Satellite sea-surface salinity (SSS) observations are a window into the ocean water cycle. However, the physical relationship between the atmospheric freshwater forcing and SSS is not linear but…

This is a grant award from the National Aeronautics and Space Administration (NASA) Shared Services Center to the Woods Hole Oceanographic Institution, a private non-profit organization focused on ocean research. The $441,148.13 grant will fund research to improve the understanding of seasonal and long-term variability in subtropical sea surface salinity, which is important for monitoring the ocean water cycle. The research will analyze satellite and Argo data to study the relationships between atmospheric forcing, ocean mixing, and the ventilation of subtropical underwater. This will help validate satellite sea surface salinity products and guide the interpretation of water cycle signals from these observations. The award does not have a set-aside designation.

Satellite sea-surface salinity (SSS) observations are a window into the ocean water cycle. However, the physical relationship between the atmospheric freshwater forcing and SSS is not linear but dictated by ocean dynamical processes including mixing, horizontal and vertical advections. We can fully appreciate the values of satellite SSS observations in monitoring ocean water cycle only when we are able to fully understand the processes that give rise to SSS variability. This proposal aims to gain an improved understanding of the mechanisms that govern the variability of subtropical sea surface salinity maximum (SSS-max) on seasonal and longer timescales. IT describes a research that will address the following three hypotheses: (i) the net evaporation flux is not sufficient as a sole forcing of SSS-max variability in the subtropical region, because ocean current advection and, particularly, vertical entrainment across the base of the mixed-layer are important oceanic contributors. (II) the water entrained from the base of the mixed layer is sourced from the subtropical underwater (stuw) that spreads west- and equatorward from its ventilation site, and hence, the SSS-max interacts with stuw formed upstream at the surface. We suggest that the salinification of the stuw in recent decades is due to the poleward expansion of the SSS-max in association with a widening of the hadley cell. (III) lastly, the RE-emergence of stuw through entrainment has the potential to influence SSS-max from one winter to next, and the effect can either enhance or diminish the effect of the evaporation-minus-precipitation (e-p) flux on SSS-max variability. The proposed work will include observation analysis that relies on satellite and argo, and a modeling study that helps diagnose the connections between various processes. Satellite salinity observations from aquarius, smap and smos are the core surface salinity datasets, but the three sets of products differ considerably from each other and from argo in mapping the ventilation region for the stuw. Therefore, validation of satellite SSS products for both the SSS-max and stuw ventilation will be a central component of the technical activities needed to support our science investigations. We propose to develop validation metrics using the conservation of the stuw properties along isopycnals and argo observations. The significance of the proposed research is in its potential to (i) gain an improved understanding of the interactions among the atmospheric forcing, the ocean mixed-layer processes, and ventilation of the subtropical ocean; (II) establish a physical check for the consistency between satellite SSS and argo subsurface observations and use IT to guide the validation and improvement of satellite products in the SSS-max regime; and (III) provide a comprehensive analysis of the SSS dynamics in the subtropical ocean and use IT to guide the interpretation of the water cycle signals in SSS observations.