Glider’s work during hurricane reveals unique insights about storm dynamics
Three days before Hurricane Gonzalo hit Bermuda in October, the Bermuda Institute of Ocean Sciences (BIOS) researchers launched their new glider Anna directly into the storm's path for a rare look at hurricane dynamics below the sea surface.
During the storm, Anna — a six-foot-long, yellow glider that resembles an airplane — dived and climbed in the water column offshore Bermuda, sending data via satellite back to BIOS researchers who were monitoring Anna's progress from the safety of their shore-based homes.
“Although not unprecedented, it is extremely rare to obtain measurements like this during a hurricane,” said marine researcher Ruth Curry, who heads the glider programme being developed at BIOS. Previous observations have come from moorings that just happened to be in the path of passing storms.
Gliders like Anna can travel thousands of miles on a single journey, providing a detailed picture of ocean dynamics. The glider dives from the sea surface to 3,000 feet depth, covering about 15 miles per day and communicating with shore-based scientists via satellite when at the sea surface.
Before the storm, Ms Curry and her team equipped the glider to measure ocean temperature, salinity, currents, oxygen and chlorophyll levels — aspects of the ocean that change dramatically during hurricanes.
Anna's unique perspective from beneath the waves yielded insights into the interplay between the ocean and the storms. For example, the upper layer of the ocean cooled by about eight degrees in one week, first as Tropical Storm Fay passed on October 12, followed by Gonzalo.
“Fay actually did Bermuda a huge favour by reducing surface temperatures to less than 79°F (the minimum temperature needed to maintain a hurricane's strength),” Ms Curry said. This caused Gonzalo to weaken from a Category 4 storm to a Category 2 before reaching the Island
At Gonzalo's peak, surface waves built to about 30 feet in height, while 200 feet below, the storm generated deep internal waves measuring 150 feet high. These internal ocean waves created turbulent mixing that brought nutrients to the surface layer, spurring a bloom of organisms relied upon by the local food web, from microbes to whales.
There were a few surprises.
At the height of the hurricane, violent winds and waves sheared off Anna's tail rudder. Although this meant that the vehicle could no longer be steered, its diving and climbing functions were not affected, and Anna was nevertheless able to take measurements of the ocean.
Remarkably, the glider continued to gather data even as it was caught up in strong currents below the surface that reached speeds of more than two knots. This gave Ms Curry an unanticipated record of these undersea flows that formed clockwise loop-de-loops for several days after the storm.
Forced to chase Anna, BIOS used the research vessel Atlantic Explorer to follow the glider's westward drift from Bermuda and recovered it two days later, on October 20.
Anna — which heads out from Bermuda again in January — now has a new rudder, as well as a decal bestowed by its Massachusetts-based developers, Teledyne Webb Research. It shows an image of King Neptune blowing on the sea below a deserving title: “Storm Glider”.
Ms Curry and fellow BIOS scientist Mark Guishard will be giving a lecture about Hurricane Gonzalo and it impact on the oceans on Thursday, December 11, 7.30pm at the Bermuda Underwater Exploration Institute (BUEI) in Hamilton.