Testing Munk's hypothesis for submesoscale eddy generation using observations in the North Atlantic

Christian E Buckingham,Clément Vic,Alberto C Naveira Garabato,Adrian P Martin,Andrew F Thompson,Ayah Lazar,Zammath Khaleel,John T Allen

doi:10.1002/2017jc012910

Abstract

AbstractA high‐resolution satellite image that reveals a train of coherent, submesoscale (6 km) vortices along the edge of an ocean front is examined in concert with hydrographic measurements in an effort to understand formation mechanisms of the submesoscale eddies. The infrared satellite image consists of ocean surface temperatures at m resolution over the midlatitude North Atlantic (48.69°N, 16.19°W). Concomitant altimetric observations coupled with regular spacing of the eddies suggest the eddies result from mesoscale stirring, filamentation, and subsequent frontal instability. While horizontal shear or barotropic instability (BTI) is one mechanism for generating such eddies (Munk's hypothesis), we conclude from linear theory coupled with the in situ data that mixed layer or submesoscale baroclinic instability (BCI) is a more plausible explanation for the observed submesoscale vortices. Here we assume that the frontal disturbance remains in its linear growth stage and is accurately described by linear dynamics. This result likely has greater applicability to the open ocean, i.e., regions where the gradient Rossby number is reduced relative to its value along coasts and within strong current systems. Given that such waters comprise an appreciable percentage of the ocean surface and that energy and buoyancy fluxes differ under BTI and BCI, this result has wider implications for open‐ocean energy/buoyancy budgets and parameterizations within ocean general circulation models. In summary, this work provides rare observational evidence of submesoscale eddy generation by BCI in the open ocean.

Highlights

Submesoscale processes are believed to play important roles in ocean turbulence, stratification, and primary productivity [Boccaletti et al, 2007; Fox-Kemper et al, 2008a, 2008b; Thomas et al, 2008; Klein and Lapeyre, 2009; Fox-Kemper et al, 2011; Levy et al, 2012; Mahadevan et al, 2012; Omand et al, 2015; Bru€ggemann and Eden, 2015; Gula et al, 2016]
To help with future analysis with the Ocean Submesoscale Interaction Study (OSMOSIS) record, we list in supporting information all periods classified as clear-sky according to our definition and which exceed 2 h in duration
Sea Surface Temperature We manually cross-referenced these periods with the availability of Visible and Infrared Imaging Radiometer Suite (VIIRS) imagery, a subset of which we provide as supporting information

Summary

Introduction

Submesoscale processes are believed to play important roles in ocean turbulence, stratification, and primary productivity [Boccaletti et al, 2007; Fox-Kemper et al, 2008a, 2008b; Thomas et al, 2008; Klein and Lapeyre, 2009; Fox-Kemper et al, 2011; Levy et al, 2012; Mahadevan et al, 2012; Omand et al, 2015; Bru€ggemann and Eden, 2015; Gula et al, 2016] Despite this fact, observations of submesoscale phenomena are scarce. Examining repeated satellite infrared (IR) images of an ocean front off the Californian coast, Flament et al [1985] found evidence of submesoscale eddy generation resulting from horizontal shear instability Another useful example is the analysis of optical imagery collected from the Apollo shuttle missions [Scully-Power, 1986; Munk et al, 2000; Munk, 2001]. McWilliams [2016] for a modern-day review of oceanic submesoscale currents

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Discussion

Conclusion