Major ocean current could warm greatly
A new study led by researchers at Binghamton University, State University of New York found that the Kuroshio Current Extension is sensitive to global climate change and has the potential to warm greatly with increased carbon dioxide levels.
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The major western boundary current in the northern Pacific Ocean, the Kuroshio Current and Extension, is analogous to the Gulf Stream, which flows along North America's east coast. Driven by the wind, boundary currents are the workhorses of the ocean, moving heat, salt and gases from the equatorial seas to the middle latitudes, Lam explained.
"In other words, these currents help distribute heat from the tropics to higher latitudes. In fact, corals occur at their highest latitude of anywhere in the world within the Kuroshio Current because the waters are so warm," she said.
That warmth stems from the surface waters that collect in the western Pacific Ocean along the equator, called the Western Pacific Warm Pool. The Kuroshio Current takes these waters north, past the Japanese coast, and then eastward at the 36°N latitude, where it joins the open Pacific Ocean. At this point, it becomes the Kuroshio Current Extension.
The current and extension vent vast amounts of heat and moisture evaporating from the warm water into the lower atmosphere in the Northern Hemisphere. Because of this, they help shape precipitation patterns over Japan and North America's West Coast, as well as the paths of typhoons, which feed off warm waters. In addition to affecting the weather, the Kuroshio also likely affects the climate, although its impact on thousand- and million-year time scales is still unclear.
... Kuroshio details ...
Because of their impact on biodiversity, weather and the climate, understanding how boundary currents such as the Kuroshio will respond to climate change and increasing CO2 levels in the atmosphere is critical. Today, these currents are warming two to three times faster than other areas of the ocean, Lam said.
Ocean model studies and observational data also show that the Kuroshio Current Extension is shifting northward and increasing its transport capacity, but researchers don't yet know how these changes will affect the organisms that live there, or local and regional weather and climate patterns.
The recently published research is the first of its kind to reconstruct the Kuroshio as it was 2.5 to 5 million years ago, a time that spanned both periods of global warming and cooling, as well as the closure of a major seaway in what is now Central America. Looking at the current's distant past may answer some of the questions about its future.
Past and future oceans
During the Pliocene, which spans 2.5 to 5.3 million years ago, atmospheric CO2 levels were near those we face today: about 350 to 450 parts per million. Today's atmosphere has about 415 parts per million of CO2.
"The fun part of this time period is that the continents were arranged similar to today, which makes the Pliocene a great time period to use as an analogue as to how the Earth system will respond to increased CO2 concentrations and warming," Lam said.
There were some differences in regard to landmasses, she noted: Until about 2.5 million years ago, a waterway existed between North and South America that allowed surface waters from the Pacific and Atlantic oceans to mingle. When the Central American Seaway closed, it may have brought the Kuroshio Current Extension into its current configuration.
The Pliocene included a period from 3 to 3.3 million years ago known as the mid-Piacenzian Warm Period (mPWP), which saw increased carbon dioxide levels and global warming. Once that period ended, cooling resumed, accompanied by the growth of glaciers and sea ice in the Northern Hemisphere's high latitudes.
In the recently published study, the researchers reconstructed the Kuroshio throughout the mPWP, using chemical signatures from the fossilized shells of marine plankton that once lived in the Kuroshio region's surface waters.
"Our data indicate that during the first phase of mPWP warming in the Pliocene, the current warmed up and potentially shifted its latitudinal position northward. It then cooled back down and perhaps shifted its position back south during a brief period of global cooling," she said.
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https://www.sciencedaily.com/releases/2021/09/210928102228.htm