On Stratified Flow over a Ridge Intersecting Coastlines

Abstract

The adjustment between two interconnecting basins of stratified fluid, resulting from the formation of a pool of dense water in one, is investigated with an 18-level numerical general circulation model (CYCM). It is found that basic features of the adjustment, i.e., (i) the generation of low-frequency, coastally trapped Kelvin waves, which carry information about the alongshore pressure gradient, created by the denser water, toward the dividing ridge; (ii) the subsequent generation of barotropic double Kelvin waves by the JEBAR (Joint Effect of Baroclinicity and Relief) effect, which propagate infinitely rapidly along the ridge setting up an anticyclonic gyre; (iii) the generation of further coastally trapped Kelvin waves at the opposite coast due to an alongshore pressure gradient, created by upwelling at the ridge edge associated with the closure of the barotropic gym; (iv) the generation of a coastally confined cyclonic gyre at the opposite ridge ~ by the JEBAR effect and viscous and diffusive processes as the coastally trapped Kelvin waves crow off the ridge, are reproduced by an equivalent linear, f-plane, two-layer model with the topography confined to the lower layer. Features, which arise from the increased number of vertical degrees of freedom in the GCM, are found to include (i) a further sheltering of the upper levels from the presence of the midge due to energy in the coastally trapped Kelvin waves being redistributed between various vertical modes; (ii) a baroclinic adjustment at the ridge face to eradicate any horizontal density gradients there, which takes the form of a Kelvin-type wave trapped against the face not previously described; and (iii) penetration of the circulation below the top of the ridge in the second basin due to vertical viscous and diffusive effects, as well as the dispersion of coastally trapped Kelvin waves of differing vertical modes. First-order estimates of the magnitude of the baroclinic signals propagating on and away from the ridge are found by consideration of continuity of man flux. Assuming that the fields may be decomposed into local vertical normal modes, a simple model with uniform stratification is used to derive analytic estimates of the relative amplitudes of the vertical modes which compare favorably with those calculated from the GCM. Assuming that the energy of the coastally trapped Kelvin wave initially incident on the ridge is chiefly in the first baroclinic mode, then it is found that the higher the ridge, the low the energy scattered into the first-order mode propagating across the ridge (that in the higher-order modes remains small), and the greater the energy scattered into a first-order mode propagating along the face of the ridge and along the opposite coast. Also, a higher ridge results in more energy being scattered into higher-order mode, coastally trapped Kelvin waves propagating in the second basin. The magnitude of the barotropic response is found to be such as to cancel the vertical average of the baroclinic flow over the ridge face associated with the incident coastally trapped Kelvin waves for the anticyclonic gyre, and dependent on viscous and diffusive processes for the cyclonic gyre. As well as ridges of top-hat cross section and differing heights, ridges with a pyramid cross section, i.e., ?sloping? sides and with an island (cf. Iceland) on top, are considered. The implications of features of the adjustment for modeling the Atlantic thermohaline circulation and the transport of passive tracers am discussed, as these highlight aspects of the formulation of topography in Gems, which merit further consideration in the general development of GCMs.