Observations of an unstable submesoscale front

A single satellite image of an ocean front coincident with in situ measurements of density from a towed, undulating vehicle is examined for evidence of mixed layer (ML) baroclinic instability (BCI). The image was taken over the North Atlantic on 19 September 2012 at 03:34 UTC by the Visible and Infrared Imaging Radiometer Suite (VIIRS) on the Suomi/National Polar-orbiting Partnership (NPP) spacecraft and consists of brightness temperatures collected within the infrared band at 390-m pixel resolution. The submesocale front (1) is characterized by a 0.5 to 0.7°C change in temperature over a 10 to 25-km distance, (2) results from cool, northern waters advected in a southeasterly direction by mesoscale eddies and (3) is populated by several submesoscale eddies along the frontal boundary. While horizontal shear is one possible mechanism for generating such eddies (Munk et al., 2000), here we consider an alternative hypothesis: that observed spiral eddies are the result of ML BCI. Vertical and lateral buoyancy gradients, along with ML depth, are estimated from in situ measurements. Theoretical eddy sizes corresponding to the most unstable mode of an Eady model are then computed and compared with observed eddy sizes. Sensitivities to model parameters are discussed. In all cases, we find good agreement between predicted and observed eddy sizes. Growth rates have not been estimated due to severe cloud cover before and after the event. Given observed Rossby numbers are less than 0.5, our study demonstrates that ML BCI might better explain the occurrence of submesoscale eddies in the open ocean.