Global climatology of abundance and solar absorption of oxygen collision complexes
Abstract
To improve our understanding of the absorption of solar radiation in the atmosphere we have characterized the spectral absorption, the spatial and temporal abundance, and the radiative forcing of the oxygen collision pairs O2·O2 and O2·N2 (O2·X ≡ O2·O2 + O2·N2). The regional, vertical, seasonal, and annual patterns of O2·X abundance and radiative forcing are obtained from a general circulation model. We estimate the mean absorption by O2·X, heretofore neglected in large scale atmospheric models, is 0.75–1.2 W m−2, or 1–2% of total atmospheric solar absorption. O2·X absorption reduces surface insolation by 0.48–0.78 W m−2 and increases the net radiative flux at the tropopause by 0.32–0.52 W m−2. These ranges bracket the uncertainties due to spectral absorption cross sections, O2·N2 efficiency, O2·X abundance, and cloud distribution. Globally averaged, O2·X enhances absorption equally in clear and in cloudy skies. We create a global climatology of well-mixed collision complex abundances by scaling O2·X abundance to other complexes such as N2·N2 and O2·Ar. Collision complex abundance depends quadratically on the concentrations of the constituents. This dependence causes a 20% increase in O2·X abundance in the Arctic relative to the Tropics for the same sea level pressure. The variations in zonal mean O2·X abundance due to surface elevation, the annual mean meridional temperature gradient, and to seasonal temperature variations are 40, 15, and 10%, respectively. O2·X heating obeys the weakly absorbing, linear limit so it peaks at the surface in clear skies, but clouds shift this peak up by 200–300 mb on seasonal timescales. Surface albedo and clouds strongly modulate the solar forcing efficiency of O2·X (forcing per unit abundance) by altering the mean photon path length. These factors produce annual mean forcing maxima in the subtropics over bright deserts and regions of marine stratus. Inclusion of O2·X in models is likely to reduce cold biases in the summertime polar atmosphere, where O2·X contributes 2–4% of total solar heating.