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From the fall of 2013 through the end of 2015, “the Blob” took over the eastern North Pacific. This mass of warm water threw the ocean into chaos, and its appearance was blamed for everything from disease outbreaks to drought to snowstorms in Texas. The Blob is probably the first “marine heatwave” you’ve heard about, and there’s a good reason for that: the term wasn’t coined until 2011, after the temperature of the water in the Indian Ocean off Australia’s west coast spiked 5 °C above average.
The study of marine heatwaves like the Blob is a field in its infancy. For those of us hearing about them for the first time, it’s easy to assume that these events are extraordinarily rare. But according to new research led by University of Washington oceanography graduate student Hillary Scannell, a marine heatwave should hit the eastern North Pacific about once every five years.
“One in five years is probably a bit higher than what we would have thought originally,” she says. But “that’s the natural frequency.”
But one-in-five is just a statistical average. The Blob, specifically, “was not something we could have anticipated,” she says; it was out of the normal range, even for a marine heatwave. The Blob caused the water along much of the west coast of North America to be 3 °C above average for more than two years. Scannell’s minimum definition of a heatwave is 1.5 °C of warming over 7.6 million square kilometers (about three-quarters the size of Canada), persisting for 17 months. Because of its size and magnitude, Scannell says, the Blob “was unprecedented since about the 1980s.”
Scannell says that the 2003 European heat wave was the catalyst for the first big studies of sea surface temperature extremes. In the years since, scientists studying the Ligurian and Tyrrhenian Seas, in the northern Mediterranean, have helped define the symptoms and consequences of marine heatwaves, including prolonged sea surface temperatures up to 2 to 3 °C warmer than expected and die-offs of some heat-sensitive species.
With a growing understanding of marine heatwaves, it is possible that the world will be ready the next time a big one like the Blob or its ilk appears. But to do that it’s important to understand what sets them off, and what role anthropogenic climate change may be playing in intensifying the natural warming and cooling cycles. This is the target of future work by Scannell and her colleagues.
The temperature of the eastern North Pacific, where the Blob hit, for example, is dominated by a decades-long temperature fluctuation known as the Pacific Decadal Oscillation. Many scientists think the oscillation is entering its warm phase, which, combined with climate change, could have amplified the warming attributable to the heatwave itself.
If the scientists can pick apart what is doing what in the ocean, says Scannell, “we can better prepare for really extreme events in the future.”