Has something to do with the way the various larger currents shift a bit as the season changes. The result is that waters that were approaching about 68 suddenly drop about 10 degrees. The other result is a significant increase in nutrients from the deep... followed by a plankton bloom, and often a red tide.

This sort of effect can cycle on and off as the currents wobble similar to the way the jet stream wobbles and alters the atmospheric weather in an area. I remember one year not long ago that was one red tide after another... makes the water look chocolate brown as much as couple miles off shore.

This year they are calling for el nino... which means that the warmer currents will tend to dominate, with perhaps a wetter winter in the west. But the whole thing starts well south of here and takes place over several months, with the actual shift in temp locally only being a couple of degrees. The shift in rainfall however is dramatic. The whole thing has a many year cycle... I don't recall, something like 7 or 14 years... I really don't remember. But the bottom line is that it is a long cycle.

And of course there are other longer cycles... seems to me there is something of a saline shift in the north Atlantic that affects things like the Gulf Stream and temps along the eastern seaboard and the north seas.