Impact of land convection on troposphere-stratosphere exchange in the tropics
1Laboratoire d'Aérologie, CNRS, Université de Toulouse III, Toulouse, France
2Centre National de Recherche Météorologique, Météo-France, Toulouse, France
3NASA Jet Propulsion Laboratory, Pasadena, California, USA
4CNRS, Service d'Aéronomie, Verrières-le-Buisson, France
Abstract. The mechanism of troposphere-stratosphere exchange in the tropics was investigated from space-borne observations of the horizontal distributions of nitrous oxide (N2O), methane (CH4) and carbon monoxide (CO) at 17 km in March-April-May by the ODIN/Sub-Millimeter Radiometer (SMR), the Upper Atmosphere Research Satellite (UARS)/Halogen Occultation Experiment (HALOE) and the TERRA/Measurements Of Pollution In The Troposphere (MOPITT) instruments in 2002–2004, completed by recent observations of the AURA/Microwave Limb Sounder (MLS) instrument during the same season in 2005. At the top of the Tropical Tropopause Layer (TTL), all gases show significant longitudinal gradients with maximum amounts primarily over Africa and, depending on the species, secondary more or less pronounced maxima above northern South America and South-East Asia. The Maritime continent in the Western Pacific never appears as a source region for the stratosphere. The large longitudinal gradient at latitudes where the circulation is essentially zonal, and the co-location of the maximum tropospheric trace gases concentrations with the overshooting features reported by the Tropical Rainfall Measuring Mission (TRMM) satellite precipitation radar, strongly supports that rapid uplift over land convective regions is the dominating process of troposphere-stratosphere exchange. Calculations carried out with the MOCAGE-Climat chemical transport model well capture the location of the maximum gas concentration in the TTL but of lesser amplitude. Although there are obvious misrepresentations of some of the sources in the model, i.e. CH4 emissions by evergreen forests, the main reason for discrepancy appears to be the underestimation of the maximum altitude reached by land convective transport in MOCAGE.