• Bachman, S. D., , and J. R. Taylor, 2014: Modelling of partially-resolved oceanic symmetric instability. Ocean Modell., 82, 1527, doi:10.1016/j.ocemod.2014.07.006.

    • Search Google Scholar
    • Export Citation
  • Belcher, S. E., and et al. , 2012: A global perspective on Langmuir turbulence in the ocean surface boundary layer. Geophys. Res. Lett., 39, L18605, doi:10.1029/2012GL052932.

  • Boccaletti, G., , R. Ferrari, , and B. Fox-Kemper, 2007: Mixed layer instabilities and restratification. J. Phys. Oceanogr., 37, 22282250, doi:10.1175/JPO3101.1.

    • Search Google Scholar
    • Export Citation
  • Brannigan, L., , D. P. Marshall, , A. C. N. Garabato, , and A. J. G. Nurser, 2015: The seasonal cycle of submesoscale flows. Ocean Modell., 92, 6984, doi:10.1016/j.ocemod.2015.05.002.

    • Search Google Scholar
    • Export Citation
  • Buckingham, C., and et al. , 2016: Seasonality of submesoscale flows in the ocean surface boundary layer. Geophys. Res. Lett., 43, doi:10.1002/2016GL068009, in press.

    • Search Google Scholar
    • Export Citation
  • Callies, J., , R. Ferrari, , J. M. Klymak, , and J. Gula, 2015: Seasonality in submesoscale turbulence. Nat. Commun., 6, 6862, doi:10.1038/ncomms7862.

    • Search Google Scholar
    • Export Citation
  • Capet, X., , J. C. McWilliams, , M. J. Molemaker, , and A. F. Shchepetkin, 2008: Mesoscale to submesoscale transition in the California Current System. Part I: Flow structure, eddy flux, and observational tests. J. Phys. Oceanogr., 38, 2943, doi:10.1175/2007JPO3671.1.

    • Search Google Scholar
    • Export Citation
  • D’Asaro, E., , C. Lee, , L. Rainville, , R. Harcourt, , and L. Thomas, 2011: Enhanced turbulence and energy dissipation at ocean fronts. Science, 332, 318322, doi:10.1126/science.1201515.

    • Search Google Scholar
    • Export Citation
  • Dee, D. P., and et al. , 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553597, doi:10.1002/qj.828.

    • Search Google Scholar
    • Export Citation
  • Ferrari, R., , and D. L. Rudnick, 2000: Thermohaline variability in the upper ocean. J. Geophys. Res., 105, 16 85716 883, doi:10.1029/2000JC900057.

    • Search Google Scholar
    • Export Citation
  • Ferrari, R., , and C. Wunsch, 2009: Ocean circulation kinetic energy: Reservoirs, sources, and sinks. Annu. Rev. Fluid Mech., 41, 253282, doi:10.1146/annurev.fluid.40.111406.102139.

    • Search Google Scholar
    • Export Citation
  • Fox-Kemper, B., , R. Ferrari, , and R. Hallberg, 2008: Parameterization of mixed layer eddies. Part I: Theory and diagnosis. J. Phys. Oceanogr., 38, 11451165, doi:10.1175/2007JPO3792.1.

    • Search Google Scholar
    • Export Citation
  • Fox-Kemper, B., and et al. , 2011: Parameterization of mixed layer eddies. III: Implementation and impact in global ocean climate simulations. Ocean Modell., 39, 6178, doi:10.1016/j.ocemod.2010.09.002.

    • Search Google Scholar
    • Export Citation
  • Haine, T. W. N., , and J. Marshall, 1998: Gravitational, symmetric, and baroclinic instability of the ocean mixed layer. J. Phys. Oceanogr., 28, 634658, doi:10.1175/1520-0485(1998)028<0634:GSABIO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., 1974: The role of potential vorticity in symmetric stability and instability. Quart. J. Roy. Meteor. Soc., 100, 480482, doi:10.1002/qj.49710042520.

    • Search Google Scholar
    • Export Citation
  • Jia, F., , L. Wu, , and B. Qiu, 2011: Seasonal modulation of eddy kinetic energy and its formation mechanism in the southeast Indian Ocean. J. Phys. Oceanogr., 41, 657665, doi:10.1175/2010JPO4436.1.

    • Search Google Scholar
    • Export Citation
  • Koltermann, K. P., , V. V. Gouretski, , and K. Jancke, 2011: Atlantic Ocean. Vol. 3, Hydrographic Atlas of the World Ocean Circulation Experiment (WOCE), International WOCE Project Office. [Available online at http://www-pord.ucsd.edu/whp_atlas/atlantic_index.html.]

  • Lévy, M., , R. Ferrari, , P. J. S. Franks, , A. P. Martin, , and P. Rivière, 2012: Bringing physics to life at the submesoscales. Geophys. Res. Lett., 39, L14602, doi:10.1029/2012GL052756.

  • Mahadevan, A., , A. Tandon, , and R. Ferrari, 2010: Rapid changes in mixed layer stratification driven by submesoscale instabilities and winds. J. Geophys. Res., 115, C03017, doi:10.1029/2008JC005203.

  • Mahadevan, A., , E. D’Asaro, , C. Lee, , and M. J. Perry, 2012: Eddy-driven stratification initiates North Atlantic spring phytoplankton blooms. Science, 337, 5458, doi:10.1126/science.1218740.

    • Search Google Scholar
    • Export Citation
  • Mensa, J. A., , Z. Garraffo, , A. Griffa, , T. M. Özgökmen, , A. Haza, , and M. Veneziani, 2013: Seasonality of the submesoscale dynamics in the Gulf Stream. Ocean Dyn., 63, 923941, doi:10.1007/s10236-013-0633-1.

    • Search Google Scholar
    • Export Citation
  • Omand, M. M., , E. A. D’Asaro, , C. M. Lee, , M. J. Perry, , N. Briggs, , I. Cetini, , and A. Mahadevan, 2015: Eddy-driven subduction exports particulate organic carbon from the spring bloom. Science, 348, 222223, doi:10.1126/science.1260062.

    • Search Google Scholar
    • Export Citation
  • Rosso, I., , A. M. Hogg, , P. G. Strutton, , A. E. Kiss, , R. Matear, , A. Klocker, , and E. van Sebille, 2014: Vertical transport in the ocean due to sub-mesoscale structures: Impacts in the Kerguelen region. Ocean Modell., 80, 1023, doi:10.1016/j.ocemod.2014.05.001.

    • Search Google Scholar
    • Export Citation
  • Sallée, J.-B., , E. Shuckburgh, , N. Bruneau, , A. J. S. Meijers, , T. J. Bracegirdle, , and Z. Wang, 2013: Assessment of Southern Ocean mixed-layer depths in CMIP5 models: Historical bias and forcing response. J. Geophys. Res. Oceans, 118, 18451862, doi:10.1002/jgrc.20157.

    • Search Google Scholar
    • Export Citation
  • Sasaki, H., , P. Klein, , B. Qiu, , and Y. Sasai, 2014: Impact of oceanic-scale interactions on the seasonal modulation of ocean dynamics by the atmosphere. Nat. Commun., 5, 5636, doi:10.1038/ncomms6636.

    • Search Google Scholar
    • Export Citation
  • Shcherbina, A. Y., , E. A. D’Asaro, , C. Lee, , J. M. Klymak, , M. J. Molemaker, , and J. C. McWilliams, 2013: Statistics of vertical vorticity, divergence, and strain in a developed submesoscale turbulence field. Geophys. Res. Lett., 40, 47064711, doi:10.1002/grl.50919.

    • Search Google Scholar
    • Export Citation
  • Stone, P. H., 1966: On non-geostrophic baroclinic stability. J. Atmos. Sci., 23, 390400, doi:10.1175/1520-0469(1966)023<0390:ONGBS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Stone, P. H., 1970: On non-geostrophic baroclinic stability: Part II. J. Atmos. Sci., 27, 721726, doi:10.1175/1520-0469(1970)027<0721:ONGBSP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Swart, S., , S. Thomalla, , and P. Monteiro, 2015: The seasonal cycle of mixed layer dynamics and phytoplankton biomass in the Sub-Antarctic Zone: A high-resolution glider experiment. J. Mar. Syst., 147, 103115, doi:10.1016/j.jmarsys.2014.06.002.

    • Search Google Scholar
    • Export Citation
  • Tandon, A., , and C. Garrett, 1994: Mixed layer restratification due to a horizontal density gradient. J. Phys. Oceanogr., 24, 14191424, doi:10.1175/1520-0485(1994)024<1419:MLRDTA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Taylor, J. R., , and R. Ferrari, 2009: On the equilibration of a symmetrically unstable front via a secondary shear instability. J. Fluid Mech., 622, 103113, doi:10.1017/S0022112008005272.

    • Search Google Scholar
    • Export Citation
  • Taylor, J. R., , and R. Ferrari, 2010: Buoyancy and wind-driven convection at mixed layer density fronts. J. Phys. Oceanogr., 40, 12221242, doi:10.1175/2010JPO4365.1.

    • Search Google Scholar
    • Export Citation
  • Thomas, L. N., 2005: Destruction of potential vorticity by winds. J. Phys. Oceanogr., 35, 24572466, doi:10.1175/JPO2830.1.

  • Thomas, L. N., , A. Tandon, , and A. Mahadevan, 2008: Sub-mesoscale processes and dynamics. Ocean Modeling in an Eddying Regime, Geophys. Monogr., Vol. 177, Amer. Geophys. Union, 17–38.

  • Thomas, L. N., , J. R. Taylor, , R. Ferrari, , and T. M. Joyce, 2013: Symmetric instability in the Gulf Stream. Deep-Sea Res. II, 91, 96110, doi:10.1016/j.dsr2.2013.02.025.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 283 283 46
PDF Downloads 263 263 54

Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

View More View Less
  • 1 Environmental Science and Engineering, California Institute of Technology, Pasadena, California
  • | 2 University of Southampton, National Oceanography Centre, Southampton, United Kingdom
  • | 3 School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
© Get Permissions
Restricted access

Abstract

The importance of submesoscale instabilities, particularly mixed layer baroclinic instability and symmetric instability, on upper-ocean mixing and energetics is well documented in regions of strong, persistent fronts such as the Kuroshio and the Gulf Stream. Less attention has been devoted to studying submesoscale flows in the open ocean, far from long-term, mean geostrophic fronts, characteristic of a large proportion of the global ocean. This study presents a year-long, submesoscale-resolving time series of near-surface buoyancy gradients, potential vorticity, and instability characteristics, collected by ocean gliders, that provides insight into open-ocean submesoscale dynamics over a full annual cycle. The gliders continuously sampled a 225 km2 region in the subtropical northeast Atlantic, measuring temperature, salinity, and pressure along 292 short (~20 km) hydrographic sections. Glider observations show a seasonal cycle in near-surface stratification. Throughout the fall (September–November), the mixed layer deepens, predominantly through gravitational instability, indicating that surface cooling dominates submesoscale restratification processes. During winter (December–March), mixed layer depths are more variable, and estimates of the balanced Richardson number, which measures the relative importance of lateral and vertical buoyancy gradients, depict conditions favorable to symmetric instability. The importance of mixed layer instabilities on the restratification of the mixed layer, as compared with surface heating and cooling, shows that submesoscale processes can reverse the sign of an equivalent heat flux up to 25% of the time during winter. These results demonstrate that the open-ocean mixed layer hosts various forced and unforced instabilities, which become more prevalent during winter, and emphasize that accurate parameterizations of submesoscale processes are needed throughout the ocean.

Corresponding author address: Andrew F. Thompson, Environmental Science and Engineering, California Institute of Technology, MC 131-24, Pasadena, CA 91125. E-mail: andrewt@caltech.edu

Abstract

The importance of submesoscale instabilities, particularly mixed layer baroclinic instability and symmetric instability, on upper-ocean mixing and energetics is well documented in regions of strong, persistent fronts such as the Kuroshio and the Gulf Stream. Less attention has been devoted to studying submesoscale flows in the open ocean, far from long-term, mean geostrophic fronts, characteristic of a large proportion of the global ocean. This study presents a year-long, submesoscale-resolving time series of near-surface buoyancy gradients, potential vorticity, and instability characteristics, collected by ocean gliders, that provides insight into open-ocean submesoscale dynamics over a full annual cycle. The gliders continuously sampled a 225 km2 region in the subtropical northeast Atlantic, measuring temperature, salinity, and pressure along 292 short (~20 km) hydrographic sections. Glider observations show a seasonal cycle in near-surface stratification. Throughout the fall (September–November), the mixed layer deepens, predominantly through gravitational instability, indicating that surface cooling dominates submesoscale restratification processes. During winter (December–March), mixed layer depths are more variable, and estimates of the balanced Richardson number, which measures the relative importance of lateral and vertical buoyancy gradients, depict conditions favorable to symmetric instability. The importance of mixed layer instabilities on the restratification of the mixed layer, as compared with surface heating and cooling, shows that submesoscale processes can reverse the sign of an equivalent heat flux up to 25% of the time during winter. These results demonstrate that the open-ocean mixed layer hosts various forced and unforced instabilities, which become more prevalent during winter, and emphasize that accurate parameterizations of submesoscale processes are needed throughout the ocean.

Corresponding author address: Andrew F. Thompson, Environmental Science and Engineering, California Institute of Technology, MC 131-24, Pasadena, CA 91125. E-mail: andrewt@caltech.edu
Save