The Characterization of a Midocean Front with a Doppler Shear Profiler and a Thermistor Chain

Abstract

It is demonstrated that a data set collected by the combined use of a Doppler shear profiler and a towed thermistor chain can provide a detailed description of a midocean near-surface front. The front described was encountered in the subtropical convergence zone in the Sargasso Sea, near 31°N and 72°W, in July 1981. A cross-front temperature change of 4°C in 29 km (at a depth of 40 m) was measured with the cooler water, indicating a highly convoluted local structure, occurring on the south side of the front. The temperature difference was not apparent at the surface as the front was covered with a relatively isothermal cap in the form of a ?25 m thick seasonal mixed layer. The Doppler data indicated that the frontal interface ran from northeast (39°T) to southwest (219°T) with southwestward flow of up to 81 cm s?1 occurring north of the front. The front was convergent with a net cross-front, flow of 16 cm s?1. The frontal interface was delineated by a shear zone with a difference of horizontal velocity of ?50 cm s?1 occurring over a depth interval of ?40 m (shear ?0.0125 S?1) that sloped down from the southeast toward the northwest. The slope of the frontal interface is estimated by several methods to have been ? ?0.0035 = ?1/290. The shear zone coincided with sloping zones of high temperature gradient and of low Richardson number. Comparing the velocity and density fields geostrophically, by use of the Margules equation and the thermal wind equation, proved successful over the broad area of the front. Small-scale features were present in both the velocity and temperature which probably reflect internal wave activity.