Publication Search Results

Now showing 1 - 4 of 4
  • (2022) Dutta, Deepashree
    Paleoproxy data suggest that past warm climates, such as the early Eocene (~56–48 million years ago), had considerably warmer Northern Hemisphere high latitudes and a weaker equator-to-pole temperature gradient compared to the pre-industrial period. However, general circulation models (GCMs) underestimate the magnitude of Arctic warming inferred from proxies, suggesting that certain physical processes might be poorly represented in these models. Most previous modelling studies use coupled models, thus making it difficult to isolate the roles of different atmospheric processes in driving Arctic amplification. Therefore, in this thesis, I first conduct an atmosphere-only model intercomparison in which four models are forced with idealised polar amplified sea surface temperature (SST) anomalies (Chapter 2). The models show largely similar Arctic high cloud, surface albedo and poleward atmospheric heat transport responses, suggesting that physical processes other than these must be responsible for the large intermodel spread in Arctic amplification seen in coupled models. Previous studies hypothesised that a large increase in Arctic polar stratospheric clouds (PSCs) caused by methane oxidation might explain the discrepancy in simulated and reconstructed Arctic amplification for past warm climates. I revisit this hypothesis in Chapter 3 using the previously applied polar amplified SST pattern but using a stratosphere resolving chemistry-climate model with a range of greenhouse gas concentrations. The water vapour concentration in the Arctic stratosphere increases monotonically with increases in methane, thereby resulting in increased PSCs. I find that the radiative effect of Arctic PSCs is approximately of the same magnitude as the radiative effect of methane. However, the greenhouse effect of the Arctic PSCs is small compared to the outgoing longwave radiation loss caused by the polar amplified SST. I also find that the Brewer-Dobson circulation (BDC) strength is sensitive to the imposed boundary conditions (SSTs and greenhouse gases). Finally, in Chapter 4, I build on the previous experimental set-up by integrating the chemistry-climate model with early Eocene topography and more realistic SSTs for that period to understand how topography and SST changes during the Eocene might have changed the behaviour of planetary waves and thus caused a BDC change. I show that changes in topography weaken the BDC, therefore causing a reduction in the Arctic stratospheric temperature and an increase in PSCs. This result reveals the key role played by the Eocene topography in increasing the fractional coverage of Arctic PSCs, which was not seen in the simulations with modern-day topography. This thesis has advanced our understanding of the roles of clouds and the large-scale atmospheric circulation, both in the troposphere and stratosphere, in Arctic amplification in a warmer-than modern-day climate. It provides insights into what might be missing in the GCMs to provide reliable projections of future changes in the Arctic.

  • (2022) Orihuela Pinto, Luis
    The 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, Mengyuan
    Droughts 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.

  • (2022) Ayat, Hooman
    Climate change is expected to change the intensity and frequency of heavy storms. Thus, understanding different characteristics of this phenomena (i.e., intensity, size, speed, direction, etc.) is vital for the effective climate adaptation. Many extreme storms have small areas and short lifetimes (sub-daily/hourly) and can have destructive impacts, especially over urban areas. Therefore, it is vital to understand the nature of changes in these extremes to reduce the risk of their destructive impacts on cities. The overarching goal of this thesis is to quantify various storm characteristics, including their changes, using radar and satellite observations. Using an object-based technique, I compare the Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG) and ground radar based Multi-Radar Multi-Sensor Quantitative Precipitation Estimates (MRMS) over the United States and show that the object-based storm properties are not sensitive to the observational platforms. However, there are differences that are statistically significant. Secondly, I investigate the error sources associated with different types of contributing data in the IMERG during the hurricane days occurred in 2016-2018 with MRMS as the reference. The results show that IMERG have better agreement with MRMS during the passive microwave (PMW) observations compared to rainfall estimates come from the combination of the interpolation techniques and infrared observations (morph/IR). Also, the quality of morph/IR estimates deteriorates with the longer absence of PMW observations. Thirdly, I establish an object-based climatology of rain systems using radar data near Sydney, Australia. The results show that rain systems in different seasons have distinct object-based characteristics, and these differences are dependent on their source of origins and also their positions over land and ocean. Using a two-step clustering algorithm, I have found five system types over Sydney peaking in different seasons. While overall rainfall statistics don't show any link to climate modes, links do appear for some system types using a multivariate approach. Finally, I show that there is a robust increasing trend of 20% per decade in sub-hourly extreme rainfall in the Sydney region over 20 years, despite no evidence of trends on hourly or daily scales. I am able to obtain this new result via a novel analysis of long-term radar data, including cross-checking between neighboring radars.