Environ Fluid Mech (2012) 12:161–183 DOI 10.1007/s10652-011-9223-2 ORIGINAL ARTICLE On the fine structure of the thermal bar front Natalia Demchenko · Irina Chubarenko · GertJan van Heijst Received: 13 May 2011 / Accepted: 24 September 2011 / Published online: 13 October 2011 © Springer Science+Business Media B.V. 2011 Abstract The thermal bar—a hydrodynamic phenomenon, arising in natural basins due to successive changes of the water temperature across the temperature of maximum density (T m , which is close to 4 ◦ C)—has been studied in laboratory experiments and by numerical simulations. The experiments were performed in a rectangular tank with an inclined bottom, filled with water with initial temperature T 0 < T m and then heated at the surface. During the heating a basin-wide circulation develops, consisting of down-slope cascades in regions where T < T m , a subsurface off-shore jet in the region where T > T m , and a compensating flow at intermediate depths towards the shallow part of the tank, supplying both off-shore flows with waters from deeper regions. Analysis of the water temperature and density fields as well as the currents has revealed that the location of the convergence zone of the surface current (when formed) does not coincide with that of the Tm-isotherm. The thermal bar front is typically understood as a convergence zone near the 4 ◦ C-isotherm, formed due to the effect of cabbeling. Our experiments demonstrate, however, that the front is associated with the leading edge of the subsurface current. The increasing distance between the 4 ◦ C- isotherm and the subsurface jet has been recorded in the laboratory experiments. Numerical simulation results corroborate the laboratory experiments. A scaling analysis predicts the speed of propagation of this frontal zone to be U ∼[g × ρ/ρ ×H] 1/2 , where H is the depth (increasing with time) of the upper thermo-active layer, ρ 0 a reference density, and ρ is the characteristic horizontal density difference across the front. A combined analysis of laboratory, field and numerical data has corroborated this law. N. Demchenko (B ) · I. Chubarenko P.P.Shirshov Institute of Oceanology RAS, Atlantic Branch, Prospect Mira, 1, Kaliningrad, Russia 236022 e-mail: ndemchenko@mail.ru G. van Heijst Department of Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands 123