Archive for December, 2010

Skidaway Institute scientist discovers important new eddies off southeast US coast

December 16, 2010

Skidaway Institute of Oceanography professor Dana Savidge aims much of her research efforts towards observing the way ocean currents move in the Gulf Stream and across Georgia’s continental shelf. Those efforts have paid off in the discovery of a series of previously unidentified eddies which may have a significant influence on the way energy and material are transported between the Gulf Stream and the shelf waters.

Savidge’s findings were published in a recent issue of Geophysical Research Letters. Jonathan Norman from Armstrong Atlantic State University also contributed substantially to the analysis, along with several members of the Skidaway Institute team, including Trent Moore, Julie Amft and Catherine Edwards.

These researchers observed Shelf Edge Tide Correlated (SETC) eddies by using low-power, high-frequency radar systems that monitors surface ocean currents across the continental shelf and out into the Gulf Stream. The small, short-lived eddies spin up along the western edge of the Gulf Stream twice daily, as the tide turns. They move southwesterward, along the shelf edge — sometimes drifting towards shore — before dissipating two or three hours later.

In this graphic depiction of radar-derived coastal currents, the color, length and direction of the arrows represent the velocity of surface currents. The Gulf Stream is clearly evident in red and yellow. A Shelf Edge Tide Correlated eddy can be seen in the box on the left edge of the Gulf Stream.

According to Savidge, the importance of the SETC eddies lies in their influence on the Gulf Stream. “The Gulf Stream transports an incredible amount of heat and kinetic energy,” she said. “It has a strong influence on the global climate.”

In addition, fluctuations or meanders in the Gulf Stream’s flow pull nutrient-rich cold water from the deeper ocean and onto the shelf, providing a basic food source for marine life. The newly-discovered eddies apparently affect both those processes.

Skidaway Institute scientist Dana Savidge standing next to a research radar antenna.

“These are processes we didn’t know were happening that affect how the Gulf Stream transports heat and energy,” Savidge said. “This is information that must be put into a model if you want the model to accurately reflect what the Gulf Stream is doing in terms of transporting heat and energy.”

The nutrients from the deep ocean that the SETC eddies lift across the shelf edge and into the sunlit upper regions of the coastal ocean may contribute to phytoplankton growth, thereby potentially contributing resources for the shelf’s wider range of marine life.

“Similar eddies may occur along other continents where currents like the Gulf Stream exist, so it will be important to determine how important they actually are in the complex interaction between the Gulf Stream and the coastal waters.” Savidge said.

The research was funded with grants from the National Science Foundation and the National Oceanic and Atmospheric Administration (NOAA). The radars were purchased with funding from the Georgia Research Alliance and NOAA.

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Skidaway Institute scientists cruise Florida’s ‘Big Bend’

December 7, 2010

“It’s a really exciting place to do oceanography, because you can throw almost any kind of instrument over the side, and it will come up with observations that lead to new science,” said Skidaway Institute scientist Catherine Edwards. A former postdoctoral fellow at Florida State University, Edwards describes her recent research cruise to the little-studied “Big Bend” section of the northeastern Gulf of Mexico.

One of Skidaway Institute’s instrument packages is lowered into the water.

In an effort to extend FSU’s coastal ocean observatory in the Florida Big Bend, Edwards deployed two self-contained bottom-mounted sensors that measure temperature, salinity, currents, and how they vary from the seafloor to the surface. The sensor packages are moored on the outer shelf to help Edwards and FSU scientists learn more about how the Gulf wind and tidal currents transport material from the shelf edge to the shore. Edwards was assisted by Austin Todd, a graduate student in physical oceanography at Florida State University.

Catherine Edwards with FSU graduate student Austin Todd and one of the instrument packages.

Many fish are spawned at the shelf break, but spend their juvenile stages in the salt marshes and estuaries. Distances of 50 to 75 miles are too far for fish larvae to swim on their own and physical models, by themselves, do not fully explain how larvae are able make the journey.

“Whether you’re tracking fish larvae or oil, the science question is the same,” Edwards said. “We are trying to develop a clearer picture of how the physics and biology interact.”

Edwards does have an idea. Coastal sea breezes shift on- and off-shore between day and night during spawning season in the Gulf of Mexico. The winds push the surface water in one direction, while deeper waters compensate with currents in the opposite direction.

“Fish larvae don’t swim far horizontally, but they do migrate up and down the water column on day-night cycles fundamentally tied to the timing of the solar cycle and thus sea breeze,” Edwards said. “Depending on the larval migration, they may simply shift their position in the water column to ride the diurnal shifts in the current to shore.”

The cruise wasn’t easy to arrange. Edwards had access to the needed instruments, but no money for ship-time, which often runs thousands to tens of thousands of dollars a day for capable oceanographic vessels. She was able to hitch a ride on a NOAA National Marine Fisheries Service (NMFS) vessel that was conducting a twice-annual cruise to studying fish biology throughout the Gulf of Mexico.

“I’m a physicist by training, so I really enjoyed the chance to ‘play biologist’ for the two week leg of the cruise,” she said. “That interaction was really valuable for planning future work with NMFS scientists.”

Edwards set up two sets of instruments very near a NOAA weather buoy. While the weather buoy collects data on the conditions above the surface, Edwards’ instruments will do the same for the conditions in the water column. Since weather conditions often drive water movement, the ability to combine the two data sets will provide valuable information.

Edwards will return in six months to collect her instrument packages and the data they have recorded.