The warmest swimming ever

“Usually I have to wade in and get numb,’’ said Joanne Smith, a Littleton resident taking an extended swim with her 11-year-old son, Chris. “Today, I was able to just get right in.’’

Up and down the New England coast, beachgoers this summer have reveled in unusually warm water, with even ankle-waders taking the once-bracing plunge. From York Beach in Maine to Horseneck Beach in Westport, vacationers and day-trippers marveled at the sudden absence of what Smith called the “pain factor,’’ the numbing sting that on the warmest days would make a quick dip a gasping ordeal.

Worldwide ocean surface temperatures reached a record high this summer, according to the National Climatic Data Center, rising to more than 1 degree above average. But the coast of New England, namely the Gulf of Maine, saw a far more dramatic rise in temperatures - to more than 64 degrees, some 3 degrees higher than the 20th-century norm.

At times this summer, water temperatures off the coast of Portland, Maine, and Gloucester were more than 10 degrees above normal, the center found. In Maine and New Hampshire, waters often topped 70 degrees, an unheard-of threshold, and surfers who normally wore wet suits braved the waters in bare skin.

“During the month in which the planet’s ocean surface temperatures set a record, this region was even more anomalously warm than the rest of the planet,’’ said Derek Arndt, chief of the climate monitoring branch of the National Climatic Data Center.

Water temperatures can change quickly and vary sharply from beach to beach, and are often monitored on shore only intermittently, making precise comparisons tricky. Some beaches, including Crane Beach in Ipswich, reported that the water in recent weeks had been no warmer than usual.

But anecdotal evidence - from scientists to sunbathers - strongly suggests a broad warming trend, particularly north of Boston. Monitors for the Maine Healthy Beaches Program found a pattern of higher temperatures from southern Maine up the coast to Penobscot Bay.

At Maine’s Scarborough Beach, infamous for its chilly waters, surfing students felt comfortable enough to shed their wet suits.

“I don’t remember putting my booties on since the second week in June, and I haven’t heard any complaints from the kids,’’ said David Turin, who runs a surf camp at the beach.

At Hampton Beach in New Hampshire, water temperatures reached 70 at least five times this summer, a mark the chief of lifeguards had rarely seen in his 50 years there.

Just why the water has been so warm is difficult to untangle. Keri Lindberg, who coordinates the healthy beaches program, says global and regional forces may be at play, but so might local circulation patterns, tides, and weather. Arndt attributed the worldwide rise in temperatures to global warming and El Niño, but said any warming of shore waters may be unrelated.

“The smaller-scale patterns are very chaotic and it’s hard to predict how they fit into the big picture,’’ he said.

Don Perkins, president of the Gulf of Maine Research Institute, a marine science center in Portland, said he swims three times a week in the ocean, and agrees it has been noticeably warmer this summer. But the exact cause is elusive, at least for now.

“It’s a hard thing to get a simple answer to,’’ he said. “It takes a while to sort out causality.’’

Researchers offer a range of theories, but many point to short-term, regional factors, such as the heavy rains earlier this summer. Fresh water tends to be warmer than ocean water and floats on top, closer to the sun, researchers say.

“I think we’re talking about a climatological phenomenon,’’ said Graham Shimmield, executive director for Bigelow Laboratory for Ocean Sciences in West Boothbay Harbor, Maine.

At the same time, most researchers say it is possible that global warming is a contributing factor, although no definitive link has been established.

Among countless other potential consequences, the warmer waters may have driven away fish that prefer cooler temperatures, said Bruce Berman, spokesman for the Boston-based advocacy group Save the Harbor Save the Bay. Using a sonar system, Berman has noticed a dearth of fish at the mouth of the Charles River.

“June and July were very different than any other year that I’ve been on the harbor,’’ he said. “Usually, I see a mix of herring, mackerel, butterfish, menhaden - all the things that big fish feed on. This year, there is almost nothing.’’

In the last few weeks, after the remnants of two tropical storms brought cooler water, Boston Harbor has “exploded with life,’’ he said, but he cautioned that other factors may be at play.

At the same time, other fish have traveled farther north than usual.

Molly Lutcavage, director of the Large Pelagics Research Center at the University of New Hampshire, said that for the last few years, she has noticed an influx of smaller bluefin tuna to the area - fish normally seen in the warmer waters off New York’s Long Island or North Carolina. Also, she said, plankton have shifted to blooming later in the year. Plankton are a major diet source for herring.

“The warming temperatures shift the whole system, and that affects bottom-up aspects of marine life,’’ said Lutcavage.

Conducting research off the coast of Provincetown, she measured the temperature at an astounding 72 degrees.

“It was unbelievably warm!’’ she said. “It was really quite something.’’

Ronnie Lees, of Northeastsurfing.com, a surf sport website, said the temperatures in August were the warmest people had ever reported, especially in Maine.

“Maine is one of the coldest waters we have in New England,’’ he said. “I’ve never heard of Maine being 72 degrees in my life.’’

In Salisbury last week, the debate focused less on the how, and more on how long it will last.

“I remember coming here when I was a kid and it was numbing,’’ recalled Kerry Abdella of Boylston. “Today it’s cool, but it’s refreshing.’’

“For September, to be able to go in the water like this, it’s nice,’’ Abdella said, as she dangled the toes of her 1-year-old daughter, Zoe, in the surf.

Milton Valencia of the Globe staff contributed to this report.