Abstract. The behavior of the Extra-tropical Transition Layer (ExTL) in the lowermost stratosphere is investigated using a Chemistry Transport Model (CTM) and analyses derived from assimilation of MLS (Microwave Limb Sounder) O3 and MOPITT (Measurements Of Pollution In The Troposphere) CO data. We use O3-CO correlations to quantify the effect of the assimilation on the height and depth of the ExTL. We firstly focus on a Stratosphere-Troposphere Exchange (STE) case study which occurred on 15 August 2007 over the British Isles (50° N, 10° W). We also extend the study at the global scale for the month of August 2007. For the STE case study, MOPITT CO analyses have the capability to sharpen the ExTL distribution whereas MLS O3 analyses provide a tropospheric expansion of the ExTL distribution with its maximum close to the thermal tropopause. When MLS O3 and MOPITT CO analyses are used together, the ExTL shows more realistic results and matches the thermal tropopause. At global scale, MOPITT CO analyses still show a sharper chemical transition between stratosphere and troposphere than the free model run. MLS O3 analyses move the ExTL toward the troposphere and broaden it. When MLS O3 analyses and MOPITT CO analyses are used together the ExTL matches the thermal tropopause poleward of 50°. This study shows that data assimilation can help overcome the shortcomings associated with a relatively coarse model resolution. The ExTL spread is larger in the Northern Hemisphere than the Southern Hemisphere suggesting that mixing processes are more active in the UTLS in the Northern Hemisphere than in the Southern Hemisphere. This work opens perspectives for studying the seasonal variations of the ExTL at extra-tropical latitudes.
Diagnosing the transition layer in the extra-tropical lowermost stratosphere using MLS O<sub>3</sub> and MOPITT CO analyses