Viable explanations for equable climates of the Cretaceous and early Cenozoic (from 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, the authors 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 that 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 m-2 for the heating intensity, the average rate for the zonal mean Arctic surface warming is 0.88C per (10 W m-2) increase in the heating for the runs with an atmospheric CO2 level of 4 × 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×PAL CO2, which show an increase of 0.38C per (10 W m-2). Further increase of the heating intensity beyond 150 W m-2 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 × PAL CO2 level, raising the heating from 120 W m-2 (estimated warm-pool convective heating value for the present-day climate) to 150 and 180 W m-2 (estimated values for the Cretaceous and early Cenozoic) produces a warming of 4°-8°C 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.
All Science Journal Classification (ASJC) codes
- Atmospheric Science