Air‐sea fluxes and surface layer turbulence around a sea surface temperature front

Abstract

The response of the lower marine atmospheric boundary layer to sharp changes in sea surface temperature was studied in the Frontal Air‐Sea Interaction Experiment (FASINEX) with aircraft and ships measuring mean and turbulence quantities, sea surface temperature, and wave state. Changing synoptic weather on 3 successive days provided cases of wind direction both approximately parallel and perpendicular to a surface temperature front. For the wind perpendicular to the front, both wind over cold‐to‐warm and warm‐to‐cold surface temperatures occurred. For the cold‐to‐warm case, the unstable boundary layer was observed to thicken, with increased convective activity on the warm side. For the warm‐to‐cold case, the surface layer buoyant stability changed from unstable to neutral or slightly stable, and the sea state and turbulence structure in the lower 100 m were immediately altered, with a large decrease in stress and slowing of the wind. Measurements for this case with two aircraft in formation at 30 and 100 m show a slightly increased stress divergence on the cold side. The turbulent velocity variances changed anisotropically across the front: the streamwise variance was practically unchanged, whereas the vertical and cross‐stream variances decreased. Model results, consistent with the observations, suggest that an internal boundary layer forms at the sea surface temperature front. The ocean wave, swell, and microwave radar backscatter fields were measured from several aircraft which flew simultaneously with the low‐level turbulence aircraft. Significant reductions in backscatter and wave height were observed on the cold side of the front.

Air–sea interaction in the Gulf of Guinea at intraseasonal time‐scales: wind bursts and coastal precipitation in boreal spring

“…Indeed, wind acceleration over warmer SST has been observed in the North Atlantic or Pacific (e.g. Businger and Shaw, 1984;Friehe et al, 1991;Kwon et al, 1998;Hashizume et al, 2002). Two main mechanisms were proposed to explain the influence of an SST front on the wind in the case of wind blowing from cold to warm SST (Xie, 2004;Small et al, 2008).…”

Observation of the marine atmospheric boundary layer in the Gulf of Guinea during the 2006 boreal spring

“…This agrees with the clear spatial and temporal coherence found in the previous section between the surface wind and SST patterns. Several previous studies of the SST front in the Pacific or North Atlantic also emphasized the same characteristics (Businger and Shaw, 1984;Friehe et al, 1991;Kwon et al, 1998;Hashizume et al, 2002;Small et al, 2008). To obtain a wide view of the differences between both sides of the SST front, ECMWF operational analyses are used to compute the mixed layer and the MABL heights.…”

“…Several previous experiments were conducted in the Pacific and North Atlantic oceans in the vicinity of such SST gradients, such as the Joint Air-Sea Interaction (JASIN; Pollard et al, 1983), the Frontal Air-Sea Interaction Experiment (FASINEX; Stage and Weller, 1985) and the 'Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale' (SEMAPHORE; Eymard et al, 1996). For a wind blowing from cold to warm SST, a strengthening of the wind, an increase of the turbulent fluxes and a thickening of the MABL above the warm side of the SST front were mainly observed (Businger and Shaw, 1984;Friehe et al, 1991;Kwon et al, 1998). These characteristics now need to be studied in the frontal zone in the Gulf of Guinea.…”

The Rhine Outflow Plume Studied by the Analysis of Synthetic Aperture Radar Data and Numerical Simulations

“…Among them, remote sensing has proved to be a powerful tool for the study of oceanic surface fronts because detecting their signatures synoptically, it can deliver a detailed description of their temporal and spatial evolution often contributing significantly to the understanding of their dynamics achieved using in situ measurement techniques. Perhaps the most widely used remote sensing sensors for the observation of sea surface manifestations of oceanic surface fronts are radiometers and radars (Friede et al 1991;Johannessen et al 1991Johannessen et al , 1996Askari et al 1993;Graber et al 1996;Vogelzang et al 1997;Sletten et al 1999). While using radiometric measurements (radiometers are passive sensors which, in general, are strongly weather-dependent) a classification of different water masses can be achieved by determining their sea surface temperature, their concentrations in phytoplankton, gelbstoff, or suspended matter, radar measurements provide a nearly weather-independent view of the sea surface roughness that depends, among other things, on surface flow divergence and shear and surfactants concentration.…”