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Maes, C., O'Kane, T., and Monselesan, D.P. (16-Dec-16)

Despite recent advances in satellite sensors, it remains great uncertainty in the large-scale spatial variations of upper ocean salinity across seasonal through interannual to decadal timescales. Consonant with both broad-scale surface warming and the amplification of the global hydrological cycle, observed global multidecadal salinity changes typically have focused on the linear response to anthropogenic forcing, but not on salinity variations due to changes in the static stability and or variability due to the intrinsic ocean or internal climate processes. Here, we examine the static stability and spatiotemporal variability of upper ocean salinity across a hierarchy of models and reanalyses. In particular, we partition the variance into time bands via application of singular spectral analysis, considering sea surface salinity (SSS), the Brunt Vaisala frequency (N2), and ocean salinity stratification (OSS) in terms of the stabilizing effect due to the haline part of N2 over the upper 500m. We identify regions of significant coherent SSS variability, either intrinsic to the ocean or in response to the inter-annually varying atmosphere. In the tropics, the OSS contributes 40–50% to N2 as compared to the thermal part and exceeds it for a few months of the seasonal cycle. Away from the tropics, near the centers of action of the subtropical gyres, there are regions characterized by the permanent absence of OSS. Based on consistency across models (CMIP5 and forced experiments) and reanalyses, we identify the stabilizing role of salinity, and the role of salinity in destabilizing upper ocean stratification in the subtropical regions where large-scale density compensation typically occurs. Finally, a survey of the small scales of observed variability of the SSS field with typical range from 10 to 100 km, that has implications for the validation of satellite-based measurements characterized by a spatial footprint of 50-150 km, would be also discussed.