How permeable is the tropopause in the vicinity of the subtropical jet ?: Seasonal variability derived from model simulations (CLaMS) and observations (Invited Speaker)

The composition of air in the vicinity of the extratropical tropopause, in particular within the lowermost stratosphere, is strongly influenced by the two-way isentropic transport across the subtropical jet (STJ). Although evidence of mixing in the vicinity of the STJ is often observed, quantifying this effect has been a significant challenge. By using a new version of the Chemical Lagrangian Model of the Stratosphere (CLaMS), we present some new results describing transport across the tropopause in mid and high latitudes.

In this new version, the stratospheric model has been extended to the Earth surface. Above the tropopause, the isentropic and cross-isentropic advection in CLaMS is driven by meteorological analysis winds and heating/cooling rates derived from a radiation calculation. Below the tropopause the model smoothly transforms from the isentropic to hybrid-pressure coordinate and, in this way, takes into account the effect of large-scale convective transport as implemented in the vertical wind of the meteorological analysis. As in previous CLaMS simulations, the irreversible transport, i.e.~mixing, is controlled by the local horizontal strain and vertical shear rates.

The multi-annual (2001-2006), global model simulations of CH4, CO2, CO and (passive) ozone are used to quantify mixing in the vicinity of the subtropical jet (STJ). Comparison with available in situ data in this region of the atmosphere (part of the Geophysica campaigns and SPURT data) are used for model validation. The observed mixing lines in the ozone/tracer correlations indicate irreversible transport processes and allow to check the validity of the mixing algorithm used in CLaMS. Asymmetric signatures of mixing on the cyclonic and anticyclonic side of the STJ could be identified with relatively long-lived tropospheric remnants in the lowermost stratosphere. We quantify the seasonal dependence of the mixing layer at the extratropical tropopause that roughly follows the seasonal cycle of the mean tropopause with strongest flux of the stratospheric air into the troposphere occurring in late spring and summer across a weak STJ.