Understanding the Super Greenhouse Effect through OLR Variability during ENSO events

At times in the deep tropics there is a remarkable anti-correlation between the clear sky outgoing longwave radiation (OLR) and the sea surface temperature (SST), an anomaly, considering the rest of the Earth displays a positive correlation between these two quantities. This phenomenon, the “Super Greenhouse Effect”, and its causes are not clearly understood, and there is a degree of ambiguity from earlier studies of the phenomenon. We investigate the causes and quantify the relative roles of factors important for generating the super greenhouse effect using ERA Reanalysis climatological data as input for a radiative transfer code, and CERES clear sky OLR data. We focus on the Central and Eastern tropical Pacific since it exhibits this phenomenon during El Niño and La Niña events during the 2000-2016 period. We show that enhanced moistening of the middle troposphere results in the largest contribution to OLR relative to other layers and is a dominant factor in producing the super greenhouse effect. Increases in water vapor, by increasing the temperature profile of the column while holding the relative humidity fixed, does not produce the super greenhouse effect and therefore large shifts in relative humidity are required to produce the phenomenon. In addition to the dominance of the middle troposphere, the lower troposphere via absorption in the window region plays a non-negligible role in determining the magnitude of the super greenhouse effect.