The structure of a weak thermohaline front

Abstract

A three-dimensional description of a near-surface thermohaline front is made possible by using data from a coordinated observational period (Second Multiship Experiment) in the Joint Air-Sea Interaction Experiment (JASIN 1978). The data are from current meters on buoys and high accuracy conductivity-temperature-depth probes used from fixed and mobile ships. The front is located in an eddy, which has length scales of order 100 km, and lies north-south marking the boundary of warm salty and cool fresh water masses. The thermoclinicity of the front is compressed into a narrow region often 500 m or less in width, and the vertical structure of thermoclinicity agrees remarkably well with the results of the numerical model of frontogenesis of MacVean & Woods (1980). The temperature and salinity structure are to a large extent mutually compensating in density with the result that the baroclinicity is too small to be convincingly detected in these data, which contain strong internal wave signals. Modulation of the static stability across the front is clearly seen and upwelling results in a drop of sea surface temperature of about 0.1 K above the front. The front is advected through the observational array at about 0.75 km h -1 and, by using the current meter measurements, it is possible to use repeated sections across the front to investigate the along-front structure. Modulation of the thermoclinicity is observed resulting in local inversions, but extensive cross-frontal intrusions are not seen. The front appears to meander and the data are compared with the results of a numerical model of frontal instability (MacVean 1980).