On the role of atmospheric dynamics in forming the climatic response to doubling CO2

In the present paper we analyze the sensitivity to doubling of CO2 of an idealized climate system consisting of an atmospheric GCM with aquaplanet boundary conditions coupled to an oceanic upper mixed layer. There is no seasonal cycle in the solar radiation which is taken to be symmetric about the equator. The system is run until it reaches a quasi-equilibrium climate. To study the sensitivity we apply an external forcing to the system in the form of doubling the concentration of CO2 and integrate the system until it reaches a new equilibrium climate. For the purposes of linear analysis we assume that the atmosphere is always in quasi-equilibrium (typical atmospheric time scales are much shorter than that of the oceanic upper mixed layer). We introduce a linear surface budget response operator which represents a matrix of Jacobian of the surface budget with respect to the surface temperature [1]. A special technique has been developed for calculating this operator. The introduced operator is used for constructing the linear estimate of the surface temperature change as a result of the increase of the concentration of CO2. The temperature response obtained from the full 3D run compares well with the corresponding linear estimate. The shapes of the response and of the least stable mode of the linear surface budget response operator look very similar. The importance of different components of the initial forcing in contributing to the response of the system is studied. In particular, we estimate the role of atmospheric dynamics in interaction between the top of the atmosphere and the surface budgets. Looking at the role of atmospheric moisture transport in forming the forcing at the surface is another subject under investigation. The analysis is done at two different rotation rates, corresponding to 24- and 48 hours length of the day. One very surprising finding of the paper is that the typical polar amplification pattern of the global warming almost disappears at the lower rotation rate. This tells about the dynamical nature of the pattern in the present climate.

Reference

[1] Alexeev, V.A. Sensitivity to CO2 doubling of an atmospheric GCM coupled to an oceanic mixed layer: a linear analysis, accepted to Climate Dynamics.