CURRENTS AT LINDENKOHL SILL IN THE SOUTHERN GULF OF MAINE

From June 22 to September 3, 1986, current speeds and directions were measured at depths of 76, 127 and 212 m over Lindenkohl Sill (225 m), which controls an important deepwater spreading path from Georges Basin into Wilkinson Basin in the Gulf of Maine. Most (70%) of the low-frequency (subtidal) variability occurred in a barotropiclike mode in which the inward currents were coherent with alongshore wind stress for approximately-5-day periods early in the record and for approximately-3-days periods late in the record; the coherence weakened during the central part of the record when dense Atlantic slope water from Georges Basin moved over the mooring site. Most of the remaining variance appeared in a baroclinic mode that added constructively to produce surprisingly large (approximately 1 m s-1) near-bottom currents during the wind-coherent intervals; at other times the modes interfered to produce near-zero currents over the sill while middepth and upper level currents were intensified. The relationship between the wind stress and the currents suggests the action of a density-modulated coastal upwelling mechanism in which the deep inward currents over the sill respond directly to northeastward alongshore wind stress at times of weak stratification; increased density stratification caused by water mass adjustments or seasonal heating produces a more complex baroclinic response. It is suggested that the springtime onset of northeastward winds over the Gulf of Maine may play an indirect but important role in the yearly intensification of the baroclinic circulation in the gulf by controlling or modulating the indraft of slope water into the inner basins.