Journal of Geophysical Research: Oceans

The front on the Northern Flank of Georges Bank in spring: 2. Cross-frontal fluxes and mixing

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Abstract

[1] Hydrographic observations made with an undulating vehicle carrying a CTD and concurrent shipboard ADCP velocity observations over a 12-day period are combined to investigate vertical mixing and cross-frontal fluxes on the Northern Flank of Georges Bank. The CTD density time series is analyzed to detect the presence of vertical overturns, the statistics of which are used to infer vertical mixing parameters. Vertical turbulent buoyancy fluxes are downward and most intense, reaching values of 5 × 10−7 W/kg, near the bottom at the edge of the bank and decrease both on- and off-bank. Horizontal, cross-bank buoyancy fluxes are partitioned into mean, tidal pumping, and nontidal eddy components and are computed as a function of cross-isobath/vertical position by averaging in the along-isobath direction. The tidal pumping component is dominant over most of the cross-bank section with a peak value of ∼1 × 10−4 W/kg, directed off-bank near the bank edge. A diagnosed tidal vertical velocity field is used with mean buoyancy gradients to compute the along-isopycnal skew flux. The horizontal component of this skew flux has similar spatial structure and magnitude to that of the observed tidal pumping flux. The divergent component of the skew flux, at depths above the bottom boundary layer, appears to be convergent north of the bank edge and divergent at the bank edge, suggesting that tidally driven advective processes drive buoyant bank water downward and off-bank at mid-depth and force the upwelling of deep, dense water near the bottom at the bank edge.

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