El Niño is “not a heat wave, it’s not heavy rain, it’s not something that comes to California,” says Swain, who studies extreme events and climate change as a California Institute for Water Resources researcher.

It is, by definition, a large patch of warm water near the equator, way out in the middle of the Pacific Ocean. When that patch gets to be at least 0.5° Celsius (0.9° Fahrenheit) warmer than the average sea surface temperatures of the past 30 years, it contains enough energy to influence weather conditions all over the globe. It’s the warm phase of a weather cycle—El Niño and the Southern Oscillation, or ENSO—that occurs normally every 2–5 years. La Niña is El Niño’s cool-water counterpart.

We don’t have comparable maps yet for 2026, but here’s what ocean temperatures looked like in January 2015—before the start of the 2015–2016 Nino—and in November, once it was underway. NASA Earth Observatory map

Researchers try to predict this anomaly in sea surface temperatures ahead of time using statistical models—and they collect ocean data to track what actually happens.

Location of El Niño monitoring zones, Niño 3.4 is the one most scientists use now. NOAA

Even if the warm water anomaly is way out in the tropical Pacific, El Niño is felt throughout the globe. El Niño heats the air above it, ultimately weakening trade winds and moving the Pacific jet stream down from the Pacific Northwest to California and the Southwest. 

Every El Niño is unique, but its impacts are typically felt during winter months in California. 

From the archives: Why is it called El Niño? Here’s how scientists explained this weather phenomenon.

Nope. They’re totally different patches of warm water.

A marine heatwave is a stretch of time during which part of the ocean is warmer than usual. There’s been a major one off California’s coast since May 2025 that scientists say will likely continue and intensify until the end of the year. Like El Niño, this California marine heatwave (sometimes called a MHW) is very disruptive for marine ecosystems—the Blob of 2014–2016 (which also coincided with a big El Niño) caused unprecedented toxic algal blooms, for example. These heat waves seem to be happening more frequently as our climate warms:  Off the West Coast, we’ve been seeing enormous MHWs for the past seven years. They usually begin in the spring and end in the winter. And the warm waters are much closer to our California shores than El Niño’s warming of the tropical Pacific (way out by the coast of Peru.) 

Anomalously warm water in the Pacific, as of August 3, 2026. El Niño is upon us. NOAA Coral Reef Watch

Sea surface temperature anomalies in the California Current ecosystem. The black outline shows the current marine heat wave. U.S. National Oceanographic and Atmospheric Administration

El Niño, meanwhile, usually lasts around 9 to 12 months, forming around early summer and tapering off after winter or spring. 

“In the summertime, there’s not really a strong connection between what’s happening in the tropical Pacific, where El Niño is sourced, and what’s going on in our weather over here,” says Art Miller, oceanographer at Scripps Institution of Oceanography. 

Right now, this massive marine heatwave is the major factor battering local marine life, but as El Niño continues to intensify in late summer and fall, it’ll play more of a role—likely, they won’t be able to catch a break.

Even though the MHW is a separate phenomenon from El Niño, we’ll see their impacts compound. According to NOAA Fisheries, they might mean, for example, more harmful algal blooms—which can poison seabirds or marine mammals, and make crab and shellfish toxic for us to eat. They could bring tough times for salmon, which have a harder time looking for prey in warm waters, while rockfish, which don’t mind warmer water, might thrive.  

Remember how we said El Niño is defined as a patch of water way out in the Pacific that’s 0.5° Celsius (0.9° Fahrenheit) above expected? Well, currently, NOAA is predicting an 81 percent chance that this patch will exceed 2.0° Celsius (3.6° Fahrenheit) above average during October, November, and December, which would make this El Niño “very strong.” And right now, that patch is sitting at around 1.5°C (2.7° Fahrenheit) above average for the end of July. 

But 81 percent is not 100 percent sure. What researchers are nearly totally sure about (97 percent, in fact) is that whatever El Niño we get, it is going to stick around through early spring 2027, close to a typical length for an El Niño.

Global warming could be making the El Niño/La Niña cycle shorter and stronger. But even though El Niño is one of the most studied weather events on the planet, researchers still don’t have enough years of data to say definitively how global warming influences it. Still, its fingerprints are likely at the crime scene. “There is definitely a signature of global warming in this current, expected to be as large as ever, maybe the largest ever event,” says Miller. 

NOAA scientists have been trying to distinguish what warm water anomalies come from El Niño versus what comes from global warming. El Niño has long been defined by a deviation from a 30-year average of ocean temperatures. But as the oceans have warmed, that 30-year reference is increasingly behind the times. So NOAA scientists this year came up with a new way to track El Niño and La Niña called the Relative Oceanic Niño Index (RONI) that compares temperatures in that patch to the rest of the tropics, which means it’s less sensitive to that 30-year reference. Hopefully, this new index will track the phenomenon in a way that better aligns with the real-world impacts it causes.

As noted above, the Godzilla predicted for the 2015-2016 El Niño didn’t really turn up—at least in terms of rainfall in northern California. (Were scientists wrong? Not really. Predictions and probabilities for wet winters are just that—probabilities—leaving a chance for California to see average precipitation or less in an El Niño year. “El Niño ‘tilts the odds’ for weather and climate impacts,” as NOAA has put it, but doesn’t guarantee them.)

An example (not an official forecast!) of how El Niño can tilt the odds toward a wetter winter. Climate.gov

Yet that El Niño was still considered one of the strongest in decades—bringing sea temperatures more than 2°C (3.6° Fahrenheit) above average. That came with serious impacts, including a record number of stranded Guadalupe fur seals and thousands of dead auklets and murres along the West Coast.

This 2026-2027 El Niño will likely be stronger than the ‘Godzilla’ El Niño ten years ago. That earlier event’s impacts offer some preview of what we could see this year. “But just because one strong El Niño event did not yield a wet winter absolutely does not mean that this one will not be wet,” says Swain.

Map of typical El Niño winter impacts from NOAA. See how the San Francisco Bay Area is outside the zones with more predictable patterns. NOAA

More rain. Odds are we’ll see a wetter winter and spring in California. But that isn’t guaranteed. The Bay Area is in an area with less predictable El Niño patterns, unlike some other parts of the U.S. “We can’t generalize and say El Niño means X for California,” Swain says.

That general pattern holds true in NOAA’s latest outlook: the Bay Area is in a zone with equal chances of being wetter or drier in the next three months. NOAA

Warm weather. Usually, El Niño brings warmer weather to northern and central California during fall and winter. These trends are less reliable for California than the rest of the U.S., but this June was the second warmest on record globally since 1940. 

That general pattern holds true in NOAA’s latest outlook: the Bay Area is in a zone where it’ll likely be warmer than usual in the next three months. NOAA

Higher sea levels, bigger king tides, and more coastal flooding. This is a big one. “I think coastal flooding is going to probably be an issue almost no matter what else happens,” Swain says. 

Because hot water expands, El Niño temporarily raises sea levels around six to twelve inches. A strong El Niño will likely increase coastal water levels even higher than it usually does. That will come on top of the long-term sea level rise that global warming has already caused—another six to twelve inches since the early 1900s. So king tides—the biggest tides of the year, next coming in November, December, and January—are likely to be bigger than usual as a result. And when they happen—or when storms bring more water—we may have big problems in coastal areas that are already prone to flooding. When extra-high king tides and rain joined forces this past winter, Corte Madera in Marin County temporarily turned into a lagoon.

A map of potential flood zones from a 2-foot sea level rise in the Bay Area. NOAA

The combination of high sea levels, king tides, and storms have the potential to be disastrous, and weather experts are warning cities to prepare for coastal flooding now. “In my experience, sometimes it does take a bit of a kick in the pants to really motivate actually going through these exercises,” Swain says. “Thinking through, what is your actual response plan, and what mitigations would you put into place?”

Potential flood zones from high tides. NOAA

In 2024, the state began to require every city to come up with a shoreline adaptation plan to prepare for rising sea levels. Many of these coastal resilience projects in the Bay area are currently underway, like the South San Francisco Bay Shoreline Project, which involves building a tidal flood protection levee. 

More shoreline erosion. If bigger storms and waves arrive, they’ll eat into shorelines that are already eroding. Another reason why strengthening seaside infrastructure will become more important in the coming months. Scientists found that the 2015–2016 El Niño erased hundreds of feet of California beaches. 

More fire risk? Perhaps unintuitively, more rainfall doesn’t always mean less fire overall. Yes, rain can put a quick end to dangerous fire conditions. But thunderstorms can also bring lightning that sparks wildfires. 

“The determining factors in lightning ignition potential are the stress and dryness of the fuels (grass, brush, trees) and how much, if any, rain accompanies the thunderstorms,” Robyn Heffernan, fire weather senior advisor for NOAA’s National Weather Service, told Bay Nature via email. “Even if significant rain accompanies thunderstorms, lightning frequently reaches far outside where the rain falls.” 

A rainy winter or spring can also encourage plants to grow more lushly. Then when a heat wave comes next summer, they may dry out—leaving more fuel sitting around, ready to burn.

Maybe less fog? Bay Area fog is at its peak during the summer months (see: Bay Nature’s ode to summer fog). Fog is created when warm air moves over the cool California waters, condensing moisture into fog. Both the marine heat wave and El Niño could suppress that process and create some fog-free windows. As of late July, fog has already been a bit patchy.

Every summer, California ecosystems benefit from the life-giving phenomenon of upwelling. Driven by persistent northerly winds, deep, nutrient-rich cold water moves to the surface and creates grand fishing grounds. El Niño events typically reduce this upwelling, which motivates fish to move to areas with more meals. When the fish go, marine animals either have to follow or starve. The whole marine food web is affected. 

This year’s marine heatwave has already put a strain on several species, and El Nino is likely to compound the strain. Brown pelicans, Brandt’s cormorants, and common murres have been the first victims to show up dead on shores, according to Tammy Russell, a marine ornithologist at Scripps.

The 2015–2016 marine heatwave/El Niño wallop of marine ecosystems offers some clues on what to expect. Seabirds, sea lions, Dungeness crabs, salmon, krill, and baleen whales all suffered. Rockfish, tuna, and orcas did well. Commercial fishermen targeting squid got a windfall, because lots of squid showed up closer to the coast. Seabirds like the Cassin’s auklets and common murres struggled. 

Seabirds are easier to monitor than most marine animals, so marine biologists like Russell use them to learn about the broader health of marine ecosystems. “If you’re visiting the beach, you may see an increase in dead seabirds or sick seabirds,” she says. “Having the number to your local bird seabird rescue or marine mammal rescue is a good idea.”

Another likely phenomenon: More ocean-dwelling species visiting from southern climes, enjoying the warmer-than-usual waters off our shores. The last El Niño brought sightings of large sunfish, whale sharks, tropical brown boobies, and other rarer tropical species to the coast of California. And there’s been some talk that sharks and venomous yellow-bellied sea snakes may be feeling more welcome in these warmer waters.