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

Abstract

A 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 ∼390 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.

Wind‐Forced Symmetric Instability at a Transient Mid‐Ocean Front

“…As indicated by Buckingham et al. (), our study domain is commonly cloud‐free every few weeks. During 3–12 April 2013, one informative VIIRS SST image could be obtained (Figure a). Reanalysis data sets .…”

“…Thermal imagery from the Visible and Infrared Imaging Radiometer Suite (VIIRS) is employed for the operational sea surface temperature (SST) product, owing to its high spatial resolution (750 m) under cloud-free conditions (Schloesser et al, 2016). As indicated by Buckingham et al (2017), our study domain is commonly cloud-free every few weeks. During 3-12 April 2013, one informative VIIRS SST image could be obtained ( Figure 1a).…”

Subduction by Submesoscales

“…The first parameterization of was suggested by Fox‐Kemper et al (, , FK), who constructed it using results from numerical simulations and linear stability analysis under the assumption of a baroclinic instability (BI) only (BI is also referred to as vertical shear instability, Buckingham et al, ). In that case, the variables that characterize SM are the horizontal buoyancy gradient , the Coriolis parameter f , and the depth h of the SM regime.…”

“…Recently, it has come to our attention that Buckingham et al (2017) section (4.3.2) also suggested the existence of lower and upper bounds for h called H 1,2 . As in our case, the upper limit was taken to be the depth of peak stratification but the lower limit was still taken to be the mixed layer depth.…”

SST Dynamics at Different Scales: Evaluating the Oceanographic Model Resolution Skill to Represent SST Processes in the Southern Ocean

“…(That is, the EO data resolve eddy‐like anomalies with diameters of 15–20 km or equivalently wavelengths twice these values.) Thus, our use of the term submesoscale is not very different from the more classical definition based on nondimensional numbers–i.e., flows characterized by order 1 Rossby and Richardson numbers (McWilliams, ; Thomas, ) and which often corresponds to spatial scales 10 km and time scales of hours to a day (Buckingham et al, , ; Erickson & Thompson, ; McWilliams, ; Thompson et al, ). This understanding of submesoscales should be distinguished from another widely used definition, which is based on the scale at which geostrophy begins to break down, approximately below the radius of deformation (Sasaki et al, ; Su et al, ; Torres et al, ).…”