Simple Model Representation of Atmosphere-Ocean GCMs and Estimation of the Time Scale of C02-Induced Climate Change

Abstract

A simple atmosphere-Ocean model is developed to represent the 20-year 1 × C0₂ and 2 × C0₂ simulations obtained with a coupled atmosphere-ocean general circulation model for the purpose of obtaining a new estimate of the characteristic response time of the climate system that accounts for oceanic upwelling. The simple atmosphere-generalized ocean model consists of a zonally averaged energy balance climate model and a zonally averaged multilayer ocean model. The high latitudes of both hemispheres are combined into a single polar region, and the low and middle latitudes into a single nonpolar region. The atmosphere is treated as a single layer and the ocean as an arbitrary number of layers. The simple model includes the meridional transport of heat between the nonpolar and polar regions for both the atmosphere and each ocean layer. The ocean model includes the vertical advective heat transfer by the vertical velocity, the latter of which is prescribed and can vary with depth in both the polar and nonpolar regions. The unknown parameters of the simple model are the meridional heat fluxes between the nonpolar and polar regions, the coefficients of heat transfer within the ocean, the heat transfer coefficient between the ocean and atmosphere, an additional ocean-atmosphere heat transfer parameter, and the climate sensitivity parameter. The parameters of the simple model are determined from the 1 × C0₂ and 2 × CO₂ simulations of the coupled atmosphere-ocean general circulation model. The simple atmosphere-ocean model is then used to project the response of the coupled atmosphere-ocean GCM from year 20 to year 100, and the resulting 2 × C0₂−1 × C0₂ differences are normalized by the estimated equilibrium temperature changes. From these projections it is estimated that the characteristic response time is between 40 and 60 years, in close agreement with the estimates of Schlesinger et al.