical mean level.