Blocking and frontogenesis by two-dimensional terrain in baroclinic flow. Part II: Analysis of flow stagnation mechanisms

Abstract

Numerical solutions presented in a companion paper show that two-dimensional mesoscale terrain becomes a much stronger barrier to a continuously stratified flow when the flow contains warm advection. Here it is shown that this baroclinic enhancement is a strictly nonlinear phenomenon. The linear analysis indicates a weakening of the upstream response in warm advection. However, a weakly nonlinear analysis shows that baroclinicity facilitates blocking in warm advection by strengthening the nonlinearity in the cross-mountain momentum equation in such a way as to amplify the vertical shear on the windward flank of the ridge. This is enough to send the flow past the blocking threshold even when conditions over the mountain are too linear to produce wave breaking. A more intuitive mechanism whereby the upstream static stability is increased by the nonlinearity in the temperature equation is found to be much less important.