Circulation and Dispersion on Browns Bank

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Abstract
Year-long current measurements reveal a permanent clockwise gyre over the western cap of Browns Bank. Maximum mean currents (0.25 m .cntdot. s-1) over the steep northern flank are uniform in the upper water column, except when pulses of low salinity water arrive from the Scotian Shelf producing stratification and vertical shear. Significant off-bank Eulerian mean currents, found near the bottom on the 100-m isobath, are consistent with the models for topographic interactions of the semidiurnal tides. A multiple regression analysis in the spectral domain reveals most low-frequency energy is concentrated in the 14-28-d bands where wind- and tidally-forced currents make roughly equal contributions (30-40%) to the total variance. In the synoptic bands (2-10-d), wind forcing alone contributes 60-80% of the variance. The temporal and spatial variability of the gyre is supported by satellite-tracked drogue studies on the Bank in three seasons. April and November 1983 observations show that the gyre circulation may be broken down by wind-forced currents. In July/September 1983, drogues consistently exited the western cap of the Bank from the northern flank after a residence time of 14 d, comparable with the near-surface recirculation time. Isotropic dispersion coefficients (30 .ltoreq. KH .ltoreq. 160 m2 .cntdot. s-1) are found to follow Okubo''s (1971) empirical formulae.