Mean age of air and transport in a CTM: Comparison of different ECMWF analyses

Abstract

A comparison of the stratospheric circulation achieved by various data assimilation winds has been performed using multiannual simulations of the TOMCAT/SLIMCAT off-line 3-D chemical transport model (CTM). Data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the U.K. Met Office (UKMO) have been used to drive the CTM. We find that important improvements have been made in the ECMWF stratospheric winds during recent years. Therefore, a more realistic Brewer-Dobson (B-D) circulation, and subtropical mixing are achieved when ECMWF operational or new interim reanalyses are used instead of ERA-40 analyses. Age-of-air and trajectory calculations show that more realistic vertical and horizontal transport is achieved with the new ECMWF assimilated winds. The modelled tape recorder signal further shows the improvement in the tropical transport with the new winds. Overall, these results show that with the recent ECMWF reanalyses, off-line CTMs can produce stratospheric tracer transport over multiannual timescales more realistically than with other previous (re)analyses. Reasons for the improvements in the new reanalysis are discussed. Copyright 2007 by the American Geophysical Union.