The surface temperature field and dynamical structure of the Almeria-Oran front from simultaneous shipboard and satellite data

Abstract

A comparative analysis is presented of data sets gathered simultaneously from satellite and field measurements during the “Almofront-1” cruise of April–June 1991 in the Almeria-Oran front region of the Western Mediterranean. The synoptic fields of temperature, salinity, density and dynamic height are found to be well correlated at depths greater than 50–75 m. All are dominated by strong meridional gradients associated with a zonal jet at approximately 35°45′N flowing eastward from Cape Tres Forcas to Oran. Two regions of anticyclonic vorticity are also observed at these depths, one between the jet and the African coast, and the other to the north, lying off the Spanish coast. The synoptic surface parameter fields show strong coupling between the geostrophic velocity fields at the surface and at depth, relatively homogeneous salinity and density fields and a surface temperature field that appears to be unrelated to the other fields. At the position of the zonal jet, the remotely-sensed thermal data show regions of anticyclonic vorticity associated with Cape Tres Forcas, which are anomalously cool and show evidence of entrainment of warm coastal water in their structure. They are also observed to propagate along the African coast during the time of the survey. The in situ derived surface temperature field, however, is characterized by small scale patchiness. This is considered to be due to the time scale over which the data used to compile this map were gathered. The surface velocity field measured in situ is found to be more closely correlated with the surface temperature field than the dynamic height field. Fine scale comparisons of the in situ and remotely-sensed temperature fields show them to correlate well, although significant discrepancies were found when the temperature variations were of small amplitudes and length scales. The theory put forward to explain the vertical variations observed is that the weakening of the density gradients in the mixed layer leads to the increased dominance of the boundary layer separation process at Cape Tres Forcas, which leads, in turn, to a lack of coupling between the surface temperature and velocity fields and the surface dynamic height field.