Investigating the Formation of Submesoscale Structures along Mesoscale Fronts and Estimating Kinematic Quantities Using Lagrangian Drifters

John Lodise,Edward H Ryan,Cedric Guigand,Pierre-Marie Poulain,Björn Lund,Mohamed Iskandarani,Tamay Özgökmen,Rafael C Gonçalves,Jochen Horstmann,Jody Klymak

doi:10.3390/fluids5030159

Abstract

Much of the vertical transport near the surface of the ocean, which plays a critical role in the transport of dissolved nutrients and gases, is thought to be associated with ageostrophic submesoscale phenomena. Vertical velocities are challenging not only to model accurately, but also to measure because of how difficult they are to locate in the surface waters of the ocean. Using unique massive drifter releases during the Lagrangian Submesoscale Experiment (LASER) campaign in the Gulf of Mexico and the Coherent Lagrangian Pathways from the Surface Ocean to the Interior (CALYPSO) experiment in the Mediterranean Sea, we investigate the generation of submesoscale structures along two different mesoscale fronts. We use a novel method to project Lagrangian trajectories to Eulerian velocity fields, in order to calculate horizontal velocity gradients at the surface, which are used as a proxy for vertical transport. The velocity reconstruction uses a squared-exponential covariance function, which characterizes velocity correlations in horizontal space and time, and determines the scales of variation using the data itself. SST and towed CTD measurements support the findings revealed by the drifter data. Due to the production of a submesoscale instability eddy in the Gulf of Mexico, convergence magnitudes of up to ∼20 times the planetary vorticity, f, are observed, the value of which is almost 3 times larger than that found in the mesoscale dominated Western Mediterranean Sea.

Highlights

In recent decades, submesoscale surface currents have received added attention due to their importance pertaining to the energy cascade from large to small scales in the ocean, the transport and dispersion of oil spills and plastics, and the strong vertical velocities associated with these smaller scale ageostrophic motions [1,2,3,4]
The CALYPSO program is being conducted in the Western Mediterranean Sea, centered on the Alboran Gyre systems, which are characterized by large persistent mesoscale eddies in the confluence region of Atlantic and Mediterranean water masses
We investigate the mechanisms responsible for such submesoscale formation observed along the mesoscale front sampled in the Gulf of Mexico, and the lack of such submesoscale dynamics along the mesoscale front in the Western Mediterranean Sea

Summary

Introduction

Submesoscale surface currents have received added attention due to their importance pertaining to the energy cascade from large to small scales in the ocean, the transport and dispersion of oil spills and plastics, and the strong vertical velocities associated with these smaller scale ageostrophic motions [1,2,3,4]. The Lagrangian Submesoscale Experiment (LASER), which took place in the Northern Gulf of Mexico in 2016, and the ongoing Coherent Lagrangian Pathways from the Surface. We compare two mesoscale frontal systems, one observed during the LASER experiment in 2016 and one observed through the CALYPSO effort in 2019, to study the causality of submesoscale formation along mesoscale fronts, as well as the dynamics within the observed submesoscale surface flows. These novel measurements of submesoscale kinematics in the proximity of larger mesoscale fronts can serve as a much needed baseline comparison to help improve operational ocean model performance

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Conclusion