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First direct evidence of the oceanic mix through the Gulf Stream

First direct evidence of the mixing of the ocean through the gulf flow

The “Triaxus”

; towing platform breaks through the rippling surface of the ocean during a storm. By towing such a platform with monitoring tools across the water, changing its depth into a “yo-yo” pattern as it traveled, the scientists created high-resolution snapshots of how a tint released upstream evolved across the front of the Gulf Stream. Credit: Craig M. Lee, UW APL

New research provides the first direct evidence of the effect of the Gulf Stream mixer, identifying a new mechanism for mixing water through the rapidly moving stream. The results have important implications for the climate, climate and fishing as mixing the oceans plays a fundamental role in these processes. The Gulf Stream is one of the major drivers of climate and biological productivity from Florida to Newfoundland and along the western coast of Europe.

The multi-institutional study conducted by a University of Maryland researcher revealed that fidgeting along the edges of the Gulf Stream through areas as small as a kilometer could be one of the main sources of ocean that mix between the waters on both sides of the current. The study was published in Proceedings of the National Academy of Sciences on July 6, 2020.

“This long-standing debate over whether the Gulf Stream acts as a blender or barrier to ocean mixing has primarily taken into account large ocean vortices, tens of kilometers a hundred kilometers wide,” said Jacob Wenegrat, assistant professor in the Department of Atmosphere of UMD and Oceanic Science and lead author of the study. “What we are adding to this debate is this new evidence that variability on a kilometer scale seems to mix a lot. And those scales are really difficult to monitor and model.”

As the Gulf Stream rises along the eastern coast of the United States and Canada, it brings warm salt water from the tropics to the North Atlantic. But the current also creates an invisible wall of water that divides two distinct oceanic regions: the coldest and freshest waters along the northern edge of the Gulf Stream that swirl counterclockwise and the warmest and saltiest waters on the southern edge of the current that circulate clockwise.

First direct evidence of the mixing of the ocean through the gulf flow

A team of researchers deployed a Knorr R / V float before releasing a fluorescent dye into the water. Scientists then monitored the drift of both dyes and floats across the Gulf Stream, revealing significant mixing of the waters through the rapid current. Credit: Craig M. Lee, UW APL

How much oceanic mixing occurs across the Gulf Stream has been a matter of scientific debate. Consequently, oceanic models predicting climate, climate and biological productivity have not fully taken into account the contribution of mixing between the two different types of water on both sides of the stream.

To conduct the study, the researchers had to bring their tools to the source: the Gulf Stream edge. Two teams of scientists aboard two world-class research ships faced winter storms on the Atlantic Ocean to release a fluorescent tint along the northern face of the Gulf Stream and trace its course over the following days.

The first team released the dye along with a float containing an acoustic beacon. Downstream, the second team followed the float and monitored the dye concentration along with water temperature, salinity, chemistry and other characteristics.

On the coast, Wenegrat and his coauthors developed high resolution simulations of physical processes that could cause the dye to disperse through the water in the way recorded by the teams on the field. Their results showed that turbulence in areas of just one kilometer exerted an important influence on the colorant path and caused a significant mixing of water properties such as salinity and temperature.

First direct evidence of the mixing of the ocean through the gulf flow

Fluorescent dye offers a unique way to monitor the evolution and mixing of water across the Gulf Stream. In a recent study, fluorescein dye (as pictured here) was released along the north face of the Gulf Stream and followed by the ship as it mixed horizontally through the current. Credit: Lance Wills, WHOI

“These findings underscore the role of variability on very small scales that are currently difficult to observe using standard methods, such as satellite observations,” said Wenegrat. “Variability on this scale is not currently resolved in global climate models and will not be solved for decades to come, so it leads us to wonder, what have we been missing?”

Showing that mixing on a small scale through the Gulf Stream can have a significant impact, the new study reveals an important and poorly recognized contribution to ocean circulation, biology and potentially the climate.

For example, the Gulf Stream plays an important role in the so-called oceanic biological pump, a system that traps excess carbon dioxide, buffering the planet from global warming. In the surface waters of the Gulf Stream region, mixing of the oceans affects the growth of phytoplankton, the basis of the oceanic food web. These phytoplankton absorb carbon dioxide near the surface and subsequently sink to the bottom, taking the carbon with it and trapping it in the deep ocean. Current models of the oceanic biological pump do not take into account the large effect that mixing on a small scale through the Gulf Stream could have on the growth of phytoplankton.

“To make progress in this area, we need to find ways to quantify these processes on a finer scale using theory, cutting edge numerical models and new observation techniques,” said Wenegrat. “We need to be able to understand their impact on large-scale circulation and ocean biogeochemistry.”

The research paper, “Improving mixing across the gyre border on the Gulf Stream front”, Jacob O. Wenegrat, Leif N. Thomas, Miles A. Sundermeyer, John R. Taylor, Eric A. D’Asaro, Jody M Klymak, R. Kipp Shearman and Craig M. Lee were published in the July 6, 2020 issue of Proceedings of the National Academy of Sciences.

New opportunities for ocean and climate modeling

More information:
Jacob O. Wenegrat el al. “Improvement of mixing across the gyre border on the Gulf Stream front” PNAS (2020). www.pnas.org/cgi/doi/10.1073/pnas.2005558117

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University of Maryland

First direct evidence of mixing the oceans across the Gulf Stream (2020, July 6)
recovered on July 7, 2020
from https://phys.org/news/2020-07-evidence-ocean-gulf-stream.html

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