Western Boundary Currents (WBCs) play a critical role in regulating the global climate, carrying large amounts of heat north from tropical waters. While there have been many studies conducted on the horizontal motions of WBCs and how they transport heat and carbon laterally in the ocean, there have been few studies on how WBCs move vertically in the water column. 

A new paper from University of Delaware researchers published in the Journal of Geophysical Research: Oceans examined the vertical motions associated with WBCs, with the data revealing robust and intense upwelling in the WBCs. This upwelling contributes significantly to the vertical transport of ocean properties and materials such as heat and carbon.

The authors on the paper include Xinfeng Liang and Yun Li, both assistant professors in the School of Marine Science and Policy, and Michael Spall, a senior scientist at the Woods Hole Oceanographic Institution. Fanglou Liao, a postdoctoral researcher in Liang’s lab, is the first author on the paper. 

Eastern and Western Boundary Ocean Currents

Ocean currents are streams of water that move in a definite path and direction from one part of the ocean to another. There are two main categories of boundary currents that are based on geographic locations: western boundary currents and eastern boundary currents. 

Western boundary currents are the currents found on the western side of all the major ocean basins, while eastern boundary currents are found on the eastern side of these basins.  

While eastern boundary currents in the subtropical ocean basins are relatively shallow, cover a wide region and are often associated with upwelling, WBCs tend to be narrow, deep-reaching, fast flowing and their role in the climate system is typically discussed in terms of horizontal transport. The effects of vertical motions of the WBCs have been generally neglected.

While physical oceanography textbooks explain upwelling systems in places like the Southern Ocean and eastern boundary currents, there is little to no mention of upwelling in the WBCs. 

“In the literature, we don’t find a lot of studies talking about the vertical motions in western boundary currents, specifically on the western side of the subtropical ocean basins, which was very significant in the data we examined,” said Liang. “It seems that people generally think there’s no upwelling in those regions or never thought about that possibility.” 

The researchers analyzed six publicly available datasets of vertical velocity in the ocean and looked at WBCs in five subtropical regions such as Kuroshio, the Gulf Stream, the Agulhas Current, the East Australian Current, and the Brazil Current. They also examined the Peruvian upwelling region to look at contrasting eastern boundary upwelling.

Looking at an 18-year average of time, the research unveiled intense upwelling around the WBC in all the products. 

Satellite discovery

One catalyst for this research came when Li’s research group used satellite images to look at well-known upwelling systems in the ocean by looking at global chlorophyll distribution. Those distributions show up in satellite images as “hotspots” and represent high concentrations of phytoplankton, which signifies the possibility of the ocean’s vertical motion induced nutrient supply to the euphotic, or uppermost, layer. 

Though there were subtropical ocean margins well-known for high phytoplankton production activity, Li’s group found that they couldn’t immediately attribute all these regions shown in the satellite images to the traditionally well-recognized upwelling zones, which surprised the team because conventional wisdom has been that they should. 

What was even more surprising is that the east coast of the United States — which is impacted by the Gulf Stream WBC — looked similar to the west coast of the United States, especially around California, a well-known hot spot for upwelling. 

“Humans can impact nearshore nutrient concentrations,” said Li. “We have river inputs and anthropogenic nutrient inputs, but when we’re talking about such a large band along the coast, it’s almost impossible, quantitatively, for it to all come from human impact. So that told us there must be some other, additional oceanic process that sustains the high primary production.” 

Overlooked ocean feature

One of the potential reasons why this has been overlooked for many years is that the WBCs have a lot of strong eddies — circular currents of water. The upwelling signals in the WBCs could have been covered up by these eddies, as an upwelling signal like cold, nutrient-rich water coming up from the bottom would be consistently erased by WBCs warm water flowing from the tropical region northward. 

“Those northward horizontal signals are much stronger than and dilute the vertical signals, so you can’t see those cooling features very clearly,” said Liang. “Generally speaking, the horizontal motion is so strong, it just kind of covers the vertical signal.” 

Now that the researchers have discovered this upwelling in the WBCs, they said that it could be important information for biological and chemical oceanographers as the upwelling in the WBCs reach deep in the ocean and also the reach fairly close to the surface. 

The upwelling brings up a lot of nutrients and plays a role in the heat transport, salt transport and water volume transport from the deep ocean to the surface layer. 

“It brings the water up and provides the nutrients and carbon a way to exchange between the surface and the deeper ocean,” said Liang. “We can see that for the subsurface, basically 100 meters below sea surface, the upwelling is a dominant branch of the upward movement of water.” 

Although their study did not directly observe WBC upwelling, the researchers are confident in their results due to the variety of ocean data products providing similar evidence, as well as the fact that when they used those products on the well-studied vertical motions in other regions of the global ocean — such as the eastern boundary currents — the results were consistent with previous theoretical and observational studies. 

Impact of newly discovered upwelling

Li said that the next steps are to take this research and assess how this upwelling affects the carbon cycle and biological production. 

Thomas Daley, an undergraduate at Bowdoin College and one of the students that Li hosted for last summer’s Research Experience for Undergraduates at UD, gave a talk at the 2022 Ocean Sciences Meeting that looked at Hidden Upwelling for Nutrient Transport (HUNT) in the Southeastern U.S. Shelf Region. 

His work showed that — similar to the coast of California — nutrients were transported through upwelling from the deep ocean basin in the southeastern United States to the continental shelf and induced primary production of carbon.

“Once again, this confirms that if the continental shelf on the southeastern United States is not more productive than California, they are at least comparable,” said Li.