Observations of Interannual Equatorial Fresh Water Jets in the Western Equatorial Pacific

Abstract:

Using upper ocean monthly salinity and temperature data from the moored TAO/TRITON array in the western equatorial Pacific since the late 1990s, we found, consistent with previous work, that the region experiences large interannual fluctuations in salinity. On the equator at 147 degrees E, 156 degrees E and 165 degrees E the interannual sea surface salinity (SSS) has peak to peak amplitudes that often exceed 1 psu. The salinity variability, which matches well the comparatively short record of overlapping SSS estimated by the Aquarius satellite, changes little over the top 50 m of the water column. Beneath this mixed layer depth the amplitude of the salinity variability steadily decreases over the remaining part of the order 100 m thick isothermal layer. Corresponding hydrostatic estimates of dynamic height over the isothermal layer lead to interannual sea level variability of only a few cm amplitude. However, the sea level due to the fresher water is associated geostrophically with a strong fresh water zonal equatorial interannual jet that at 156 degrees E has an amplitude of about 27 cm/s. Along-track altimeter data give a geostrophic equatorial zonal interannual flow that agrees well with this, suggesting that the near-surface interannual flow in the region is due to the shallow fresh jet. A zonal momentum balance indicates that this jet is mostly due to zonal wind stress forcing. The fresh water jet is maximally correlated with the Nino3.4 El Nino index when the jet leads by 3 months.