The Subinertial Momentum Balance of the North Atlantic Subtropical Convergence Zone

Abstract

The upper-ocean to forcing by pressure gradients and wind stress is examined using observations from the Frontal Air?Sea Interaction Experiment. A moored way acquired time series of winds. upper-ocean currents, temperatures, and salinities between winter and late spring in a region of the Sargasso Sea known for the presence of upper-ocean fronts. These fronts have timescales of 10 days and dominate current variance, while winds varied with the 4-day timescale of passing weather systems. Employing a frequency domain regression model. it is found that gestrophy accounts for most of the low-frequency (>100 h)current variance in the seasonal pycnoclino, but wind-forced shear becomes important nearer the surface. In particular, currents oriented in the typical NE?SW alonfront direction display geostrophic balance, while those perpendicular to them do not. Wind forcing can produce geostrophic currents indirectly through Ekman pumping. and knowledge of the geostrophic shear is required to distinguish between this and currents driven directly by the wind through turbulent shear stress. Previous investigations rely on the assumption that no wind-driven stress penetrates below the mixed layer to remove the wind-coherent geostrophic flow. Baroclinic pressure gradients are calculated using estimates of density across the moored array. A linear regression model uses pressure gradients record to explicitly remove the geostrophic shear and isolate the directly wind-driven acceleration at timescales longer than 10 days. The resulting response satisfies the Ekman transport relation, penetrates well into the stratified fluid spirals to the right, and decays with depth.