Better Predictions of Secondary - and More Subtle - Tides

By Andrea Cohen, MIT Sea Grant

"The tides are about the only thing we can predict," says Wendell Brown. But the professor at UMASS Dartmouth's School for Marine Science and Technology (SMAST) is quick to point out that this is only the case for the flood-ebb tide. Secondary tides, which result from the seafloor's curvature and depth change, are another animal.

"The tides are probably the most predictable aspect of ocean variability, and secondary flow associated with the ebb and flood flows is predictable as well," he says. "Because secondary flows are so much more subtle, we need to discover what they are, so that we can know what to predict."

CTD deployment

Photo by Chris Jakubiak

Toward that end, Brown and colleagues are conducting research to better understand such transient tidal eddies east of Cape Cod, and how those flows, which spiral along with the primary flow, may affect scallop populations. MIT Sea Grant funds the research, with additional support from Rutgers University and the National Park Service.

Brown first got the idea for the project while working in Eastham with a high-frequency radar technology called Coastal Oceans Dynamic Radar (CODAR). This land-based, high-frequency radar system measures sea surface current by transmitting a low-power signal that backscatters off the ocean waves. That data can then be used to calculate the speed and direction of those surface currents.

"We noticed this eddy kind of motion just east of Chatham and that intrigued us," notes Brown. "We know that the secondary flows are related to the strong primary tidal flows and are undoubtedly influenced by the presence of the eddy motion. When you get the secondary flow, you get upwelling and downwelling."

While Brown's career has focused on ocean physics, he also knew that the site off Chatham was home to rich scallop beds, and the question of how those physics affected the sea life there piqued his curiosity.

"We need to understand the full scale of current variability that scallop spat are dealing with," he explains. "Do the physics help the scallop spat do what they do, which is to sink to the bottom and hook onto the seafloor? Or is it something they have to combat?" he wonders.

The answer, he explains, lies in the physics. And to get at that answer, Brown and SMAST graduate students are employing a combination of fieldwork and modeling. The CODAR, installed at the National Park Service's Nauset Beach in Eastham, works in tandem with a Rutgers CODAR installation at the Coast Guard station on Nantucket and produces hourly surface current maps that reveal the motion of tidal eddies. From a research vessel a few miles off the coast of Chatham, Brown and colleagues also conducted CTD (conductivity, temperature and depth) and ADCP (acoustic Doppler current profiler) surveys. In addition, they deployed a moored ADCP/bottom pressure instrument to collect data on the currents over a three-month period, as well as a drifter, which sent data back on the speed and direction of currents for a month in their area of interest.

Brown's students are now busy with the data collected. For his master's thesis, Gustavo Marques is assimilating the information from the CODAR maps to create models for an ocean observing system. Along with water properties, those models also take stratification into consideration. "The ocean is sensitive to stratification, and the models we use can depict that sensitivity," notes Brown.

To date, he and colleagues have used a high resolution, finite-element ocean circulation model, called QUODDY by its Dartmouth College developers. This model simulates the external tidal variability in the greater Gulf of Maine region from Halifax, Nova Scotia to Watch Hill, RI, and offshore to the continental slope seaward of Georges Bank.

Thus far, says Brown, the model appears to simulate the primary tidal flows in the Great South Channel east of Cape Cod quite well. In addition, he says, "We are trying to answer the questions related to how real are the model-simulated eddy motion and secondary flows."

Brown says that the models are like weather forecasts - only better. "Simple ocean model tidal forecasts are much better than weather forecasts because the primary forcing is the astronomy, not the 'fickle' winds," he explains. "However, the weaker secondary flows may be affected by other more variable ocean conditions, such as stratification, and thus may be less predictable." He notes that CODAR-derived current maps may be used in the future to help make secondary flows more predictable.

The CODAR installation in Eastham is one of 30 such installations helping with mapping in the Mid-Atlantic Regional Coastal Ocean Observation System (MARCOOS). Running from Cape Cod to North Carolina, those radar systems provide a data stream of hourly maps.

The Coast Guard is currently testing a new CODAR-related modeling approach to better define search and rescue operations. Developed by Jim O'Donnell (UCONN), Dave Ullman (URI) and colleagues, the approach employs successive CODAR-derived surface current maps to produce forecasts of the trajectories of "objects" floating on the ocean's surface.

The problem, explains Brown, is the uncertainty in the CODAR-derived surface currents being used. To address that problem, SMAST graduate student Shawna King is analyzing the data collected by the drifter off of Chatham. She hopes to reveal the level of uncertainty for these search and rescue operations by comparing surface drifter observations with model drifter trajectories.

Brown sees many applications for the models this research can generate, including better forecasts for rip currents, improved information for ship navigation, and better forecasting for power companies looking to understand to properly position their repair equipment for storm response. He notes that both commercial and recreational fishermen would be keen to have detailed, reliable information about waves, currents, and water temperature to make decisions about where and when to fish. Those planning and executing clean-ups for oil spills would also benefit from such data, as would surfers looking for that perfect barreling surf.

And the scallops? Brown and his colleagues are still examining the data on that, but he's confident of the connection: Looking out the window into New Bedford Harbor, home to the largest wild scallop fishery in the world, he muses: "The physics and biology are quite symbiotic."