San Francisco Chronicle

Winds are making bay water more chilly

- By Jack Lee and Danielle Echeverria Chronicle climate editor Kate Galbraith contribute­d to this report. Reach Danielle Echeverria: danielle.echeverria@sfchronicl­; Twitter: @DanielleEc­hev. Reach Jack Lee: jack.lee@sfchronicl­

The waters around the Bay Area are known for being chilly and unforgivin­g. But swimmers have had to endure even more unpleasant conditions than usual, the National Weather Service says.

“It has been frigid,” said Brian Garcia, a meteorolog­ist with the National Weather Service who likes to surf in Santa Cruz. Garcia estimated that the water temperatur­es are the coldest for this time of year in about a decade.

Readings on water sensors have plummeted along with air temperatur­es, according to Garcia, and register about 5 degrees below normal for March.

“People definitely notice,” said Zach Margolis, a board member with the South End Rowing Club, whose members regularly swim in San Francisco Bay. Not only has the water felt colder than usual, it’s also been chilly later in the year than normal, Margolis said.

“It’s been especially hard to get into the water,” Margolis said.

Diane Walton, the president of the Dolphin Club, South End’s neighbor and friendly rival, said the cold temperatur­es aren’t keeping swimmers away. Her club is in the midst of its Polar Bear Challenge, which is to swim 40 miles between mid-December and mid-March.

“While the drop in temperatur­e may mean shorter swims, it doesn’t keep us out of the water,” she said.

On a frigid and rainy Saturday morning, her words rang true. Dozens of swimmers bobbed in and out of the water ahead of the annual Polar Plunge, when dozens more jumped into the bay, many in costume, as a fundraiser for Special Olympics.

A group of eighth-graders from Los Cerros Middle School in Danville was among the first to take the plunge, and their screams revealed just how cold the water was.

“It felt freezing,” said Nina Indelicato, shivering in a soaked tie-dye blue shirt that matched her classmates.

But some said the cold of the bay was welcome after standing in the wind and the rain.

“It was better than it was outside,” said Jamie Meza. “It honestly felt warm.”

“It felt really good,” added Davaa Russell. “It was refreshing.”

The water temperatur­e at Fort Point, at the mouth of the bay, has been about 4 degrees below normal in recent days, according to data from the California Ocean Observing Systems Data Portal. Measuremen­ts on March 1 and March 2 averaged around 50 degrees Fahrenheit, slightly chillier than normal temperatur­es, around 54.

It’s a similar picture deeper in the Bay. The waters off Alcatraz Island are about 3 degrees colder than normal. At 12:45 p.m. on Saturday, for example, temperatur­es there hovered around 50 degrees Fahrenheit, noticeably below mean temperatur­es closer to 53 degrees.

The cause of the low water temperatur­es is cold winds coming from the Yukon and northern Canada, Garcia said. They push surface waters from north to south, bringing more cold water toward the Bay Area.

This weather pattern, in conjunctio­n with what is called the Coriolis effect that pertains to how water lags the spinning earth, have effectivel­y pushed water from the coast, leading to an upwelling of cold subsurface water, Garcia said. Upwelling also happens in the summer, but in winter there is less sunlight to warm the water, he said. The entire West Coast is affected, Garcia said: “It’s just cold.”

Different parts of the Bay may be responding differentl­y to the spate of cold weather.

“It’s 2 degrees colder in the Berkeley Marina right now,” Margolis said, based on comparison­s of water temperatur­es from swimmers’ watches.

John Largier, director of the Bodega Marine Laboratory in Bodega Bay, said that while the water in the bay and ocean has been chilly on some dates this year, it has sometimes been colder in other years.

Water in the bay, he said by email, can get cold “due to the inflow of freshwater from the rivers.” At the Martinez-Amorco Pier in Suisun Bay, water temperatur­es have been around 49 degrees Fahrenheit, he said, “but this is typical in winter and has dropped to 48F or even 47F there in prior winters.” Water takes a lot longer to cool than air, he noted.

In the area of the Explorator­ium, water temperatur­es may not be significan­tly lower than normal for this time of year, said Ron Hipschman, an Explorator­ium scientist, by email.

Fortunatel­y, for the brave swimmers and surfers venturing into frigid Bay Area waters, weather patterns are expected to shift: Water temperatur­es may rise mid- to late this week, Garcia said.

And for the Los Cerros students, who may not take the Polar Plunge again until next year, the cold didn’t bother them for long. Madeline Taylor, still shivering, suggested they wrap up their morning with some ice cream.

All of them enthusiast­ically agreed.

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