National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1990

Surface equatorial flow anomalies in the Pacific Ocean during the 1986–87 ENSO using GEOSAT altimeter data

Picaut, J., B. Camusat, T. Delcroix, M.J. McPhaden, and A.J. Busalacchi

In Proceedings of the Western Pacific International Meeting and Workshop on TOGA COARE, ORSTOM, Nouméa, New Caledonia, 24–30 May 1989, 301–309 (1989)

Estimates of surface geostrophic zonal flow in the equatorial Pacific are deduced from the 17-day exact repeat orbit GEOSTAT measurements for the period November 1986–November 1987. This period coincides with the height of the 1986-87 ENSO. Along-track altimeter height anomalies are first smoothed using a combination of linear and nonlinear filters. By combining several tracks in the zonal direction and filtering in time, we are able to obtain low frequency sea surface height at any point of the tropical Pacific. Currents are calculated from the differentiated form of the meridional momentum equation at the equator and from the classic first derivative of the meridional pressure field away from the equator. Comparisons of low frequency near-surface zonal current directly measured from equatorial moorings at 165°E, 140°W and 110°W yield a correlation of 0.83, 0.84, and 0.50 respectively with a mean rms difference of 0.23 m s. Sea level and zonal velocity solutions from a tropical Pacific numerical model are used as proxy data sets in order to quantify errors induced to quantify the geostropic calculation by the GEOSAT space-time sampling. In December 1986, a downwelling equatorial Kelvin wave is generated in the western Pacific and shows up, near the forcing area, as an intense local 1 m s eastward equatorial surface flow anomaly. This Kelvin wave propogates into the eastern equatorial Pacific with a phase speed of about 2.5 m s and is associated with eastward equatorial current anomalies of 0.3–0.5 m s. In February 1987, an upwelling equatorial Kelvin wave is excited near the date line and propogates eastward. This wave, characterized by a westward flow anomaly of 0.3–0.8 m s, reaches the eastern Pacific boundary in March 1987 where it forces apparently an upwelling first meridional mode equatorial Rossby wave. This Rossby wave propogates westward in the ocean interior at about 0.8 m s as a patch of equatorially trapped eastward flow (0.6–0.8 m s maximum) flanked, in both hemispheres, by 0.2–0.4 m s westward flow anomalies which decreased the South Equatorial Current and the North and South Equatorial Countercurrents. The equatorial Rossby wave propogation could be traced sequentially through the eastern, central and western Pacific from April to September 1987. Thus GEOSAT altimeter data indicate that equatorial Kelvin waves and possible eastern reflection as equatorial Rossby waves are an important component of basin scale surface current variability during the 1986–87 ENSO.

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