One of the most common weather questions we’ve been receiving lately comes from residents along the North Shore and Cape Ann: why is the ocean water still so cold?
It’s not your imagination – ocean water temperatures from Boston Harbor points north have been exceptionally cold this summer, after a fairly typical start to the warmer water season in the spring. As of this writing on July 19, the water temperature at Wells Beach, Maine, is 63 degrees but Portsmouth is only 59 degrees and the ocean water is similarly at or below 60 degrees all the way into Boston Harbor! Believe it or not, Portsmouth averages 60 degrees water this time of the year, but Boston Harbor should be running either side of 67 degrees, which is why North Shore and Cape Ann residents are especially sensitive to the cold water this late in the season.
The cooling ocean water really became noticeable after the first week of July as two phenomena occurred back-to-back to send water temperatures spiraling. First, a feed of colder water sliding south down the Maine coast surged along the Cape Ann shoreline and entered Boston Harbor from the north. Interestingly, ocean currents helped the water miles offshore recover fairly quickly over the last two weeks, but closer to shore, not only was a cooler pocket of water trapped, but a new phenomenon took hold: upwelling. Upwelling is a common occurrence for any coastal community near the open ocean – when the wind blows from off the land, usually a northwest, west or southwest wind in New England, that wind pushes the skim of relatively shallow, milder ocean water out to sea. The void left behind near the shoreline has to be replaced, so new water rises up from deeper, colder ocean levels to replace it, causing a drop in water temperature near the shoreline.
This July has been exceptional because right after we saw the push of a cold pool of water from the northeast, a steady supply of warm air has come surging into New England, arriving on a southwest and west wind. Even when cold fronts come through and break the warmth, the wind still blows from the northwest. All of this means the surface skin of ocean water keeps getting blown away and upwelling continues repeatedly along the coast. If the wind relaxes, or sea breezes set up for a few days, this would allow much milder water sitting miles offshore to blow back into our beaches…that change in wind just hasn’t happened yet. As you might expect, there are many New England coastal locales that benefit from warmer water in a pattern like this: the South Coast! From New Haven, Connecticut, all the way to the South Coast of Cape Cod, water temperatures in the middle 70s are running 1-3 degrees warmer than normal, as the shallow, mild water over Long Island Sound, Block Island Sound and both Vineyard and Nantucket Sounds is blown into the shoreline by the persistent wind. Chances are good that when the weather pattern allows for a relaxed wind and sea breezes develop – or if we get a few days of an easterly wind – you’ll see those North Shore and Boston Harbor water temperatures rebound fairly quickly.
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We’ll keep an eye out for those changes and keep you posted in our weather broadcasts on-air and online on NBC10 Boston, NECN and Telemundo New England’s First Alert Team.