THE ROLE OF MOMENTUM FLUXES IN SHAPING THE LIFE-CYCLE OF A BAROCLINICWAVE

The wide disparities in barcolinic wave development between spherical and Cartesian geometry are investigated with the purpose of assessing the role of the eddy momentum fluxes. Differences are already signific ant at the linear stage, as momentum fluxes are predominantly poleward in spherical geometry and predominantly equatorward in Cartesian geom etry. More important, the low-level flux convergence is displaced pole ward on the sphere and equatorward on the plane. On the sphere, these circumstances lead to rapid poleward movement of the low-level zonal-m ean jet. The anticyclonic horizontal shear region expands as the jet f eeds back on the momentum flux. The wave breaks anticyclonically and q uickly zonalizes. In the Cartesian life cycle, the equatorward displac ement of the flux convergence is counteracted by the mean meridional c irculation and there is consequently a weaker feedback with the horizo ntal shear. The wave breaks, in this case cyclonically, but then takes much longer to zonalize. On the sphere, the angular velocity gradient in uniform westerly or easterly Bow adds a separate mechanism for con verting eddy kinetic energy to zonal mean, further hastening the zonal ization process. It is possible to change the sign of the eddy momentu m flux and the sense of the breaking in either geometry by slightly ch anging the basic flow. For example, cyclonic roll-up on the sphere can be obtained by adding weak cyclonic barotropic shear, as highlighted in a recently published study. Similiary, the addition of anticyclonic barotropic shear in a Cartesian simulation leads to anticyclonic wave breaking. An easterly jet on the sphere allows cyclonic breaking, but the wave still zonalizes rapidly, as in the case of a westerly jet. T he persistence of the nonlinear eddies in these diverse experiments is not well correlated with the minimum value of the refractive index fo r Rossby waves, as suggested in the referenced study. It is proposed t hat the longevity of residual vortices after wave breaking is determin ed not by the sign of the vorticity or the breadth of the waveguide, b ut by the sign of the momentum flux and the geometry of the model.