Gulf Stream Species in Cold North Waters Spur Scientific Discovery

In early 2017, something strange was going on in the waters off the coast of Rhode Island.  Fishers were pulling up a very unusual bycatch of Gulf Stream flounder and juvenile Black Sea bass. The Gulf Stream flounder are typically found in warmer waters, and while black sea bass are a very common fish, they are typically not present in cold water in the middle of winter, particularly not their juveniles. The fish coming up in their nets were so unusual for this time of the year, that a member of the Commercial Fisheries Research Foundation (CFRF) sent a photograph to Woods Hole Oceanographic Institution (WHOI) scientist Glen Gawarkiewicz asking what he thought might be causing these tropical fish to end up in cold New England waters in the dead of winter.

Anna Mercer from CFRF sent this photo of Gulf Stream flounder and juvenile Black Sea Bass to Glen Gawarkiewicz, launching an investigation that resulted in three research papers.

“One fun thing about my relationship with the fishing community now is if they see unusual things, they send them to me. And then I’ll usually go right to the Pioneer Array website,  look at the data and see if there’s some kind of a story, I can tell about the oceanographic conditions that might have given rise to unusual features, or unusual fish,” said Gawarkiewicz.

This query about an unusual catch prompted a scientific investigation whose results were recently published in the Journal of Geophysical Research: Oceans. The authors, WHOI colleagues Ke Chen, Glen Gawarkiewicz, and Jiayan Yang, identified for the first time the cause and multi-faceted dynamics at play in a subsurface marine heat wave (a high temperature anomaly event), expanding views of contributing factors to such ocean-altering events. They pinpointed the interplay between smaller scale cyclonic eddies and warm water intrusions that created an anomalous marine heat wave.

The research team started with the traditional hypothesis that warm core rings at the shelf break were pushing warm and salty water onto the shelf towards shore, generating a marine heat wave. But  after analyzing the fishing data and oceanographic data from the Pioneer Array, the team was not able to pinpoint the exact process causing the marine heat wave.  Chen took up the challenge and created a numerical model simulating ocean conditions during November 2016-February 2017.

The model, which was at an exceptionally fine scale of 1 kilometer resolution, captured what the team saw from the data. The model revealed warm core rings were not solely responsible for the increase in temperature of water moving into shore, but rather the heated, salty water was being moved by a combination of previously overlooked smaller scale cyclone-like eddies (circular currents of water) in the periphery of the warm core ring. The model also revealed a very persistent wind blowing from west to east in late January 2017 that worked jointly with the cyclonic eddies that had already changed the outer shelf conditions.

The winds brought the warm and salty water from the shelf break to around 50 meters depth for a distance larger than 100 kilometers over five-six days. The intrusion was localized. It moved along the slope,  climbed up the shelf, and moved onshore.

“So this remarkable intrusion was really different from what has been known of the dynamics contributing to onshore intrusions for this region, “explained Chen. “It is a very new and exciting finding that such intrusions can be a combination of smaller scale eddies and wind causing warm water intrusions at a particular location.”  The model shows the complexity of such intrusions that may result from multiple sources and conditions.  “It provides a more complete picture of conditions and how they might ultimately impact what fish are caught and where they are caught,” added Chen.

Gawarkiewicz concluded, “The Pioneer Array has just worked wonders for our relationship with the fishing industry and has also allowed us to see ongoing changes in this ocean region that keep accelerating.  The Pioneer Array just brilliantly combines both the cutting-edge research, like the modeling Ke is doing, with the real dire societal need of identifying and ultimately understanding changing ocean conditions. I only wish that we could have a Pioneer Array, basically in every region of the country.”


See Boston Globe article on December 28, 2021 for other ways OOI data are contributing to scientific understanding of the changing ocean.