Mean circulation of the upper layers of the western equatorial Pacific Ocean

Abstract

Hydrographic (0–1000 dbar) and direct current measurements (0–600 dbar) along 165°E (20°S‐10°N) between January 1984 and July 1991 are used to investigate the mean circulation and its relationships with the distributions of salinity and potential vorticity on isopycnal surfaces. Less well documented mean sections along 142°E and 137°E are used to complement the 165°E analysis. The Equatorial Undercurrent is centered on the equator at 165°E and on 0.5°N at 142°E. No variation in its transport (15.0 × 106 m3 s−1) is found between those longitudes. We find indication of the Equatorial Undercurrent at 137°E‐0.75°N in the geostrophic field. The northern and southern Subsurface Countercurrents are clearly identified by extrema of eastward velocity at 165°E around 3°N and 3°S (250 dbar). No evidence of a southern Subsurface Countercurrent is found at 142°E. At 137°E the northern Subsurface Countercurrent is not characterized by a local extrema of eastward velocity: the North Equatorial Countercurrent seems to extend from the surface to 400 dbar with a southward shift of its core. Analysis of the seasonal variability at 165°E indicates that the Equatorial Undercurrent transport increases by a factor 2 between January (10.7 × 106 m3 s−1) and July (21.5 × 106 m3 s−1) and the Equatorial Intermediate Current transport is halved (6.3 × 106 m3 s−1 in January, 3.5 × 106 m3 s−1 in July). In contrast, the transport of the Subsurface Countercurrents does not vary substantially between those two months. The meridional distributions of salinity and potential vorticity show that the axes of the main eastward currents are associated with strong meridional property gradients, not with property extrema. The eastward currents thus represent a barrier to the northward extension of the high salinity Tropical Water. Relatively weak meridional gradients of salinity and potential vorticity are observed in the westward directed South Equatorial Current and Equatorial Intermediate Current.