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Coastal ice influences North Pacific circulation.

When warm surface water flows north from the Southern Hemisphere, it becomes more salty, and when it reaches the North Atlantic, it cools and sinks. The thick, deep layer that then circulates outward, known as North Atlantic Deep Water, has a strong influence on global ocean circulation and climate. This global movement of ocean waters of variable temperatures and densities is known as the thermohaline circulation, and its dynamics in the North Atlantic have been extensively studied. Circulation in the North Pacific, which is driven by North Pacific Intermediate Water, has received less attention, but new research published in Paleoceanography has found that ice formation along the coastline plays an important role in circulation in that region, subsequently influencing both regional and global climate.

Karla Knudson and Christina Ravelo of the University of California, Santa Cruz, studied sediment cores taken from the floor of the Bering Sea, looking specifically at carbon and oxygen isotopes in the calcium carbonate shells of tiny marine organisms known as foraminifera, which are indicators of past water temperatures and other oceanographic information. The cores depict 1.2 million years of ocean circulation and climate in the region, spanning both glacial and interglacial periods. Past modeling has suggested that during glacial periods when water is frozen in ice sheets and sea levels decline enough that the Bering Strait closes, the flow of fresher water from the North Pacific into the saltier North Atlantic is shut off by the newly formed land bridge. This causes stronger overturning circulation in the Atlantic due to the greater salinity there. Meanwhile, the circulation of the North Pacific in such modeling becomes weaker due to its fresher water, creating an oceanic "seesaw" that cools one ocean and warms the other.

But analysis of the cores showed that "the overturning circulation actually strengthens in both oceans when the Bering Strait is closed," according to Knudson. The researchers discovered that the models did not account for the strong brine production generated from the formation of sea ice in the Bering Sea. The ice deposits salt into the water, which increases the water's density and causes it to sink to the depths of the ocean. The cold, deep water is transferred to the equator while the warmer surface water moves toward the poles, and this overturning circulation "helps to modulate the climate by transferring heat from the equator to the poles," notes Ravelo.

Only high-resolution regional climate models are able to capture the process of coastal sea ice formation, a finding that could be useful in understanding future climate change impacts. As global temperatures warm, "we could see reduced sea ice formation in the Sea of Okhotsk and the Arctic," says Ravelo. "So it would be nice for the climate models to have

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Publication:Bulletin of the American Meteorological Society
Geographic Code:0PACI
Date:Feb 1, 2016
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