Title: Projected Changes in the Annual Cycle of Surface Temperature and Precipitation Due to Greenhouse Gas Increases
Description: When forced with increasing greenhouse gases, global climate models project changes to the seasonality of several key climate variables. These include delays in the phase of surface temperature, precipitation, and vertical motion indicating maxima and minima occurring later in the year. The changes also include an increase in the amplitude (or annual range) of low-latitude surface temperature and tropical precipitation and a decrease in the amplitude of high-latitude surface temperature and vertical motion. The aim of this thesis is to detail these changes, understand the links between them and ultimately relate them to simple physical mechanisms. At high latitudes, all of the global climate models of the CMIP3 intercomparison suite project a phase delay and amplitude decrease in surface temperature. Evidence is provided that the changes are mainly driven by sea ice loss: as sea ice melts during the 21st century, the previously unexposed open ocean increases the effective heat capacity of the surface layer, slowing and damping the temperature response at the surface.

In the tropics and subtropics, changes in phase and amplitude are smaller and less spatially uniform than near the poles, but they are still prevalent in the models. These regions experience a small phase delay, but an amplitude increase of the surface temperature cycle, a combination that is inconsistent with changes to the effective heat capacity of the system. Evidence suggests that changes in the tropics and subtropics are linked to changes in surface heat fluxes. The next chapter investigates the nature of the projected phase delay and amplitude increase of precipitation using AGCM experiments forced by SST perturbations representing idealizations of the changes in annual mean, amplitude, and phase as simulated by CMIP5 models. A uniform SST warming is sufficient to force both an amplification and a delay of the annual cycle of precipitation. The amplification is due to an increase in the annual mean vertical water vapor gradient, while the delay is linked to a phase delay in the annual cycle of the circulation.

A budget analysis of this simulation reveals a large degree of similarity with the CMIP5 results. In the second experiment, only the seasonal characteristics of SST are changed. For an amplified annual cycle of SST there is an amplified annual cycle of precipitation, while for a delayed SST there is a delayed annual cycle of precipitation. Assuming that SST changes can entirely explain the seasonal precipitation changes, the AGCM simulations suggest that the annual mean warming explains most of the amplitude increase and much of the phase delay in the CMIP5 models. However, imperfect agreement between the changes in the SST-forced AGCM simulations and the CMIP5 coupled simulations suggests that coupled effects may play a significant role. Finally, the connections between changes in the seasonality of precipitation, temperature and circulation are studied in the tropics using models of varying complexity. These models include coupled model simulations with idealized forcing, a simple, semi-empirical model to describe the effect of land-ocean interactions, an aquaplanet model, and a dry, dynamical model.

Each gives insights into the projected CMIP changes. Taken together they suggest that changes in the amplitude of vertical motions are consistent with a weakening of the annual mean circulation and can explain part of the changes in the amplitude of precipitation over both ocean and land, when combined with the thermodynamic effect described previously. By increasing the amplitude of the annual cycle of surface winds, the changes in circulation may also increase the amplitude of the surface temperature via the surface energy balance. The delay in the phase of circulation directly leads to a delay in the phase of precipitation, especially over ocean.