One of the noticeable trends this week at the coast is a drop in sea surface temperatures along the Jersey coastline. This happens from time to time during the summer and it's known as upwelling. You can see the difference in temperatures along the Shore from Monday (first map below) to Thursday (second map below), with the reds and oranges (temperatures that are above 77, 78 degrees in the surface water) replaced by blues and greens (temperatures below 72 and in the case of the blue shading, below 70 degrees).
Upwelling occurs locally when warmer surface water is pushed out to sea and replaced by cooler water that comes up from lower depths of the sea below. Upwelling can be caused by a strong wind push offshore, such as the one we dealt with on Monday and Tuesday along the coastline, and can drop temperatures in the immediate ocean along the coast by ten or more degrees in a matter of 12-24 hours. This can turn what feels like great ocean water one day into an ice box the next. Delaware Bay can also be victimized to an extent (see above) as water gets pushed out into the Atlantic (typically on northwest winds) and cooler water below the surface replaces the moved out warmth. However, the Bay is shallower and upwelling does not last as long in the Bay as it does along the coast.
The cool water will not last -- the sun's energy will warm this water back up and ocean temperatures along the coast for the weekend will slowly warm, perhaps not to the upper 70's as we saw a week ago, but back into the low/mid 70's for the weekend. This will make swimming a bit more tolerable although some local beaches may have an issue or two with cooler temperatures through the weekend.
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