A Spectral Perspective of ENSO Driven OLR Variability

Abstract. The study of short-term unforced variability of the Earth radiative budget can provide much information for the understanding of the long-term effect of external radiative forcing, related to the present climate change. In this regard, inter-annual variability of the Outgoing Longwave Radiation (OLR) is strongly shaped by El-Niño Southern Oscillation (ENSO). So far, the relationship between the OLR and ENSO has been investigated using broadband satellites-based observations, such as those of the Clouds and Earth Radiant Energy System (CERES), finding that the peak of the OLR response lags the peak of ENSO activity. However, such analysis cannot inform on the individual processes that drive the radiative response to ENSO. Here, we exploit the spectrally-resolved clear-sky OLR fluxes – measured by the Infrared Atmospheric Sounding Interferometer (IASI) and the Atmospheric Infrared Sounder (AIRS) instruments – to expand the observational analysis of ENSO's radiative response. The observed signal is then decomposed using a spectral kernel analysis into water vapor, surface and air temperature, and ozone feedback, to evaluate the role of individual processes building the overall response. Results show a strong contribution coming from the ozone absorption band, along with a contribution of opposite sign coming from the the core of the carbon dioxide band, which is mainly affected by stratospheric temperature. This analysis confirms the important role of the spectral dimension to study climate processes. In this regard, it sets the basis for a spectral diagnostic to evaluate how ENSO driven variability is reproduced by climate models.