Role of Rossby wave breaking in the variability of large-scale atmospheric transport and mixing

Abstract

We demonstrate that Rossby wave breaking (RWB) plays an important role in both horizontal and vertical large-scale transport/mixing in both observations and idealized general circulation models. In the horizontal direction, RWB contributes to a substantial fraction of transient moisture flux into the Arctic. In the vertical direction, RWB modifies thermal stratification near the tropopause which leads to enhanced mass exchange across the tropopause. In understanding the variability of RWB related transport and mixing, we show that it is essential to separate the two types of RWB – anticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB) – for two fundamental differences between them. The first difference is the opposite relationship between jet positions and their frequencies of occurrence. For both horizontal transport of moisture into the Arctic and vertical mixing of ozone across the tropopause, the robust relationship between jet position and AWB/CWB frequency is of first order importance in explaining the large-scale transport/mixing anomaly patterns influenced by climate variabilities involving jet shifting, such as the El-Nino Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). The second robust difference is the mixing strength exhibited by individual AWB and CWB events. In idealized lifecycle and climate simulations, as well as reanalysis data, CWB consistently exhibits stronger mixing strength than AWB. Combined with the robust relationship between jet variability and AWB/CWB frequency, such a difference is demonstrated to translate into a decrease in total upper troposphere diffusivity as the jet shifts poleward in an idealized climate simulation.