Dynamic partition on the isentropic stratosphere-troposphere exchange (STE)

The stratosphere-troposphere exchange (STE) flux provides as an essential bridge communicating the chemically distinct stratosphere and troposphere. With a projected accelerating Brewer-Dobson circulation (BDC), such a communication is expected to increase in the future. Besides the BDC, multiple processes are suggested to influence the STE, including the baroclinic wave growth over mid-latitudes peaking in the wintertime, and the summertime monsoon system over (sub)tropics. However, their relative contributions to the STE are less understood.

This present study quantifies the STE of air mass across the potential vorticity (PV) tropopause along individual isentrope. This PV-based Lagrangian approach derives comparable STE flux from the Eulerian eddy flux approach. Moreover, the diagnostics not only distinguishes the meridional location of STE, but also enables a dynamic partition that attributes the STE flux to variabilities in isentropic eddy mixing and diabatic PV source separately. The specified dynamics (SD) version of the Whole Atmosphere Community Climate Model (WACCM) and the Canadian Middle Atmosphere Model (CMAM) are used for the estimate of isentropic STE flux. It is found that, the STE flux associated with the isentropic eddy mixing, primarily from stratosphere to troposphere, offsets largely with the troposphere-to-stratosphere STE flux associated with the diabatic PV source. The former slightly overwhelms over the latter, resulting in the net STE flux with a similar spatiotemporal pattern as the flux component associated with the isentropic mixing but with a much smaller magnitude. The relative contributions from both wintertime baroclinic waves and summertime monsoon to the STE flux are discussed, and their connections with the wave breaking-induced eddy mixing are elucidated.