Global Distribution and Governing Dynamics of Submesoscale Density Fronts

Abstract While the dynamics at submesoscales (on the order of 0.1–10 km) are thought to be important globally for a range of processes near the air–sea interface, few observational studies sufficiently span scales to include both the submesoscale and global scales, leaving many questions concerning the coupling between the scales unexplored. To address this gap, we use a global dataset of ship-based thermosalinograph and satellite sea surface temperature data to identify over 250 000 submesoscale density fronts throughout the ocean. Globally, we find that the mean submesoscale frontal dynamics can be characterized by a scaling based on the hypothesis that the Rossby number and Froude number are proportional, Ro ∼ Fr. Our results also show that the large-scale ocean characteristics play a role in setting the spatial variability of submesoscale frontal horizontal buoyancy gradients (i.e., frontal “sharpness”). If the large-scale background density gradient is large and/or dominated by salinity as opposed to temperature variability, then submesoscale fronts tend to be sharper. We show that globally, shallow mixed layers are also associated with sharper submesoscale fronts, in contrast to previous regional-scale findings. This global perspective on the variability and dynamics of submesoscale fronts raises many additional questions and, hopefully, will inspire the formation of new scale-spanning avenues for future studies. Significance Statement Fronts in the ocean are important since they affect the exchange of heat, energy, and other properties between the ocean and atmosphere and have many impacts on climate, fisheries, and biology. Our study focuses on fronts 1.5–30 km wide, which are challenging to observe over large areas because they are so small. This study is novel because we use global measurements to understand how small fronts vary in different areas of the ocean. We show that large-scale ocean conditions alter key properties of small fronts. These results are of value because they provide a first glimpse of how these relatively small fronts vary globally and open many potential avenues for future studies that quantify the global impacts of these fronts.