Lagrangian Reconstruction to Extract Small‐Scale Salinity Variability From SMAP Observations

Abstract

AbstractAs the resolution of observations and models improves, emerging evidence indicates that ocean variability on 1–200‐km scales is of fundamental importance to ocean circulation, air‐sea interaction, and biogeochemistry. In many regions, salinity variability dominates over thermal effects in forming density fronts. Unfortunately, current satellite observations of sea surface salinity (SSS) only resolve scales ≥40 km (or larger, depending on the product). In this study, we investigate small‐scale variability (≲25 km) by reconstructing gridded SSS observations made by the Soil Moisture Active Passive (SMAP) satellite in the northwest Atlantic Ocean. Using altimetric geostrophic currents, we numerically advect SMAP SSS fields to produce a Lagrangian reconstruction that represents small scales. Reconstructed fields are compared to in‐situ salinity observations made by a ship‐board thermosalinograph, revealing a marked improvement in small‐scale salinity variability when compared to the original SMAP fields, particularly from the continental shelf to the Gulf Stream. In the Sargasso Sea, however, both SMAP and the reconstructed fields contain higher variability than is observed in situ. Enhanced small‐scale salinity variability is concentrated in two bands: a northern band aligned with the continental shelfbreak and a southern band aligned with the Gulf Stream mean position. Seasonal differences in the small‐scale variability appear to covary with the seasonal cycle of the large‐scale SSS gradients resulting from the freshening of the coastal waters during periods of elevated river outflow.