Quantifying Isentropic Mixing in a Modified Lagrangian Coordinate:

Isentropic mixing is an important process for the distribution of chemical constituents in the mid-to-high latitudes. A new framework is introduced to quantify both symmetric and asymmetric isentropic mixing in a modified Lagrangian coordinate. The framework is applied to quantify the mixing associated with the two types of Rossby wave breaking (i.e. cyclonic and anticyclonic) in two sets of idealized numerical simulations – eddy lifecycle simulations and a long climate simulation. In both sets of simulations, cyclonic wave breaking (CWB) exhibits either comparable or stronger mixing than anticyclonic wave breaking (AWB). Since CWB occurs less frequently while AWB occurs more frequently when the jet shifts poleward, this difference in mixing strength between the two types of Rossby wave breaking translates into a change in total isentropic mixing when the jet shifts. Specifically, it is found in the climate simulation that as the jet shifts poleward, symmetric mixing is weakened in the underworld and both symmetric and asymmetric mixing across the tropopause is weakened in the mid-to-high latitudes. The dynamical relationship above is relevant not only for internal climate variability such as the El-Niño Southern Oscillation (ENSO) and the annular modes, but also, for future climate change that may drive changes in the jet position. 8