Impact of the Asian monsoon anticyclone on the variability of mid-to-upper tropospheric methane above the Mediterranean Basin

摘要

The space and time variabilities of methane (CH₄) total column and uppertropospheric mixing ratios are analysed above the Mediterranean Basin (MB) aspart of the Chemical and Aerosol Mediterranean Experiment (ChArMEx)programme. Since the analysis of the mid-to-upper tropospheric CH₄distribution from spaceborne sensors and model outputs is challenging, wehave adopted a climatological approach and have used a wide variety ofdata sets. We have combined spaceborne measurements from the Thermal And Nearinfrared Sensor for carbon Observations – Fourier Transform Spectrometer(TANSO-FTS) instrument on the Greenhouse gases Observing SATellite (GOSAT)satellite, the Atmospheric InfraRed Spectrometer (AIRS) on the AURA platformand the Infrared Atmospheric Sounder Interferometer (IASI) instrument aboardthe MetOp-A platform with model results from the Chemical Transport Model(CTM) MOCAGE, and the Chemical Climate Models (CCMs) CNRM-AOCCM andLMDz-OR-INCA (according to different emission scenarios). In order tominimize systematic errors in the spaceborne measurements, we have onlyconsidered maritime pixels over the MB. The period of interest spans from2008 to 2011 considering satellite and MOCAGE data and, regarding the CCMs,from 2001 to 2010. Although CH₄ is a long-lived tracer with lifetime of~12 years and is supposed to be well mixed in the troposphere, aneast–west gradient in CH₄ is observed and modelled in the mid-to-uppertroposphere with a maximum in the Western MB in all seasons except in summerwhen CH₄ accumulates above the Eastern MB. The peak-to-peak amplitude ofthe east–west seasonal variation in CH₄ above the MB in the uppertroposphere (300 hPa) is weak but almost twice as great in the satellitemeasurements (~25 ppbv) as in the model data (~15 ppbv).The maximum of CH₄ in summer above the eastern MB can be explained by aseries of dynamical processes only occurring in summer. The Asian monsoontraps and uplifts high amounts of CH₄ to the upper troposphere wherethey build up. The Asian Monsoon Anticyclone redistributes these elevatedCH₄ amounts towards North Africa and the Middle East to finally reach anddescend in the eastern MB. In the lower troposphere, the CH₄ variabilityis mainly driven by the local sources of emission in the vicinity of the MB.