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AbstractPrevious studies on semi‐arid ecosystems have shown high values of soil moisture variability (SMV) primarily induced by the combined effects of non‐uniform precipitation, incoming solar radiation, and soil and vegetation properties. However, the relative impact of these various factors on SMV has been difficult to evaluate due to limited availability of field data. In addition, only a limited number of studies have analysed the role of landscape morphology on SMV. Here we use numerical simulations of a simple hydrological model, the Bucket Grassland Model, to systematically analyse the effect of each contributing factor on SMV on two different landscape morphologies. The two different landform morphologies represent landscapes dominated respectively by either diffusive erosion or fluvial erosion processes. We conducted various simulations driven by a stochastically generated 100‐year climate time series, which is long enough to capture climatic fluctuations, in order to understand the effect of various soil moisture controlling factors on the spatiotemporal SMV. Our modelling results show that the fluvial dominated landscapes promote higher spatial SMV than the diffusive dominated ones. Further, the role of landform morphology on SMV is more pronounced in regions where the spatial variability of incoming solar radiation and precipitation is high.

Future climate change is projected to increase the frequency and severity of droughts in many drought-prone regions of the world. However, coarsely-resolved and sometimes opposing rainfall changes across global climate models, and a lack of understanding of the weather-scale processes that cause droughts to begin and end, create uncertainties around these future changes in many regions. Here, we reveal that the co-occurrence of a deep low-pressure system over southeast Australia with an adjacent high-pressure system in the Tasman Sea is a key dynamic mechanism responsible for extreme and drought-breaking rainfall in southeast Australia. Regional climate models project a decline in the frequency of these events by 9% for every degree of global warming, with an 11.6% per degree decline in associated rainfall, increasing the risk of the region entering and staying in drought in the late 21st century. This critical dynamical context brings confidence to the projected increase in drought frequency and duration in southeast Australia and provides an opportunity to robustly identify future drought changes in parts of the world where similar mechanisms may also play a role in extreme, drought-breaking rainfall.

Ensemble simulations, using both coupled ocean–atmosphere (AOGCM) and atmosphere only (AGCM) general circulation models, are employed to examine the austral winter response of the Hadley circulation (HC) and stationary Rossby wave propagation (SRW) to a warming climate. Changes in the strength and width of the HC are firstly examined in a set of runs with idealized sea surface temperature (SST) perturbations as boundary conditions in the AGCM. Strong and weak SST gradient experiments (SG and WG, respectively) simulate changes in the HC intensity, whereas narrow (5°S–5°N) and wide (30°S–30°N) SST warming experiments simulate changes in the HC width. To examine the combined impact of changes in the strength and width of the HC upon SRW propagation two AOGCM simulations using different scenarios of increasing carbon dioxide (CO2) concentrations are employed. We show that, in contrast to a wide SST warming, the atmospheric simulations with a narrow SST warming produce stronger and very zonally extended Rossby wave sources, leading to stronger and eastward shifted troughs and ridges. Simulations with SST anomalies, either in narrow or wide latitude bands only modify the intensity of the troughs and ridges. SST anomalies outside the narrow latitude band of 5°S–5°N do not significantly affect the spatial pattern of SRW propagation. AOGCM simulations with 1 %/year increasing CO2 concentrations or 4 times preindustrial CO2 levels reveal very similar SRW responses to the atmospheric only simulations with anomalously wider SST warming. Our results suggest that in a warmer climate, the changes in the strength and width of the HC act in concert to significantly alter SRW sources and propagation characteristics.

AbstractTheory relating changes in area‐average evaporation with changes in the evaporation from pans or open water is developed. Such changes can arise by Type (a) processes related to large‐scale changes in atmospheric concentrations and circulation that modify surface evaporation rates in the same direction, and Type (b) processes related to coupling between the surface and atmospheric boundary layer (ABL) at the landscape scale that usually modify area‐average evaporation and pan evaporation in different directions. The interrelationship between evaporation rates in response to Type (a) changes is derived. They have the same sign and broadly similar magnitude but the change in area‐average evaporation is modified by surface resistance. As an alternative to assuming the complementary evaporation hypothesis, the results of previous modelling studies that investigated surface–atmosphere coupling are parametrized and used to develop a theoretical description of Type (b) coupling via vapour pressure deficit (VPD) in the ABL. The interrelationship between appropriately normalized pan and area‐average evaporation rates is shown to vary with temperature and wind speed but, on average, the Type (b) changes are approximately equal and opposite. Long‐term Australian pan evaporation data are analyzed to demonstrate the simultaneous presence of Type (a) and (b) processes, and observations from three field sites in southwestern USA show support for the theory describing Type (b) coupling via VPD. Copyright © 2009 Royal Meteorological Society

The sparse nature of observational records across the mid- to high latitudes of the Southern Hemisphere limits the ability to place late-twentieth-century environmental changes in the context of long-term (multidecadal and centennial) variability. Historical records from subantarctic islands offer considerable potential for developing highly resolved records of change. In 1905, a whaling and meteorological station was established at Grytviken on subantarctic South Georgia in the South Atlantic (54°S, 36°W), providing near-continuous daily observations through to present day. This paper reports a new, daily observational record of temperature and precipitation from Grytviken, which is compared to regional datasets and historical reanalysis. The authors find a shift toward increasingly warmer daytime extremes commencing from the mid-twentieth century and accompanied by warmer nighttime temperatures, with an average rate of temperature rise of 0.13°C decade−1 over the period 1907–2016 ( p < 0.0001). Analysis of these data and reanalysis products suggest a change of pervasive synoptic conditions across the mid- to high latitudes since the mid-twentieth century, characterized by stronger westerly airflow and associated warm föhn winds across South Georgia. This rapid rate of warming and associated declining habitat suitability has important negative implications for biodiversity, including the survival of key marine biota in the region.

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 379 (2195) ISSN:1364-503X ISSN:1471-2962

Assessments of climate change impacts on freshwater ecosystems are generally based on global climate models (GCMs) and ecologically relevant “time-averaged” hydrological indicators derived from long-term records. Although uncertainties from GCMs have been recognized, the influence of downscaling methods remains unclear. This paper evaluates the influence of applying different downscaling methods of increasing complexity (annual scaling, monthly scaling, quantile scaling, and weather generator method) on the assessment of ecological outcomes. In addition to time-averaged indicators, “sequence-dependent” metrics which involve ecological dynamics by considering the impacts of flow sequencing are also adopted. In a case study in Australia, the condition of river red gum forest was assessed. Results show that the choice of downscaling methods can be of similar importance as that of GCMs in ecological impact studies. Where sequence-dependent metrics are adopted, more sophisticated downscaling techniques should be used to better represent changes in the frequency and sequence of flow events.

Abstract. Knowledge of past, current, and future hydroclimatic risk is of great importance. However, like many other countries, Australia's observed hydroclimate records are at best only ∼ 120 years long (i.e. from ∼ 1900 to the present) but are typically less than ∼ 50 years long. Therefore, recent research has focused on developing longer hydroclimate records based on palaeoclimate information from a variety of different sources. Here we review and compare the insights emerging from 11 published palaeoclimate records that are relevant for annual to sub-decadal hydroclimatic variability in eastern Australia over the last ∼ 1000 years. The sources of palaeoclimate information include ice cores, tree rings, cave deposits, and lake sediment deposits. The published palaeoclimate information was then analysed to determine when (and where) there was agreement (or uncertainty) about the timing of wet and dry epochs in the pre-instrumental period (1000–1899). The occurrence, frequency, duration, and spatial extent of pre-instrumental wet and dry epochs was then compared to wet and dry epochs since 1900. The results show that instrumental records (∼ 1900–present) underestimate (or at least misrepresent) the full range of rainfall variability that has occurred, and is possible, in eastern Australia. Even more disturbing is the suggestion, based on insights from the published palaeoclimate data analysed, that 71 % of the pre-instrumental period appears to have no equivalent in the instrumental period. This implies that the majority of the past 1000 years was unlike anything encountered in the period that informs water infrastructure, planning, and policy in Australia. A case study, using a typical water storage reservoir in eastern Australia, demonstrates that current water resource infrastructure and management strategies would not cope under the range of pre-instrumental conditions that this study suggests has occurred. When coupled with projected impacts of climate change and growing demands, these results highlight some major challenges for water resource management and infrastructure. Though our case study location is eastern Australia, these challenges, and the limitations associated with current methods that depend on instrumental records that are too short to realistically characterise interannual to multi-decadal variability, also apply globally.