The spring transition from horizontal to vertical thermal stratification on a midlatitude continental shelf

Abstract

The spring transition from wintertime horizontal thermal stratification to the summertime vertically stratified state on a wide, sloping midlatitude continental shelf is investigated. The sigma z‐level model (SZM), a new three‐dimensional primitive equation numerical ocean model employing a hybrid sigma coordinate and z‐level coordinate in the vertical, simulates the continental shelf region. A simple analytical model to describe the heating rate of a well‐mixed water column is applied to the problem to illustrate the importance of bottom slope in determining the evolution of the horizontal thermal stratification. The applicability of this simple model to continental shelf dynamics forced by realistic surface fluxes is studied with a “pseudo” two‐dimensional (x‐z) form of the SZM forced with daily heat flux and wind stress records from the West Florida Shelf. A term‐by‐term analysis of the depth‐averaged time‐integrated temperature equation quantifies the contribution of each physical process to the evolution of the horizontal temperature field. The simple model used to describe the heating rate of a homogeneous water column appears as the dominant balance in the integrated temperature equation. Cross‐shelf temperature advection associated with upwelling and downwelling alters the horizontal depth‐integrated temperature over the shelf predicted by the simple model. Two processes describe the spring transition. The erosion of the wintertime horizontal thermal gradient occurs on a timescale of a few months when the water column is being heated and mixed periodically. The second process is the formation of the seasonal thermocline following the last strong mixing event. This occurs on a timescale of less than a week under a stabilizing heat flux and light winds completing the spring transition from horizontal to vertical thermal stratification.