Estimating and Understanding the Energy Decay of Coherent Eddies from In situ Observations and Satellite Altimetry.

Abstract

Quantifying and understanding the mixing and decay properties of oceanic mesoscale coherent eddies is crucial, as coupled climate models become eddy-permitting. Measuring experimentally the decay of coherent eddies is a difficult and expensive task. As of now, only a very limited number of repeated field surveys across the same eddy have been achieved. On the other hand, satellite altimetry offers a nearly synoptic two-dimensional view of the evolution of ocean vortices, and the record is now 30 years-long, allowing for the computation of solid statistics. But most coherent eddies are essentially baroclinic and knowledge of their vertical structure is crucial to understand the decay of their energy, or heat and salt contents. Here, we take advantage of the dense array of ARGO float profiles in the Gulf of Mexico (GoM) to reconstruct the three-dimensional structure of all Loop Current Rings (LCR) detached since 1993, using the Gravest Empirical Modes (GEM) method and a gridded altimetry product. The 3D reconstruction method was validated using independent glider observations and exhibits a striking accuracy in estimating the kinetic and available potential energy of LCRs, as well as their heat and salt contents. The decay of LCRs in terms of energy and thermohaline properties is then studied in details and it is shown that, despite their longevity of 6 to 15 months, they decay continuously with time at an inverse exponential rate, and have lost 80 % of their total energy as they reach the western GoM’s continental slope, where they were previously suspected to decay. We studied the impact of the wind and the feed back of the current on the wind on the energy decay of LCRs, and found that wind stress work (using relative wind stress), is responsible for half of the total energy loss during the eddy’s life time, while available potential energy decay is entirely driven by Ekman buoyancy fluxes and the barotropic vorticity decay is driven by wind stress curl. This suggests that wind forcing (when considering the feed-back of ocean currents on wind stress) is the leading-order mechanism in the decay of these coherent eddies.