On zonal asymmetry and climate sensitivity

Abstract

The role of zonal asymmetry in climate sensitivity is studied with an annual energy-balance climate model of the Northern Hemisphere. Energy balances are formulated for oceanic and continental regions separately, and coupled through zonal energy transports. Dependent variables are Θ and T, representing sea-level temperature in the oceanic and continental part, respectively. In this model zonal asymmetry is defined as Θ - T. It is forced by differences between the oceanic and continental part in (i) the nature of the radiation budget, and (ii) the capacity of transporting energy polewards. The implications of the latter are first illustrated with a simple box model, which can be treated analytically.

The major conclusion of this study is that the sensitivity of the model climate to insolation variations is hardly affected by zonal asymmetry. This result does not depend on the particular set of transport constants used. Temperature drops caused by a 1% decrease in solar constant are in the 1.5 to 2°C range. Experiments are discussed that reveal how the model climate responds to changes in transport constants. It is found that in the 60–70°N latitude belt the present zonal asymmetry (≃4°C) is about 1/4 of the value it would have in the absence of zonal energy transport. For the hemispheric mean asymmetry this ratio is about 1/6. Those results indicate that changes in the transport capacities of the ocean-atmosphere system may be of considerable importance with regard to climate sensitivity.