A Smooth Lattice construction of the Oppenheimer-Snyder spacetime

We present test results for the smooth lattice method using an Oppenheimer-Snyder spacetime. The results are in excellent agreement with theory and numerical results from other authors.Comment: 60 pages, 28 figure

Global mean surface temperature response to large-scale patterns of variability in observations and CMIP5

Global mean surface temperature (GMST) fluctuates over decadal to multidecadal time-scales. Patterns of internal variability are partly responsible, but the relationships can be conflated by anthropogenically-forced signals. Here we adopt a physically-based method of separating internal variability from forced responses to examine how trends in large-scale patterns, specifically the Interdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Variability (AMV), influence GMST. After removing the forced responses, observed variability of GMST is close to the central estimates of Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations, but models tend to underestimate IPO variability at time-scales >10 years, and AMV at time-scales >20 years. Correlations between GMST trends and these patterns are also underrepresented, most strongly at 10- and 35-year time-scales, for IPO and AMV respectively. Strikingly, models that simulate stronger variability of IPO and AMV also exhibit stronger relationships between these patterns and GMST, predominately at the 10- and 35-year time-scales, respectively

Relationships between high temperatures and Pacific Oyster disease and mortality in southeast Tasmania, Australia

Warm ocean temperature extremes, including marine heatwaves, have profound impacts on natural marine systems and aquaculture industries across the globe. In Tasmania, Australia, one aquaculture industry that has been significantly impacted by warm temperatures is Pacific Oyster (Magallana gigas, previously named Crassostrea gigas) farming, due to recurring outbreaks of the virus Ostreid herpesvirus 1. Such viral outbreaks are understood to be driven by high seawater temperatures, but the temperature threshold or duration for triggering disease and mortalities remain unclear. This study investigates the relationship between in-situ farm temperatures and oyster disease and mortality on the southeast coast of Tasmania, Australia using daily observations from three oyster growing areas (Pipe Clay Lagoon, Upper Pittwater, and Lower Pittwater) over three seasons. It is found that a 12-day averaged daily mean temperature is an excellent measure of the occurrence of high mortality. Specifically, a 21-day mean of 23.7 °C resulted in a 70% likelihood of high mortality, which is defined here as oyster losses of >15%. On the other hand, for lower levels of disease and mortality, a 12-day average of daily mean temperature gave the strongest relationship. A 12-day mean of 19.7 °C led to 70% probability of some disease and low mortality. The analysis also found in-situ farm temperature generally correlates well with remotely sourced temperature observations, indicating their potential usability for operational management. This study demonstrates a statistical risk analysis framework for the oyster farming industry, helping to improve the understanding of the detrimental impact of high temperatures on Pacific Oysters

A stakeholder-guided marine heatwave hazard index for fisheries and aquaculture

Marine heatwaves pose an increasing threat to fisheries and aquaculture around the world under climate change. However, the threat has not been estimated for the coming decades in a form that meets the needs of these industries. Tasmanian fisheries and aquaculture in southeast Australia have been severely impacted by marine heatwaves in recent years, especially the oyster, abalone, and salmon industries. In a series of semi-structured interviews with key Tasmanian fishery and aquaculture stakeholders, information was gathered about the following: (i) the impacts they have experienced to date from marine heatwaves, (ii) their planning for future marine heatwaves, and (iii) the information that would be most useful to aid planning. Using CMIP6 historical and future simulations of sea surface temperatures around Tasmania, we developed a marine heatwave hazard index guided by these stakeholder conversations. The region experienced a severe marine heatwave during the austral summer of 2015/16, which has been used here as a reference point to define the index. Our marine heatwave hazard index shows that conditions like those experienced in 2015/16 are projected to occur approximately 1-in-5 years by the 2050s under a low emissions scenario (SSP1-2.6) or 1-in-2 years under a high emissions scenario (SSP5-8.5). Increased frequency of marine heatwaves will likely reduce productivity by both direct (mortality) and in-direct (ecosystem change, greater incidence of disease) impacts on target species. The illustrative hazard index is one step towards a marine heatwave risk index, which would also need to consider aspects of exposure and vulnerability to be of greater utility to stakeholders

Smooth lattice general relativity, and SPH simulations of swimming linked bodies

In this thesis, two quite different bodies of work are presented. The first is the study of the smooth lattice approach to numerical relativity which has been developed by Brewin (1998a,b, 2002). The aim of the research was to further test the viability of the method. Previously, it had been used to model a Kasner cosmology and a Schwarzschild black hole. Both were shown to produce excellent results. In this thesis, the details and results for a long-term, stable integration of a (one-dimensional) symmetric gravitational collapse are given. In the second body of work, smoothed particle hydrodynamics (SPH) was used to model swimming linked bodies. These simulations are an attempt to model the motion of real swimming animals, such as fish or eels. The calculations were for three linked, rigid bodies in two-dimensions. However, the set of equations provided are easily generalised for more bodies, and for three dimensions. The results are shown for a range of body and fluid parameters. The common link between the two areas of research is that they are computational in nature. Both involve the study and development of specific numerical algorithms, and their application to particular problems. Whilst a significant amount of time was spent on developing the mathematical formulae, the vast majority was spent on the numerical implementation, i.e. writing and testing code, using both C and Fortran programming languages. The calculations were all done on a standard Apple eMac desktop computer. The thesis has been split into two parts to reflect that the two bodies of work are separate. In each part, the relevant mathematical and numerical details are described, and the associated results are presented

Predictability of marine heatwaves off Western Australia using a linear inverse model

Marine heatwaves (MHWs) off Western Australia (110°E−116°E, 22°S−32°S; hereafter, WA MHWs) can cause devastating ecological impacts, as was evidenced by the 2011 extreme event. Previous studies suggest that La Niña is the major large-scale driver of WA MHWs, while Indian Ocean Dipole (IOD) may also play a role. Here, we investigate historical WA MHWs and their connections to these large-scale climate modes in an ocean model (ACCESS−OM2) simulation driven by a prescribed atmosphere from JRA55−do over 1959–2018. Rather than analysing sea surface temperature, the WA MHWs and climate mode indices were characterized and investigated in vertically averaged temperature (VAT) to ~300m depth to afford the longer ocean dynamic time scales, including remote oceanic connections. We develop a cyclostationary linear inverse model (CS-LIM; from 35°S−10°N, across the Indo-Pacific Ocean), to investigate the relative contributions of La Niña VAT and positive IOD VAT to the predictability of WA VAT MHWs. Using a large ensemble of CSLIM simulations, we found that ~50% of WA MHWs were preceded about 5 months by La Niña, and 30% of the MHWs by positive IOD about 20 months prior. While precursor La Niña or positive IOD, on their own, were found to correspond with increased WA MHW likelihood in the months following (~2.7 times or ~1.5 times more likely than by chance, respectively), in combination these climate mode phases were found to produce the largest enhancement in MHW likelihood (~3.2 times more likely than by chance). Additionally, we found that stronger and longer La Niña and/or positive IOD tend to lead stronger and longer WA MHWs

Impacts of marine heatwaves on tropical western and central Pacific Island nations and their communities

Marine heatwaves can have devastating impacts on marine species and habitats, often with flow-on effects to human communities and livelihoods. This is of particular importance to Pacific Island countries that rely heavily on coastal and ocean resources, and for which projected increases in future marine heatwave (MHW) frequency, intensity, and duration could be detrimental across the Pacific Island region. In this study, we investigate MHWs in the tropical western and central Pacific Ocean region, focusing on observed MHWs, their associated impacts, and future projections using Coupled Model Intercomparison Project phase 6 (CMIP6) simulations under a low (SSP1–2.6) and a high (SSP5–8.5) greenhouse gas emissions scenario. Documented impacts from “Moderate” mean intensity MHW events in Fiji, Samoa, and Palau, that were categorised as “Strong” at their peak, included fish and invertebrate mortality and coral bleaching. Based on CMIP6 multi-model mean estimates, and relative to current baselines, “Moderate” intensity MHWs are projected to increase from recent historical (1995–2014) values of 10–50 days per year (dpy) across the region to the equivalent of >100 dpy by the year 2050 under the low emissions scenario, and > 200 dpy nearer the equator. Under the high emissions scenario, 200 dpy of Moderate MHW intensities are projected across most of the region by 2050, with >300 dpy nearer the equator. For the most intense “Extreme” category of MHW, estimates range from 50 dpy projected under the high emissions scenario by 2050. In contrast, “Extreme” MHWs are projected to increase to <5 dpy by 2050 under the low emissions scenario, highlighting the importance for Pacific Island nations that global emissions more closely follow the low emissions scenario trajectory

Model under-representation of decadal Pacific trade wind trends and its link to tropical Atlantic bias

The strengthening of the Pacific trade winds in recent decades has been unmatched in the observational record stretching back to the early twentieth century. This wind strengthening has been connected with numerous climate-related phenomena, including accelerated sea-level rise in the western Pacific, alterations to Indo-Pacific ocean currents, increased ocean heat uptake, and a slow-down in the rate of global-mean surface warming. Here we show that models in the Coupled Model Intercomparison Project phase 5 underestimate the observed range of decadal trends in the Pacific trade winds, despite capturing the range in decadal sea surface temperature (SST) variability. Analysis of observational data suggests that tropical Atlantic SST contributes considerably to the Pacific trade wind trends, whereas the Atlantic feedback in coupled models is muted. Atmosphere-only simulations forced by observed SST are capable of recovering the time-variation and the magnitude of the trade wind trends. Hence, we explore whether it is the biases in the mean or in the anomalous SST patterns that are responsible for the under-representation in fully coupled models. Over interannual time-scales, we find that model biases in the patterns of Atlantic SST anomalies are the strongest source of error in the precipitation and atmospheric circulation response. In contrast, on decadal time-scales, the magnitude of the model biases in Atlantic mean SST are directly linked with the trade wind variability response