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(2022) Orihuela Pinto, LuisThesisThe importance of the Atlantic Meridional Overturning Circulation (AMOC) on Earth’s climate is primarily via its role in regulating global ocean heat transport. Past climatic states were characterised by a collapsed AMOC and future climate projections suggest that, under a global warming scenario, the AMOC will undergo a weakening that could end in a potential collapse. An AMOC collapse would have major ramifications for Atlantic Ocean heat transport, Arctic sea-ice coverage and regional climate. However, it remains unclear how an AMOC shutdown might impact other regions of the globe. In particular, the AMOC’s connection to tropical climate processes and variability remains unclear and requires further study. Here a global climate model is used to show how an AMOC collapse alters the tropical Pacific atmospheric circulation. This occurs by virtue of the reduced northward oceanic heat transport which leaves an excess of heat in the South Atlantic that triggers atmospheric convection over the area and causes anomalous subsidence in the east Pacific. Subsequently, the Pacific Walker circulation accelerates and cools the tropical Pacific Ocean. Furthermore, the tropical Pacific mean state change due to the AMOC collapse causes a weakening in the atmosphere-ocean coupling locally, which alters the governing El Niño Southern Oscillation (ENSO) feedbacks. Consequently, ENSO events feature a damped growth rate and decreased variability which reduces the frequency of extreme El Niño events and shifts the maximum warming of the majority of El Niño events towards the central Pacific. Finally, an analysis is performed to determine how the tropical Pacific cooling caused by an AMOC collapse could in turn feedback on the AMOC strength. To do this, the resulting Pacific cooling is imposed in a set of model runs and find that the AMOC transport increases. The AMOC strengthening occurs due to an atmospheric teleconnection via Rossby waves moving from the tropical Pacific to the North Atlantic which alter the surface circulation locally, ultimately favouring ocean deep convection through surface heat extraction in the North Atlantic. These results shed light on the mechanisms behind possible future global climate changes in relation to potential changes in the AMOC and may help interpret past climate reconstructions associated with the AMOC, the tropical Pacific and ENSO variations, along with their interactions.
(2022) Mu, MengyuanThesisDroughts and heatwaves impact human and natural systems in Australia, and groundwater helps ecosystems survive these extremes. However, how groundwater affects land-atmosphere interactions during droughts and heatwaves has rarely been examined. This thesis explores the influence of groundwater on the impact and the intensity of heatwaves and droughts, focusing on southeast Australia, by using the Community Atmosphere-Biosphere Land Exchange (CABLE) land surface model (LSM). First, this thesis evaluates multiple ways to represent key processes in CABLE using a comprehensive set of observations in an Australian water-limited site. CABLE simulates the land hydrology during droughts and heatwaves well, if high-resolution observations of evaporation and root zone processes are available to configure the model and to select appropriate parameterizations. The results highlight both the opportunity and the challenge in improving LSMs for simulating droughts and heatwaves well. Second, using the most realistic model configuration from the model evaluation step, this thesis examines how groundwater influences ecosystems during co-occurring heatwaves and droughts. Results demonstrate the importance of groundwater in sustaining transpiration for the first 1–2 years of multi-year droughts. Results also demonstrate how the lack of deep roots or stomatal closure under high vapour pressure deficit or high temperature can reduce the role of groundwater. Given these are not always represented in LSMs, these results indicate the potential for overestimating the impact of droughts and heatwaves in climate model simulations. Third, coupled experiments using Weather Research and Forecasting (WRF) and CABLE examined the influence of groundwater on heatwave intensity in southeast Australia. Results show that groundwater moistens and cools both the land surface and atmospheric boundary layer during heatwaves. Groundwater reduces maximum air temperatures near the surface by up to 3 °C, and by up to 1 °C through the atmospheric boundary layer, but only where the water table depth was shallow, and overlain by forests. Overall, this work quantifies the impact of groundwater on heatwave intensity and identifies the impacted regions over southeast Australia. The thesis concludes with areas for future model development, with the goal of further improving the simulations of heatwaves and droughts which are projected to increase in many regions of the world due to climate change.