Quantifying Isentropic Transport and Mixing in a Modified Lagrangian

Isentropic transport and mixing are important processes for the distribution of chemical constituents in the mid-to-high latitudes. A new framework is introduced to quantify both isentropic transport and isentropic mixing in a modified Lagrangian coordinate. The framework is applied to quantify the transport and 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 transport and mixing than anticyclonic wave breaking (AWB). Since CWB occurs less frequently when the jet shifts poleward while AWB occurs more frequently, this asymmetry in the transport/mixing associated with the two types of Rossby wave breaking translates into a change in total isentropic transport/mixing when the jet shifts. Specifically, it is found in the climate simulation that as the jet shifts poleward, isentropic eddy mixing (quantified by eddy diffusivity) decreases in the underworld and both eddy mixing and transport across the tropopause decrease 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