Ocean frontal variability in the Frontal Air‐Sea Interaction Experiment

Abstract

Oceanographic observations during the Frontal Air‐Sea Interaction Experiment (FASINEX) in the Sargasso Sea indicate that upper ocean fronts in the subtropical convergence are strongly surface‐intensified coherent flow structures which have a preferred orientation and translation speed. Fronts grow as they propagate in this region such that currents roughly double in strength in a time that they translate by twice their width. While currents are dominated by geostrophic momentum balance, the Rossby number of these flows can reach a few tenths. Mixed layer water from the cold side of fronts appears to subduct under warm‐side near‐surface waters, since isolated lenses of water with cold‐side temperature and salinity characteristics are found embedded in the seasonal thermocline on the warm side of fronts. Although plausible vertical speeds were observed, no actual rates of subduction could be estimated. Typical frontal widths, temperature contrasts, horizontal currents, vertical shears, vertical currents, and translation speeds of fronts in the region studied are 20 km, 1°C, 0.5 m s−1, 0.005 s−1, 0.0005 m s−1, and 0.2 m s−1, respectively.