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.

Oceanic fronts in the Sargasso Sea control the early life and drift of Atlantic eels

“…The range of temperature between the sharp changes along 648 W (21.58C and 25.58C, from south The ship-mounted current profilers (ADCP) supplied us with estimates of current directions and velocities at each sampling station (figures 1 and 2a, arrows). The estimates showed a high variability, which is to be expected in this dynamic area where the hydrography is influenced by eddies and undulations of the frontal bands (Voorhis 1969;Eriksen et al 1991). However, when crossing the frontal structures, we observed distinctive changes in current patterns.…”

“…, have been found during other studies of the same subtropical fronts (Eriksen et al 1991;Fernández & Pingree 1996). However, such high velocities are probably not sustained for the entire migration because frontal characteristics change with seasons (Ullman et al 2007), and the meandering of frontal bands leads to a route that is significantly longer than a direct distance.…”

Species assemblages of leptocephali in the southwestern Sargasso Sea

“…In the STCZ, 2 or 3 latitudinal bands of fronts are evident in satellite imagery of sea surface temperature (Halliwell et al 1991), but their locations and movements vary. Surface water converges toward the fronts where strong frontal jets occur (Eriksen et al 1991). During summer, these fronts dissipate as the surface layer warms, and a general westward flow is thought to occur in the southern Sargasso Sea, as has been seen in some drifter tracks (Reverdin et al 2003).…”

“…These fronts form in the upper 200 m, where colder water in the north meets the warmer mixed layer of lower density water in the upper 100 m in the south (Kleckner & McCleave 1988, Eriksen et al 1991, Halliwell et al 1991. In the STCZ, 2 or 3 latitudinal bands of fronts are evident in satellite imagery of sea surface temperature (Halliwell et al 1991), but their locations and movements vary.…”

“…This raises the question of how some species that do not migrate offshore to spawn become so widely distributed across the southwest Sargasso Sea. In addition to Gulf Stream recirculation, some leptocephali could be transported eastward into the Sargasso Sea by the frontal jets present in the STCZ from late autumn until spring (Eriksen et al 1991). Two or 3 bands of fronts form there, and because water converges into the fronts on both sides, many leptocephali likely become entrained and transported eastward .…”