Interannual Variability of the Subsurface High Salinity Tongue South of the Equator at 165°E

Abstract

A 14-yr time series of salinity at thermocline level was constructed from repeated meridional CTD sections (averaging about 110 days apart) spanning the equator along 165°E during 1984?97. A tongue of high salinity water extends along the isopycnal σt = 24.5 from its surface outcrop in the southeast Pacific to 175-m depth near 5°?10°S along the section at 165°E. In the west, the tongue moves vertically with the thermocline, mostly as part of the ENSO cycle, while salinity in the tongue varied interannually over a range of 0.4 psu. Most of this variability was due to zonal advection along the isopycnal tongue, with similar changes observed at other longitudes in the west and central Pacific. Part of the interannual salinity signal can be attributed to changes in the near-zonal flow of the South Equatorial Current associated with El Niño, but a general 0.3 psu rise occurring during the 1990s (probably starting even earlier) was not apparently consistent with this explanation. An attempt was made to trace the source of these changes to surface fluxes at the outcrop, where the variation of evaporation and precipitation suggested salinity anomalies of the same magnitude as the subsurface changes. However, the implied surface salinity changes were of the wrong sign to explain the subsequent downstream subsurface variability, and therefore present observations do not demonstrate any influence of subduction of surface properties on salinity in the southern high salinity tongue.