Variability of the Tropical Atlantic and Pacific SSS Minimum Zones in the Last Three Decades and Their Relations to the ITCZ and NECC

Abstract:

This study focuses on the time-space variability of the low Sea Surface Salinity (SSS) waters located from the West to the East within about 2°N-12°N in the Atlantic and Pacific oceans. The analysis is based on a combination of in situ SSS observations collected in the last three decades from voluntary observing ships, TAO/TRITON and PIRATA moorings, Argo floats and (few) CTD profiles. We show that the mean position of the Atlantic and Pacific low SSS waters is tightly related to the local minimum in Evaporation minus Precipitation (E-P) budget linked to the Inter Tropical Convergence Zones (ITCZ) and to salt transport by the eastward flowing North Equatorial Counter Current (NECC). We also show via EOF analyses that the meridional position of this SSS minimum varies both at seasonal time scale, with a northernmost position in boreal summer, and at interannual time scale in relation with ENSO and the Atlantic meridional mode, with however subtle differences in timing between the western, central and eastern basins. The role of the ITCZ-related E-P budget and NECC-related salt advection in these seasonal and interannual changes is examined. We further document the long-term meridional migration of these low SSS waters in the last three decades and discuss whether or not it is consistent with the expected global change effects.