Global Large-Scale Stratosphere-Troposphere Exchange in Modern Reanalyses

Abstract

Stratosphere-troposphere exchange (STE) has important and signi cant impacts on the chemical and radiative properties of the upper troposphere and lower stratosphere. This study presents a 15-year climatology of global large-scale STE from four modern reanalyses: ERA-Interim, JRA-55, MERRA-2, and MERRA-1. STE is separated into four categories for analysis to identify the signi cance of known transport mechanisms: 1) vertical stratosphere-to-troposphere transport (STT), 2) vertical troposphere-to-stratosphere transport (TST), 3) lateral STT (that occurring between the tropics and the extratropics and across the tropopause "break"), and 4) lateral TST.

In addition, this study employs a method to identify STE as that crossing the lapse-rate tropopause (LRT), while most previous studies have used a potential vorticity (PV) isosurface as the troposphere-stratosphere boundary. PV-based and LRT-based STE climatologies are compared using the same reanalysis output (ERA-Interim). The comparison reveals quantitative and qualitative differences, particularly in the geographic representation of TST in the polar regions.

Based upon spatiotemporal integrations among the reanalysis models, we find STE to be STT-dominant in ERA-Interim and JRA-55 and TST-dominant in the MERRA reanalyses. Time series analysis over the 15-year period show long-term changes in STT and TST, which are argued to correspond with changes in the Brewer-Dobson circulation. Ultimately, differences occur as a result of physical and dynamical differences between the four reanalysis models.