Physical Oceanography

Long Island Sound is an estuary; a protected coastal body of water with open connections to the sea in which saline seawater is measurably diluted by fresh water. Ocean water flows in and out of the Sound through the opening in its eastern end, known as the Race.

Fresh water, from the rivers entering the Sound dilutes the seawater. Estuarine properties are repeated on a smaller scale in the many harbors and river mouths in the Sound. Thus, Long Island Sound is a large estuary with many smaller estuaries along its edges, such as New Haven Harbor and the mouth of the Connecticut River.

Tides

On the shores of Long Island Sound, the waters rise and advance, then fall and retreat with a regular rhythm. The tides are produced by the gravitational attraction of the moon and sun on the water. The rotation of the earth causes the tide to be semi-diurnal; in other words, the Sound experiences high tide about twice a day (once every 12 hours and 25 minutes, to be more exact) resulting in two high and two low tides in a 24 hour period. The difference in the water depth between high and low tide is called the tidal range.

The tides in Long Island Sound are driven by the oceanic tide outside the Race. Because of the Sound’s size and shape, it is particularly tuned to amplify the oceanic tide.  Consequently, the tidal range in the western Sound is larger than the tidal range in the east. At Stamford, for example, the average tidal range is about 2.2 meters (7.2 feet) while at New London the range is only 0.9 meters (3.0 feet). Strong tidal currents are associated with the tidal changes in water level.  As the water level falls after high tide, water flows eastward out of the Sound through the Race. This is the ebb (outgoing) tide. When the water level begins to rise again after low tide, the flood (incoming) tidal currents bring water westward, into the Sound. The tidal currents rush in and out (flood and ebb) with the current speeds at their minimum near times of high and low tides.  The tidal currents reach their greatest speeds in the eastern Sound. The current speeds diminish to the west.

Long Island Sound is an arm of the sea.  However, the saline sea water in the Sound is diluted with fresh water from the rivers of Connecticut creating a type of water called brackish. This dilution is increasingly pronounced moving westward, away from the source of salt water, the ocean.  The salinity of the surface water at the Race is typically about 30 parts salt per 1,000 parts water, or, as it is scientifically expressed, 30ppt. The salinity diminishes to about 26 ppt at Stamford. Salt water has a greater density than fresh water.  Consequently, at any location the surface water is usually less saline than the water near the bottom by one or two parts per thousand.  

The distribution of salinity at any particular time depends on many factors, such as the magnitude of river discharge and rate of evaporation. Differences in salinity among different parts of the Sound result from a continuous flow of both fresh water and salt water through the Sound. On average, some 470 cubic meters (about 120,000 gallons) of fresh water enter the Sound every second. The Connecticut River is the major contributor; this one source accounts for more than 70 percent of the freshwater supply. Nonetheless, the inflow of sea water greatly exceeds the fresh water supply. An estimated 19,000 cubic meters per second (over 5 million gallons per second) enter the Sound from the ocean.

 Circulation

The horizontal and vertical salinity differences in Long Island Sound are typical of many estuaries, and they produce a characteristic estuarine circulation of water. The heavy, saline bottom water sinks and flows under the fresher surface water. Thus, saline bottom water flows westward into the Sound, while less saline surface water flows eastward out of the Sound. Intense mixing of surface and bottom water occurs in the shallow near-shore regions and over reefs and shoals.

The estuarine circulation flows all of the time, during both ebb and flood tides. Thus, the current observed in Long Island Sound is the sum of the estuarine circulation and the tidal currents. The estuarine circulation, however, is so slow that it can only be noticed by careful measurements. In the central Sound, the maximum tidal current speed may be 1.0 knot, while the speed of the superimposed estuarine circulation is likely to be less than 0.2 knots. The speed of this flow of bottom water diminishes to the west, and at depths less than about 20 meters (65.6 feet), bottom water flows shoreward into the near-shore mixing zone.

 Sediments

The currents sweep the bottom sediments of Long Island Sound. They have worked the sandy bottom of the eastern Sound into large underwater dunes, or sand waves. The estuarine circulation superimposed on the tidal currents produces a net westward transport of sand out of the eastern Sound into the central muddy basin.  In the central and western basins, a large amount of silt has accumulated. Fine sediment is introduced by the rivers and is carried by the estuarine circulation into the inner Sound. The accumulation of silt is aided by the feeding activity of animals inhabiting the muddy bottom. Fine grains of silt are bound into much larger fecal pellets of bottom-dwelling animals. The muddy central basin is covered with a layer of fecal pellets about 0.5 centimeters (0.2 inches) thick.

During each tidal cycle a layer of sediment one or two millimeters thick (less than a tenth of an inch) is eroded and redistributed within the central basin. Throughout the Sound, the tidal streams resuspend and redeposit more than seven million tons of sediment daily. Despite this activity, fine silt is accumulating in the central and western basins of Long Island Sound at the rate of about a millimeter per year.

Temperature

Long Island Sound is a dynamic environment of strong tidal currents and varied salinity. The water temperature is also extremely variable. In the Sound, the water temperature oscillates from 0° Celsius (32° Fahrenheit) in the winter to about 22° Celsius (71.6° Fahrenheit) in the summer, a range as great as that of any body of water in the world.