Mechanisms of Controlling of Upper Layer Salinity Variability in the Pacific Ocean

The mechanisms that control upper layer salinity variability in the Pacific Ocean are investigated using a high- resolution (1/4° x 1/4°) ocean model. The time derivative of upper layer salinity (∂ S/∂ t) from 21 years of model output (1984-2004) is correlated with specific surface salinity budget terms, namely, evaporation, precipitation, evaporation minus precipitation (E-P) and surface salinity advection on spatial scales of 1° and on separate monthly and seasonal scales. Of the four salinity budget terms considered here, surface salt advection has the highest correlation with ∂ S/∂ t at monthly timescales. However, in a narrow equatorial band in the eastern Pacific (150°W-80°W), salinity advection has no significant correlation with ∂ S/∂ t , suggesting that other physical processes such as vertical mixing might be important in this area. E-P is positively correlated with ∂ S/∂ t over most areas of the Pacific Ocean's subtropical gyres. There is no significant correlation between E-P and ∂ S/∂ t in the equatorial Pacific with the exception of a small area in the eastern equatorial Pacific where the correlation is negative. The spatial distribution of correlation of ∂ S/∂ t with negative precipitation generally follows that of E-P. Evaporation, on the other hand, has a very strong correlation with ∂ S/∂ t (>0.6) off the east coast of Japan along the Kuroshio Current and continues further north into the North Pacific Current. On seasonal scales, there are fewer areas of significant correlations with the salinity budget terms compared to the monthly scale correlations, except for evaporation, where the spatial distribution of significant correlation is similar to those on the monthly scale. These calculations help assess the viability of one of the scientific goals of NASA's Aquarius mission that will measure salinity from space, i.e., infer E-P from average monthly salinity retrievals.