Large‐scale meridional transport in the tropical Pacific Ocean during the 1986–1987 El Niño from Geosat

Abstract

Geosat altimeter data are used to obtain the first detailed basin‐wide measurement of the meridional transport of warm surface water in the tropical Pacific during El Niño. Using a combination of crossover and collinear difference techniques, continuous sea level time series are constructed on a 2°×1° grid covering the Pacific between 20°N and 20°S for the 4‐year period 1985–1989. Zonal integrations of these data are performed over three latitude bands to examine large‐scale sea level changes. Comparison with tide gauge data suggests that these zonal averages have an accuracy of better than 1 cm. The Geosat analyses show a clear pattern of water exchange involving principally the equatorial and north equatorial regions. From the onset of the warm event in late 1986 to the mature phase in mid‐1987, mean sea level in the equatorial region dropped nearly 5 cm while simultaneously in the north equatorial region it increased by about the same amount. These anomalies, equivalent to 2×104 m3 of upper layer water, persisted for nearly 2 years before both regions returned to normal. A similar pattern and amplitude of north‐south water exchange is also observed on the seasonal time scale, consistent with annual variation of the wind stress curl. Water moves northward across 8°N during boreal winter and southward during summer with a net transport of approximately 30 Sv. Thus, in terms of meridional transport, the 1986–1987 El Niño is seen as a low‐frequency modulation of the normal seasonal cycle. In contrast to the widely held view that a surplus of equatorial water is required prior to El Niño, no such buildup was observed prior to the 1986–1987 event.