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Article

Tropical cyclones (TCs) in their most intense expression (hurricanes or typhoons) are the main natural hazards known to humankind. The impressive socioeconomic consequences for countries dealing with TCs make our ability to model these organized convective structures a key issue to better understanding their nature and their interaction with the climate system. The destructive effects of TCs are mainly caused by three factors: strong wind, storm surge, and extreme precipitation. These TC-induced effects contribute to the annual worldwide damage of the order of billions of dollars and a death toll of thousands of people. Together with the development of tools able to simulate TCs, an accurate estimate of the impact of global warming on TC activity is thus not only of academic interest but also has important implications from a societal and economic point of view. The aim of this article is to provide a description of the TC modeling implementations available to investigate present and future climate scenarios. The two main approaches to dynamically model TCs under a climate perspective are through hurricane models and climate models. Both classes of models evaluate the numerical equations governing the climate system. A hurricane model is an objective tool, designed to simulate the behavior of a tropical cyclone representing the detailed time evolution of the vortex. Considering the global scale, a climate model can be an atmosphere (or ocean)-only general circulation model (GCM) or a fully coupled general circulation model (CGCM). To improve the ability of a climate model in representing small-scale features, instead of a general circulation model, a regional model (RM) can be used: this approach makes it possible to increase the spatial resolution, reducing the extension of the domain considered. In order to be able to represent the tropical cyclone structure, a climate model needs a sufficiently high horizontal resolution (of the order of tens of kilometers) leading to the usage of a great deal of computational power. Both tools can be used to evaluate TC behavior under different climate conditions. The added value of a climate model is its ability to represent the interplay of TCs with the climate system, namely two-way relationships with both atmosphere and ocean dynamics and thermodynamics. In particular, CGCMs are able to take into account the well-known feedback between atmosphere and ocean components induced by TC activity and also the TC–related remote impacts on large-scale atmospheric circulation. The science surrounding TCs has developed in parallel with the increasing complexity of the mentioned tools, both in terms of progress in explaining the physical processes involved and the increased availability of computational power. Many climate research groups around the world, dealing with such numerical models, continuously provide data sets to the scientific community, feeding this branch of climate change science.

Article

John Minnery and Iraphne Childs

Natural hazards governance varies across Australia for two critical reasons: first, the country’s large size and latitudinal range; and second, its divided federal government structure. The first feature—the magnitude and latitudinal spread—results in a number of climatic zones, from the tropical north, through the sub-tropics, to temperate southern zones and the arid central deserts. Consequently, state and local government jurisdictions must respond to different natural hazard types and variable seasonality. In addition, the El Niño-La Niña southern oscillation cycle has a strong impact. Flooding can occur throughout the continent and is the most frequent natural hazard and most extensive in scope, although extreme heat events cause the greatest number of fatalities. In summer, cyclones frequently occur in northern Australia and severe bushfires in the southeast and southwest. Hence, governance structures and disaster response mechanisms across Australia, while sharing many similarities, of necessity vary according to hazard type in different geographical locations. Climatological hazards dominate the range and occurrence of hazard events in Australia: floods, cyclones, storms, storm surge, drought, extreme heat events, and bushfire (but local landslips and earthquakes also occur). The second major reason for variation is that Australia has three formal levels of government (national, State, and local) with each having their own responsibilities and resources. The national government has constitutional powers only over matters of national importance or those which cross State boundaries. In terms of hazards governance, it can advise and support the states but is intimately involved only with major hazards. The six States have the principal constitutional responsibility for hazards planning, usually with a responsible State minister, and each can have a different approach. The strong vertical fiscal imbalance among the levels of government does give the national government powerful financial leverage. Local governments are the front-line hazards planning and management authorities, but because they represent local communities their approaches and capacities vary enormously. There are a number of ways in which the resultant potential for fragmentation is addressed. Regional groupings of local governments (usually assisted by the relevant state government) can work together. State governments collaborate through joint Ministerial meetings and policies. The intergovernmental Council of Australian Governments has produced a National Strategy for Disaster Resilience, which guides each state’s approach. Under these circumstances a clear national hierarchical chain of command is not possible, but serious efforts have been made to work collaboratively.

Article

Anshu Sharma and Sunny Kumar

India faces a very broad range of hazards due to its wide geoclimatic spread. This, combined with deep-rooted social, economic, physical, and institutional vulnerabilities, makes India one of the highest disaster-affected countries in the world. Natural hazards have gained higher visibility due to an increasing frequency and magnitude of their impact in recent decades, and efforts to manage disasters have been largely unable to keep pace with the growing incidences, scale, and complexities of disaster events. A number of mega events between 1990 and 2005, including earthquakes, cyclones, floods, and a tsunami, created momentum in decision making to look at disasters critically and to push for a shift from response to mitigation and preparedness. While efforts were put in place for appropriate legislation, institution building, and planning, these processes were long drawn out and time and resource intensive. It has taken years for the governance systems to begin showing results on the ground. While these efforts were being formulated, the changing face of disasters began to present new challenges. Between 2005 and 2015, a number of unprecedented events shook the system, underscoring the increasing variability and thus unpredictability of natural hazards as a new normal. Events in this period included cloudbursts and flash floods in the deserts, droughts in areas that are normally flood prone, abnormal hail and storm events, and floods of rare fury. To augment the shifting natural hazard landscape, urbanization and changing lifestyles have made facing disasters more challenging. For example, having entire cities run out of water is a situation that response systems are not geared to address. The future will be nothing like the past, with climate change adding to natural hazard complexities. Yet, the tools to manage hazards and reduce vulnerabilities are also evolving to unprecedented levels of sophistication. Science, people, and innovations will be valuable instruments for addressing the challenges of natural hazards in the times ahead.

Article

Tropical cyclones, also known as hurricanes or typhoons, are one of the most violent weather phenomena on the planet, posing significant threats to those living near or along coastlines where tropical cyclone–related impacts are most pronounced. About 80 tropical cyclones form annually, a rate that has been remarkably steady over the period of reliable historical record. Roughly two thirds of these storms form in the Northern Hemisphere from about June to November, while the remaining third form in the Southern Hemisphere typically during the months of November to May. Our understanding of the global and regional spatial patterns, the year-to-year variability, and temporal trends of these storms has improved considerably since the advent of meteorological satellites in the 1960s because of advances in both remote-sensing technology and operational analysis procedures. The well-recognized spatial patterns of tropical cyclone formation and tracks were laid out in a series of seminal papers in the late 1960s and 1970s and remain an accurate sketch even to this day. Concerning the year-to-year variability of tropical cyclone frequency, the El Niño Southern Oscillation (ENSO) has by far the most dominant influence across multiple ocean basins, so much so that it is typically used as the main predictor for statistical forecasts of seasonal tropical cyclone activity. ENSO has a modulating influence on atmospheric circulation patterns, even in regions remote to the tropical Pacific, which, in turn, can act to enhance or inhibit tropical cyclone formation. While the meteorological and climate community has come a long way in our understanding of the global and regional climatological features of tropical cyclones, as well as some aspects of the broader relationship between tropical cyclones and climate, we are still hindered by temporal inconsistencies within the historical record of storm data, particularly pertaining to tropical cyclone intensity. Despite recent efforts to homogenize the historical record using satellite-derived intensity data back to the early 1980s, the relatively short period makes it difficult to discern secular trends due to anthropogenic climate change from natural trends occurring on decadal to multidecadal time scales.