HYPERDIFFUSION, MAXIMUM ENTROPY PRODUCTION, AND THE SIMULATED EQUATOR-POLE TEMPERATURE GRADIENT IN AN ATMOSPHERIC GENERAL CIRCULATION MODEL Author:

Hyperdiffusion is used in atmospheric General Circulation Models to account for turbulent dissipation at subgrid scale and its intensity affects the efficiency of poleward heat transport by the atmospheric circulation. We perform sensitivity simulations with a dynamic-core GCM to investigate the effects of different intensities of hyperdiffusion and different model resolutions on the simulated equator-pole temperature gradient. We examine the different simulations using entropy production as a measure of baroclinic activity and show that there is a maximum in entropy production. In comparison to the climate at a state of maximum entropy production, every other simulated climate at a given resolution leads to an increased equator-pole temperature gradient. We then demonstrate that maximum entropy production can be used to tune low-resolution models to closely resemble the simulated climate of a high-resolution simulation. We conclude that tuning a GCM to a state of maximum entropy production is an efficient tool to tune low-resolution climate system models to adequately simulate the equator-pole temperature gradient. INTRODUCTION The equator-pole temperature gradient (DTEP) is of central importance to the global climate system. The gradient is affected by the strength of the atmospheric circulation, which compensates to some extent for the differences in radiative forcing by transporting heat. Paleoclimatological indicators suggest that DTEP has changed significantly in the past due to differences in the radiative forcing. For instance, for warm climates during the Eocene and the Cretaceous, atmospheric concentrations of carbon dioxide were significantly higher, and resulted in a much reduced temperature gradient of 19-23 K in comparison to today’s value of about 33 K (Pierrehumbert, 2002). On the other hand, during ice ages of the Pleistocene, the temperature gradient was increased to 50 K. Clearly, the snow albedo feedback and cloud feedbacks add to the causes for the difference in radiative forcing. Here, however, we solely focus on the role of the atmospheric circulation in transporting heat and how it is represented by atmospheric General Circulation Models (GCMs). Kleidon: Hyperdiffusion and the Equator-Pole Temperature Gradient page 2 of 20 _____________________________________________________________________________________________