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Climate and Weather Extremes

Humans and ecosystems struggle to cope with extreme weather and climate conditions. Research into phenomena that are extreme in their rarity, intensity, or both aims to help societies better anticipate and manage the challenges of the most impactful future weather and climate events, be they weeks or decades from now. Extreme weather and events have catastrophic impact on humans and the environment, and their prediction is essential for planning and mitigation preparation.

In this Collection, we highlight research looking at extreme events across the globe and their prediction.

Editorial

Reviews and commentaries

This paper provides a summary of the Workshop on Sub-Seasonal to Seasonal (S2S) Predictability of Extreme Weather and Climate, held at Columbia University, December 6–7, 2016. The 2-day workshop was attended by over 100 people and took stock of recent developments in Sub-seasonal to Seasonal predictability, S2S extreme weather phenomena, and real world predictions and use of forecasts. Workshop motivations, new findings, and outstanding questions discussed are described.

Meeting Report | Open Access | | npj Climate and Atmospheric Science

Research articles

Wet and dry hydroclimatic extremes can pose severe stress to human societies under global warming. A new metric of cumulative stress due to hydroclimatic extremes is introduced, expressed in “equivalent reference stress years (ERSY)” i.e. the mean annual stress (e.g. potential for damage) in present climate conditions. 21st century climate projections show that, under the high-end RCP8.5 greenhouse gas scenario, by 2100, increases in wet and dry extremes add ~155 ERSY over global land areas (~125 for wet and ~30 for dry extremes), with wet hotspots over Asia, Eastern Africa and the Americas, and dry hotspots throughout Central and South America, Europe, West Africa and coastal Australia. Consideration of population exposure yields potential stress hotspots exceeding 400 added ERSY over Africa, North America, and Australia. The hydroclimatic stress is considerably reduced under the RCP2.6 scenario.

Article | Open Access | | npj Climate and Atmospheric Science

Low Arctic sea-ice extent in July is associated with reduced tornado activity in the United States. Robert J. Trapp from the University of Illinois, and Kimberly A. Hoogewind from Purdue University, analysed sea-ice and tornado data from 1990–2015 to examine a possible correlation between Arctic conditions and tornado activity in North America in summer. When Arctic sea-ice is low in July, tornado activity in the United States in July is also low. The decreased activity during this month is attributed to the presence of unfavorable atmospheric conditions for tornado formation over central North America. Such a robust link could be used in forecasts of tornado activity. Further research is needed to determine whether Arctic sea-ice loss is the driver of anomalous atmospheric conditions over North America and if so, how.

Article | Open Access | | npj Climate and Atmospheric Science

Pyrocumulonimbus—thunderstorms spawned from fire—inject 10× the mass of smoke particles into the upper atmosphere than once thought. When hot enough, wildfires can trigger convective updrafts, the depths of which extend well into the lower stratosphere. David Peterson and colleagues from the Naval Research Laboratory use satellite observations to quantify the impact of pyrocumulonimbus on troposphere-to-stratosphere exchange of smoke aerosols, focusing on five events that occurred in the Pacific Northwest in August 2017. The mass of smoke aerosols injected into the lower stratosphere is estimated to be 0.1–0.3 Tg, an order of magnitude larger than previous assessments, and roughly equal to that expected from a medium-sized volcanic eruption. With observed and projected increases in wildfires, any subsequent intrusions of smoke particles into the stratosphere could have considerable impacts on the global climate.

Article | Open Access | | npj Climate and Atmospheric Science

The proportion of North Pacific tropical cyclones—hurricanes and typhoons—leading to official storm warnings is rising, in line with an increased threat from severe storms, but the increasing frequency of warnings may be making them less effective. Nam-Young Kang at the Korea Meteorological Administration and James Elsner at Florida State University, USA, studied the changing intensity of tropical cyclones, the classification of the storms and the incidence of storm warnings. The classification systems used by different agencies are inconsistent and can cause confusion when communicating risks. With global warming, categories such as ‘very strong typhoon’ or ‘hurricane category 4-5’ now occur so frequently that their implications need to be reconsidered. The authors review issues that could be used to improve the validity of warning systems to prepare more effectively for catastrophic storms.

Article | Open Access | | npj Climate and Atmospheric Science

While the number of tornadoes has remained fairly static in the United States over the past 40 years, strong geographic contrasts are apparent. Tornadoes have the potential to cause severe damage, yet understanding their changes in time—particularly the impact of anthropogenic warming—has been hampered by sparse observations. Vittorio Gensini and Harold Brooks, from Northern Illinois University and the National Severe Storms Laboratory, respectively, therefore use a proxy of tornado activity—the significant tornado parameter, STP—to investigate regional trends since 1979. Tornado counts have increased in the Southeast, Midwest, and Northeast, as represented by a robust upward trend in the STP. In the southern Great Plains, by contrast, negative trends are apparent. These regional differences emphasise the need to consider geographic variability when assessing projected shifts in tornado hazards.

Article | Open Access | | npj Climate and Atmospheric Science

Cold spells in the Northern Hemisphere mid-latitudes are influenced by the stratospheric polar vortex in two different ways. Marlene Kretschmer from the Potsdam Institute for Climate Impact Research, Germany, and collaborators use cluster analysis to show there are two dominant patterns associated with a weak polar vortex. The first is zonally-symmetric, associated with absorbed upward wave activity fluxes and is linked to a negative North Atlantic Oscillation and cold spells over Eurasia. Whilst the second is zonally-asymmetric, associated with downward reflected waves over Canada and is linked to a negative Western Pacific Oscillation and cold spells over North America. This knowledge could allow better prediction of winter weather on sub- to seasonal scales.

Article | Open Access | | npj Climate and Atmospheric Science

Long-term drought severely reduces the size and richness of the soil seed bank beneath calcareous grassland. Sofía Basto at the Universidad Javeriana, Colombia, and collaborators worlwide, demonstrated that negative effects of drought are larger on seed banks than on vegetation by sampling soils after 14 years of experimental drought, and simulating future climate change scenarios. Although drought-sensitive species decreased both above and below ground, several species disappeared from the seed bank. This is of concern because seed banks help to maintain vegetation diversity, reduce extinction risk, and allow recovery after disturbance. Understanding the mechanisms responsible for this decline is crucial to predict the vulnerability of grasslands under the increase in drought frequency and intensity detected worldwide.

Article | Open Access | | npj Climate and Atmospheric Science

Satellite observations and Earth system model simulations reveal that marine heatwaves have increased in recent decades and will increase further in terms of frequency, intensity, duration and spatial extent.

Letter | | Nature

Will extremely wet winters like that of 2016/2017, which marked one of the wettest for the western United States, become more frequent in the warming climate? With the assistance of a column water vapor (CWV) contour-following diagnostic (known as wave activity budget), this study unraveled the unique characteristics of atmospheric rivers in terms of transport function and high hydrological cycling rate upon making landfalls. Under climate warming, the local CWV wave activity was found to increases by ~40% over the northeastern Pacific and western United States by the end of the 21st century, indicating lengthening and more frequent landfalling ARs with a consequence of a ~20% increase in the related hydrological extremes in the west coast. Therefore, the answer to the question is that yes, there will be wetter winters in the west.

Article | Open Access | | npj Climate and Atmospheric Science

A validation window that broadens with time is developed to provide seamless verification of extreme heat forecasts from days to weeks. Trent Ford (Southern Illinois University), Paul Dirmeyer (COLA-George Mason University), and David Benson (George Mason University) applied several skill metrics with a Poisson function weighting strategy to verify NOAA coupled forecast system model extreme heat forecasts over the United States. The model fails to persist heat waves as readily as is observed, and this inconsistency worsens with longer forecast lead times. Land surface–atmosphere interactions appear to influence heat wave persistence, but the model misrepresents these interactions, leading to poor skill in the Southwest and Midwest regions. The Poisson weighting strategy provides a versatile framework for verifying forecasts across subseasonal timescales. Continued verification and improvement of model forecasts contributes to reducing the risk of extreme climate events.

Article | Open Access | | npj Climate and Atmospheric Science