Design and Critical Appraisal of an Accelerated Integration Procedure for Atmospheric GCM/Mixed-Layer Ocean Models

Abstract

An accelerated integration procedure (AIP) is developed for the OSU atmospheric GCM/mixed-layer ocean model. In this AIP the depth of the mixed-layer ocean is reduced by an acceleration factor f(a) = 12 from 60 m to 5 m and the length of a solar cycle is correspondingly reduced to eliminate the increase in the amplitude of the annual cycle of oceanic temperature which would otherwise occur. Furthermore, the ground bulk heat capacity, ground water field capacity and heat of fusion for sea ice and for snow on sea ice are reduced by f(a) to accelerate the equilibration of the ground temperature, soil water and sea ice, respectively. The AIP was used for 1 X CO2 and 2 X CO2 simulations with the OSU AGCM/mixed-layer ocean model. The AIP attained the equilibrium climates in these simulations with the computer-time equivalent of about 2.5 unaccelerated solar cycles, but after the switch from the AIP to the normal unaccelerated integration procedure (NIP), the temperatures increased to new equilibrium values. Although additional computer time was required to achieve these new equilibria, the overall 1 X CO2 and 2 X CO2 simulations with the AIP/NIP required respectively only 55% and 28% of the computer time which would have been required with the NIP alone. Thus the AIP was successful in saving a significant amount of computer time. The success of the AIP notwithstanding, an analysis was undertaken to determine the cause of the change in the equilibrium climate following the AIP/NIP switch. Diagnosis of the 1 X CO2 simulation by the OSU AGCM/mixed-layer ocean model and tests with a latitudinally dependent energy balance model show that it is the increase in the amplitude of the annual cycle of atmospheric temperature from the AIP to the NIP which, acting through the ice-albedo/temperature feedback mechanism, causes the change in the equilibrium climate following the AIP/NIP switch. It is therefore concluded that while the AIP can save a significant amount of computer time in achieving equilibrium with an AGCM/mixed-layer ocean model, caution in its use is warranted.