An Analysis of the Hydrological Cycle and Poleward Heat Transports Simulated by Two Climate Models

Abstract

Heat and Freshwater transport by the oceans and atmosphere are an integral part of the climate system, aiming to cool the Tropics and warm the Extra-Tropics. General Circulation Models (GCMs) are used to simulate the climate system, however a key weakness to them is the uncertainty associated with model predictions. One component of this uncertainty is due to the model structural bias associated with the choice of ocean model vertical coordinate type, which can have substantial feedback within a coupled ocean-atmosphere model. This thesis aims to investigate the Heat and Freshwater transport in the climate system with specific relevance to three main topics: 1. sensitivity of heat and freshwater transport to model numerics 2. coupling between heat and freshwater transport 3. changes to heat and freshwater transport to increasing CO2 concentration Firstly, the choice of ocean vertical coordinate on the computed heat and freshwater transport in the oceans and atmosphere was investigated. By comparing the models CHIME (isopycnal level ocean model) and HadCM3 (z-level ocean model) in a control climate, it was found that variations to the atmospheric latent and dry static energy transports were much larger than those induced from anthropogenic emission scenarios predicted by the latest IPCC report (AR4, 2007). Secondly, a new theory that constrained the ratio of ocean to atmospheric heat transport Ho/Ha as a function of ocean temperature and salinity was examined. This theory was tested using a control scenario from the HadCM3 model for mid-latitudes, finding good agreement over the Northern Hemisphere, though poorer performance over the Southern Hemisphere. Finally, climate snapshots in CHIME were analysed under an increasing CO2 environment. An examination of heat and freshwater transport for the ocean - atmosphere and the atmospheric dry static - latent energy components, showed significant compensation within each pair. Further investigation into the ocean overturning circulation and atmospheric moisture transport revealed: a salinification (freshening) of the Atlantic (Pacific), increased zonal moisture transport through Central America, and a weakening of the Atlantic meridional overturning circulation, validating CHIME's anthropogenic responses with predictions from AR4.