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.
Modeling Tropical Cyclones in a Changing Climate
Hurricanes and Health
Caleb Dresser, Satchit Balsari, and Jennifer Leaning
Hurricanes, also referred to as tropical cyclones or typhoons, are powerful storms that originate over warm ocean waters. Throughout history, these storms have had lasting impacts on societies around the world. High winds, rain, storm surges, and floods affect lives, land, and livelihoods and have a variety of effects on human health. The direct health impacts of hurricanes include drowning due to flooding and trauma resulting from storm surges, blown debris, and structural collapse. Systems for detection, forecasting, early warning, and communications can give populations time to make preparations before hurricane landfall. Evacuation, shelter use, and other preparedness efforts have reduced mortality from hurricanes in many parts of Asia and the Americas. Engineered defenses such as sea walls, flood barriers, and raised structures provide added protection in some settings. While effective in the medium term, such approaches are costly and require dedicated resources, and therefore they have not been implemented in many at-risk sites around the world. Indirect health impacts of hurricanes arise from damage to housing, electricity, water, and transportation infrastructure, and from effects on social supports, economies, and healthcare systems. Indirect health impacts can include infectious diseases, carbon monoxide poisoning, trauma sustained during cleanup, mental health effects, exacerbations of chronic disease, and increases in all-cause mortality. Indirect and long-term health consequences are poorly understood because dedicated study of specific impacts has occurred in only a handful of settings, and, given the diverse array of societies and geographies affected by hurricanes, it is unclear how generalizable the results of these studies may be. Policy makers face three interlinked challenges in protecting human health from hurricanes. First, climate change is leading to increased hazards in many locations by altering hurricane dynamics and contributing to sea-level rise. Second, patterns of intensifying coastal settlement and development are expected to increase population exposure. Third, unequal patterns of exposure and impact on specific populations will continue to raise issues of climate and environmental injustice. Situationally appropriate strategies to protect health from future storms will vary widely, as they must both address the locally relevant manifestations of hurricane hazards and adapt to the cultural and economic context of the affected population. In some areas, inexorable ocean encroachment may lead to consideration of managed retreat from high-risk coastlines; in others, the presence of very large coastal urban populations that cannot feasibly evacuate may lead to design and use of vertical shelters for temporary protection during storms. New ideas and programs are urgently needed in many settings to address hazards associated with extreme rainfall, rising seas on floodplains and low-lying islands, landslide risk in areas undergoing rapid deforestation, and structurally unsound housing in some urban settings. Policies to reduce greenhouse gas emissions will help reduce long-term risk from hurricanes and sea-level rise. Without concrete actions to address both hurricane hazards and population vulnerabiliy, the 21st century may be marked by increasingly dangerous hurricanes affecting growing coastal populations that will be left with few viable options for seeking safety.
The Global Climatology of Tropical Cyclones
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.