O3 variability/trends in the troposphere from IASI observations in 2008-2017
Résumé
In this study, we describe the recent changes in the tropospheric ozone (O3) columns (TOCs) measured by the Infrared Atmospheric Sounding Interferometer (IASI) onboard the Metop satellites during the first ten years of the IASI operation (2008-2017). The instrument provides a unique dataset of vertically-resolved O3 profiles with a twice daily global coverage and a fairly good vertical resolution allowing us to monitor the year-to-year variability in the troposphere. The retrievals are performed using the FORLI software, a fast radiative transfer model based on the optimal estimation method, set up for near real time and large scale processing of IASI data.
We differentiate trend characteristics from the seasonal and non-seasonal O3 variations captured by IASI in the troposphere by applying appropriate annual and seasonal multivariate regression models, which include important geophysical drivers of O3 variation (e.g. quasi biennial oscillations - QBO, El Niño/Southern Oscillation - ENSO, North Atlantic Oscillation-NAO) and a linear trend term, on time series of spatially gridded averaged O3. The performances of the regression models (annual vs seasonal) are first investigated. Given the large contribution of the interannual variability, we will then describe the effects of the main contributing O3 proxies (e.g. positive - or negatives - ENSO indexes measured during moderate to intense El Niño - or La Niña - episodes in the tropics) in addition to the adjusted O3 trend patterns. A special focus will be given over the Northern Hemisphere which is characterized by decreasing O3 precursor emissions (mainly over Europe and the US). FORLI O3-CO correlations patterns will also be discussed to evaluate the continental influence on the tropospheric O3 trends.