Variability and Trends in Ozone since 2002 from the GOMOS/ENVISAT IPF V6 Reprocessing

Following the Montreal protocol and its amendments, the concentration of Ozone Depleting Gases (ODP) is decreasing slowly in the stratosphere. We expect a progressive recovery of the ozone layer during the next decades. However, due to complex interactions between radiative, dynamical and chemical processes in the atmosphere, the rate at which the recovery will take place is still very uncertain. Furthermore, the ozone layer is perturbed by external forcings like solar variability and volcanic eruptions. Accurate measurements of the 3D distribution of stratospheric ozone are strongly required to understand all causes of the ozone layer variability in order to better predict its future evolution. The global coverage can be only obtained from satellite observations and a validation with ground-based NDACC instruments is needed to assess the quality of the data. The GOMOS instrument on board ENVISAT (launched in 2002 in a helio-synchronous polar orbit) measures the vertical profiles of stratospheric and mesospheric constituents using the stellar occultation technique with a UV-visible spectrometer (250 to 680 nm) for O3, NO2, NO3 and aerosols and two dedicated near IR spectrometers centred at 760 nm for O2 and 940 nm for H2O. The atmospheric transmission is computed by the ratio between a star spectrum measured through the atmosphere and a reference star spectrum measured outside the atmosphere. Vertical profiles of constituents are obtained by a global spectral inversion followed by a vertical inversion using a Tikhonov regularization. GOMOS is by principle an auto-calibrated instrument because it does not depend on radiometric calibration. However, due to the ageing of the CCD detector, creating an increase in the dark charge not perfectly corrected in Version 5 algorithms, a bias was detected in the difference between GOMOS and NDACC ozone LIDARs during recent years (Keckhut et al., ACP, 2010). A full reprocessing of GOMOS data since 2002 using V6 algorithms is in progress. This new version handles correctly the CCD dark charge increase and the bias is expected to disappear. The GOMOS V6 ozone climatology for the period 2002-2011 from 15 to 100 km will be presented. A comparison will be made with NDACC ozone LIDARs for the common altitude range (15-40 km). The ozone evolution during this period and the attribution to various forcings (ODP, Brewer-Dobson circulation, solar cycle, QBO, ENSO, ...) will be discussed