Do planetary wave dynamics contribute to equable climates?

Viable explanations for equable climates of the Cretaceous and early Cenozoic (about 145 to 50 million years ago), especially for the above-freezing temperatures detected for high-latitude continental winters, have been a long-standing challenge. In this study, we suggest that enhanced and localized tropical convection, associated with a strengthened paleo warm pool, may contribute toward high-latitude warming through the excitation of poleward propagating Rossby waves. This warming takes place through the poleward heat flux and an overturning circulation that accompany the Rossby waves. This mechanism is tested with an atmosphere-mixed layer ocean general circulation model (GCM) by imposing idealized localized heating and compensating cooling, a heating structure which mimics the effect of warm pool convective heating.

The localized tropical heating is indeed found to contribute to a warming of the Arctic during the winter. Within the range of 0-150 W/m² for the heating intensity, the average rate for the zonal mean Arctic surface warming is 0.8^oC per 10 W/m² increase in the heating for the runs with an atmospheric CO₂ level of 4 X PAL (Preindustrial Atmospheric Level, 1 PAL = 280 ppmv), the Cretaceous and early Cenozoic values considered for this study. This rate of warming for the Arctic is lower in model runs with 1 X PAL CO₂, which show an increase of 0.3^oC per 10 W/m². Further increase of the heating intensity beyond 150 W/m² produces little change in the Arctic surface air temperature. This saturation behavior is interpreted as being a result of nonlinear wave-wave interaction which leads to equatorward wave refraction.

Under the 4 X PAL CO₂ level, raising the heating from 120 W/m² (estimated warm pool convective heating value for the present-day climate) to 150 W/m² and 180 W/m² (estimated values for the Cretaceous and early Cenozoic) produces a warming of 4^oC-8^oC over northern Siberia and the adjacent Arctic Ocean. Relative to the warming caused by a quadrupling of CO₂ alone, this temperature increase accounts for about 30% of the warming over this region. The possible influence of warm pool convective heating on the present-day Arctic is also discussed.