why are eddies (and gyres, for that matter) circular?
mostly because the earth rotates.
this effects prevailing winds, winds act on water currents, and since the oceans are 'closed loops (ie, most of the water in the Atlantic ocean stays in the atlantic ocean, and doesn't flow-- river like--into the pacific ocean or the indian ocean!), when water is moved by the wind NORTH AND EAST by the winds, it can't just flow(somewhere else!) , but a must move in a circular 'path' back to the south and west.
if you have water in a bath tub, you can set up a circular currnent. it isn't self sustaining. earths currents are more or less self sustaining by wind action.
eddies are often located were there are conflicting currents adjecnt to each other, and the eddies are almost like small bearing that take the presure of equalizing the two conflicting flows.
(for reason i don't fully understand,) High tide in the bronx (north end of 'EAST RIVER') is not at the same time as hightide in manhatan (south end of EAST RIVER)--(obviously the east river isn't really a river but a straight!)
Hell Gate is were the two conflicting tides 'meet' --it is marked by strong currents, whirlpools and eddies. the uneven levels of the tidal waters are 'evened out'--messily!
