Scientists say they have the first direct evidence of a “blender effect” in the Gulf Stream, which is one of the main drivers of climate and biological productivity on the east coast of North America and the west coast of Europe.
A multi-institutional study published in the Proceedings of the National Academy of Sciences reveals that churning in areas as small as a kilometre long could be a source of mixing between the waters on either side of the current, which brings warm salty water from the tropics into the north Atlantic.
The current also creates two distinct ocean regions: colder, fresher water along its northern edge that swirls in a counter-clockwise direction, and warmer, saltier water on the southern edge that circulates in a clockwise direction.
How much ocean mixing occurs across the Gulf Stream has been a matter of scientific debate, says lead author Jacob Wenegrat, from the University of Maryland, US, but until now that debate has mainly considered big ocean eddies up to a hundred kilometres across.
“What we’re adding… is this new evidence that variability at the kilometre scale seems to be doing a lot of mixing. And those scales are really hard to monitor and model.”
To do that, scientists aboard two research vessels released a fluorescent dye along the northern front of the Gulf Stream and traced its path over the following days.
The first team released the dye and a float containing an acoustic beacon. Downstream, the second tracked the float and monitored the concentration of dye along with water temperature, salinity, chemistry and other features.
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 manner the field teams recorded.
Their results suggest that turbulence across areas as small as a kilometre long exerted an important influence on the dye’s path and resulted in significant mixing of water properties such as salinity and temperature.
“Variability at this scale is not currently resolved in global climate models and won’t be for decades to come, so it leads us to wonder, what have we been missing?” Wenegrat says.
The researchers believe the findings reveal an under-recognised contributor to ocean circulation, biology and potentially climate.
They note, for example, that ocean mixing in the surface waters of the Gulf Stream region influences the growth of phytoplankton, which absorb carbon dioxide near the surface then sink to the bottom, trapping it in the deep ocean.
Current models of the ocean biological pump don’t account for the large effect small-scale mixing across the Gulf Stream could have on phytoplankton growth.
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