Central Gulf of Maine

Overview

The Gulf of Maine is known as one of the most productive coastal seas in the world and is famous for its groundfisheries. Most of the productivity, however, does not occur in the Central Gulf but is focused on the shallow areas that rim the Gulf, where nutrient rich waters come into contact with sunlight. With its deeper waters, the Central Gulf is relatively depauperate. Exceptions include areas of localized upwelling, such as Cashes Ledge (to the west of Wilkinson Basin) and Truxton Swell (south of Jordan Basin). 1,2

Surface area

22,796 mi2 (59,041.37 km2)

Depth

The range of depth in the Central Gulf of Maine is from ~100 to ~200 m (not including the basins).

Bathymetry including key features

The central Gulf of Maine is a broad extension of the continental shelf. Ranging in depth from 100 to 200 m, the Central Gulf is separated from dry land on its northern, western and eastern sides by the coastal shelf. To the south, it is separated from the continental shelf break and slope by Georges and Browns Banks. The Northeast Channel, between the eastern end of Georges Bank and Browns Bank, and the Great South Channel, between the western end of Georges Bank and Nantucket Shoals, connect the Central Gulf with the Northwest Atlantic. The bathymetry of the Central Gulf is dominated by three large basins, Wilkinson to the west and Georges and Jordan to the east.

Substrate

Much of the Central Gulf of Maine, particularly its basins, is covered in thick deposits of mud. Unsorted glacial till is found in glacial moraines such as Truxton Swell to the south of Jordan Basin and Sewell Ridge to the north of Georges Basin. Much of the eastern portion of the Central Gulf is covered with modified glacial till – sand mixed with silt, clay and gravel. Gravel, with occasional boulders predominates on Platts Bank and Fippennies Ledge, high points in the northwestern portion of the Central Gulf. Cashes Ledge, and other high points in the same area, are formed by bed rock.3,4

Relationship to large-scale circulatory patterns of the Gulf of Maine

The circulation of the Gulf of Maine is comprised of a counterclockwise gyre that traces the coast from Nova Scotia to Cape Cod. This pattern is driven in large part by temperature; in spring, water in the interior of the Gulf begins to warm while water on the edge of the continental shelf and the slope outside the Gulf are cooling. As the water in the interior Gulf warms, it expands, becoming less dense and lighter. The difference in density between these more buoyant waters and cooler offshore waters creates a pressure gradient. The force of this gradient establishes a “downhill” flow of water away from the coast and the action of Coriolis forces (resulting from rotation of the earth) turns it toward the west. The resulting westerly flowing coastal current helps to draw water into the Gulf of Maine.

Relatively fresh, cold water enters the Gulf of Maine over the Scotian Shelf. It flows northeastward around the tip of Nova Scotia. After a turn through the Bay of Fundy, this current flows southwestward, setting up the coastal current that dominates circulation in the Gulf of Maine. Riverine discharge and tidal flows contribute to the coastal current. Dense, warmer and more saline water from the continental slope outside of the Gulf enters the Gulf through the Northeast Channel, a deep valley between Georges Bank and Browns Bank. It flows into Georges Basin and contributes to counterclockwise gyres that flow over both Georges and Jordan Basins. Outflow from the Gulf occurs through the Great South Channel and upper layers of the Northeast Channel.

The dynamics of the coastal current that flows southwestward along the coast of Maine divides the central Gulf of Maine into two oceanographically distinct areas. The eastern portion of this current is relatively fast moving and somewhat colder than the western portion of the current. Where the two meet, at the mouth of Penobscot Bay, the faster moving eastern current is, to a large degree, deflected offshore. These waters are, in turn, entrained in the counterclockwise gyres that flow over Georges and Jordan Basins. This process divides the interior Gulf into eastern and western portions that are relatively isolated from one another.

Smaller versions of the Gulf scale counterclockwise gyre follow the contours of the three major basins of the Gulf, Wilkinson in the eastern Gulf and Georges and Jordan Basins to the west. Clockwise gyres circulate over Georges and Browns Banks, the large underwater deposits that separate the Gulf of Maine from the Northwest Atlantic.5,6

Information on average (seasonal/annual) oceanographic conditions

The circulation of the Gulf has a distinct seasonal pattern. The counterclockwise gyre takes shape in the early spring. As the season proceeds, the discharge of freshwater from over sixty rivers in the Gulf watershed contributes to the currents. The action of the tides strengthens the summer circulation. In addition, the warming of the surface of the ocean results in stratification; a warmer layer floats on top of a mid-depth layer that preserves winter temperatures and salinities. It, in turn, is underlain by more saline “bottom water”. Stratification is most pronounced in the deeper areas of the western Gulf. The counterclockwise gyre is established in the top layer; the current picks up speed as the top layer slides over the middle layer. These currents reach their broadest extent and greatest speeds by the end of December. Then, cooling of the atmosphere results in cooling of the ocean surface. As it cools, it sinks, replacing the stratified layers with well mixed waters. As the currents mix downward, they are slowed by the friction they encounter when they reach the bottom. By February, the counterclockwise circulation pattern is no longer evident.7

There is considerable variation in the circulation patterns of the Gulf of Maine from year to year. Variations in the temperature and volume of water flowing into the Gulf, in the range of atmospheric temperatures affecting the Gulf, and in the size and timing of the spring freshet affect the scale and duration of the various gyres that define Gulf circulation patterns.

Presence of GoMOOS buoy

Buoy N is located in the Northeast Channel.

1 Thomas et al., 2003.
2 Conkling, 1995
3 Dorsey and Pederson, 1988
4 NEFMC, 2004
5 Xue, et al., 2000
6 Pettigrew, et al., in press
7 Xue, et al., 2000