The Deep Flow through the Northeast Channel, Gulf of Maine

Abstract

The flow dynamics, volume, heat, and nitrogen transport, and water mass structure in the Northeast Channel (42°17?N, 65°58?W) are investigated using two years of moored velocity and temperature data. Measurements were made from September 1976 to September 1978 at three moorings across the channel just inside the sill with instruments at 100 m, 150 m, and 16 m off the bottom. Energetic water motions occurred at three time scales: 1) tidal frequencies; 2) low-frequency motions in the 4?10-day range; and 3) mean motions with periods of three months or longer. The dominant tidal constituent was the semidiurnal (M2) tide, whose elipses all rotated clockwise, with magnitudes ranging from 61 cm s?1 at 150 m depth decreasing to 41 cm s?1 16 m off the bottom. The low-frequency currents showed strong seasonal variability: In winter, these motions took the form of strong bursts of current up to 50 cm s?1 both into and out of the channel, and were the dominant subtidal motions; in summer, these bursts were small and were superimposed on a steady inflow (northeast side) or outflow (southwest side). The low-frequency currents were strongly correlated with surface wind stress and coastal synthetic subsurface pressure (SSP), suggesting that outflows and inflows in the 4?10-day range were associated with a large-scale setup or setdown of the Gulf of Maine by the wind stress. The mean currents were partitioned year-round into an inflow and an outflow region. Outflows occurred at 100 m and 150 m depth on the southwest side of the channel with mean magnitudes of 7 and 2 cm s?1, respectively. Inflows occurred everywhere else, with magnitudes of about 10 cm s?1 at 100 m decreasing to about 3 cm s?1 toward the bottom. The total integrated volume transport below 75 cm depth at two week intervals was highly variable, but when averaged over the entire dataset, it was in-channel at a rate of 262 ± 58 (? 103 m2 s?1),which gave a replacement time of about 11 ? 2 months for deep waters of the Gulf of Maine. The temperature time series suggested that the inflowing water consisted of both Warm Slope Water (WSW) and Labrador Slope Water (LSW). The eddy heat flux added heat to the deep waters of the Gulf of Maine at a rate of 173 ? 106 kcal s?1. A highly simplified nitrogen transport estimate suggested that at least 4.05 ? 109 ?mol s?1 was advected through the channel, and possibly as much as 5.87 ? 109 ?mol s?1 if the NH4 contribution was included.