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

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 Nino. 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 Nino 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 Nino, no such buildup was observed prior to the 1986–1987 event.