Small-Scale Fronts in Ultra-High Resolution Level-4 Satellite SST: Validation and Implications
Abstract:Submesoscale oceanic fronts have been implicated in the turbulent energy cascade, and in nutrient supply into and carbon export out of the euphotic zone. However, their large-scale extent is unknown due to their characteristic small spatial (1-10km) and short time (~1 day) scales that complicate observations. Current large-scale understanding of fronts from satellite SST and ocean color is limited to a climatological view of occurrence frequency in cloud-free events. We show that useful estimates of frontal spatial coverage and structure can be derived from the recently available, merged satellite Level-4 SST product (G1SST at 1km resolution) using a gradient-based detection method in the North Atlantic subtropical gyre (28o-38oN, -75o- -45oW ). G1SST fronts are validated with in-situ fronts in continuous ship measurements from the Oleander Project. At a matching distance of Δx=5km, 79% of the G1SST fronts are in-situ fronts, and 64% of the in-situ fronts are detected by G1SST; these matches up increase with larger Δx. Comparing with in-situ velocities, ~56% of the fronts are coincident with across-track velocity jets with low-pass filter at scales > 5km，and 70% at scales > 50km, indicating that fronts are in large-scale geostrophic balance. Near-surface vertical shear predicted from thermal-wind relationship is in-phase but much smaller than (<50%) observed shear, indicating large ageostrophic shear is inclined to be co-located with surface baroclinic zones, likely due to interaction between fronts and gravity waves.
For the North Atlantic subtropical gyre, we find that submesoscale fronts comprise 57±4.6% of the total surface area. Fronts are found not only in the energetic Gulf Steam region, but also are surprisingly numerous in the quiescent subtropical gyre. This finding is consistent with previous modeling studies that indicate that submesoscale fronts could help to resolve the ‘nutrient--primary production’ balance in oligotrophic regions.