First direct evidence of ocean mixing through the Gulf Stream

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

The multi-agency study led by a University of Maryland researcher revealed that churning along the edges of the Gulf Stream through areas as small as a kilometer could be a major source of ocean mixing between the waters on both sides of the current. The study was published in the procedures of the National Academy of Sciences on July 6, 2020.

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

As the Gulf Stream moves along the east coast of the US 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 ocean regions: the coldest and coldest waters along the northern edge of the Gulf Stream that swirl counterclockwise, and the warmest and saltiest waters in the south edge of the stream. that circulate clockwise.

The amount of ocean mixing that occurs in the Gulf Stream has been a matter of scientific debate. As a result, ocean models that predict climate, climate, and biological productivity have not fully explained the contribution of mixing between the two very different types of water on either side of the stream.

To carry out the study, the researchers had to bring their instruments to the source: the edge of the Gulf Stream. Two teams of scientists aboard two world-class research ships braved winter storms in the Atlantic Ocean to release a fluorescent dye along the northern front of the Gulf Stream and track their path for the next few days.

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

Back on shore, Wenegrat and his co-authors developed high-resolution simulations of the physical processes that could cause the dye to disperse through the water in the way that field crews recorded. Their results showed that turbulence in areas as small as a kilometer had a major influence on the path of the dye and resulted in a significant mix of water properties, such as salinity and temperature.

“These results emphasize the role of variability at very small scales that are currently difficult to observe using standard methods, such as satellite observations,” Wenegrat said. “Variability at this scale is not currently resolved in global climate models and will not be in the coming decades, so it leads us to ask ourselves, what have we been missing?”

Showing that small-scale mixing across the Gulf Stream can have a significant impact, the new study reveals an important and under-recognized contributor to ocean circulation, biology, and potentially climate.

For example, the Gulf Stream plays an important role in what is known as the ocean’s biological pump, a system that traps excess carbon dioxide, protecting the planet from global warming. In the surface waters of the Gulf Stream region, the mixing of the oceans influences the growth of phytoplankton, the foundation of the ocean’s food web. These phytoplankton absorb carbon dioxide near the surface and then sink to the bottom, taking carbon and trapping it deep in the ocean. Current models of the oceanic biological pump do not take into account the great effect that small-scale mixing through the Gulf Stream could have on the growth of phytoplankton.

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

The research paper, “Improved Blending Across the Turning Limit 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 the procedures of the National Academy of Sciences.