Breakdown: What is a “marine heat wave” and why are they on the rise?

Published: Oct. 3, 2022 at 9:41 AM CDT
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MEMPHIS, Tenn. (WMC) - You’ve likely experienced — or at least heard of — a heat wave: a 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.

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.

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.
This image from September compares sea surface temperature anomolies (how much warmer or cooler...
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.(NOAA Fisheries)

Just like a heat wave in here in Memphis might have different characteristics than a heat wave in Seattle, the ocean naturally varies from place to place.

In some locations, there’s a great deal of variability from year to year. Its in these kinds of places where there is that much more of an extreme anomaly for it to be called a marine heat wave event.

However, in other locations — lower latitudes especially — it doesn’t take as much of a temperature rise to be something that happens very rarely.

What are the impacts to fish and other marine life?

Scientists have noted “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 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.

For example, “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?

It is known that much of the extra heat from the atmosphere due to increased greenhouse gas concentrations goes into the ocean. So when there are 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 there is 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.

While it has been recognized for a while that ocean temperatures and properties fluctuate, calling them marine heat waves is relatively new (roughly the last 5 years).

Still, much remains unclear about marine heat waves. However, enough is known about marine heat waves for scientists to be gravely concerned about their potential impacts.

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