On the maintenance of weak meridional temperature gradients during warm climates

Abstract

This thesis examines the dynamics of equable climates. The underlying physics of two mechanisms by which weak meridional temperature gradients might be maintained are studied. First, I examine the evolution of stratospheric dynamics and winter temperatures when the surface temperature gradient and tropospheric eddy energy decrease in order to assess whether large-scale conditions are more favorable for polar stratospheric cloud formation. Second, I examine whether the combination of high carbon dioxide and interactive, tropical cyclone dependent ocean mixing is sufficient to maintain a weak temperature gradient. I examine planetary wave generation, the energetics of the general circulation, and vertical wave propagation in a general circulation model with a resolved stratosphere forced with a weak surface temperature gradient. Compared to the present climate, transient eddy energy decreases, but stationary eddy energy does not. The polar tropopause rises, which supports a weaker temperature gradient in the lower stratosphere, a weaker stratospheric jet, and an increase in the wave activity vertically propagating into the stratosphere.

(cont.) As a result, the residual mean circulation strengthens and temperatures in the polar stratosphere change little even when the surface temperature gradient is quite weak. Temperatures in the Arctic polar vortex remain much warmer than radiative equilibrium, inhibiting large-scale polar stratospheric cloud formation. The height of the extratropical tropopause rises and the tropospheric lapse rate follows a moist adiabat when surface temperatures are warm, suggesting convection plays a significant role in setting extratropical tropospheric stratification during warm climates. The second part of the thesis addresses the role of tropical cyclone induced mixing in the oceans' general circulation. I examine the sensitivity of the oceans' meridional overturning circulation and heat flux to the locations at which mixing occurs. When confined to the tropical Atlantic, a robust single-basin circulation can be maintained, but the Indian and Pacific become quiescent, cut off from the dynamics occurring in the Atlantic. Mixing isolated in the tropical Pacific, however, can support a global circulation by mechanically lifting deep fluid parcels formed in the Atlantic, raising their potential energy.

(cont.) The oceans' total heat flux is found to be sensitive to mixing in the tropics, in both the Atlantic and the Pacific, and in the upper 400 meters of the ocean. Coupling mixing with a measure of tropical cyclone intensity and frequency creates a positive feedback between climate and the poleward energy flux. When combined with a parameterization of the background mixing that evolves with stratification, a warmer, less stratified ocean can support a stronger diapycnal mixing during warm climates with high loads of carbon dioxide. In these simulations, tropical cyclones are stronger and more frequent, providing an increased energy source for more vigorous mixing in the tropical oceans. Combined with a stratification-dependent mixing scheme, such mixing provides a sufficiently strong heat flux that is able to maintain weak sea surface temperature gradients.