250
Chapter 9: The Atlantic Ocean
in the Gulf of Maine, across the mouth of the macrotidal Bay of Fundy, around Grand
Manan Island, and also on the Nantucket Shoals south of Cape Cod (Townsend, 1992).
Within the Gulf of St. Lawrence, tidal mixing and tidal fronts occur mainly in the
channels between Anticosti, Prince Edward Island, and the mainland and also around
the Magdalen Islands (Pingree and Griffiths, 1980). Further south, in the Middle Atlantic
Bight, tidal fronts parallel the coast and separate an inner neritic from an outer open
shelf zone. Beyond Cape Hatteras, the water column over the shelf in the South Atlantic
Bight is permanently stratified except as noted below.
A shelf/slope front, marked by the 34% isohaline within the slope water, reflects the
instability between the shelf and slope water masses and is a consistent feature of this
province although, unfortunately, confusion is possible between this front and the North
Wall itself (Smith and Petrie, 1982). The shelf/slope front is periodically modified when
warm-core eddies, which have been shed from the meandering Gulf Stream, flood in
across the outer shelf and create instability and jet currents in the shelf water. At such
times, bursts of topographic Rossby waves propagate across the shelf as internal bores or
solitary waves (Smith and Sandstrom, 1986); this mechanism is responsible for significant
transport of nutrient-replete slope water up onto the shelf. Where the Gulf Stream lies
over the slope in the South Atlantic Bight, energetic frontal eddies frequently intrude
into shallow water.
Defining Characteristics of Regional Oceanography
Circulation is extremely dynamic and variable in this province, a not surprising observation in the light of the extraordinary conjunction here of polar and subtropical conditions;
in winter, the pack ice of the Gulf of St. Lawrence is only about 300 km from the north
wall of the Gulf Stream and hence of subtropical surface water. I believe that this is a
unique condition.
The Labrador Current transports low-salinity, Arctic water and separates into an
inshore and offshore component on encountering the topography of the Grand Banks.
The inshore component passes along the Avalon Channel close to the east coast of
Newfoundland and contributes to shelf-water formation on the Grand Banks; some also
enters the Gulf of St. Lawrence through the Straits of Belle Isle. The offshore component
passes around the continental slope of the outer Grand Banks and contributes to the
slope water that lies between the Gulf Stream and the shelf water.
Despite the complex geography of its continental shelf, and its important meridional
extent, the NWCS province is nevertheless unified by equatorward flow of cool water
originating in the Labrador Current, which may be followed as far as south as Cape
Hatteras at 35
N, always above the slope and with maximum velocity just beyond the shelf
break: at Newfoundland, southward flow over shelf, slope, and deep water is respectively
0.8, 5.7, and 30 Sv. The freshwater component of the flow over the shelf is relatively
minor, being measured in millisverdrups (Loder et al., 1998), despite the effluent from
the St. Lawrence River.
Thus, the entire province north of Cape Hatteras is characterized by meandering southward coastal flow throughout the year, though this responds to variability in sustained
wind direction. During winter, southerly winds frequently confine the coastal water zone
to a narrow, vertically homogenous feature along the inner shelf. Conversely, northerly
winds spread the inner shelf water seaward and thus induce stratification. Indeed, during
periods of sustained longshore wind from the southwest, local upwelling cells may form
at many points along the coast. Coastal upwelling occurs off southwest Nova Scotia
persistently, and also intermittently along the southeast-facing coast when wind forcing
is from a favorable direction.
Circulation in the Gulf of St. Lawrence passes around a complex eddy field, which
is induced by the several large islands and the complex coastline and is influenced by
Chapter 9: The Atlantic Ocean
in the Gulf of Maine, across the mouth of the macrotidal Bay of Fundy, around Grand
Manan Island, and also on the Nantucket Shoals south of Cape Cod (Townsend, 1992).
Within the Gulf of St. Lawrence, tidal mixing and tidal fronts occur mainly in the
channels between Anticosti, Prince Edward Island, and the mainland and also around
the Magdalen Islands (Pingree and Griffiths, 1980). Further south, in the Middle Atlantic
Bight, tidal fronts parallel the coast and separate an inner neritic from an outer open
shelf zone. Beyond Cape Hatteras, the water column over the shelf in the South Atlantic
Bight is permanently stratified except as noted below.
A shelf/slope front, marked by the 34% isohaline within the slope water, reflects the
instability between the shelf and slope water masses and is a consistent feature of this
province although, unfortunately, confusion is possible between this front and the North
Wall itself (Smith and Petrie, 1982). The shelf/slope front is periodically modified when
warm-core eddies, which have been shed from the meandering Gulf Stream, flood in
across the outer shelf and create instability and jet currents in the shelf water. At such
times, bursts of topographic Rossby waves propagate across the shelf as internal bores or
solitary waves (Smith and Sandstrom, 1986); this mechanism is responsible for significant
transport of nutrient-replete slope water up onto the shelf. Where the Gulf Stream lies
over the slope in the South Atlantic Bight, energetic frontal eddies frequently intrude
into shallow water.
Defining Characteristics of Regional Oceanography
Circulation is extremely dynamic and variable in this province, a not surprising observation in the light of the extraordinary conjunction here of polar and subtropical conditions;
in winter, the pack ice of the Gulf of St. Lawrence is only about 300 km from the north
wall of the Gulf Stream and hence of subtropical surface water. I believe that this is a
unique condition.
The Labrador Current transports low-salinity, Arctic water and separates into an
inshore and offshore component on encountering the topography of the Grand Banks.
The inshore component passes along the Avalon Channel close to the east coast of
Newfoundland and contributes to shelf-water formation on the Grand Banks; some also
enters the Gulf of St. Lawrence through the Straits of Belle Isle. The offshore component
passes around the continental slope of the outer Grand Banks and contributes to the
slope water that lies between the Gulf Stream and the shelf water.
Despite the complex geography of its continental shelf, and its important meridional
extent, the NWCS province is nevertheless unified by equatorward flow of cool water
originating in the Labrador Current, which may be followed as far as south as Cape
Hatteras at 35
N, always above the slope and with maximum velocity just beyond the shelf
break: at Newfoundland, southward flow over shelf, slope, and deep water is respectively
0.8, 5.7, and 30 Sv. The freshwater component of the flow over the shelf is relatively
minor, being measured in millisverdrups (Loder et al., 1998), despite the effluent from
the St. Lawrence River.
Thus, the entire province north of Cape Hatteras is characterized by meandering southward coastal flow throughout the year, though this responds to variability in sustained
wind direction. During winter, southerly winds frequently confine the coastal water zone
to a narrow, vertically homogenous feature along the inner shelf. Conversely, northerly
winds spread the inner shelf water seaward and thus induce stratification. Indeed, during
periods of sustained longshore wind from the southwest, local upwelling cells may form
at many points along the coast. Coastal upwelling occurs off southwest Nova Scotia
persistently, and also intermittently along the southeast-facing coast when wind forcing
is from a favorable direction.
Circulation in the Gulf of St. Lawrence passes around a complex eddy field, which
is induced by the several large islands and the complex coastline and is influenced by