Physical Oceanography of Georges Bank

On the large scale, Georges Bank is part of a very long coastal current system which flows southwestward from Laborador to the Mid-Atlantic Bight (Figure 4; Chapman and Beardsley, 1989). Locally, two oceanic water masses and local air/sea interaction determine the hydrographic conditions within the Gulf of Maine and on Georges Bank (Bigelow, 1926). Cold, low salinity coastal water enters the Gulf of Maine from the Scotian Shelf (Smith, 1983) and warm, saline Slope Water enters through the Northeast Channel (Ramp et al., 1985). Both of these inflows to the Gulf exhibit significant seasonal and inter-annual variability. The two water masses plus local runoff mix during a generally cyclonic (counterclockwise) movement around the Gulf which may slow down or even reverse in part during winter (Brown and Irish, 1992). During the summer, this basin-scale movement may be comprised of smaller-scale cyclonic flows around Jordan, Wilkinson, and Georges Basins (Brooks, 1985, Butman and Beardsley, 1991). In the western Gulf of Maine, near-surface waters flow both southward around Nantucket Shoals into the Mid-Atlantic Bight and eastward onto the northwest flank of Georges Bank (Hopkins and Garfield, 1981; Limeburner and Beardsley, 1982; Beardsley et al., 1991). On the northwest flank, the interaction of strong tidal currents and steep bottom topography results in a strong horizontal density front and a narrow, jet-like current flowing eastward along the northern edge of the Bank (Loder, 1980; Magnell et al., 1980; Butman, 1982). The flow then turns clockwise and slows to move southwestward along the southern flank of the Bank. The strength of this anticyclonic (clockwise) circulation increases with increasing stratification in the spring and summer (Butman et al., 1987). A permanent hydrographic front along the southern edge of the Bank separates the shelf water over the Bank from the more saline Slope Water offshore (Flagg, 1987). Much of the shelf water flowing southwestward along the southern flank of the Bank continues westward into the Mid-Atlantic Bight (Beardsley et al., 1985). During stratified conditions, some of the shelf water on the southern flank of the Bank moves northward through the Great South Channel, forming a partially closed gyre (Butman and Beardsley, 1987).

Instantaneous water movements on the Bank are dominated by strong tidal forcing and wind events associated with storms. The Gulf of Maine/Bay of Fundy basin system is near-resonance at the semi-diurnal tidal frequency, which amplifies the semidurnal tidal amplitude in the Bay of Fundy and the currents across Georges Bank (Garrett, 1972; Greenberg, 1979; Brown and Moody, 1987). The tidal currents are rotary with maximum velocities (and tidal excursions) increasing from about 30 cm/s (4.3 km) near the shelf break on the southern flank to 75 cm/s (10.7 km) and larger over the crest (Bumpus, 1976; Moody et al., 1983). Due to bottom friction, tidal currents exhibit a large decrease in magnitude and some veering in direction near the bottom (Brown, 1984). This vertical shear generates strong turbulence and vertical mixing over the shallow top of the Bank (Garrett et al., 1978). Wind driven currents, although transient, can be much larger than the mean velocities (Noble et al., 1985; Brink et al., 1987). Storm events potentially can exchange large amounts of water on/off the Bank. In addition, along the southern edge of the Bank, warm-core Gulf Stream rings can influence the flow field and entrain large amounts of shelf water off Georges Bank and into the Slope Water offshore (Flagg, 1987; Garfield and Evans, 1989).

The balance between the tendencies for recirculation of water around the Bank and for advective exchanges of water from the Bank result in an average residence time of about 50 days for a near-surface water parcel within the 100 m isobath on the Bank (Flagg et al., 1982; Loder et al., 1982; Beardsley et al., 1991). Estimates of residence time are somewhat shorter in winter and somewhat longer in summer. The increase in residence time in summer appears to result from a greater tendency for water to recirculate around the Bank during stratified conditions.

Water properties on the Bank exhibit characteristic seasonal cycles. In winter, atmospheric cooling and wind mixing keep the entire Bank region vertically well-mixed (Figure 5a). During spring, increasing solar insolation and decreasing wind mixing (Figure 6a-b) cause the water column on the southern flank to become stratified in temperature and density (Figure 5b, Figure 6c-d). Over the shallow, central region of the Bank, turbulent tidal mixing is sufficiently strong to keep the water column well mixed year round. Tidally mixed fronts develop near the 60 m isobath, separating the central Bank from the stratified waters to the north in the Gulf of Maine and to the south on the flank of the Bank (Loder and Greenberg, 1986). Seasonal warming and increasing stratification continue through the summer. In autumn and winter, cooling and increasing wind mixing again lead to well mixed conditions over most of the bank in the winter.

The seasonal range in temperature is about 12oC at the surface in the well-mixed region, and about 8oC at the bottom over the southern flank of the Bank. The currents on the southern flank of the bank also exhibit a seasonal cycle, with mid-depth (45 m) along-bank values of about 5 cm/s in March increasing to a maximum of about 11 cm/s in September. The seasonal increase in along-bank velocity is consistent with the geostrophic shear associated with the increasing across-bank density contrast between the well-mixed and stratified regions (Butman and Beardsley, 1987).

The role of inter-annual and longer fluctuations in the North Atlantic on the oceanography of Georges Bank must be noted. Recent observations show that Georges Bank has experienced some of the North Atlantic's largest changes in sea surface termperature (Gordon et al., 1992). A global analysis shows that the North Atlantic experiences the second highest amplitude variations in large-scale inter-annual surface pressure patterns (Wallace and Gutzler, 1981). These variations, identified as the North Atlantic Oscillation (Walker, 1924; Walker and Bliss, 1932), have a period of about 7 years (Rogers, 1984) and are associated with the observed pattern of wind strength, SST anomalies, changes in water masses, Labrador Sea water formation, and sea-ice coverage. There is also evidence that such variations covary with cod stocks in the Northwest Atlantic that are otherwise unrelated to fishing pressure. Longer time-scale climate fluctuations, based on the geological record and model simulations, also appear linked to swings in the dynamics of the North Atlantic and related large shifts in fisheries potential.