Exam (elaborations) Physical Activity 310987

Christian E. Buckingham1,2 , Zammath Khaleel1,3, Ayah Lazar4,5, Adrian P. Martin6 , John T. Allen7,8, Alberto C. Naveira Garabato1, Andrew F. Thompson4 , and Clement Vic 1 1 National Oceanography Centre, University of Southampton, Southampton, UK, 2 Now at British Antarctic Survey, Cambridge, UK, 3 Now at Ministry of Environment and Energy, Male, Maldives, 4 California Institute of Technology, Pasadena, California, USA, 5 Now at Israel Oceanographic and Limnological Research, Haifa, Israel, 6 National Oceanography Centre, Southampton, UK, 7 University of Portsmouth, Portsmouth, UK, 8 VECTis Environmental Consultants, LLP, Portsmouth, UK 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 (48.698N, 16.198W). 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. Plain Language Summary Here, we test Munk’s theory for small-scale eddy generation using a unique set of satellite- and ship-based observations. We find that for one particular set of observations in the North Atlantic, the mechanism for eddy generation is not pure horizontal shear, as proposed by Munk et al. (2000) and Munk (2001), but is instead vertical shear, or baroclinic instability. While by itself, this is not a globally important result, taken in the context of mesoscale eddies which are ubiquitous in the World Ocean, this suggests energy exchanges in the more ambient, open ocean are the result of the latter mechanism. In conclusion, submesoscale eddy generation is poorly understood in the ocean and we need to better constrain our geographical and temporal understanding of these processes for representation in coarseresolution models. 1. Background 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; Bruggemann and € Eden, 2015; Gula et al., 2016]. Despite this fact, observations of submesoscale phenomena are scarce. Traditional sampling strategies often fail to resolve these phenomena owing to their small-to-moderate lateral scale (0.1–10 km) and quickly evolving nature (hours to days). While expensive field campaigns have been designed to overcome this challenge—examples include the Scalable Lateral Mixing and Coherent Turbulence (i.e., LatMix) [Shcherbina et al., 2015], Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) [Buckingham et al., 2016; Thompson et al., 2016; Pe