National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2001

Equatorial Pacific Ocean horizontal velocity, divergence, and upwelling

Johnson, G.C., M.J. McPhaden, and E. Firing

J. Phys. Oceanogr., 31(3), 839–849, doi: 10.1175/1520-0485(2001)031<0839:EPOHVD>2.0.CO;2 (2001)

Upper-ocean horizontal velocity and divergence were estimated from shipboard observations taken from 1991 to 1999 in the equatorial Pacific between 170°W and 95°W. Mean transports were estimated for the zonal currents at the mean longitude of the sections, 136°W. Mean meridional currents for the entire longitude range included poleward surface flows reaching -0.09 m s−1 in the south and 0.13 m s−1 in the north as well as equatorward flow within the thermocline reaching 0.05 m s−1 in the south and 0.04 m s−1 in the north near 23°C (85 m). Vertical velocity was diagnosed by integrating horizontal divergence estimated for the entire region down from the surface. Equatorial upwelling velocities peaked at 1.9 (±0.9) × 10−5 m s−1 at 50 m. The upwelling transport in the area bounded by 3.6°S-5.2°N, 170°W-95°W was 62 (±18) × 106 m3 s−1 at 50 m. Strong downwelling was apparent within the North Equatorial Countercurrent. An asymmetry in the meridional flows suggested that on the order of 10 × 106 m3 s−1 of thermocline water from the Southern Hemisphere was upwelled at the equator and moved into the Northern Hemisphere as surface water. This interhemispheric exchange path could be part of the route for water from the Southern Hemisphere to supply the Indonesian Throughflow.

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