Impact of the Asian monsoon anticyclone on the variability of mid-to-upper tropospheric methane above the Mediterranean Basin Impact of the Asian monsoon anticyclone on the variability of mid-to-upper tropospheric methane above the Mediterranean Basin
Abstract
The space and time variabilities of methane (CH4) total column and upper tropospheric mixing ratios are analysed above the Mediterranean Basin (MB) as part of the Chemical and Aerosol Mediterranean Experiment programme. We have adopted a climatological approach and have used a wide variety of data sets.
We have combined spaceborne measurements from the Thermal And Near infrared Sensor for carbon Observations - Fourier Transform Spectrometer instrument on the Greenhouse gases Observing SATellite, the Atmospheric InfraRed Spectrometer (AIRS) on the AURA platform and the Infrared Atmospheric Sounder Interferometer instrument aboard the MetOp-A platform with model results from the Chemical Transport Mode MOCAGE, and the Chemical Climate Models CNRM-AOCCM and LMDz-ORINCA. In order to minimize systematic errors in the spaceborne measurements, we have only considered maritime pixels over the MB.
The period of interest spans from 2008 to 2011 considering satellite and MOCAGE data and, regarding the CCMs, from 2001 to 2010. Although CH4 is a long-lived tracer, an east-west gradient in CH4 is observed and modelled in the mid-to-upper troposphere with a maximum in the Western MB in all seasons except in summer when CH4 accumulates above the Eastern MB.
The peak-to-peak amplitude of the east-west seasonal variation in CH4 above the MB in the upper troposphere (300 hPa) is weak but almost twice as great in the satellite measurements (similar to 25 ppbv) as in the model data (similar to 15 ppbv). The maximum of CH4 in summer above the eastern MB can be explained by a series of dynamical processes only occurring in summer.
The Asian monsoon traps and uplifts high amounts of CH4 to the upper troposphere where they build up. The Asian Monsoon Anticyclone redistributes these elevated CH4 amounts towards North Africa and the Middle East to finally reach and descend in the eastern MB.