Resurfacing of South Pacific Tropical Water in the Equatorial Pacific and Its Variability Associated with ENSO

Analysis of results from a simulated passive tracer confirms the resurfacing of South Pacific Tropical Water in the equatorial Pacific. Over the period of integration (1993–2011), both the volume and barycenter of the South Pacific Tropical Water that resurfaces in the equatorial Pacific are tightly linked to El Niño–Southern Oscillation (ENSO), with their correlation with the Niño-3.4 index reaching −0.79 and 0.84, respectively. Their correlation (−0.75 and 0.85) with the sea surface salinity index, Niño-S34.8, is also high. Of particular interest is that both the volume and barycenter of the resurfacing South Pacific Tropical Water peak earlier than the ENSO indices by about 3 months. On the interannual time scale, the resurfacing of South Pacific Tropical Water may modulate the sea surface salinity in the equatorial Pacific at a rate equivalent to as much as 25% of the surface freshwater flux. The results suggest that the resurfacing of South Pacific Tropical Water directly contributes to the sea surface salinity variability in the equatorial Pacific and potentially plays a role in ENSO evolution.

Subduction of South Pacific Tropical Water and Its Equatorward Pathways as Shown by a Simulated Passive Tracer*

This study investigates the subduction of South Pacific Tropical Water (SPTW) and its equatorward pathways using a simulated passive tracer of the consortium Estimating the Circulation & Climate of the Ocean (ECCO). The results show that approximately 5.8 Sv (1 Sv ≡ 106 m3 s−1) of the SPTW is formed in the subtropical South Pacific Ocean within the density range between 24.0 and 25.0 kg m−3, of which about 87% is due to vertical pumping and 13% is due to lateral induction, comparing reasonably well with estimates from climatological data. Once subducted, most SPTW spreads in the subtropical South Pacific. Because of the presence of mixing, some portion of the water is transformed, and its tracer-weighted density steadily increases from an initial value of 24.4 to nearly 25.0 kg m−3 after 13 years of integration. Approximately 42% of the water makes its way into the equatorial Pacific, either through the western boundary or interior pathway. The two equatorward pathways are essentially of equal importance. A large (~70%) portion of the SPTW entering the equatorial region resurfaces in the central equatorial Pacific. The potential impacts of the resurfacing SPTW on the equatorial thermocline and surface stratification are discussed.