An Analysis of the Hydrological Cycle and Poleward Heat Transports Simulated by Two Climate Models
Author(s)
Dancel, Christopher Marlon
Type
Thesis
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.
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.
Date Issued
2012-06
Date Awarded
2012-07
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Czaja, Arnaud
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)