THE HEAT-BUDGET IN THE NORTH-ATLANTIC SUBTROPICAL FRONTAL ZONE

The heat budget in the North Atlantic subtropical frontal zone is exam ined using moored measurements of horizontal velocity, temperature, an d surface heat flux obtained during the Frontal Air-Sea interaction Ex periment (FASINEX). The three moorings used in this calculation define d a triangle with base of 19 km and height of 28 km, and were densely instrumented from the surface to 160-m depth. Empirical orthogonal fun ctions (EOFs) are employed to identify components of current and tempe rature variability with scales that are resolved by the observations. A least squares fit to the EOF-filtered data is used to derive maps of velocity and temperature. The temperature field is dominated by the p assage of fronts, with the highest variability at 160 m. Horizontal te mperature gradients at 160 m peak at values approaching 1 x 10(-4) deg rees-C m-1 and are indicative of a tilting of the seasonal thermocline . Velocities are directed primarily along isotherms and are swifter at the surface than at depth. The heat budget of the upper 160 m, for pe riods longer than 48 hours, is essentially a balance between rate of c hange of heat and horizontal advection; the correlation between these terms is 0.7. A simple explanation is the northward advection by a mea n current of a series of fronts. The effect of surface heating becomes apparent in the upper 40 m, notably at the diurnal frequency and its first harmonic. An imbalance in the mean (102 days) heat budget can be rectified by a mean downwelling velocity of 5 x 10(-5) m s-1 at 160 m .