Journal Article

Quantifying the Contribution of Ocean Advection and Surface Flux to the Upper‐Ocean Salinity Variability Resolved by Climate Model Simulations

Siqueira, Leo, Kirtman, Benjamin P., Laurindo, Lucas C., Small, R. Justin, Thompson, LuAnne

2024

Overview

This study examines the impact of ocean advection and surface freshwater flux on the non‐seasonal, upper‐ocean salinity variability in two climate model simulations with eddy‐resolving and eddy‐parameterized ocean components (HR and LR, respectively). We assess the realism of each simulation by comparing their sea surface salinity (SSS) variance with satellite and Argo float estimates. In the extratropics, the HR variance is about five times larger than that in LR and agrees with Argo. In turn, the extratropical satellite SSS variance is smaller than that from HR and Argo by about a factor of two, potentially caused by the insufficient resolution of radiometers to capture mesoscale features and their low sensitivity to SSS in cold waters. Using a simplified salinity conservation equation for the upper‐50‐m ocean, we find that the advection‐driven variance in HR is, on average, 10 times larger than the surface flux‐driven variance, reflecting the action of mesoscale processes. Plain Language Summary This study explores the importance of ocean currents, evaporation, and rainfall for driving changes in the salt concentration in the upper ocean (known as salinity) in two climate model simulations with differing ocean resolutions. The high‐resolution model (HR) simulates ocean currents with dimensions of tens of km, while the low‐resolution model (LR) can only simulate currents with hundreds of km in size. When comparing their simulated sea surface salinity variations with those captured by satellites and autonomous floats from the Argo array, the salinity variability in the high‐resolution model is similar to the Argo data at mid to high latitudes and about five times stronger than that in the low‐resolution model. The satellite data show a variability two times smaller than HR and Argo in the same regions, potentially due to their insufficient spatial resolution at higher latitudes and their low sensitivity to the surface salinity in cold waters. Using a simple equation describing the conservation of salinity in the upper ocean, we have shown that small‐scale ocean currents drive most of the salinity variability in HR, while in LR, ocean currents play a much smaller role. Key Points We investigate how advection and surface flux affect upper‐50‐m salinity variance in eddy‐resolving and eddy‐parameterized climate models The extratropical variance in the eddy‐resolving run matches Argo and is much larger than in the eddy‐parameterized run and satellite data The larger upper‐ocean salinity variance in the eddy‐resolving run is predominantly driven by mesoscale ocean processes

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Advection / Argo / Climate / Climate models / Cold water / Conservation / Conservation equations / Drifters / Evaporation / Floats / Fluctuations / Freshwater / Inland water environment / Latitude / mesoscale eddies / Mesoscale features / Mesoscale phenomena / Mesoscale processes / Microwave imagery / Ocean currents / Oceans / OISSS / Radiometers / Rainfall / Salinity / Salinity effects / Salinity variations / Satellite data / Satellites / Sea currents / Sea surface / Seasonal variability / Sensitivity / Simulation / Spatial discrimination / Spatial resolution / Surface salinity / Surface water / Upper ocean / upper‐ocean salinity / Variability / Vortices