Barry A. Klinger
George Mason University
Atmospheric, Oceanic, and Earth Sciences Dept.
One of the strangest effects of the rotation of the Earth is that it allows water to defy gravity. This is due to the Coriolis effect: whenever an object moves, the Coriolis force pushes sideways (to the right in the northern hemisphere, left in southern).
Suppose the surface of the ocean was distorted somehow so that water was piled into a mound or hill. Our intuition, based on seeing water that is moving too quickly to be affected by the Coriolis force, is that the water would flow "downhill". This way the mound would flatten out because of gravity. However, if the mound is so wide that it takes more than a day for the water to flow all the way down the mound, then the Coriolis force makes it go around the mound instead.
The same phenomenon explains why the wind whips around a low-pressure zone such as a hurricane instead of just being sucked into the low.
Satellites can actually measure the sea surface height by bouncing radar signals off the surface. The figure at right shows the sea surface height (SSH) in the North Atlantic Ocean (America is on the left, Europe and Africa on the right edge of chart). The Atlantic Ocean has water piled up nearly 2 meters high (shown in red) off the coast of Florida, and there is a depression (shown in blue) in the sea surface between Canada and Greenland in the North. The sharp boundary between high and low water off the southeast US marks a strong current: the Gulf Stream.
There is a direct relation between the sea surface slope and how fast the water is going. This is confirmed by the figure below. Both panels use arrows to estimate direction and magnitude of the surface velocity in the North Atlantic Ocean. The chart on the left calculates the velocity from the slope of the sea surface. The chart on the right calculates velocity by observing the location of drifters floating in the ocean. The two pictures are very similar even though they were created using completely different data and methods.*
*There are some other effects associated with wind pushing on the water which alter the two estimates of velocity, especially in the tropics.
References: SSH from 2006-2009 AVISO data, surface data Lumpkin and Giraffo (2005), both data sets smoothed with 3 degree wide averaging.
Last modified: 25 March 2015