Though Twitter has been abuzz with data showing that the deadly marine heatwave known as The Blob is returning, there has been little to no reporting on it so far. I am sure that will change once the N Pacific heat wave intensifies.  

But, News of the North Bay does an excellent job writing on the phenomena:

Of great interest currently is a rapid heat up of sea surface temperatures off the California coast, and across almost the entire North Pacific. The re-emergence of a warm water “blob” is not only unusual during our current La Nina conditions, but has rapidly reached levels rarely seen. 

Already the heat up has brought tropical fish species like the Mahi Mahi to the Southern California offshore waters, and other subtropical species to the Northern California coast.

A close up look at the current sea surface temperature anomoly shows unusually warm water from just north of Hawaii, extending to the California coast. Further north, SST’s are off the charts.

The ocean temperature signals are a topic of much debate in scientific circles. While the cool water near the equator is typical of a moderate La Nina, the hot water further north is not. One theory being circulated is controversial, and it holds that persistent high pressure forming over the warm sea surface of the North Pacific will force the storm track south, gracing Callifornia with substantial rain. This would be similar to the winter of 2016-2017. Other theories are more ominous, with the return of a North Pacific blocking high pressure system that keeps storms from reaching California.

Importantly, this extremely warm sea surface water has showed up only recently, and may not be around through the fall and winter. It is simply too early to tell. For now our relatively high humidities (for Northern California) in recent weeks are almost certainly due in part to the water temperatures. And, it is becoming increasingly acknowledged that climate change is altering past climate patterns to a significant degree. We will continue to follow the situation and keep you up to date.

NOAA has a great analogy on how climate change makes waters hotter.

A good analogy is steroids and home runs. One of these events is like a batter getting ahold of a pitch and hitting a long fly ball. If that batter’s on steroids, like our climate system, the ball is liable to go that much farther and go over the fence and become a heat wave.

Marine heatwaves are dominating much of Earth's oceans. The North Pacific has experienced a severe marine heatwave this July, which will have disastrous impacts on marine ecosystems. Water temperatures have been as high as 13.6°C (24.5°F) above normal. pic.twitter.com/yz2pNi8VUv

The National Audobon Society writes that heatwaves rapidly displace marine animals capable of fleeing the extraordinary temperatures. They have been found to shift 460 miles in an attempt to stay cool. Those that can’t escape, such as shellfish and corals, will die.

Up until now, most research on marine heatwaves has focused on stationary organisms like  coral reefs  that have no choice but to endure the heat—and often perish. But a new  study  conducted by researchers at the National Oceanic and Atmospheric Administration examines how mobile species like fish respond to these climatic events. Published today in  Nature, the study found that marine heatwaves can raise the temperature of hundreds or thousands of miles of cold-water habitat, forcing wildlife to quickly move to cooler areas to survive.

“In past marine heatwave events along the West Coast, there have been over 1,000 kilometers [620 miles] of displacement,” says Michael Jacox, lead author of the study. “The temperatures you would normally see in southern California might actually be found off of northern California or even Oregon.” These dramatic, temporary  shifts affect not only aquatic animals, but also the seabirds that feed on them.

The most extreme case known to science occurred in the Northeast Pacific from 2014 to 2016, when an unprecedented marine heatwave known as “the blob" shifted thermal habitats more than 460 miles, according to the study. During that period, the region experienced one of the  largest seabird die-offs in Alaskan history: Tens of thousands of emaciated alcids washed up along the coast. Since 2015, these mass seabird die-offs in the Northeast Pacific have become  nearly annual occurrences, affecting Cassin’s Auklets, Common Murres, and Short-tailed Shearwaters, as well as Alaska’s Native communities that rely on the eggs for protein.

The mass seabird death associated with the blob "is a very prominent example of thermal displacement,” says Kathy Kuletz, a U.S. Fish and Wildlife Service biologist. She recently co-authored a  study  that attributes the death of an estimated 1 million Common Murres in 2015 to the blob. In fact, the seabirds' deaths on the surface gave scientists a clue that something serious was happening underwater—and the new analysis confirms that it was a marine heatwave. Like fish populations, some seabird species have also shifted their distributions under relatively short periods of time, Kuletz says. During the the region's warmer-water years of 2017 to 2019, Thick-billed Murres moved north, appearing in higher densities in the southern Chukchi Sea than in their regular habitat of the northern Bering Sea.