A spectral perspective of the clear-sky OLR variability driven by ENSO

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 satellite-based observations, such as those of the Clouds and Earth Radiant Energy System, finding that the peak of the OLR response lags the peak of ENSO activity. However, such observations cannot directly 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 and the Atmospheric Infrared Sounder instruments – to expand the observational analysis of ENSO's radiative response, showing that its intensity and lag vary along the spectral dimension. The spectral fingerprint of water vapor, surface and air temperature, and ozone feedback is then calculated using a set of spectral kernels to evaluate the role of individual processes in 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 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.