Presenter: Kathleen Donohue (University of Rhode Island)

Description:
By measuring full water column circulation with mesoscale resolution, two mapping arrays in the Gulf of Mexico reveal key interactions between the Loop Current and deep eddies that jointly develop and intensify in the process of Loop Current Eddy separation.  During the 30 month period April 2009 through November 2011, the formation of three Loop Current Eddies, Ekman, Franklin, and Hadal, were observed by an array of moored current meters and bottom mounted pressure equipped inverted echo sounders (PIES). Ten years later as part of the Understanding Loop Current System (UGOS) program, a comparable array of PIES with attached current meters deployed for two years during June 2019 through May 2021 observed the formation of another three Loop Current Eddies, Sverdrup, Thor, and Ursa. Both experiments consistently observed the following processes. When the Loop Current is retracted, the deep ocean eddies become quiescent. When the Loop Current extends into the Gulf, large meanders develop and the deep eddy field is energized. Joint intensification is intermittent – depending upon vertical phasing and lasting tens of days. A train of upper-deep eddy interactions leads to each eddy separation and deep eddies develop in a pattern consistent with baroclinic instability. The expanded coverage of the UGOS 2019-2020 array shows the development of deep cyclones when the Loop Current interacts with the Mississippi Fan. These cyclones appear to trigger Loop Current eddies, particularly for the Thor event. This observation confirms earlier numerical modeling studies and emphasizes the role of Loop Current-topographic interactions in the eddy shedding process.