Cyclones and storms

Tropical cyclones and large storms are natural meteorological events that have significant environmental impacts. Such events typically bring high winds and large quantities of rain, which may cause storm tides, coastal erosion, flooding, landslides, vegetation die-off and other impacts. Damage may play out over different timescales; some storm elements have an immediate impact (e.g. wind breaking tree limbs), whereas others may be delayed (e.g. storm tides bringing more salty water into estuaries, which will affect ecosystems over time).

In natural environments, cyclones and storms can reshape the physical environment, and cause localised damage and disruption to habitats and species. But the effects can sometimes be positive; some species depend on extreme events to open new or reshaped environments to colonisation.

In human-modified environments such as our cities and towns, reshaping of the physical landscape and damage to structures or vegetation is typically seen as negative. Much time, effort and money are invested in reducing or eliminating the impacts of cyclones or storms.

Tropical cyclones

Tropical cyclones form over warm seas, typically when the sea surface temperature is greater than 26.5 °C. They may follow erratic paths over days or even weeks but lose strength and dissipate once over land. However, former cyclones may persist as low-pressure systems and track south, out of the tropics, and may re-form if they pass from land back to sea. Tropical cyclones are one of the costliest and most fatal weather disasters on our planet (Chu et al. 2020).

Climate change is affecting the frequency and intensity of tropical cyclones and storms (Chu et al. 2020). Current predictions are that tropical cyclones, particularly nonsevere cyclones, are likely to become less frequent. However, a greater proportion of cyclones are projected to be of high intensity over the coming decades, and to deliver more intense rainfall (Chu et al. 2020, CSIRO & BOM 2020, Pepler & Dowdy 2021). There is low confidence that some tropical cyclones may track further south (Bruyère et al. 2020, ESCC Hub 2020). (see the Climate chapter)

From January 2016 to December 2020, 35 tropical cyclones occurred in the Australian region, with 16 making landfall on mainland Australia or island territories (BOM 2021b). Notable among these are:

• severe tropical cyclone Veronica (2019), which caused about $2 billion of lost revenue due to damaged plant and port sites, and deaths of more than 2,000 cattle, when it tracked close to, but did not cross, the Pilbara coast (Paterson 2019)
• severe tropical cyclone Oma (2019), which tracked sufficiently close to the south-east Queensland coast that a cyclone watch was issued that included Brisbane for the first time in at least 20 years
• severe tropical cyclone Monica (2016), which was the strongest tropical cyclone recorded in the Australian region, and severe tropical cyclone Marcus (2018), which was the strongest cyclone to affect Darwin since tropical cyclone Tracy in 1974 (BOM 2019)
• severe tropical cyclone Debbie (2017; see case study: Severe tropical cyclone Debbie).

The impact of cyclones varies with their intensity, their forward-moving speed and the preceding conditions, but can include significant impacts on natural and altered environments. Strong and gusting winds frequently break or tip trees; break branches; and strip leaves, flowers and fruit from natural forests, woodlands and agricultural plantings. Such disturbances reduce productivity in both natural and agricultural settings, with knock-on effects on habitats and food resources for wildlife, and on human populations and economies. Disturbance can also trigger changed recruitment patterns in natural habitats, with weed species dominating recovering vegetation in some instances (Murphy & Metcalfe 2016). High winds also impact domestic buildings and infrastructure, either directly by blowing off roofing material or felling power poles, for example, or indirectly causing damage as trees fall onto buildings or across infrastructure, or as other windborne missiles damage structures.

In addition to high winds, the effects of cyclones are compounded by associated flooding rains, storm tides that cause coastal inundation and erosion, damage caused to infrastructure by timber and other debris, and, in the longer term, accumulation of vegetation debris that may subsequently pose a bushfire risk. The storm tides that typically accompany landfall of cyclones may also cause major disturbance to shallow and coastal areas (Figure 1). Beach erosion may be effectively permanent at planning timescales as a result of the slow natural accretion of sand, requiring a management response to restore beaches after cyclones (Mortlock et al. 2018). Although sea level rise itself is not an extreme event, it can exacerbate the impact of extreme events, such as tropical cyclones, severe storms and heavy rainfall in coastal regions.

Cyclones also damage coral and seagrass beds, wash fish and marine mammals ashore, and reshape coastal and intertidal communities such as mangrove forests. Cyclone Debbie was a slow-moving cyclone. Consequently, gale-force winds persisted near some of the reefs near the Whitsunday Islands, traditional land and sea Country of the Ngaro people, for up to 56 hours, resulting in an average loss of coral cover of 70% at 2 metres depth, and up to 98% loss in some areas (GBRMPA 2019).

Given that coral recovery from cyclone-derived wave action can take decades, and that coral damage can occur hundreds of kilometres from the track of a cyclone, climate change forecasts are predicting dramatic impacts on coral reef ecosystems (Puotinen et al. 2020). This modelling suggests that cyclones may become more prevalent in ecoregions where they are currently infrequent, including Shark Bay (Western Australia) and the southern and south-eastern Great Barrier Reef, and that regions such as north-western Australia may see a greater incidence of large and strong cyclones. These disturbances are in addition to increasing thermal stress and the impacts of sediment, nutrient and pesticide residues on freshwater flows (see the Coasts chapter).

East coast lows

Whereas tropical cyclones form over warm waters in the summer months, east coast lows are intense low-pressure mid-latitude cyclonic systems that occur off southern Queensland, New South Wales, eastern Victoria and north-eastern Tasmania, typically during autumn and winter, although they can happen at any time of year. Recent studies show an increase in east coast lows during warm months and a decrease in cold months (Pepler et al. 2016, Dowdy et al. 2019a).

Although east coast lows are a cause of severe weather and can cause flooding, they also make an important contribution to reservoir recharge and water security – east coast lows are responsible for 23% of the rainfall on the New South Wales eastern seaboard, and 40% of the heavy rain events (AdaptNSW 2016). The impacts of east coast lows are also delivered through extreme winds and occasionally associated tornadoes, and lightning. East coast lows are significant generators of waves and coastal storms; the direction and size of waves determine the extent to which they erode or deposit sands and sediments (OEM 2016).

Hail and convective storms

Severe thunderstorms may bring heavy rain, hail, strong winds and lightning strikes. When accompanied by hail more than 2 centimetres (cm) in diameter, these events are one of the costliest yet most common events to affect southern Australia (Allen & Allen 2016). Recent events include hailstorms in:

• Brisbane (2014; $1.05 billion insurance losses) (AIDR 2015)
• Sydney (2018; $1.35 billion) (Hussein & Langbein 2020)
• the Australian Capital Territory (see Figure 3), eastern Melbourne and parts of New South Wales (2020; $1.2 billion) (AIDR 2020).

Hail has significant impacts on agriculture. Large (more than 2 cm) and giant (more than 5 cm) hail can damage infrastructure and equipment, and large volumes of smaller hail can strip leaves from plants or cause surface imperfections on fruit. Severe hailstorms across New South Wales, South Australia and Victoria in November 2016 affected vineyards, almond and stone fruit crops, and 21,000 hectares of field crops (AIDR 2017).

There are also anecdotal reports of hail injuring wildlife, including birds and flying foxes. Netting used to reduce hail damage to fruit crops can also affect animals through problems with entanglement.

Bruyère et al. (2020) summarised predictions of hail size and incidence under climate change scenarios. The models are not certain but suggest a decrease in small hail and a slight increase in large hail. Extreme convective storms of the sort that are frequently accompanied by hail are likely to become more frequent. Indeed, records from the east coast show an increase in hailstorm hours and hail days over the past decades, and several record-breaking hail events have been recorded in recent years for Adelaide, Melbourne and Perth. National and international data suggest that the risk of damaging hail, for both infrastructure and agriculture, is likely to increase.

As human populations in affected areas increase, with a corresponding increased risk of high insurance losses, better understanding of hailstorms would benefit forecasters, industry and the public. Soderholm et al. (2017) reviewed 18 years of hail events in south-east Queensland and demonstrated that hailstorm activity is greater in El Niño years and lower in La Niña years (see the Climate chapter).

Analysis of radar data has helped to identify topographic features that increase thunderstorm initiation, and local areas that are more prone to hailstorms. This work has been developed across Australia and applied radar stations, including those supporting Adelaide, Brisbane, Canberra, Hobart, Melbourne, Perth and Sydney (Dowdy et al. 2020, Warren et al. 2020). Soderholm et al. (2017) observed that more research is required to understand the impact of multiyear drivers such as climate and land surface processes, and that there is a paucity of field observation in the areas most frequently impacted. WeatheX, an app developed by the Australian Research Council Centre of Excellence for Climate Extremes, goes some way to addressing this by encouraging citizen scientists to record and upload weather observations in a standardised manner to support weather and climate researchers.

In addition to the direct impacts of storms, there are also indirect effects such as thunderstorm asthma (see the Air Quality chapter). In spring and early summer when pollen grains that cause hayfever and asthma are present in the air, thunderstorm weather can rupture the grains into tiny pieces. These pieces are less likely to be trapped by the body’s filtering mechanisms and can thus be drawn deep into the lungs, triggering breathing problems. In the Melbourne epidemic thunderstorm asthma event in November 2016, there were 3,365 excess respiratory-related presentations at hospital emergency departments, and 476 additional hospital admissions above normal background rates (Thien et al. 2018). Only 28% had previous doctor-diagnosed asthma, and 10 people died. Epidemic thunderstorm asthma is a rare event, and only 1 verified death (in the United Kingdom in 2002) has previously been recorded (IGEM-Vic 2017). Thunderstorms are not a requirement for epidemic thunderstorm asthma events, as other convergence lines where 2 air bodies meet can cause similar effects, but thunderstorms act as ‘meteorological brooms’ that sweep up and concentrate pollen and other allergens (Bannister et al. 2020).