Why Is the Ocean Still So Warm in November?

By now, San Diego's water should be sliding toward winter. Instead, Scripps Pier set a late-season record in October and the ocean is still running far warmer than normal. Here's what's keeping it warm, what it's doing to the coast, and how long it could last.

Most years, San Diego's ocean peaks in late summer and starts cooling through the fall. This year it never really let go. On October 19, the sea surface at Scripps Pier hit 74.5°F, the highest reading that late in the year in more than a century of records, the San Francisco Chronicle reported. That's about 10 degrees above the late-October average of 65.3°F, according to Meteored.

Locals have noticed. On a hot weekend in late August, Santee resident Ben Stanger jumped in at Mission Beach after a morning run and told NBC 7 San Diego what plenty of San Diegans were thinking:

"I'm used to going numb after a few minutes. But with the water the way it is right now, I could stay in there all day."

Ben StangerSantee resident, to NBC 7 San Diego

"It doesn't feel like San Diego," he added. Two months later, it still doesn't. So what's going on?

Reason 1: A marine heat wave that never left

The warm water didn't start with El Niño. A marine heat wave formed off the West Coast in 2025 and has hung on ever since. NOAA tracks it as NEP25A, and its latest update says "very few areas have been without record-breaking temperatures."

Back in May, NOAA researcher Andrew Leising told KPBS that "all the warming we're seeing now is left over from basically the marine heat wave that started last year." That same month, Scripps' Melissa Carter told NBC 7 the heat was tied to "persistent high-pressure systems that last in the North Pacific." Those systems weaken the winds, and that matters for the next reason.

Reason 2: The ocean's air conditioning is switched off

San Diego's water normally stays cool thanks to upwelling. Northwest winds push surface water offshore, and cold water from deep below rises to replace it. It's the coast's built-in air conditioner.

This year the winds have been calm, so upwelling has largely stopped. "We have been seeing tremendously warm water, and not seeing a lot of upwelling," UC San Diego marine biologist Jennifer Smith said in a UC San Diego briefing on October 20. Without cold water coming up from below, the sun keeps heating the surface and nothing resets it. It's also why the water is so clear: no upwelling means fewer nutrients and less plankton, as CBS 8 reported. We covered that side of the story in Why San Diego's Water Is So Clear Right Now.

Reason 3: El Niño's Kelvin wave arrived in October

On top of all that, a very strong El Niño is building in the tropical Pacific. One of its effects is a Kelvin wave: a slow pulse of warm water that travels from the equator and then runs north along the coast. One reached California in October and pushed the thermocline, the boundary between warm surface water and cold deep water, hundreds of feet deeper, according to the Chronicle.

A deeper thermocline means even when the wind does blow, there's less cold water close enough to the surface to mix up. "All the changes we are seeing right now caused by El Niño are stacking on top of (an) existing marine heat wave," UC San Diego oceanographer Daniel Rudnick told the Chronicle.

How strong is this El Niño? Equatorial Pacific temperatures are running about 7°F above normal, and NOAA gives an 83% chance of a strong-to-very-strong event this winter, per UC San Diego.

What the warm water is doing

The kelp is melting

Giant kelp needs cold, nutrient-rich water. "Unfortunately, temperatures above 68 degrees, the stuff just disintegrates," Brian Zach, owner of Zach's Scuba Shack, told CBS 8. He said the thickest kelp forest near the quarter-mile buoy off La Jolla Cove is now empty. Jennifer Smith has measured about 70°F water even 50 feet down.

Tropical visitors are showing up

Giant oceanic manta rays have been spotted near San Diego, striped marlin are turning up far north of their usual range, and mahi mahi, bluefin tuna and bonito are being caught closer to shore, the Chronicle reported. In September, an olive ridley sea turtle nested in Southern California, the first documented nesting on the U.S. West Coast.

The tides are running high

Warm water expands and takes up more space. Add the Kelvin wave, and sea level along the coast has climbed. In late October, 10News reported a morning high tide above 8 feet, with water running 1 to 1.7 feet above predicted tides and a Coastal Flood Warning for San Diego County's coastline. Climate scientist Daniel Swain wrote on Weather West that those tides were "not being caused primarily by exceptionally large tidal influence from the sun and moon." The ocean itself is higher.

How long will it last?

Probably months. The European ECMWF model forecasts coastal water about 10°F above normal from November through January, per the Chronicle. Swain expects El Niño to keep strengthening for at least a few more weeks and the warm-water effects to persist for months.

The bigger near-term concern is flooding. The real king tides are still ahead, around November 24-26, December 23-25 and January 21-22, and Swain warned they could run 6 to 12 inches higher than October's. NOAA gives San Diego better than an 80% chance of high-tide flooding in November, December and January, according to Meteored.

Winter storms could cool the water locally by stirring it up, but El Niño storms bring their own problems. "We'll keep seeing unusual things all winter!" NOAA's Leising told the Chronicle.

Making the most of it, safely

• Still wear a wetsuit. Warm surface water can hide cooler layers below, and a long session at 70°F will still chill you. A 3mm suit is plenty for most people right now.
• Watch the tides. During king tides, beaches shrink, waves reach the bluffs and access stairs can flood. Check tide times before you walk out.
• Wait out the rain. San Diego County advises avoiding ocean contact for 72 hours after rain, especially near river mouths, lagoon outlets and storm drains.
• Log what you see. Unusual fish, missing kelp and water temps all help scientists track how the reef is changing.

Frequently asked questions

What's the normal water temperature in San Diego in November?

The late-October average at Scripps Pier is about 65°F, and the long-term November-through-April average is about 58 to 63°F, with the coldest water usually from late December through March, per NBC 7.

Is this the same as 2015?

It's the closest match. CBS 8 reported the only comparable October in Scripps Pier's 110-year record was 2015, when a marine heat wave was followed by El Niño. This year, El Niño looks stronger.

Is the warm water dangerous?

Not to swim in, but the side effects are real: kelp loss, hungry seabirds and marine mammals, and coastal flooding during king tides.

More from the blog: Why San Diego's water is so clear right now · Best time to dive La Jolla Cove · The "Do Not Eat" fish advisory explained