Intrusion of recent air in midlatitude stratosphere revealed by in situ tracer measurements and trajectory calculations

In the midlatitude stratosphere the mean circulation is downward leading to relatively old air. Mixing between this old air and more recent air from the troposphere or the tropical stratosphere can nevertheless occur. In particular, mixing between the lower part of the tropical stratospheric reservoir and the midlatitude stratosphere was reported in the so‐called “tropically controlled transition region” which extends from the tropics to midlatitudes between the isentropic surfaces 380 K and 450 K (15–18 km altitude). O3/CO and HCl/O3 relations derived from the measurements of the in situ tunable lasers (TDLAS) balloon‐borne instrument Spectroscopie Infrarouge par Absorption de Lasers Embarqués (SPIRALE), and 3D air parcels trajectories are used to analyze the characteristics of this region at the time and location of SPIRALE. These measurements took place above Aire sur l'Adour (43.6°N, 0°E) in October 2002. Two‐month 3D backward trajectories ending around each individual measurement recorded with a time step of 1 s are computed on the basis of European Centre for Medium‐Range Weather Forecasts (ECMWF) wind fields. The region is found to extend from 380 K to 465 K, and mixing is found to take place 1 to 2 months before the SPIRALE measurements. The region is composed of two layers with different characteristics. In the 380–405 K layer the recent air is uplifted from below the 380 K isentropic surface, and the percentage of recent air is high (minimum estimate 32%). In the 405–465 K layer the origin of recent air is the tropical stratosphere, and the percentage of the recent air is lower (minimum estimate 12%).