New York Harbor, and the surrounding waters are driven by a number of forces, all joining to create the fascinating conditions that we display on this web site.
The tide is the biggest force driving motion of the water in our area. You can see the water moving in and out, on a 6-hour, diurnal cycle. If you look carefully, you can even see Coriolis moving saltier water to the right as it enters the harbor, and fresher water to the right as it leaves the harbor.
The Hudson River is also a big factor. A huge volume of water, sometimes as high as 1000 cubic meters per second, flows out. In the summer, this quiets to as little as 500 m 3/sec. The fresh water is lighter than the heavier, salty water from the ocean and so it flows out on top. This is known as estuarine, two-layer, or stratified flow. The tide acts to mix the two liquids, but the tide here is not strong enough to do so completely. A big wind event, such as a Nor'easter can temporarily create a mixed conditions. Twice a month, when the moon is inline with the sun, there is an extra-strong tide (the spring tide) which mixes the water more. Twice a month the moon is at 90 degrees to the sun (in quadrature) and the tide is particularly weak (neap tide). You can see this by looking at the time series model output. If you look at the salinity of top and bottom, there are times each month when the difference is more pronounced, and times when it is far more mixed. Needless to say, a storm can also have a strong, though short-term effect.
By far the strongest driver of currents are the tides. In the open ocean, tides do not have much of an effect. But when a large mass of water has to move through a narrow channel, you can see the tide much more clearly. Select surface currents on the forecast and play through a tidal cycle. If you look at the coast of New Jersey, you can see that off the coast, the water is moving fairly slowly, though it moves faster over shallow areas. But at the mouth of New York Harbor, the velocity of the water is much higher. If you switch to the view of lower Manhattan, you can see that the velocity of the water in the East River is higher still. While it is outside our primary areas of interest, the highest velocity currents in our area are between Queens and 125th street in Manhattan on the East River, in an area known as HellGate. The currents there can get in the vacinity of 6 knots.
At the George Washington Bridge, salinity of the surface water can range from 5 to 12psu. The water gets progessively fresher as it goes further up the Hudson. The salt intrudes further when there is less fresh water flowing out the Hudson. We have a CTD measuring temperature and salinity at the Bridge.
At Castle Point, where Stevens is, the salinity ranges much more widely.