Observations of Interannual Equatorial Freshwater Jets in the Western Pacific

Observations of TRITON moored array salinity and temperature in the very wet western equatorial Pacific at 137 degrees E, 147 degrees E, and 156 degrees E since the late 1990s reveal the importance of rainfall to the interannual flow and El Nino- Southern Oscillation ( ENSO) dynamics. Past work has shown that in this region a fresher surface isohaline layer is embedded in a thicker isothermal layer. Array estimates of dynamic height relative to the 50-70- m isothermal layer depth ( ILD) indicate a near- surface salinity- driven contribution to the monthly sea level anomaly zeta'F that is uncorrelated with, and smaller than, monthly anomalous sea surface height ( SSH) estimated from altimeter data. Despite the smaller size of zeta' F, its meridional gradient dominates the total sea level meridional gradient. Thus, the corresponding shallow equatorially trapped interannual freshwater jet u'F dominates the near- surface zonal interannual flow. This jetlike flow has a meridional scale of only about 2 degrees- 3 degrees of latitude, an amplitude of 23 cm s 21, and is associated with the zonal back and forth displacement of the western equatorial warm/ fresh pool that is fundamental to El Nino. The jet is not directly forced by the interannual freshwater surface flux but rather by wind stress anomalies that are mostly east of the warm/ fresh pool edge during La Nina and mostly west of it during El Nino. A conceptual coupled ocean- atmosphere instability model is proposed to understand these observations. Calculations show that Aquarius satellite sea surface salinity ( SSS) data match the TRITON in situ data well and that the satellite SSS can be used to estimate zeta 0 F, and hence u0 F, geostrophically.