Straits of Florida VIIR chlorophyll a image

By: Miram Gleiber

June 11, 2015

Hello from about 12 miles offshore Sugarloaf Key aboard the R/V Walton Smith. We departed from the docks at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science (RSMAS) in Miami yesterday morning and have been steaming south chasing down an eddy!!

What’s an eddy? And why do we care about it?

An eddy is water mass moving in a circular motion. For example, a river can have eddies form along the edges where the fast moving water meets the land (image left). The eddy we are
looking for forms in a similar way, but is about 120 miles long and 60 miles wide in the Straits of Florida (top image; eddy circled in red).

The Straits of Florida is the ocean region between the Florida Keys and Cuba on the south side, and South Florida and the Bahamas on the east side. Most of the water from the Gulf of Mexico exits to the Atlantic Ocean through these narrow straits. The water moves fast, about 2-5 mph. When this fast moving water meets the land at the western-most tip of the Florida Keys, eddies form, with water moving in a counter-clockwise direction. In the Northern Hemisphere, counter-clockwise spinning eddies have upwelling at their core, bringing water to the surface that is cooler (upwelling), saltier, and more nutrient-rich compared to the surrounding water.

The reason we are all out in the ocean sampling plankton is to better understand the interactions between larval fish, their predators and prey, especially in regions of higher plankton abundance. Since these counter-clockwise eddies bring more nutrients to the surface, they create “hot-spots” for plankton to thrive, thus one of our research goals is to sample in and out of an eddy.

 

How to find an eddy

The best way to locate a large eddy is to look at satellite ocean color images, which show the higher concentrations of the pigment chlorophyll a (a proxy to measure how much

phytoplankton is present) in regions with higher nutrients. Ocean color images from NASA’s VIIRS satellite show this eddy last week as fairly defined (image top); however, the image from yesterday shows it has started to break-up.

While a whirlpool in the ocean may evoke images of Charybdis from The Odyssey, this eddy is actually proving quite difficult to find, especially since it’s breaking apart. Unfortunately we cannot go back in time and request to start our cruise when the eddy was stronger, but that’s how science goes! Even though the eddy is breaking up, we are still interested in sampling the plankton associated with the eddy waters.

Now we are out in the ocean, near where the eddy was last seen, we can use in-situ (i.e. placed in the environment) measurements from DPI-2 (formerly ISIIS) and the ship’s ADCP (Acoustic Doppler Current Profiler) to see signatures of the eddy. These include colder water temps, higher salinity, and increased fluorescence from more phytoplankton in the water.