Is the forward motion of tropical cyclones in the Australian region slowing due to anthropogenic climate change?

Thomas Mortlock and Ryan Crompton.

As the Earth’s atmosphere warms, the atmospheric circulation changes. Understanding how tropical cyclone activity may change in response to this warming is no easy task, with recent studies showing considerable dispersion in projected changes in activity for the Australian region. For example, Knutson et al. (2015) projected a decrease in tropical cyclone activity, including Cat 4-5 storms, around northeast Australia. Earlier, in 2014, the IPCC Fifth Assessment Report (Reisinger et al. 2014) summarised the projected changes as “Tropical cyclones are projected to increase in intensity and stay similar or decrease in numbers and occur further south (low confidence)”.

Identifying anthropogenic climate change influences on observational records of tropical cyclone activity is also challenging. Reliable records are relatively short and contain high year-to-year variability. Most research effort has focused on identifying changes in frequency and intensity: Callaghan and Power (2010), for example, documented a long-term decline in numbers of Cat 3-5 events making landfall over eastern Australia. Other recent studies have begun to consider changes in the distribution of events and other characteristics, including their forward motion (referred to as the translation speed). Sharmila and Walsh (2018) showed that events in the Australian region may reach further south while Kossin (2018), reported on by Risk Frontiers in Briefing Note 370 in July this year, found a global slowing of translation speeds. Here we further discuss the findings of Kossin (2018) for the Australian region.

Anthropogenic warming may also cause a general weakening of summertime tropical circulation (Vecchi et al., 2006; Mann et al., 2017) and, because tropical cyclones are carried along within their ambient environmental wind, the translation speed of tropical cyclones may slow, thereby increasing the potential for flooding and longer duration sustained high wind speeds (Kossin 2018). Tropical Cyclone Debbie (Queensland, March 2017) and Hurricane Harvey (Texas, August 2017) are two recent examples of slow-moving events.

In addition to the reported global slowdown in tropical cyclone translation speeds, Kossin (2018) also analysed trends across various regions. While those for the Northern Hemisphere were strong, those for the Australian region, both over land and over water, were only marginally significant and exhibited high multi-annual variability.

Here we present an exploratory investigation of the extent to which changes in tropical cyclone translation speeds around Australia (Kossin, 2018) are driven by internal climate variability, in addition to any possible anthropogenic warming signal. The proxy for translation speeds is the ambient winds that control the movement of tropical cyclones. We begin with the tropical Indian Ocean, < 100 ° E (Fig. 1), where Kossin (2018) reported a -0.01 km/hr/yr trend between 1949 and 2016.

Chan and Gray (1981) suggested that winds between 500 and 700 mB are the most relevant measure of ambient winds that transport tropical cyclones. We extracted the 500 mB scalar wind speed monthly means (November to April – coinciding with our tropical cyclone season) from 1980/81 to 2017/18, using the NCEP-NCAR Reanalysis, for the region between 5 and 20 °S and 50 and 100 °E. (Prior to 1980, the homogeneity of the reanalysis record is questionable.)

We then compared the year-on-year scalar wind speeds (averaged within the analysis region) to the Pacific Decadal Oscillation (PDO) Index. The PDO is the leading principal component of North Pacific monthly sea surface temperature variability and can be seen as a long-lived (multi-decadal) ENSO-like pattern of Pacific climate variability. While the PDO is a Pacific-origin index, the tropical cyclone climatologies in Queensland, Northern Territory and Western Australia are principally influenced by Pacific ENSO variability, in addition to other regional climate indices such as the Indian Ocean Dipole and the Madden-Julian Oscillation.

composite mean scalar wind speeds in the Indian Ocean
Figure 1. Map of the Indian Ocean showing; composite mean scalar wind speeds (m/s) at 500 mB for Nov-Apr 2017/18, analysis area (5 – 20 °S, 50 – 100 °E); 100 ° meridian used in Kossin (2018); and, location of northwest WA.
mean scalar wind speeds between 1981 and 2018 and correlation during period 1981 to 2000
Figure 2. Comparison between year-on-year (Nov-Apr) mean scalar wind speeds at 500 mB within analysis box (Fig. 1), and the PDO, between 1981 and 2018 (A), and correlation between the same two variables during period 1981 – 2000 (B) and 2001 – 2018 (C).

Our results show a strong correlation between the ambient environmental winds in the tropical Indian Ocean (TIO) and the PDO (average of Nov-Apr PDO values for each year)during the period 1981 – 2000 (R = 0.54, p < 0.05, Fig. 2b), but this is much diminished during the period 2001 – 2018 (R = 0.23, p < 0.05, Fig. 2c). The two time-series in Fig. 2a show a change in the relationship between the variables occurred around the year 2000. They also show that wind speeds are consistently higher post-2000.

The PDO was in a sustained ‘warm’ phase (i.e. PDO positive, or El Niño–like) from approximately 1977 to 1999, after which it has experienced less coherent polarity (Fig. 3). Our analysis suggests that during this period, ambient winds (and by inference, tropical cyclone translation speeds) in the Indian Ocean were closely related to variability in the PDO. Post-2000, a weakening of the PDO signal coincides with a much-reduced level of correlation, and a jump to higher wind speeds.

It is well known that the PDO influences interdecadal variability of tropical cyclogenesis in northern Australia (Grant and Walsh, 2001). However, the importance of the PDO on cyclone translation speeds for this region remains unclear. Our brief analysis suggests PDO positive conditions suppress wind speeds in the upper atmosphere in the TIO and, by inference, reduce tropical cyclone translation speeds in this region. This is because during PDO positive (El Niño–like) conditions, sea surface temperature anomalies occur further east in the Pacific Ocean – causing the area of cyclogenesis to move eastwards away from Australia.

When the PDO signal becomes more La Niña to ENSO neutral-like (i.e. post-2000, Fig. 3), wind speeds in the TIO increase but become less correlated to the PDO Index. This suggests a more complex relationship between upper atmosphere winds in this region and other regional climate indices (like the Indian Ocean Dipole or Madden-Julien Oscillation), during multi-decadal periods where the PDO signal is not strong.

Further work is needed to fully explore these relationships, and to extend the analysis into the Pacific. What can be concluded at this juncture is that the role of internal climate variability needs also be considered when analysing tropical cyclone records.

Observed monthly values of the PDO Index 1900 to 2014
Figure 3. Observed monthly values of the PDO Index (1900 – 2014). Note the predominantly warm phase (PDO positive or El Niño-like) between approx. 1977 and 2000, and the break-down in sustained polarity thereafter. The PDO warm phase coincides with the period of strong correlation in Fig. 2 (prior to 2000). Source: Wikipedia (2018).


Callaghan, J. & Power, S. (2010). Variability and decline in the number of severe tropical cyclones making land-fall over eastern Australia since the late nineteenth century. Clim. Dyn., 37, 647-662.

Chan, J.C. and Gray, W.M. (1981). Tropical Cyclone Movement and Surrounding Flow Relationships. Mon. Weather Rev., 110, 1354-1374.

Grant, A.P. and Walsh, K.J.E. (2001). Interdecadal variability in north-east Australian tropical cyclone formation. Atmos. Sci. Let., 1530-261X.

Kossin, J.P. (2018). A global slowdown of tropical-cyclone translation speed. Nature, 558, 104-107.

Knutson, T.R. et al. (2015). Global projections of intense tropical cyclone activity for the late twenty-first century from dynamical downscaling of CMIP5/RCP4.5 scenarios. J. Clim., 28, 7203–7224.Mann, M. E. et al. (2017). Influence of anthropogenic climate change on planetary wave resonance and extreme weather events. Sci. Rep. 7, 19831.

Reisinger, A., R.L. Kitching, F. Chiew, L. Hughes, P.C.D. Newton, S.S. Schuster, A. Tait, and P. Whetton, 2014: Australasia. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1371-1438.

Sharmila, S & Walsh, K.J.E. (2018). Recent poleward shift of tropical cyclone formation linked to Hadley cell expansion. Nature, 8, 730-736.

Vecchi, G. A. et al. (2006). Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature 441, 73–76.

QuakeAUS 6.0

Risk Frontiers’ new Australian earthquake loss model is now available.

We are excited to announce the release of our new probabilistic earthquake loss model for Australia.

The updated model incorporates Geoscience Australia’s recent revision of the Australian Earthquake Catalogue and, for the first time, the inclusion of an active fault model.

The model also includes a number of updates incorporating the latest data and methodologies.

Estimated losses have generally decreased across the country due to the update of the historical earthquake catalogue. This effect is partly mitigated at longer return periods in regions where active faults have now been modelled.

Abrupt climate change in the Anthropocene

Thomas Mortlock and Paul Somerville

This briefing contains excerpts from a recently-published article in the journal Proceedings of the National Academy of Sciences (PNAS) by Will Steffen and colleagues. The paper has sparked recent media interest and scientific discussion on the possibility of abrupt climate change that lies outside ‘likely’ projections, by surpassing climate thresholds and instigation of positive feedback loops. It calls for stronger action on climate mitigation because of this risk. Will Steffen is Emeritus Professor at the Climate Change Institute at ANU, and a Councillor for the Climate Council, an Australian climate change communications organisation.

The following are some extracts from Steffen’s paper, followed by some comments on this work. The full article and associated references can be accessed here.

Steffen et al.’s article – in short

We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a “Hothouse Earth” pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene.

We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state.

Such action entails stewardship of the entire Earth System—biosphere, climate, and societies—and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values.

Our analysis suggests that the Earth System may be approaching a planetary threshold that could lock in a continuing rapid pathway toward much hotter conditions—Hothouse Earth. This pathway would be propelled by strong, intrinsic, biogeophysical feedbacks difficult to influence by human actions, a pathway that could not be reversed, steered, or substantially slowed.

Source: Steffen et al. 2018
Source: Steffen et al. 2018

Where such a threshold might be is uncertain, but it could be only decades ahead at a temperature rise of ∼2.0 °C above preindustrial, and thus, it could be within the range of the Paris Accord temperature targets. The impacts of a Hothouse Earth pathway on human societies would likely be massive, sometimes abrupt, and undoubtedly disruptive. Avoiding this threshold by creating a Stabilized Earth pathway can only be achieved and maintained by a coordinated, deliberate effort by human societies to manage our relationship with the rest of the Earth System, recognizing that humanity is an integral, interacting component of the system. Humanity is now facing the need for critical decisions and actions that could influence our future for centuries, if not millennia.


The idea of abrupt climate change and threshold events is well established and there is evidence in the sedimentary record that such events have occurred multiple times in the past. To provide some context, we are talking about the transition from glacial to interglacial type climate (or vice-versa) within a matter of decades. These thresholds are difficult to forecast but readily identifiable in hindsight. Once a threshold is passed, a feedback loop can develop that reinforces and amplifies the climate signal – and this is the scenario that Steffen et al. explore. However, it is important to highlight that they can equally lead to an abrupt climate signal that is opposite to the initial forcing.

A well-cited example of this is the ‘8.2 event’, where a warming trend led to a sudden decrease in atmospheric temperatures, most notably over the North Atlantic and Europe, around 8,200 years before present. One theory is that warming ocean temperatures in the Arctic led to sea ice melt, which freshened and warmed the surface ocean and inhibited the sinking of salty, cold water to the ocean floor. This mechanism is required to sustain the ocean’s thermohaline circulation, of which the Gulf Stream (which transports warm water to NW Europe) is the surface signal. The slowing or closing down of this mechanism around the Arctic may have led to a slowing or deviation of the Gulf Stream, and abruptly cooler air temperatures (on the order of 3 to 4 ° C) over NW Europe. Paleo-climatic evidence suggests this all happened in the space of 20 years. Similarly, today, there is a strong ice melt and positive temperature signal around the Arctic. The climate response is highly complex and difficult to predict.

In their paper, Steffen et al. also use the term ‘Anthropocene’. This is a somewhat politically-charged term proposed for the present geological epoch dating from the commencement of significant human impact on the Earth’s environment and ecosystems, including, but not limited to, anthropogenic climate change (Waters et al., 2016). The past 10,000 years or so is known as the Holocene (the present inter-glacial period), thus the ‘Anthropocene’ would be a sub-division of this. There are suggestions that the Anthropocene should start from the beginning of the Industrial Revolution, or even the detonation of the first Atomic Bomb.

However, the International Commission on Stratigraphy (ICS), which has the prerogative of naming geological epochs, does not concur. Almost coincident with the publication of Steffen’s paper, the ICS ratified the subdivision of the Holocene and renamed the Late Holocene as the Meghalayan Epoch, snubbing the term Anthropocene. According to the ICS, the Meghalayan started about 4,250 years ago with a mega-drought that caused the collapse of a number of civilisations in Egypt, the Middle East, India and China about 2,250 years BCE. The ICS objects that the Anthropocene does not arise from geology and is not associated with a “stratigraphic unit” (rock layer); it is based more on the future than the past; is more a part of human history than the immensely long history of Earth; and is a political statement, rather than a scientific one (The Australian, August 11, 2018).

As reported by Mark Maslin (Professor of Earth System Science at University College London) in The Conversation (August 9, 2018), the ICS’s decision is a blow to those pushing for tough action on climate change, and “has profound philosophical, social, economic and political implications”. Maslin says “there is a huge difference to the story of humanity if we are living in the Meghalayan Age that makes no mention of the human impact on the environment — or in the Anthropocene Epoch, which says human actions constitute a new force of nature. The Meghalayan Age says the present is just more of the same as the past. The Anthropocene rewrites the human story, highlighting the need for planetary stewardship.”

The call to arms for stronger mitigation on climate change is a positive one, because it is unlikely any level of planning or adaptation could cope with temperature changes (and associated hazards) of 3 – 4 ° C occurring over a couple of decades. However, inertia – in both the climate system and on a political level – may result in it being too little too late.


Colin N. Waters, Jan Zalasiewicz, Colin Summerhayes, Anthony D. Barnosky, Clément Poirier, Agnieszka Gałuszka, Alejandro Cearreta, Matt Edgeworth, Erle C. Ellis, Michael Ellis, Catherine Jeandel, Reinhold Leinfelder, J. R. McNeill, Daniel deB. Richter, Will Steffen, James Syvitski, Davor Vidas, Michael Wagreich, Mark Williams, An Zhisheng, Jacques Grinevald, Eric Odada, Naomi Oreskes, Alexander P. Wolfe (2016), The Anthropocene is functionally and stratigraphically distinct from the Holocene. Science, 351, 6269.

Lloyd, G. (2018). Will Steffen’s paper gets scientists hot under the collar. The Australian, August 11, 2018.

Maslin, Mark (2018). Anthropocene vs Meghalayan: why geologists are fighting over whether humans are a force of nature. Article published in The Conversation, August 9, 2018.

Steffen, Will, Johan Rockström, Katherine Richardson, Timothy M. Lenton, Carl Folke, Diana Liverman, Colin P. Summerhayes, Anthony D. Barnosky, Sarah E. Cornell, Michel Crucifix, Jonathan F. Donges, Ingo Fetzer, Steven J. Lade, Marten Scheffer, Ricarda Winkelmann, and Hans Joachim Schellnhuber (2018). Trajectories of the Earth System in the Anthropocene. Proceedings of the National Academy of Science, August 6, 2018. 201810141.

Risk Frontiers’ new earthquake model shows reduced losses for Australia

Valentina Koschatzky and Paul Somerville

We are excited to announce we have released our new probabilistic earthquake loss model for Australia, QuakeAUS 6.0. The updated model, developed by Dr Valentina Koschatzky with input from Risk Frontiers’ Chief Geoscientist, Dr Paul Somerville, incorporates Geoscience Australia’s recent revision of the Australian Earthquake Catalogue (Allen et al., 2017), which has more than halved the rate of earthquakes exceeding 4.5 in magnitude.  The main features of the new model are:

  • New Distributed Earthquake Source Model (based on RF analysis of the new GA catalogue – 2018)
  • Inclusion of an Active Fault Model
  • Updated Soil Classification (McPherson 2017)
  • Updated Soil Amplification Model (Campbell & Bozorgnia 2014)
  • Updated Variable Resolution GRid (VRG), Exposure & Market Portfolio (Gnaf 2018 + Nexis 9)
Distributed Earthquake Source Model

A new distributed earthquake source model was implemented using the revised Geoscience Australia earthquake catalogue from the National Seismic Hazard Assessment (NSHA18) project (Allen et al., 2017), which will be released by GA in September 2018.

Active Fault Model

The active fault model incorporates earthquakes on potentially active faults based on GA’s Neotectonic Feature Database (Clark, 2012).  These geologically identified rare and large prehistorical events are not represented in the short historical record of earthquakes in Australia.

Updated Soil Class and Soil Amplification Models

We implement the Australian Seismic Site Conditions MAP (ASSCM) released by GA in June 2017 in the calculation of site amplification. This is a significant revision and upgrade of the previous map published in 2007. The site amplification model has also been updated (Campbell & Bozorgnia, 2014).

Variable Resolution Grid

We implemented the latest GNAF (2018) and Nexis (2018 V.9) data in the design of an updated variable resolution grid (VRG) and market portfolio to best reflect the current property exposure across all lines of business.

Effects on Losses

Compared with the previous version of Risk Frontiers’ QuakeAUS model, losses have generally decreased across the country (average annual loss is 80% and the 200-year return period loss is 63% of former values on a testing national portfolio) due to the update of the historical earthquake catalogue. This effect is partly mitigated at longer return periods in regions where active faults have now been modelled. The changes in losses are not uniform spatially or temporally. Sydney, for example, shows a drastic reduction in losses at every return period, while the losses for Melbourne show a slight increment. In other areas such as Adelaide the losses are lower than in the previous model for short return periods, but that trend is reversed for return periods greater than 1,000 years


Allen, T., J. Griffin, M. Leonard, D. Clark and H. Ghasemi (2017). An updated National Seismic Hazard Assessment for Australia: Are we designing for the right earthquakes? Proceedings of the Annual Conference of the Australian Earthquake Engineering Society in Canberra, November 24-26, 2017.

Campbell, Kenneth & Bozorgnia, Yousef. (2014). NGA-West2 Ground Motion Model for the Average Horizontal Components of PGA, PGV, and 5% Damped Linear Acceleration Response Spectra. Earthquake Spectra. 30. 1087-1115.

McPherson, A. A. (2017). A Revised Seismic Site Conditions Map for Australia. Record 2017/XX. Geoscience Australia, Canberra. DOI

Clark, D. (2012). Neotectonic Features Database. Commonwealth of Australia (Geoscience Australia).


Flood levee influences on community preparedness: a paradox?

Australian Journal of Emergency Management. July 2018 edition.

Flood levees are a commonly used method of flood protection. Previous research has proposed the concept of the ‘levee paradox’ to describe the situation whereby the construction of levees leads to a lowered community awareness of the risks of flooding and increased development in the ‘protected’ area. The consequences of this are the risks of larger losses in less frequent but deeper floods when levees overtop or fail. This paper uses the recent history of flooding and levee construction to investigate the ‘levee paradox’ through a study of flood preparedness and floodplain development in Lismore, NSW.

Read more.