Latitudinal differences of changes in Northern Hemisphere clear-sky outgoing longwave radiation over the past 4 decades (1979-2021)

Abstract Earth’s energy budget is modulated by climate forcings and feedbacks. Outgoing longwave radiation (OLR) is a critical component of the Earth’s energy budget, and the balance between absorbed solar radiation and OLR determines the net gain of energy of the entire climate system. Radiation computations using 43 years of ERA5 reanalysis profiles show an increase in clear-sky OLR in the polar regions and a decrease in the tropics. To understand this contrast, we analyze zonal- and annual-mean clear-sky OLR changes in the Northern Hemisphere from 1979 to 2021. We identify a transition in temporal clear-sky OLR changes from low to high latitudes, with a “crossover“ point where the sign changes around 30ºN. We improve upon previous studies by employing infrared spectrally-detailed line-by-line radiative transfer calculations, to decompose zonal-mean trends caused by forcing and feedback mechanisms. Our diagnostic investigation shows that the increase in water vapor exerts a dominant negative contribution to the OLR temporal change in the tropics due to a larger OLR sensitivity to water vapor increase there, while the increase in surface temperature (Ts) dominates the positive temporal change in the Arctic, both due to Arctic amplification and a larger OLR sensitivity to Ts increase. Greater GHGs’ forcings and smaller OLR sensitivity to Ts change in the tropics, as well as an imperfect cancellation between atmospheric temperature and water vapor contribution, delineates the crossover point at around 30ºN.