A River In The Sea: Examining Impacts of Winter's Coastal Currents

Jeffrey Brodeur, WHOI Sea Grant

Physical oceanographer Glen Gawarkiewicz admits he could have selected a research topic more conducive to warm days in the bright blue waters of the Caribbean, drifting along with the lazy currents in and amongst the coral reefs.

But when you're interested in how seasonal cooling and storms affect the density of seawater, the seawater you're interested in is just off Cape Cod and the season you're interested in is winter, you prepare for the worst and hope for the best.

Al Plueddeman and Andrey Shcherbina recovering the REMUS after a successful mission.
Photo by Chris Linder.

For Gawarkiewicz, a senior scientist at Woods Hole Oceanographic Institution, that meant 12-hour days beginning and ending in the dark, rough seas, and pitching decks.

The reward for his efforts in the WHOI Sea Grant-funded project, though, are some surprising insights into how the seawater off the Cape reacts in the winter, especially in the impact freshwater currents have on the salinity and density of near-coastal waters. During this project Gawarkiewicz worked closely with a former post-doc, Andrey Shcherbina, and research associate Chris Linder.

Utilizing the autonomous underwater vehicle REMUS (Remote Environmental Monitoring Units) and several moorings, Gawarkiewicz expected to see wintertime "mixing" of salinity and temperatures, thanks to factors such as atmospheric cooling, strong winds, and storms. That, he believed, would make the area next to the coast cooler and denser, wiping out the coastal current.

Instead he saw a current that fought back - and hard.

There was a wedge of "lighter" freshwater coming from rivers in Maine, like the Penobscot and Kennebec, driven south by buoyancy differences, and that freshwater hugged the beaches on the eastern end of Cape Cod. At times, Gawarkiewicz says, it was so apparent he could even see a visible line of foam and re-circulating debris between the two water densities.

"Freshwater comes out of Maine and takes a right turn, and when that current hits the area off the Cape, it hits a choke point," Gawarkiewicz says.

And that's where things get interesting.

Over the course of the winter, he found, there is a narrower band of freshwater, as the current decreases in strength and width because of less input from northern sources. As a result, the fresher water creeps up against the coast until about March. Later in the spring, there is a major influx of freshwater due to snowmelt, and the current widens and strengthens.

However, the persistent coastal current has a big effect on seawater near the coast. Because that current is fresh and less dense, it prevents the winter cooling from making dense water along the coast. But the cooling does tend to increase density and drive those denser waters into deeper water offshore of the coastal current.

"So there's a battle going on over the shelf," Gawarkiewicz says.

The information gleaned from the Cape research, he says, has wide-ranging implications, both scientific and societal. The Gulf of Maine coastal current often carries harmful algae blooms, the infamous red tide that can decimate shellfishing areas. In 2005, an HAB did nearly $30 million in damage in the Bay State alone. While red tides don't occur in winter, the winter processes affect stratification in the central Gulf of Maine, which in turn impacts the structure of the coastal current in the spring and summer.

"Understanding this coastal current throughout the year is important because it ties into so many ecosystems," says Gawarkiewicz, adding that working on the project has allowed him to meet with the general public, harbormasters and decision-makers. "It's a project with local impact at its best."

But the impact goes beyond the Gulf of Maine, especially in this time of concern about climate change.

"The same type of processes that we're seeing here on the Outer Cape are going on in the Arctic," Gawarkiewicz says, since freshwater melt from rivers and glaciers creates a similar current and environment to that coming out of rivers in Maine. "It's a nice natural lab for what occurs [in the Arctic]."

Gawarkiewicz says there are many opportunities for additional research in the currents, which are nutrient rich. Finding takers to join him, however, was nearly impossible - for this project, at least.

"Oddly enough, no one wants to go out with us," he says with a laugh. "Now, my Belize project - there was lots of interest in that."