Ocean Science www.ocean-sci.net 1812-0784 1812-0792 6 1 2010 10.5194/os-6-285-2010 http://www.ocean-sci.net/6/285/2010/ http://www.ocean-sci.net/6/285/2010/os-6-285-2010.html http://www.ocean-sci.net/6/285/2010/os-6-285-2010.pdf 285 299 2010-02-26 On contribution of horizontal and intra-layer convection to the formation of the Baltic Sea cold intermediate layer I. Chubarenko irina_chubarenko@mail.ru N. Demchenko P. P. Shirshov Institute of Oceanology RAS, Atlantic Branch, 236 000 Prospect Mira, 1, Kaliningrad, Russia Seasonal cascades down the coastal slopes and intra-layer convection are considered as the two additional mechanisms contributing to the Baltic Sea cold intermediate layer (CIL) formation along with conventional seasonal vertical mixing. Field measurements are presented, reporting for the first time the possibility of denser water formation and cascading from the Baltic Sea underwater slopes, which take place under fall and winter cooling conditions and deliver waters into intermediate layer of salinity stratified deep-sea area. The presence in spring within the CIL of water with temperature below that of maximum density (Tmd) and that at the local surface in winter time allows tracing its formation: it is argued that the source of the coldest waters of the Baltic CIL is early spring (March–April) cascading, arising due to heating of water before reaching the Tmd. Fast increase of the open water heat content during further spring heating indicates that horizontal exchange rather than direct solar heating is responsible for that. When the surface is covered with water, heated above the Tmd, the conditions within the CIL become favorable for intralayer convection due to the presence of waters of Tmd in intermediate layer, which can explain its well-known features – the observed increase of its salinity and deepening with time. Bennett, J. R.: Thermally driven lake currents during the spring and fall transition periods, Proc. 14th Conf. Great Lakes Res., Intl. Assoc. Great Lakes Res. Michigan, USA, 535–544~pp., 1971. Britter, R. E. and Linden, P. F.: The motion of a front of a gravity current travelling down an incline, J. Fluid Mech., 99, 531–543, 1980. Brooks, I. and Lick, W.: Lake currents associated with the thermal bar, J. Geophys. Res., 77(30), 6000–6013, 1972. Browand, F. K., Guyomar, D., and Yoon, S. C.: The behaviour of a turbulent front in a stratified fluid: Experiments with an oscillating grid, J. Geophys. Res., 92, 5329–5341, 1987. Carmack, E. C.: Combined influence of inflow and lake temperatures on spring circulation in a riverine lake, J. Phys. Oceanogr. 9, 422–434, 1979. Chubarenko, I. and Hutter, K.: Thermally driven interaction of the littoral and limnetic zones by autumnal cooling process, J. Limnol., 64(1), 31–42, 2005. Chubarenko, I. P., Demchenko, N. Y., and Hutter, K.: Horizontal convection induced by surface cooling over incline: laboratory experiment, Proc. International Conference "Fluxes and Structures in Fluids", Moscow, Russia, 27–29, 2005. Chubarenko, I. and Palij, A.: 3-D numerical modeling of cooling process over incline: comparison of hydrostatic and nonhydrostatic simulations, Proc of Int. Conf. "The complex study of the Atlantic ocean", Kaliningrad, Russia, 23–25, 2006. Chubarenko, I. and Demchenko, N.: Laboratory modeling of the thermal bar structure and the associated circulation in a basin with slopping bottom, Oceanology, 48(3), 349–361, 2008. Chubarenko, I., Esiukova, E., and Koutitonsky, V.: Simulation of horizontal convection induced by surface cooling over sea slope, Proc. Int. Conf. BSSC, Rostock, Warnemuende, Germany, 35, 2007. Ellison, T. H. and Turner, J. S.: Turbulent entrainment in stratified flows, J. Fluid Mech., 6, 423–448, 1959. Farrow, D. E. and Patterson, J. C.: On the response of a reservoir sidearm to diurnal heating and cooling, J. Fluid Mech. 246, 143–161, 1993. Farrow, D. E.: Periodically forced natural convection over slowly varying topography, J. Fluid Mech., 508, 1–21, 2004. Fedorov, K. N.: Fine thermohaline structure of ocean water masses, Hydrometeoizdat, Leningrad, 184~pp., 1976. Fer, I., Lemmin, U., and Thorpe, S. A.: Winter cascading of cold water in Lake Geneva, J. Geophys. Res., 107, 2236–2569, 2002a. Fer, I., Lemmin, U., and Thorpe, S. A.: Observations of mixing near the sides of a deep lake in winter, Limnol. Oceanogr., 47(2), 535–544, 2002b. Forel, F. A.: La congélation des lacs Suisses et Savoyards pendant l'hiver 1879–1880, 11 – Lac Léman, L'\"Echo des Alpes, 3, 149–161, 1880. Foster, T. D. and Carmack, E. C.: Frontal zone mixing and Antarctic Bottom Water formation in the southern Weddell Sea, Deep-Sea Res., 233(4), 301–318, 1976. Garrett, C.: Marginal mixing theories, Atmos. Ocean., 29, 313–339, 1991. Hagen, E. and Feistel, R.: Synoptic changes in the deep rim current during stagnant hydrographic conditions in the Eastern Gotland Basin, Baltic Sea, Oceanologia, 49(2), 185–208, 2007. Hinrichsen, H. H., Lehmann, A., Petereit, C., and Schmidt, J.: Correlation analyses of Baltic Sea winter water mass formation and its impact on secondary and tertiary production, Oceanologia, 49(3), 381–395, 2007. Horsh, G. M. and Stefan, H. G.: Convective circulation in littoral water due to surface cooling, Limnol. Oceanogr., 33(5), 1068–1083, 1988. Horsh, G. M., Stefan, H. G., and Gavali, S.: Numerical simulation of cooling-induced convective currents on a littoral slope, Int. J. Numer. Method H., 19, 105–134, 1994. Hydrometeorology and hydrochemistry of the seas of the USSR: Volume~3: The Baltic Sea, Hydrometeoizdat, St Petersburg, 450~pp., 1992. Imboden, D. M. and Wüest, A.: Mixing mechanisms in lakes, in: Physics and Chemistry of Lakes, edited by: Lerman, A., Imboden, D., and Gat, J., Springer-Verlag, Germany, 83–138, 1995. IOW long term temperature, salinity, oxygen observations in a frame of HELCOM program: 2001–2008 yy, http://www.io-warnemuende.de/, 2009. Jacobs, P. and Ivey, G.: The influence of rotation on shelf convection, J. Fluid Mech., 369, 23–48, 1998. Killworth, P. D.: Mixing on the Weddell Sea continental Slope, Deep-Sea Res. 24, 427–448, 1977. Leaman, K. D. and Schott, F. A.: Hydrographic structure of the convection regime in the Gulf of Lions: winter 1987, J. Phys. Oceanogr., 21, 575–598, 1991. Mamayev, O. I.: Temperature-salinity analysis of world ocean waters, Elsevier, Amsterdam, 374~pp., 1975. Meincke, J.: On the distribution of low salinity intermediate waters around the Farores, Deutsche Hydr. Zeitschrift, 31(2), 50–64, 1978. Morozov, E. G., Shchuka, S. A. , Golenko, N. N., Zapotylko, V. S., and Stont, J. I.: Temperature Structure in the Coastal Zone of the Baltic Sea, Dokl. Akad. Nauk, 416(7), 1066–1070, 2007. Mortimer, C. H.: Lake hydrodynamics, Mitteilungen Internationale Vereiningung fuer Limnologie, 20, 124–197, 1974. R/v "Professor Stokman": Scientific reports of cruises no 52 (24–29~May~2003), no 59 (3–9~March~2004) and no 67 (2–7~March~2005), AO~IO~RAN, Kaliningrad, 2003–2005. Sea and Coast: The National Atlas of Sweden, Swedich Meteorological and Hydrological Institute, edited by: Sjöberg., B., SNA Publishing, Stockholm, ISBN 91-87760-16-9 128~p., 1992. Shimaraev, M. N. and Granin, N. G.: Temperature stratification and the mechanism of convection in Lake Baikal, Dokl. Akad. Nauk, 321, 381–385, 1991. State and Evolution of the Baltic Sea: 1952–2005, A Detailed 50-Year Survey of Meteorology and Climate, Physics, Chemistry, Biology, and Marine Environment, edited by: Feistel, R., Naush, G., and Wastmund, N., J. Wiley & Sons, 2008. Sturman, J. J., Oldham, C. E., and Ivey, G. N.: Steady convective exchange flow down slopes, Aquat. Sci., 61, 260–278, 1999. Thomsen, C., Blaume, F., Fohrmann, H., Peeken, I., Zeller, U.: Particle transport processes at slope environments – event driven flux across the Barents Sea continental margin, Mar. Geol., 175, 237–250, 2001. Thorpe, S. A. and White, M.: A deep intermediate nepheloid layer, Deep Sea Res., 35, 1665–1671, 1988. Wüest, A., Ravens, T. M., and Granin, N. G.: Cold intrusions in Lake Baikal: Direct observational evidence for deep-water renewal, Limnol. Oceanogr., 50(1), 184–196, 2005. Zhmur, V. V. and Yakubenko, M. V.: Dynamics of density currents on incline, Physics of Atmosphere and the Ocean, Proc. Rus. Acad. Sci., 37(4), 1–10, 2001.