Global Analysis of Coastal Gradients of Sea Surface Salinity

Alina N Dossa,Alex Costa Da Silva,Gaël Alory,Adeola M Dahunsi,Arnaud Bertrand

doi:10.3390/rs13132507

Abstract

Sea surface salinity (SSS) is a key variable for ocean–atmosphere interactions and the water cycle. Due to its climatic importance, increasing efforts have been made for its global in situ observation, and dedicated satellite missions have been launched more recently to allow homogeneous coverage at higher resolution. Cross-shore SSS gradients can bear the signature of different coastal processes such as river plumes, upwelling or boundary currents, as we illustrate in a few regions. However, satellites performances are questionable in coastal regions. Here, we assess the skill of four gridded products derived from the Soil Moisture Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) satellites and the GLORYS global model reanalysis at capturing cross-shore SSS gradients in coastal bands up to 300 km wide. These products are compared with thermosalinography (TSG) measurements, which provide continuous data from the open ocean to the coast along ship tracks. The comparison shows various skills from one product to the other, decreasing as the coast gets closer. The bias in reproducing coastal SSS gradients is unrelated to how the SSS biases evolve with the distance to the coast. Despite limited skill, satellite products generally agree better with collocated TSG data than a global reanalysis and show a large range of coastal SSS gradients with different signs. Moreover, satellites reveal a global dominance of coastal freshening, primarily related to river runoff over shelves. This work shows a great potential of SSS remote sensing to monitor coastal processes, which would, however, require a jump in the resolution of future SSS satellite missions to be fully exploited.

Highlights

Sea surface salinity (SSS) is an essential climate variable that bears the signature of the water cycle at the ocean–atmosphere interface, where most water fluxes occur [1]
Except for the bias of Soil Moisture Active Passive (SMAP) RSS and the standard deviation (STD) of Soil Moisture Ocean Salinity (SMOS) Laboratoire d’Oceanographie et du Climat (LOCEAN), there is no obvious loss in data quality when approaching the coast farther than 100 km offshore
GLORYS shows an increasing bias and STD from the largest to the narrowest coastal bands, which are on average between those found for the SMAP products

Summary

Introduction

Sea surface salinity (SSS) is an essential climate variable that bears the signature of the water cycle at the ocean–atmosphere interface, where most water fluxes occur [1]. The large-scale spatial patterns of SSS mostly result from the balance between evaporation and precipitation. High SSS is found in the subtropical gyres, where evaporation dominates [2,3], while lower SSS is found around the intertropical convergence zones and at high latitudes, where precipitation dominates. SSS is affected at high latitudes by the formation or melting of sea ice, which has much smaller salt content than seawater [4]. Ocean circulation and mesoscale eddies contribute to the transport of water properties and to the spatial distribution of SSS (e.g., [5]). Ocean circulation and mesoscale eddies contribute to the transport of water properties and to the spatial distribution of SSS (e.g., [5]). 4.0/).

Methods

Results

Conclusion