Vast Pacific marine heat wave bringing humid weather to California

A vast marine heat wave recently arrived to the California coast after slowly migrating across the Pacific Ocean. A buoy just off the coast of San Francisco measured 64 degree ocean water Sunday, the warmest measurement there in three years.

Sea-surface temperatures are as much as 7 degrees above normal in parts of the northeast Pacific. It’s the fourth-largest marine heat wave since 1982 in a region extending from just north of Hawaii to California and Alaska, according to Andrew Leising, a research oceanographer at the National Oceanic and Atmospheric Administration.

“It’s hot out there!” Leising said in an email. “Pretty much the entire coast is a heat wave now.” 

Scientists don’t agree on a single marine heat wave threshold, although it is generally defined as ocean temperatures above the seasonally-varying 90th percentile. The Southern California Bight from Point Conception to the Mexico border is the only area in the northeast Pacific not meeting heat wave criteria, according to Leising.

If the current heat wave follows past similar events, according to Leising, it’ll continue to expand and encroach shoreward throughout September and fade by November. Scientists don’t yet know how deep into the ocean the anomalously warm water extends, but even a shallow region of warmth can impact coastal weather.

San Francisco’s dew point — the temperature that water vapor condenses to liquid — climbed to the mid-60s Tuesday morning. These higher dew points are a direct impact of evaporation of the warm ocean water, creating muggy conditions. Typical dew points in San Francisco this time of year are in the mid-50s.

Temperatures aren’t able to drop as far at night due to the increased humidity. San Francisco’s lows continue to bottom out around 60 degrees, a few degrees warmer than normal.

If the marine heat wave persists into the wet season, storms could bring more lightning and heavier downpours “due to … greater instability created by cold air over the warmer-than-average water,” said Alex Tardy, author of the Weather Echo blog and a retired National Weather Service meteorologist.

Marine heat waves have impacts beyond meteorology. Marine animals change their feeding and migration patterns, moving into a narrower zone of cooler water, which increases the risk of whale-ship entanglements and harmful algal blooms.

Water along the California coast follows an annual temperature cycle, reaching its coldest point in spring and warmest in early fall. But marine heat waves can magnify this cycle.

From 2014 to 2016, scientists tracked a massive area of water along the West Coast, coined “The Blob,” that was unusually warm as far as hundreds of feet below the water’s surface. The Blob lasted two years before eventually fading.

Since 2016, anomalously warm sea-surface temperatures have frequently appeared in summer, but none have lasted as long or penetrated as deep as The Blob. These marine heat waves often start far offshore in spring and move toward the coast by fall, and some years are more pronounced than others.

This year’s marine heat wave is big for this time of year, and could grow. It rivals 2019 as the northeast Pacific’s largest through early September, Leising said.

“We’re still about one month away from when these heat waves typically reach their maximum area,” Leising said. “There’s a moderate probability that it might get even larger.”

It’s already the fourth-largest marine heat wave on record in the northeast Pacific, according to Leising’s dataset. He calculates daily sea-surface temperature anomalies back to 1982, when satellite-based ocean measurements began. By other definitions, which average months of data, the current marine heat wave isn’t as severe.

Scientists say the marine heat wave is being caused by weak winds over the northeast Pacific. Calmer winds whip up fewer waves that overturn cold water from deeper in the ocean. This weather feature is known as the North Pacific High, which reaches its peak strength in summer.

Other parts of the Pacific have been unusually warm the past several years, too. 

Changing currents along the coast of Japan have pushed tropical water into the central North Pacific, where ocean water is typically colder. Any shifts to water temperatures in this region can impact the strength and position of the jet stream, which guides storms such as atmospheric rivers from Japan toward the U.S. West Coast.

Climate change is heating up oceans worldwide as marine heat waves become more common. The North Pacific is a hot spot. Scientists like Leising are concerned marine heat waves there could continue to accelerate.

“That just blows my mind that we’ve already gotten to that point where we’re just accepting that it’s almost like this is the new normal,” Leising said.