The past 10 years have seen a series of "warm blobs" in the northeast Pacific Ocean. These marine heat waves do widespread damage to ecosystems and marine life in the area, but the mechanisms by which they develop and are sustained are still uncertain. Now a research group has found that they are caused by climate "teleconnections" from wave trains that originate in the Mediterranean Sea and North Atlantic region.

The first warm patch discovered in the northeast Pacific Ocean was the "Blob" event of 2013–2016, followed by another warm blob in 2019–2020.

The Blob stretched from coastal Alaska to the Baja region of California, with sea surface temperatures as much as 6°C above normal. Vital fish stocks such as sockeye salmon and Pacific cod were impacted, and the event saw geographical shifts of a number of species, including phytoplankton, as well as the closures of important fisheries and mass strandings of marine mammals and seabirds. But some species increased in numbers, such as pyrosomes, bioluminescent colonies of millimeter-sized individuals and commonly called "sea pickles," which were attracted to the warm water.

With global warming these marine heat waves are expected to become more frequent, larger in magnitude and longer in duration. The fundamental problem is that warmer waters hold less carbon dioxide and offer fewer nutrients for the plants and animals that exist there.

The 2013 Blob was spawned by a long-lasting ridge of high pressure that formed over the Gulf of Alaska in the fall of that year. While some details are uncertain, it is known that these high-pressure systems maintained the warm blobs, while simultaneously causing abnormal cold temperatures over North America during colder seasons.

Previous studies have linked this high-pressure ridge to teleconnections from tropical sea surface temperature anomalies, but it's been unclear whether such teleconnections from the extratropical region—the midlatitudes—could also contribute to the ridge.

The group, led by Professor Jian Shi of the Ocean University of China and consisting of scientists from four different continents, looked at 13 warm blob events in the northeast Pacific that peaked from November to March. The research is published in the journal Nature Communications.

By closely examining details of the atmospheric patterns that developed near the region in the months before the blobs began, they found that nine of the 13 warm blob events occurred during the positive phase of the North Atlantic Oscillation (NAO), a weather pattern over the North Atlantic ocean consisting of fluctuations in pressure between an area near Iceland and one near the Azore Islands off the coast of Portugal.

They also observed that Rossby waves, also called planetary waves, contributed to the northeast Pacific blobs. Rossby waves are inertial waves caused by a restoring force, which is the Coriolis force. Such waves, observed in fluids (both water and planetary atmospheres), travel east to west across the planet, often taking years when far from the equator.

Through a long chain of causal reasoning, wave trains from the Mediterranean and northern Atlantic regions were found to contribute to the northeast Pacific blobs.

Such wave train dynamics are a potential source for predicting the anomalous ridge and associated warm blobs, and the resulting cold season blobs elsewhere. But the group cautions that this mechanism, which is prominent in November, may not apply to other winter months because the background states (outside the northeast Pacific) play a crucial role in the generation and guidance of Rossby waves, thus influencing the establishment of teleconnection patterns.

Physically, these planetary wave trains are triggered by increased rainfall and latent heat release over the Mediterranean Sea and are accompanied by decreased rainfall over the North Atlantic. This scenario can transport wave energy to the northeast Pacific Ocean, guided by the westerly jet stream, and induce the anomalous pressure ridge there.

Ten of the 13 warm blob events, about 77%, corresponded with higher-than-normal rainfall over the Mediterranean region. Variations in that region could play an important role in sustaining the Rossby wave train and hence the anomalous ridge over the northeast Pacific.

In order to confirm the driving force of the Mediterranean and North Atlantic regions in exciting the wave trains and anomalous ridge over the northeast Pacific, the team conducted atmospheric model experiments using version 5.0 of the Community Atmosphere Model; (CAM5).

Their results showed that when sea surface temperature anomalies are superimposed onto the climatology in the northeast Atlantic, the anomalous high-pressure ridges have a strong association.

Such extratropical teleconnections offer potential predictability for warm blobs in the northeast Pacific Ocean and temperature fluctuations over North America. There is not much scientists and policymakers can do about the former, but knowing what's coming in regions on land could help prepare for fuel needs, snow removal, assistance for the poor and homeless, and even whether regional residents need special tires on their winter vehicles or added insulation in houses.

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