Michael Alan Barnes

As renewable energy capacity in Australia’s power system increases, questions arise about whether supply can meet demand on days with high electricity consumption. We examine the meteorology of very hot days in eastern Australia when there is also sufficient cloud cover to reduce electricity supply from solar photovoltaics. We show that these days are relatively rare, with hot, cloudy conditions occurring on 1.5%–3.5% of days during November–April, although they may have potential impacts on the electricity system on hourly time-scales. We find that hot, cloudy conditions in southeastern Australia generally occur on days preceding a cold front. Increased moisture is the main difference between hot-cloudy and hot-sunny days in southeastern Australia, while more local aspects of the meteorology may drive hot, cloudy conditions further north. Maps of satellite-derived surface global irradiance suggest that there can be spatially coherent regions of hot, cloudy conditions in the southeast, with a higher likelihood of temporally and spatially compounding events in this region. Differences in wind energy availability on hot-cloudy and hot-sunny days suggest that deficits in solar energy on hot-cloudy days may be mitigated by favourable wind resources. This work demonstrates that a scenario-based approach that matches the meteorological drivers of both renewable energy supply and electricity demand is necessary for understanding risks and opportunities in the current and future energy system. This is particularly evident when considering rare high-impact weather events arising from the spatial or temporal confluence of meteorological conditions that may not be considered individually as severe.

In this study, we describe the Southern Annular Mode (SAM) as a function of regional extratropical variability in order to understand the synoptic-dynamic features associated with it. The SAM is often considered to be the leading mode of variability in the extratropics and is used to interpret changes and impacts in the large-scale mid-latitude circulation and surface weather in the Southern Hemisphere. This interpretation of the SAM has however been questioned in recent years, given the SAM's lack of annularity and weak correlation to surface weather and synoptic features. From this regional perspective, we once again highlight the non-annularity of the SAM and show explicitly how these asymmetries can result in the misinterpretation and misattribution of regional surface weather impacts. Through analysing a set of weather features including Rossby wave breaking (RWB) zones, cyclones and jets, we show the SAM can be interpreted as a function of the RWB characteristics across the hemisphere, in line with similar perspectives of the North Atlantic Oscillation. In fact, changes in the storm track and jets occur locally and non-annularly across the hemisphere in response to regional RWB, in contrast to the annular view assumed by the SAM. This interpretation provides opportunity for further understanding of present and future Southern Hemispheric extratropical variability and its associated surface weather impacts.

In this study, we describe the Southern Annular Mode (SAM) as a function of regional extratropical variability in order to understand the synoptic-dynamic features associated with it. The SAM is often considered to be the leading mode of variability in the extratropics and is used to interpret changes and impacts in the large-scale mid-latitude circulation and surface weather in the Southern Hemisphere. This interpretation of the SAM has however been questioned in recent years, given the SAM's lack of annularity and weak correlation to surface weather and synoptic features. From this regional perspective, we once again highlight the non-annularity of the SAM and show explicitly how these asymmetries can result in the misinterpretation and misattribution of regional surface weather impacts. Through analysing a set of weather features including Rossby wave breaking (RWB) zones, cyclones and jets, we show the SAM can be interpreted as a function of the RWB characteristics across the hemisphere, in line with similar perspectives of the North Atlantic Oscillation. In fact, changes in the storm track and jets occur locally and non-annularly across the hemisphere in response to regional RWB, in contrast to the annular view assumed by the SAM. This interpretation provides opportunity for further understanding of present and future Southern Hemispheric extratropical variability and its associated surface weather impacts.

In this study, we describe the Southern Annular Mode (SAM) as a function of regional extratropical variability in order to understand the synoptic-dynamic features associated with it. The SAM is often considered to be the leading mode of variability in the extratropics and is used to interpret changes and impacts in the large-scale mid-latitude circulation and surface weather in the Southern Hemisphere. This interpretation of the SAM has however been questioned in recent years, given the SAM's lack of annularity and weak correlation to surface weather and synoptic features. From this regional perspective, we once again highlight the non-annularity of the SAM and show explicitly how these asymmetries can result in the misinterpretation and misattribution of regional surface weather impacts. Through analysing a set of weather features including Rossby wave breaking (RWB) zones, cyclones and jets, we show the SAM can be interpreted as a function of the RWB characteristics across the hemisphere, in line with similar perspectives of the North Atlantic Oscillation. In fact, changes in the storm track and jets occur locally and non-annularly across the hemisphere in response to regional RWB, in contrast to the annular view assumed by the SAM. This interpretation provides opportunity for further understanding of present and future Southern Hemispheric extratropical variability and its associated surface weather impacts.

Rossby waves are fundamental meteorological phenomena in the extratropics. When these waves amplify and break, they often lead to extreme weather events, including heatwaves, heavy rainfall, and strong winds. Here we apply an objective classification method to identify equatorward anticyclonic and cyclonic Rossby wave breaking morphologies, analogous to the LC1 and LC2 types identified in previous research. Anticyclonic Rossby wave breaking zones are shown to evolve as expected, representing the barotropic decay of baroclinic Rossby wave packet. Composite analysis of the evolution of cyclonic Rossby wave breaking morphologies however shows that these morphologies develop from the debris of preceding anticyclonic Rossby wave breaking. Cyclonic morphologies are further linked to Rossby wave packet generation and downstream development. The role of Rossby wave breaking in extreme weather is illustrated through the example of heavy rainfall along Australia’s east coast, emphasizing its importance in the generation of such extremes.

Using three‐hourly ERA5 and ERA5‐Land reanalyses from 1979 to 2024, we determine the characteristics of wet spells in Australia in summer (October–March) and winter (April–September), focusing on northern and southeast Australia. Wet spells in summer account for up to 90% of seasonal precipitation in northern Australia, with a frequency of 20%–30% and a mean duration of 8–10 hours. In winter, wet spells of similar frequency contribute up to 96% of precipitation in southeast Australia, with a mean duration of 12–17 hours. Wet spells lasting six hours to one day account for the largest fraction (50%–60%) of seasonal precipitation in summer, whereas 12‐hour to two‐day wet spells contribute the most (50%–70%) in winter. Wet spells longer than 12 hours account for nearly 90% of the three‐hourly extreme precipitation events with a mean intensity of 3–4 mm·3‐h ⁻¹ . Shorter spells are associated with light showers with a mean intensity of 0.5–2 mm·3‐h ⁻¹ and contribute 10%–30% of extreme events. The increase in seasonal precipitation during wet years is primarily due to an increase in the frequency of wet spells in northern Australia. In contrast, increases in both wet‐spell frequency and intensity are important in southeast Australia. In both regions, wet spells longer than 12 hours contribute the most to the change in precipitation between wet and dry seasons. The synoptic environments for subdaily wet spells in northern Australia are tropical convection and monsoon low‐pressure systems. Longer wet spells show patterns similar to active monsoon bursts with a well‐developed monsoon trough. In southeast Australia, the synoptic patterns for subdaily spells resemble extratropical lows and fronts, while longer wet spells are mostly associated with cut‐off lows.

Synoptic weather typing is a common technique for categorizing regional circulation patterns over a region. Defined and commonly used sets of synoptic weather types (SWTs) have the potential to simplify the analysis and interpretation of the regional circulation on the weather time and spatial scales and create a common language between the often segregated weather and climate research communities. However, circulation classifications are potentially complicated over Australia owing to its position in both the tropics and extratropics. Here we present a set of 30 SWTs over Australia by k means clustering the 850 hPa wind field in the ERA5 data set. We show that the Australian SWTs represent recognizable weather patterns over the continent throughout the year. Importantly, the SWTs capture features in the extratropics together with characteristics associated with the tropical monsoon. An initial analysis of the relationship of the SWTs with the modes of climate variability, high impact weather and renewable energy related variables showing the utility of the SWTs in a wide range of applications across disciplines within earth system sciences and beyond.

Rossby waves are fundamental meteorological structures in the extratropics of both hemispheres. Several extremes and weather regimes have been linked to the amplification and breaking of Rossby waves propagating along the extratropical waveguide. The morphology and evolution of Rossby wave breaking (RWB) on the Southern Hemisphere dynamical tropopause is studied here through an objective classification algorithm. Although the well-known classifications of RWB morphologies (LC1, LC2, P1, and P2) work well, the objective algorithm identifies important distinctions between RWB events with higher and lower amplitude structures and more meridional and zonal orientations. The different morphologies reflect the differences in the structure of the Rossby wave packets, the degree of phase locking with the low levels, the strength of the diabatic processes, and the amplitude of the nonlinearities in the flow. Anticyclonic RWB morphologies are associated with the decay of a Rossby wave packet, often producing a cyclonic–anticyclonic potential vorticity (PV) dipole in its wake. The cyclonic PV cutoff in this PV debris field can be stirred back into the extratropical waveguide resulting in a cyclonically overturned PV contour. Unlike anticyclonic RWB morphologies, cyclonic RWB morphologies in the Southern Hemisphere upper troposphere are associated with Rossby wave packet generation and downstream development. Significance Statement: This work aims to determine how well the well-known and much-used two- and four-type classifications of Rossby wave breaking (RWB) represent the full range of morphologies and analyze the differences in their evolution. An objective clustering technique identifies important distinctions between high- and low-amplitude patterns and more meridional and zonal orientations that the two- (anticyclonic, cyclonic) and four- (P1, P2, LC1, and LC2) type classifications do not. Anticyclonic RWB leads to the decay of a Rossby wave packet, whereas cyclonic RWB generates another Rossby wave packet and downstream development. This fundamental difference in the evolution, together with differences in the physical processes shaping these morphologies, deepens our understanding of RWB in the Southern Hemisphere upper troposphere.

Synoptic weather typing is a common technique for categorising regional circulation patterns over a region. Defined and commonly used sets of synoptic weather types have the potential to simplify regional circulation on the weather timescale and create a common language between the often segregated weather and climate research communities. However, circulation classifications are potentially complicated over Australia owing to its position in both the tropics and extratropics. Here we present a set of 30 synoptic weather types over Australia based on clustering the circulation by the low-level wind field. We show that the resulting synoptic weather types represent recognisable weather patterns over the continent throughout the year. Importantly, they capture features in the extratropics together with characteristics associated with the tropical monsoon. A cursory analysis of their relationship with the modes of variability, high impact weather and renewable energy related variables show that this set of Australian synoptic weather types have utility in a wide range applications across disciplines within earth system sciences and beyond.

In the austral summer, parts of southeastern Australia are prone to heavy rainfall that causes major riverine flooding and fatalities. Easterly flow associated with an anticyclone in the Tasman Sea, large moisture transports from the Coral Sea, and upper tropospheric cyclonic disturbances all contribute to these heavy rainfall episodes. However, questions regarding their synoptic dynamics remain, including which of these ingredients are the most critical. These questions are addressed by comparing composite pressure and moisture fields of heavy rainfall days over selected regions with non‐heavy rainfall days that have a similar synoptic pattern. A synoptic climatology is constructed for this purpose by ‐means cluster analysis of 500 hPa geopotential height anomalies from the European Centre for Medium‐range Weather Forecasts Reanalysis v5, for all December to March days over a period of 40 years. Heavy rainfall days in the wettest clusters have negative 500 hPa geopotential height anomalies immediately west of the affected region that are stronger on average than those of non‐heavy rainfall days. Their accompanying distributions of surface pressure, precipitable water, and vertical motion are consistent with cyclonic baroclinic development and are preceded by anticyclonic Rossby wave breaking. Heavy rainfall days also show an increased frequency of blocking near E; however, this peaks 1–2 days after the onset of heavy rain. Regional rainfall in these clusters shows strong sensitivity to lower pressure immediately westward but little sensitivity to high pressure in the Tasman Sea until after the commencement of rain. A companion study using the same cluster analysis illustrated the link between anticyclonic Rossby wave breaking and heatwaves in southeastern Australia. These latest results highlight the upper cyclonic anomalies that often form on the equatorward flank of anticyclonic Rossby wave breaking as the key ingredient separating days with a favourable synoptic‐scale pattern of surface high pressure into those that rain heavily and those that do not.