Past warm climates represent one extreme of Earth's known climate states. Here, we study warm climates in both idealized simulations and full-complexity general circulation model (GCM) simulations of the early Eocene epoch, approximately 50 million years ago.

In increasingly warmer idealized aquaplanet simulations, the amplitude of intra-seasonal tropical variability is enhanced. The anomalies propagate eastward in the tropics and resemble the observed Madden-Julian Oscillation (MJO). The strong MJO anomalies drive a momentum convergence on the equator that causes westerly winds in the troposphere, a state known as superrotation. The results in this thesis show that superrotation further enhances the MJO by affecting the penetration of midlatitude eddies into the deep tropics. An additional question is how a super-rotating atmosphere, a dramatically different general circulation regime compared to today, will affect the climate, potentially via changes in cloud distributions and ocean circulation. If the superrotation extends down to the surface near the equator, surface westerly winds will drive equatorial downwelling in the eastern equatorial Pacific Ocean, rather than upwelling as in the present climate. Here, we show that surface superrotation is unlikely in past warm climates, although this in part depends on the intensity of the vertical momentum transfer associated with cumulus convection and how this process is represented in a specific GCM. 

There is, currently, no consensus on what the specific mix of forcings was that caused the warm climates of the early Eocene. High greenhouse gases likely played a significant role, but simulations with reasonable greenhouse gas concentrations cannot reproduce the high temperatures estimated by proxy data. Here, we investigate both an early Eocene climate forced by high greenhouse gas concentrations and one forced by optically thinner clouds, with artificially increased cloud droplet radius that causes increased solar radiation at the surface. Both alternative warming scenarios produce nearly identical zonal mean temperatures, but the hydrologic cycle differs; the thinner clouds scenario has 11% larger global mean precipitation. Moreover, the results in this thesis indicate that a reasonable estimate of vegetation, based on the model simulation, is likely necessary to evaluate alternative warming scenarios with proxy data.