NPG Nonlinear Processes in Geophysics NPG Nonlin. Processes Geophys. 1607-7946 Copernicus Publications Göttingen, Germany 10.5194/npg-17-37-2010 Vertical mixing and coherent anticyclones in the ocean: the role of stratification Koszalka I. 1 Ceballos L. 2 Bracco A. 2 Institute of Geosciences, University of Oslo, 1022 Blinden, 0315 Oslo, Norway School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, GA 30332, USA 22 01 2010 17 1 37 47 Copyright: © 2010 I. Koszalka et al. 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://npg.copernicus.org/articles/17/37/2010/npg-17-37-2010.html The full text article is available as a PDF file from https://npg.copernicus.org/articles/17/37/2010/npg-17-37-2010.pdf

The role played by wind-forced anticyclones in the vertical transport and mixing at the ocean mesoscale is investigated with a primitive-equation numerical model in an idealized configuration. The focus of this work is to determine how the stratification impacts such transport. <br><br> The flows, forced only at the surface by an idealized wind forcing, are predominantly horizontal and, on average, quasigeostrophic. Inside vortex cores and intense filaments, however, the dynamics is strongly ageostrophic. <br><br> Mesoscale anticyclones appear as "islands" of increased penetration of wind energy into the ocean interior and they represent the maxima of available potential energy. The amount of available potential energy is directly correlated with the degree of stratification. <br><br> The wind energy injected at the surface is transferred at depth through the generation and subsequent straining effect of Vortex Rossby Waves (VRWs), and through near-inertial internal oscillations trapped inside anticyclonic vortices. Both these mechanisms are affected by stratification. Stronger transfer but larger confinement close to the surface is found when the stratification is stronger. For weaker stratification, vertical mixing close to the surface is less intense but below about 150 m attains substantially higher values due to an increased contribution of both VRWs, whose time scale is on the order of few days, and of near-inertial motions, with a time scale of few hours.

 References Arai, M. and Yamagata, T.: Asymmetric evolution of eddies in rotation shallow water, Chaos, 4, 163–173, 1994. Arbic, B.&nbsp;K., Flierl, G.&nbsp;R., and Scott, R.&nbsp;B.: Cascade inequalities for forced-dissipated geostrophic turbulence, J. Phys. Oceanogr., 37, 1470–2177, 2007. Babiano, A. and Provenzale, A.: Coherent vortices and tracer cascades in two-dimensional turbulence, J. Fluid Mech., 574, 429–448, 2007. Benitez-Nelson, C.&nbsp;R., Bidigare, R.&nbsp;R., Dickey, T.&nbsp;D., Landry, M.&nbsp;R., Leonard, C.&nbsp;L., Brown, S.&nbsp;L., Nencioli, F., Rii, Y.&nbsp;M., Maiti, K., Becker, J.&nbsp;W., Bibby, T.&nbsp;S., Black, W., Cai, W.-J., Carlson, C.&nbsp;A., Chen, F., Kuwahara, V.&nbsp;S., Mahaffey, C., McAndrew, P.&nbsp;M., Quay, P.&nbsp;D., Rappe, M.&nbsp;S., Selph, K.&nbsp;E., Simmons, M.&nbsp;P., and Yang, E.&nbsp;J.: Mesoscale Eddies Drive Increased Silica Export in the Subtropical {P}acific {O}cean, Science, 316, 1017–1021, 2007. Bracco, A. and Pedlosky, J.: Vortex generation by topography in locally unstable baroclinic flows, J. Phys. Oceanogr., 33, 207–219, 2003. Bracco, A., von Hardenberg, J., Provenzale, A., Weiss, J.&nbsp;B., and McWilliams, J.&nbsp;C.: Dispersion and mixing in quasigeostrophic turbulence, Phys. Rev. Lett., 92(9), 084501, 1–4, 2004. Brown, J.&nbsp;N. and Fedorov, A.&nbsp;V.: Mean energy balance in the tropical Pacific Ocean, J. Mar. Res., 66(1), 1–23, 2008. Cho, J. and Polvani, L.&nbsp;M.: The emergence of jets and vortices in freely evolving, shallow- water turbulence on a sphere, Phys. Fluids, 8, 1531–1552, 1996. Danioux, E. and Klein, P.: A resonance mechanism leading to wind-forced motions with a 2f frequency, J. Phys. Ocean., 38, 2322–2329, 2008. Danioux, E., Klein, P., and Riviere, P.: Propagation of wind energy into the Deep ocean through a fully turbulent mesoscale eddy field, J. Phys. Oceanogr., 38, 2224–2241, 2008. Gent, P.&nbsp;R. and McWilliams, J.&nbsp;C.: Isopycnal mixing in ocean circulation models, J. Phys. Ocean., 20, 150–155, 1995. Graves, L.&nbsp;P., Mc{W}illiams, J.&nbsp;C., and Montgomery, M.&nbsp;T.: Vortex evolution due to straining: a mechanism for dominance of strong, interior anticyclones, Geophys. Astro. Fluid, 100(3), 151–183, 2006. Griffies, S.&nbsp;M.: Development in ocean climate modelling, Ocean Model., 2, 123–192, 2000. Hay, W.&nbsp;W., Flögel, S., and Söding, E.: Is the initiation of glaciation on Antartica related to a change in the structure of the ocean?, Global Planet. Change, 45, 23–33, 2004. Klein, P., Lapeyre, G., and Large, W.&nbsp;G.: Wind ringing of the ocean in presence of mesoscale eddies, Geophys. Res. Lett., 31, L15306, https://doi.org/10.1029/2004GL020274, 2004. Koszalka, I., Bracco, A., McWilliams, J.&nbsp;C., and Provenzale, A.: Dynamics of wind-forced coherent anticyclones in open ocean, J. Geophys. Res., 56, C08011, https://doi.org/10.1029/2009JC005388, 2009. Kunze, E.: Near-inertial wave propagation in geostrophic shear, J. Phys. Oceanogr., 15, 544–565, 1985. Large, W.&nbsp;G., Mc{W}illiams, J.&nbsp;C., and Doney, S.: Oceanic vertical mixing: a review and a model with a non-local {K}-profile boundary layer parameterization, Rev. Geophys., 32(4), 363–403, 1994. Larichev, V.&nbsp;D. and McWilliams, J.&nbsp;C.: Weakly decaying turbulence in an equivalent-barotropic fluid, Phys. Fluids, 3(5), 938–950, 1991. Lee, D.-K. and Niiler, P.: The inertial chimney: the near inertial energy drainage from the ocean surface to the deep layers, J. Geophys. Res., 103, 7579–7591, 1998. Li, X., Chao, Y., Mc{W}illiams, J.&nbsp;C., and Fu, L.-L.: A comparison of two vertical-mixing schemes in a {P}acific {O}cean general circulation model, J. Climate, 14, 1377–1398, 2001. McGillicuddy, D.&nbsp;J., Anderson, L.&nbsp;A., Bates, N.&nbsp;R., Bibby, T., Buesseler, K.&nbsp;O., Carlson, C.&nbsp;A., Davis, C.&nbsp;S., Ewart, C., Falkowski, P.&nbsp;G., Goldhwait, S.&nbsp;A., Hansell, D.&nbsp;A., Jenkins, W.&nbsp;J., Johnson, R., Kosnyrev, V.&nbsp;K., Ledwell, J.&nbsp;R., Li, Q.&nbsp;P., Siegel, D.&nbsp;A., and Steinberg, D.&nbsp;K.: Eddy/wind interactions stimulate extraordinary mid-ocean plankton blooms, Science, 316, 1021–1026, 2007. Mc{W}illiams, J.&nbsp;C. and Weiss, J.&nbsp;B.: Anisotropic geophysical vortices, Chaos, 4(2), 305–311, 1994. Oort, A.&nbsp;H., Ascher, S.&nbsp;C., Levitus, S., and Peixoto, J.&nbsp;P.: New estimates of available potential energy in the world ocean, J. Geophys. Res., 94, 3187–3200, 1989. Pasquero, C., Bracco, A., and Provenzale, A.: Coherent vortices, Lagrangian particles and the marine ecosystem, in: Shallow Flows, edited by: Jirka, G.&nbsp;H. and Uijttewaal, W. S.&nbsp;J., A. A. Balkema Publishers, 339–412, 2004. Pasquero, C., Bracco, A., Provenzale, A., and Weiss, J.&nbsp;B.: Particle motion in the sea of eddies, in: Lagrangian Analysis and Prediction of Coastal and Ocean Dynamics, edited by: Griffa, A., Kirwan, A. D., Mariano, A. J., Ozgökmen, T., and Rossby, T., Cambridge University Press, chapter&nbsp;4, 89–118, 2007. Petersen, M.&nbsp;R., Julien, K., and Weiss, J.&nbsp;B.: Vortex cores, strain cells, and filaments in quasigeostrophic turbulence, Phys. Fluids, 18, 026601, https://doi.org/10.1063/1.2166452, 2006. Polvani, L.&nbsp;M., Mc{W}illiams, J.&nbsp;C., Spall, M.&nbsp;A., and Ford, R.: The coherent structures of shallow-water turbulence: deformation radius effects, cyclon/anticyclone asymmetry and gravity-wave generation, Chaos, 4(2), 177–186, 1994. Provenzale, A.: Transport by coherent barotropic vortices, Annu. Rev. Fluid Mech., 31, 55–93, 1999. Shchepetkin, A. and Mc{W}illiams, J.&nbsp;C.: The regional oceanic modelling system ({ROMS}): a split-explicit, free-surface, topography-following-coordinate oceanic model, Ocean Dynam., 9, 347–3404, 2005. Smith, K.&nbsp;S. and Vallis, G.&nbsp;K.: The scales and equilibration of midocean eddies: freely evolving flow, J. Phys. Oceanogr., 31, 554–571, 2001. Smith, K.&nbsp;S. and Vallis, G.&nbsp;K.: The scales and equilibration of midocean eddies: Forced-Dissipative flow, J. Phys. Oceanogr., 32, 1699–1720, 2002. Spall, M.&nbsp;A., Pickart, R.&nbsp;S., Fratantoni, P.&nbsp;S., and Plueddeman, A.&nbsp;J.: Western Arctic Shelfbreak eddies: formation and transport, J. Phys. Ocean., 38(8), 1644–-1668, 2007. Stammer, D.: Global characteristics of ocean variability estimated from regional TOPEX/POSEIDON altimeter measurements, J. Phys. Ocean., 27, 1743–1769, 1997. Vallis, G. K.: Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-scale Circulation, Cambridge University Press, New York, US, 2006. Weiss, J.: The dynamics of enstrophy transfer in two-dimensional turbulence, Physica D, 48, 273–294, 1981. Wunsch, C. and Ferrari, R.: Vertical mixing, energy and the general circulation of the oceans, Annu. Rev. Fluid Mech., 36, 281–314, 2004. Yavneh, I., Shchepetkin, A.&nbsp;F., McWilliams, J.&nbsp;C., and Graves, L.&nbsp;P.: Multigrid solutions of rotating, stably stratified flows: the balance equations and their turbulent dynamics, J. Comp. Phys., 136, 245–262, 1997.