Simulations of the effects of interannual and decadal variability on the clear-sky outgoing long-wave radiation spectrum

Using atmospheric profiles derived from the Hadley Centre atmosphere climate model version 3 (HadAM3) as input to a radiative transfer code, the sensitivity of the resolved spectrum of clear-sky outgoing long-wave radiation to both interannual and longer-term atmospheric variability has been analysed. A comparison of the simulated spectra with available observations from two satellite-based instruments indicates a reasonable match, although consistent differences are present. These may be explained by a combination of uncertainties in the atmospheric state. and in the relative calibration of the two instruments. Focusing on the simulations: if HadAM3 is forced by the observed sea surface temperature (SST) record alone, and long-term alterations in the well-mixed greenhouse gases are imposed in the radiance simulations, the changes seen within the major absorption bands are robust. Under a second scenario, where the effects of solar variability, volcanic aerosol, ozone changes and increases in the well-mixed greenhouse gases are also included in the forcing of HadAM3, the long-term profile changes tend to show an enhanced upper-tropospheric warming and low/mid-stratospheric cooling, with increased near-surface humidities compared to the SST-only case. However, the tropospheric response of the system, manifested in the spectral change pattern over the atmospheric window and water vapour bands, falls within the range of year-to-year variability.