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So what are marine heat waves?
Katie Valentine

So what are marine heat waves?

A NOAA scientist explains

It’s likely you’ve experienced — or at least heard of — a heat wave on land: those prolonged periods when temperatures are unusually high. But in recent years, marine scientists have been turning their sights on another kind of heat wave — one that occurs in the ocean.

This image from September compares sea surface temperature anomolies (how much warmer or cooler the water is compared to normal levels) for September 2014, when the Blob emerged, and September 2019. Credit: NOAA Fisheries

From 2014 to 2016, the ocean waters off the West Coast were hit with hotter-than-usual temperatures in a marine heat wave that came to be known as “the Blob.” This stretch of warm water had big impacts on the West Coast marine environment and economy, and stands as the largest marine heat wave since NOAA satellites started keeping track in 1981. Now, three years after the last Blob, another marine heat wave has surfaced off the West Coast, and scientists say it’s the second-largest one they’ve seen.

NOAA Research sat down with Nick Bond, Washington state climatologist and a scientist with the University of Washington's Joint Institute for the Study of the Atmosphere and Ocean (a NOAA cooperative institute), to learn more about marine heat waves. 

What is a marine heat wave? 

Different folks have used different thresholds, but basically it is the ocean’s equivalent to what a heat wave in the weather is, which is very unusual temperatures that make a difference in the marine ecosystem. 

Just like a heat wave in Phoenix might have different characteristics than a heat wave in Seattle, the natural variability of the ocean is different from place to place. There are some places where there is a great deal of variability from year to year. In those kinds of places, you have to have that much more of an extreme anomaly for it to be called a marine heat wave event. In other places — lower latitudes especially — it doesn’t take as much of a temperature rise to be something that happens very rarely. 

How long have scientists been documenting marine heat waves? 

Calling them marine heat waves – that’s been the last roughly five years. It has been recognized for a while that ocean temperatures and properties fluctuate, but the focus on events rather than slower changes in the ocean is newer. 

What causes marine heat waves?

Almost always it’s unusual weather patterns that either cause more heat than usual to go into the ocean, warming up the surface, or in some cases suppress the amount of heat coming out of the ocean.

The Blob that got going in the winter of 2013 and 2014 was a case where the normal cooling of the ocean in the winter months was reduced. There was a very persistent ridge of higher-than-normal atmospheric pressure over the northeast Pacific Ocean, and it blocked the usual tracks of storms across that region, in the Gulf of Alaska and off the West Coast of the U.S. With those storms not going across, there was less wind to mix up cold water from below, which usually happens each winter.

Is climate change affecting the development of marine heat waves?

We know a lot of extra heat from the atmosphere due to increased greenhouse gas concentrations has gone into the ocean. So when we’ve gotten unusual weather patterns due to natural variability, they’ve caused fluctuations in ocean temperatures that are on top of that global warming. That means when we had a big fluctuation — like the Blob of 2014-2016 — that was expressed on top of that upward trend. It meant that the temperatures were that much warmer as a result. 

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.

Have marine heat waves become more frequent or more intense in recent years?

We’re still looking into that. I’ve gone back and looked at historical records and, for example, found a very intense heat wave in the Pacific in 1958. It was a cooler climate then, and it really stands out above the background of that period. 

We’re already seeing some tentative evidence that there are more of these events coming along and certainly our climate models are suggesting that the frequency of our marine heat waves, gauged with respect to historical temperatures, is going to go up markedly as the climate continues to warm.



Krill and copepods, types of zooplankton, from the Bering Sea. In warmer water, zooplankton tend to be smaller and less calorie-dense than in colder water. This image was captured by Jeff Napp of the NOAA Alaska Fisheries Science Center. Credit: NOAA

What are the impacts to fish and other marine life?

We do see substantial to even severe impacts on marine ecosystems from marine heat waves. We see those impacts everywhere from the base of the food chain — plankton, which everything in the ocean relies on — to higher trophic levels.

What we see off the West Coast in particular, in warmer conditions, is the zooplankton are smaller and don’t have as much fat in them. For a lot of seabirds, marine mammals and small fish, these zooplankton are just not as good as the ones that are bigger and have more fat and calories. 

We also see a link between marine heat waves and harmful algal blooms. In 2015, there was an unbelievably massive harmful algal bloom up the West Coast, and it shut down the commercial harvest of Dungeness crab in Oregon and Washington for a period and recreational harvest of shellfish. There was also some evidence of marine mammals that were eating contaminated fish and other things were suffering from the poisoning. We’re seeing some of that in the Puget Sound region late this past summer. 

What is your research focusing on? 

One of the regions I’ve been most interested in is what’s going on in Alaskan waters. In this present event, it turns out there’s not just warmer surface waters but some warmer waters quite deep in the northern Gulf of Alaska. What caused that? Is it something that could have been predicted? I’m starting to work with biologists, looking at larval fish abundances and how this event is playing out in the northern Gulf of Alaska. 

Elsewhere in the climate community, there’s a lot of curiosity about what really caused the weather patterns we’ve seen this past summer. Was it just a random thing that popped up or was there was a role in the subtropical ocean in forcing those weather patterns? That’s an area that some folks at Scripps and NOAA’s Southwest Fisheries Science Center are working on.

Anything else you want to add about marine heat waves?

One thing to remember is, it’s not like when this happens everything goes down the tubes, never to come back. But on the other hand, obviously if we get lots more of these kinds of events, happening back to back, there are going to be changes to the system that will be permanent too. 

Check out these resources for more information on marine heat waves:

Marine Heatwave Reminiscent of the Blob Lingers off West Coast

Looking Back at The Blob: Record Warming Drives Unprecedented Ocean Change

Looking Back at The Blob - Chapter 2: Marine Heat Wave Intensifies, “Completely Off the Chart”

Looking Back at the Blob - Chapter 3: Unexpected Effects Conspire in Increased Entanglements

NOAA research shows promise for predicting marine heat waves

Previous Article Heat waves could increase substantially in size by mid-century, says new study
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Oceanic and Atmospheric Research (OAR) - or "NOAA Research" - provides the research foundation for understanding the complex systems that support our planet. Working in partnership with other organizational units of the NOAA, a bureau of the Department of Commerce, NOAA Research enables better forecasts, earlier warnings for natural disasters, and a greater understanding of the Earth. Our role is to provide unbiased science to better manage the environment, nationally, and globally.


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