Laboratory Modeling of the Downwelling Coastal Current and the Related Bottom Ekman Layer above the Inclined Bottom in the Rotating Homogeneous and Stratified Fluid

D. N. Elkin✉, A. G. Zatsepin

Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russian Federation

✉ e-mail: dmelkin@mail.ru

Abstract

Laboratory experiments on studying the downwelling coastal current above the inclined bottom are conducted in the tank placed at a rotating platform. The current is formed by a source of constant flow of water with the same density (barotropic case) or the lower one (baroclinic case) as compared to the water in the tank. It is revealed that both in the barotropic and the baroclinic (under certain conditions) cases, the bottom Ekman layer formation implies sinking of the downwelling coastal current water. Some regularities in dynamics of the downwelling coastal current and the associated bottom Ekman layer in the barotropic and baroclinic cases are determined. It is found that in the barotropic case, the water in the bottom Ekman layer spreads from the source down the slope. It is shown that in the baroclinic case when the densities of water in the source and in the tank do not differ significantly, the water with lower density from the source enters the bottom Ekman layer which undergoes convective instability. The criteria for transition from the barotropic current regime to the baroclinic one, i. e. to the convective instability of the bottom Ekman layer are determined. It is revealed that convection in the bottom layer develops at the Rayleigh number values considerably exceeding its critical values characteristic of the laminar boundary layer in a rotating fluid. Actually, the bottom Ekman layer in the experiments is not purely laminar; its thickness is much higher than that of the laminar flow. Consequently, the coefficient of effective viscosity in this layer is also higher than the kinematic viscosity value which is used for calculating the critical Rayleigh number. When the densities of water in the source and in the tank differ significantly, the water with lower density propagates near the surface, does not enter the bottom Ekman layer and does not undergo convective instability.

Keywords

downwelling coastal currents, laboratory modeling, inclined bottom, water stratification, rotating platform, bottom Ekman layer, convective instability, mixing

Acknowledgements

The present study was carried out under the framework of the State Order No. 0149-2018-0003. It is supported by RSF Grant No. 14-50-00095 concerning the analysis of the results and by RFBR Grant No. 17-05-00381 – concerning the article preparation for the publication.

Original russian text

Original Russian Text © D. N. Elkin, A. G. Zatsepin, 2018, published in MORSKOY GIDROFIZICHESKIY ZHURNAL, Vol. 34, Iss. 3, pp. 179–191 (2018)

For citation

Elkin, D.N. and Zatsepin, A.G., 2018. Laboratory Modeling of the Downwelling Coastal Current and the Related Bottom Ekman Layer above the Inclined Bottom in the Rotating Homogeneous and Stratified Fluid. Physical Oceanography, 25(3), pp. 165-176. doi:10.22449/1573-160X-2018-3-165-176

DOI

10.22449/1573-160X-2018-3-165-176

References

  1. Munk, W.H., 1966. Abyssal Recipes. Deep Sea Research and Oceanographic Abstracts, [e-journal] 13(4), pp. 707-730. https://doi.org/10.1016/0011-7471(66)90602-4
  2. Zatsepin, A.G., Golenko, N.N., Korzh, A.O., Kremenetskii, V.V., Paka, V.T., Poyarkov, S.G. and Stunzhas, P.A., 2007. Influence of the Dynamics of Currents on the Hydrophysical Structure of the Waters and the Vertical Exchange in the Active Layer of the Black Sea. Oceanology, [e-journal] 47(3), pp. 301-312. https://doi.org/10.1134/S0001437007030022
  3. Zhurbas, V.M., Oh, I.S. and Park, T., 2006. Role of the Beta-Effect in the Decay of the Alongshore Baroclinic Jet Associated with Transient Coastal Upwelling and Downwelling: Numerical Experiments. Oceanology, [e-journal] 46(2), pp. 170-177. https://doi.org/10.1134/S0001437006020032
  4. Schaeffer, A., Roughan, M. and Wood, J.E., 2014. Observed Bottom Boundary Layer Transport and Uplift on the Continental Shelf Adjacent to a Western Boundary Current. Journal of Geophysical Research, [e-journal] 119(8). doi:10.1002/2013JC009735
  5. Moum, J.N., Perlin, A., Klymak, J.M., Levine, M.D., Boyd, T. and Kosro, P.M., 2004. Convectively Driven Mixing in the Bottom Boundary Layer. J. Phys. Oceanogr., [e-journal] 34(10), pp. 2189-2202. https://doi.org/10.1175/1520-0485(2004)034<2189:CDMITB>2.0.CO;2
  6. Ostrovskii, A.G. and Zatsepin A.G., 2016. Intense Ventilation of the Black Sea Pycnocline due to Vertical Turbulent Exchange in the Rim Current Area. Deep-Sea Res. I, [e-journal] 116, pp. 1-13. https://doi.org/10.1016/j.dsr.2016.07.011
  7. Elkin, D.N., Zatsepin, A.G., Podymov, O.I. and Ostrovskii, A.G., 2017. Sinking of Less Dense Water in the Bottom Ekman Layer Formed by a Coastal Downwelling Current over a Sloping Bottom. Oceanology, [e-journal] 57(4), pp. 478-484. https://doi.org/10.1134/S0001437017040051
  8. Zatsepin, A.G. and Semenov, A.V., 1997. Barotropnye Vikhri-Intruzii nad Ploskim Dnom vo Vrashchayushcheysya Zhidkosti [Barotropic Vortex-Intrusions over a Flat Bottom in a Rotating Fluid]. In: Intruzionnye i Plotnostnye Techeniya (Pamyati V. N. Anuchina) [Intrusive and Dense Currents (memory of Anuchin V.N.)]. Kaliningrad: Kaliningrad State Technical University, pp. 55-69 (in Russian).
  9. Boubnov, B.M. and Golitsyn, G.S., 1995. Convection in Rotating Fluids. Dordrecht: Kluwer Academic Publishers, 224 p.
  10. Didkovskii, V.L., Semenov, A.V. and Zatsepin, A.G., 2000. Mesoscale Currents upon the Smooth Sloping Bottom and in the Presence of Ridges and Canyons. In: UNESCO, 2000. Oceanic Fronts and Related Phenomena: Konstantin Fedorov International Memorial Symposium, Pushkin, Saint Petersburg, 18–22 May 1998: Proceedings. IOC Workshop report N 159. UNESCO, pp. 89-94.
  11. Ginsburg, A.I. and Fedorov, K.N., 1978. O Kriticheskom Pogranichnom Chisle Releya pri Okhlazhdenii Vody cherez Svobodnuyu Poverkhnost' [On the Critical Value of the Boundary Rayleigh Number Assotiated with the Cooling of Water through Free Surface]. Izvestiya of the Academy of Sciences of the USSR. Atmospheric and Oceanic Physics, 14(4), pp. 433-436 (in Russian).

Download the article (PDF)