Experimental Simulation of the ‘Retreat’ of the Seasonal Thermocline by Surface Heating

Abstract

Towards the end of the winter-cooling period, the seasonal thermocline in bodies of water such as the upper ocean, lakes, fjords and estuaries has, in general, been eroded by the combined action of convection and mechanical mixing to such an extent that the upper mixed layer is rather deep. But with the start of the spring heating period, turbulence in the lower regions of the layer is suppressed by the stabilizing heat flux at the surface and successively shallower mixed layers are often observed to form. The process culminates in a comparatively shallow mixed layer, which as time evolves is then heated and further eroded by the wind and other mixing processes later in the spring and summer. During the winter, the seasonal thermocline advances even more rapidly under the combined action of convection due to surface-cooling and mechanical mixing. In ice-covered bodies such as arctic fjords, salt-extrusion due to ice-growth also contributes to convective erosion. The process of deepening of the thermocline has been studied extensively in recent years (for example, see Kraus, 19 77) but the `retreat' at the beginning of the spring season has not received the attention it deserves since the important contribution of Kraus and Turner (1967). The `retreat' is germane to the problem of the determination of the mixed layer depth and surface temperatures that result later during summer heating and winter cooling because this process establishes the initial conditions for the depth at the start of the erosion cycle.