Why is the sea level higher off the coast of Bermuda than New York?
Any current which flows for longer than a day is, effectively, turning in circles as the Earth rotates. In the same way that an object moving in circles requires a force at right angles to its motion, like the Moon orbiting the Earth, any mean current in the ocean also requires a force (to the left, in the northern hemisphere) to balance its steady motion. This balance, between motion and a pressure force to the left, is known as geostrophy. It as also the reason why winds tend to blow along isobars, rather than directly from regions of high to low pressure, as you see every day on the weather. The strong northward current, the Gulf Stream, which flows between Bermuda and New York results in sea level at Bermuda being about 1 metre higher than say Charleston (that is to say ‘higher’ with respect to a surface called the ‘geoid’, see 1. above).
