Interannual Variability in Warm Water Volume Transports in the Equatorial Pacific during 1993–99

Abstract

Previous studies have indicated that the volume of warm water (WWV) in the equatorial Pacific is intrinsically related to the dynamics of the El Niño?Southern Oscillation (ENSO) cycle. A gridded subsurface temperature dataset, which incorporates temperature measurements from expendable bathythermograph profiles as well as measurements from the moorings in the Tropical Atmosphere and Ocean Array, was used along with historical hydrography to quantify the variability of the WWV (with temperatures greater than 20°C) within the region 8°S?8°N, 156°E?95°W during 1993?99. These data were also used to estimate geostrophic transports of warm water relative to a level of no motion at 1000 dbar (1 dbar = 104 Pa). Ekman transports were estimated using wind data from gridded observations, assimilating model output, and satellite scatterometer measurements. The results indicate that the buildup of WWV between the weak 1994?95 El Niño event and the onset of the strong 1997?98 El Niño event resulted primarily from an anomalous decrease in the net southward transport (geostrophic plus Ekman) across 8°S in the western and central Pacific combined with an anomalous increase in eastward flow across 156°E. Afterward, beginning in April 1997, WWV in the equatorial region was reduced by about 26% coincident with the 1997?98 El Niño. Approximately half of the warm water lost during this period is accounted for by poleward transport across a wide range of longitudes, while the remaining half is accounted for by vertical water mass transformations. The implications of the results for understanding and modeling the ENSO cycle are discussed.