A global climatology of stratosphere-troposphere exchange using the ERA-interim dataset from 1979 to 2011

Abstract

Abstract. In this study we use the ERA-Interim reanalysis dataset from the European Centre for Medium-Range Weather Forecasts (ECMWF) and a refined version of a previously developed Lagrangian methodology to compile a global 33 year climatology of stratosphere-troposphere exchange (STE) from 1979 to 2011. Fluxes of mass and ozone are calculated across the tropopause, pressure surfaces in the troposphere, and the top of the planetary boundary layer (PBL). This climatology provides a state-of-the-art quantification of the geographical distribution of STE and the preferred transport pathways, and insight into the temporal evolution of STE during the last 33 yr. We confirm the distinct zonal and seasonal asymmetry found in previous studies using comparable methods. The subset of "deep STE", where stratospheric air reaches the PBL within 4 days or vice versa, shows especially strong geographical and seasonal variations. The global hotspots for deep STE are found along the west coast of North America and over the Tibetan Plateau, especially in boreal winter and spring. An analysis of the time series reveals significant positive trends of the net downward mass flux and of deep STE in both directions. The downward ozone flux across the tropopause is dominated by the seasonal cycle of ozone concentrations at the tropopause and peaks in summer, when the mass flux is nearly at its minimum. For the subset of deep STE events, the situation is reversed and the downward ozone flux into the PBL is dominated by the mass flux and peaks in early spring. Thus surface ozone concentration along the west coast of North America and around the Tibetan Plateau are likely to be influenced by deep stratospheric intrusions. Quantitatively, all our results depend on the minimum residence time τ used to filter out transient STE trajectories. This dependence is shown to be a~power law with exponents ranging between −0.44 and −0.87 for mass and ozone fluxes in both directions.