LOS ANGELES — Southern California was built on many myths — but its weather was not one of them.

Generations were lured to the region with promises of perfect weather, that "mild Mediterranean-type climate" that could heal the sick and grow practically everything. The railroads advertised the L.A. climate as a last resort for the world's sickly. It was the place where rained-out sporting events and umbrellas at the Rose Parade were so rare they were occasions for big news.

Sure, it could get hot. But, as the boosters always said, "at least it's a dry heat."

So what went so very wrong this year?

The oppressive moisture in Southern California's air this week has felt more like Florida and the Gulf Coast, with many remarking that this is the worst, most powerful humidity they've ever felt. Scientists suspect that the combination of high heat and a suffocating deep-tropical air mass will be considered the worst humidity the region has experienced in the modern era.

"These are some of the highest humidities we've ever experienced in coastal Southern California. There's already some evidence that this summer has been, at least at some locations in Southern California, the most humid summer on record," said UC climate scientist Daniel Swain.

And it could be a grim preview of things to come. Climate change has been heating up Southern California for some time, heightening fire danger and worsening droughts. With a very strong El Niño climate pattern predicted this winter, experts expect significant — and possibly historic — damage on the California coast.

"And the reality is, in some of these cases, we may not be able to do much at this point to engineer a way out of quite-serious coastal flooding, beach erosion, bluff collapses later this winter," Swain said. "It's going to be a real problem."

Before this week's searing humidity, using data through the end of August, Swain had already calculated that this summer had the highest humidity as measured by the dew point — a reliable indicator of how dry or humid it feels. His analysis found that, since 1980 — using dew points averaged at the Santa Barbara, Los Angeles, Long Beach and San Diego airports — this summer's humidity was the highest over the last 46 years.

Record-breaking humidity

According to the weather service office in Tampa, Fla., a dew point of below 60 degrees is comfortable; above 65 feels muggy; above 70, very humid and uncomfortable; and above 75, oppressive. The dew point reached 76 on Tuesday in Long Beach, Huntington Beach and Newport Beach shortly after noon — a similar dew point to that found in New Orleans and Orlando, Fla., at the exact same time.

The humidity makes heat feel even hotter.

And that's what makes this summer feel so different.

Coastal Southern California enjoys its relatively mild summer climate thanks to several factors, including a cool ocean's air-conditioning effect, protective mountains, proximity to deserts, and a weather pattern that usually avoids drawing in tropical moisture.

Southern California's dry summers typically exist because of its land-to-sea breezes in the morning and sea-to-land breezes in the afternoon, according to meteorologist Kristan Lund of the weather service's Oxnard office. The morning cycle brings humidity-lowering dry air from the deserts into the L.A. Basin, while the afternoons see air over the chillier ocean water move inland. High pressure over Southern California in the summer tends to result in warm weather that's dry and doesn't pull in tropical moisture.

The ocean off of Southern California is also much chillier than along the Gulf Coast and East Coast. The average coastal water temperature in Santa Monica in September is 67.9 degrees, but is 86.3 degrees in Tampa Bay in Florida, and moisture from the Gulf of Mexico influences the humidity all around the region, as well as up the East Coast.

All of those protections against humidity spectacularly broke down this summer in Southern California.

"This is not the same thing as you might say, 'Yeah, Woodland Hills, they always get to 110 every summer,'" Swain said, which might typically feel cooler, because, as you sweat, a dry breeze cools you, especially in the shade. But when the air is humid, people can't shed heat as quickly, Swain said.

"The other problem here, too, is that the very highest heat-index values I'm seeing are in Central and South Central L.A.," where a number of people don't have air conditioning at home, Swain said. "This is, you know, right at the upper edge of humid heat that we've ever really observed in the L.A. area."

L.A. County health officials urged people to be on the lookout for symptoms of heat stroke or heat exhaustion, such as dizziness, nausea, rapid heartbeat and confusion.

"Heat causes more deaths in the U.S. annually than floods, storms, and lightning combined. Those most likely to get ill during the heat include older adults, young children, outdoor workers, athletes, and individuals with chronic medical conditions," said Dr. Muntu Davis, the L.A. County health officer.

Causes for the extreme humidity

The extreme humidity is being fueled by the remnants of Hurricane Marie, which formed during an unusually active hurricane season in the eastern Pacific Ocean, likely related to El Niño, a climate pattern that refers to a warming of the tropical central and eastern Pacific Ocean.

"[Marie] was able to funnel its moisture over our area from the southwest, and that brought more humid conditions because it's tropical moisture," Lund said. The tropical moisture is combining with monsoonal moisture that can come into Southern California from the southeast at this time of year.

Marie's remnants also fueled the thunderstorms and rain that have washed over the region, and is contributing to the heat being felt across Southern California, the Central Coast and the San Francisco Bay Area, Swain said.

An extraordinary marine heat wave is further adding to the humidity. According to the National Oceanic and Atmospheric Administration's marine heat wave tracker, it has spread all along the California coastline.

There's also a stretch of exceptionally warm ocean water from the shores of the Galápagos all the way to Alaska, according to Swain.

"It's really warm off the coast of Baja [California] now — it is ridiculous. It is off-the-charts warm," Swain said. "Right now, off the coast of Baja near Ensenada, we're talking about water temperatures that are essentially 10 degrees Fahrenheit above average."

And off California, ocean-water temperatures are well above average, Swain said. "The water off of L.A. is at or above its warmest-ever-observed levels. And the water west of San Francisco is currently right around those highest-observed levels as well — relative to what's typical west of San Francisco."

Swain expects future waves of humidity are probable in coming years.

"This will probably not be the most extreme humid heat event we see even this decade in Southern California, even though it is at the very upper edge or even beyond what we've seen historically," he said.

Concerns about the winter

With Southern California still assessing the significant coastal damage from big waves and high tides in recent days, forecasters warned that more erosion was expected. Through Thursday, the weather service warned of significant beach erosion, coastal flooding of beaches and parking lots, "shallow flooding of seawater over vulnerable south-facing shores," as well as a higher risk of ocean drownings due to strong and dangerous rip currents.

The big waves are being fueled by the energy produced by Marie, and propelled by an elevated sea level due to warming ocean waters, according to Swain.

"The sea level off the coast of Southern California is now six to 10 inches above average," Swain said. "It might not sound like a lot, but ... you add on top of it high tide, you add on top of it big waves from Marie, and we're already getting significant coastal flooding."

Scientists are also expecting the arrival of what's known as a Kelvin wave in California later this month, which is traveling up the North American coast; it is currently off Mexico's west coast. Such Kelvin waves, which are generated during El Niños and move warm water across the ocean, raise sea surface temperatures and sea levels, according to NOAA.

One or two more Kelvin waves are expected through the fall and winter, Swain said.

"This is what we see during 'very strong,' or extreme, El Niño events, which is what we have this year — [this] progressive propagation of Kelvin waves. And they can stack on top of each other: So it might be that the temporary sea level rise from one of these Kelvin waves doesn't fully decline before the next peak arrives," Swain said. "This is why I'm so concerned about what November, December and January look like," when super king tides are expected.

On top of all that is the record warmth of the oceans, fueled by human-caused global warming, which are at higher levels that during previous very strong El Niños.

"The oceans are rising with each passing year. As that water becomes warmer, it thermally expands. More of the continental ice sheets melt. And it raises the sea level," Swain said.

"It's a little bit wild that we're talking about [how] coastal flooding in San Diego during an El Niño event is influenced by how much Greenland and Antarctica have melted in the last few decades," he added.

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