The atmospheric response to Arctic and Antarctic sea ice changes typical of the present day and coming decades is investigated using the Hadley Centre global climate model (HadGEM3). The response is ...diagnosed from ensemble simulations of the period 1979 to 2009 with observed and perturbed sea ice concentrations. The experimental design allows the impacts of ocean–atmosphere coupling and the background atmospheric state to be assessed. The modeled response can be very different to that inferred from statistical relationships, showing that the response cannot be easily diagnosed from observations. Reduced Arctic sea ice drives a local low pressure response in boreal summer and autumn. Increased Antarctic sea ice drives a poleward shift of the Southern Hemisphere midlatitude jet, especially in the cold season. Coupling enables surface temperature responses to spread to the ocean, amplifying the atmospheric response and revealing additional impacts including warming of the North Atlantic in response to reduced Arctic sea ice, with a northward shift of the Atlantic intertropical convergence zone and increased Sahel rainfall. The background state controls the sign of the North Atlantic Oscillation (NAO) response via the refraction of planetary waves. This could help to resolve differences in previous studies, and potentially provides an “emergent constraint” to narrow the uncertainties in the NAO response, highlighting the need for future multimodel coordinated experiments.
DEFINING SUDDEN STRATOSPHERIC WARMINGS Butler, Amy H.; Seidel, Dian J.; Hardiman, Steven C. ...
Bulletin of the American Meteorological Society,
11/2015, Letnik:
96, Številka:
11
Journal Article
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Sudden stratospheric warmings (SSWs) are large, rapid temperature rises in the winter polar stratosphere, occurring predominantly in the Northern Hemisphere. Major SSWs are also associated with a ...reversal of the climatological westerly zonal-mean zonal winds. Circulation anomalies associated with SSWs can descend into the troposphere with substantial surface weather impacts, such as wintertime extreme cold air outbreaks. After their discovery in 1952, SSWs were classified by the World Meteorological Organization. An examination of literature suggests that a single, original reference for an exact definition of SSWs is elusive, but in many references a definition involves the reversal of the meridional temperature gradient and, for major warmings, the reversal of the zonal circulation poleward of 60° latitude at 10 hPa.
Though versions of this definition are still commonly used to detect SSWs, the details of the definition and its implementation remain ambiguous. In addition, other SSW definitions have been used in the last few decades, resulting in inconsistent classification of SSW events. We seek to answer the questions: How has the SSW definition changed, and how sensitive is the detection of SSWs to the definition used? For what kind of analysis is a “standard” definition useful? We argue that a standard SSW definition is necessary for maintaining a consistent and robust metric to assess polar stratospheric wintertime variability in climate models and other statistical applications. To provide a basis for, and to encourage participation in, a communitywide discussion currently underway, we explore what criteria are important for a standard definition and propose possible ways to update the definition.
The Brewer–Dobson Circulation in CMIP6 Abalos, Marta; Calvo, Natalia; Benito-Barca, Samuel ...
Atmospheric chemistry and physics,
09/2021, Letnik:
21, Številka:
17
Journal Article
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The Brewer–Dobson circulation (BDC) is a key feature of the stratosphere that models need to accurately represent in order to simulate surface climate variability and change adequately. For the first ...time, the Climate Model Intercomparison Project includes in its phase 6 (CMIP6) a set of diagnostics that allow for careful evaluation of the BDC. Here, the BDC is evaluated against observations and reanalyses using historical simulations. CMIP6 results confirm the well-known inconsistency in the sign of BDC trends between observations and models in the middle and upper stratosphere. Nevertheless, the large uncertainty in the observational trend estimates opens the door to compatibility. In particular, when accounting for the limited sampling of the observations, model and observational trend error bars overlap in 40 % of the simulations with available output. The increasing CO2 simulations feature an acceleration of the BDC but reveal a large spread in the middle-to-upper stratospheric trends, possibly related to the parameterized gravity wave forcing. The very close connection between the shallow branch of the residual circulation and surface temperature is highlighted, which is absent in the deep branch. The trends in mean age of air are shown to be more robust throughout the stratosphere than those in the residual circulation.
This article analyses the annual mean vertical and latitudinal structure of the Brewer–Dobson circulation in the CMIP5 models. The strength of the tropical mass upwelling is found to increase at all ...altitudes throughout the stratosphere due to climate change. However, the width of the tropical upwelling region narrows below about 20 hPa, and widens above 20 hPa, suggesting different physical mechanisms may play a role in this change above and below 20 hPa. In the lower stratosphere, an equatorward shift in the stationary wave critical line allows waves to propagate further into the Tropics. However, in the upper stratosphere, where the behaviour is dominated by what happens during the winter, an increase in the extratropical zonal mean westerly jet leads to a reduced equatorward refraction of planetary waves. The seasonal cycle of the change in the Brewer–Dobson circulation is also considered, and differences are found in the latitudinal structure of the increased extratropical downwelling between the Northern and Southern Hemispheres in winter.
Long-range prediction and the stratosphere Scaife, Adam A; Baldwin, Mark P; Butler, Amy H ...
Atmospheric chemistry and physics,
02/2022, Letnik:
22, Številka:
4
Journal Article
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Over recent years there have been concomitant advances in the
development of stratosphere-resolving numerical models, our understanding of
stratosphere–troposphere interaction, and the extension of ...long-range
forecasts to explicitly include the stratosphere. These advances are now
allowing for new and improved capability in long-range prediction. We present an
overview of this development and show how the inclusion of the stratosphere
in forecast systems aids monthly, seasonal, and annual-to-decadal climate
predictions and multidecadal projections. We end with an outlook towards the
future and identify areas of improvement that could further benefit these
rapidly evolving predictions.
The Southern Ocean is a pivotal component of the global climate system yet it is poorly represented in climate models, with significant biases in upper-ocean temperatures, clouds and winds. Combining ...Atmospheric and Coupled Model Inter-comparison Project (AMIP5/CMIP5) simulations, with observations and equilibrium heat budget theory, we show that across the CMIP5 ensemble variations in sea surface temperature biases in the 40-60°S Southern Ocean are primarily caused by AMIP5 atmospheric model net surface flux bias variations, linked to cloud-related short-wave errors. Equilibration of the biases involves local coupled sea surface temperature bias feedbacks onto the surface heat flux components. In combination with wind feedbacks, these biases adversely modify upper-ocean thermal structure. Most AMIP5 atmospheric models that exhibit small net heat flux biases appear to achieve this through compensating errors. We demonstrate that targeted developments to cloud-related parameterisations provide a route to better represent the Southern Ocean in climate models and projections.
The Brewer-Dobson circulation inferred from ERA-Interim Seviour, William J. M.; Butchart, Neal; Hardiman, Steven C.
Quarterly journal of the Royal Meteorological Society,
April 2012 Part B, Letnik:
138, Številka:
665
Journal Article
Abstract
Skilful predictions of near-term climate extremes are key to a resilient society. However, standard methods of analysing seasonal forecasts are not optimised to identify the rarer and most ...impactful extremes. For example, standard tercile probability maps, used in real-time regional climate outlooks, failed to convey the extreme magnitude of summer 2022 Pakistan rainfall that was, in fact, widely predicted by seasonal forecasts. Here we argue that, in this case, a strong summer La Niña provided a window of opportunity to issue a much more confident forecast for extreme rainfall than average skill estimates would suggest. We explore ways of building forecast confidence via a physical understanding of dynamical mechanisms, perturbation experiments to isolate extreme drivers, and simple empirical relationships. We highlight the need for more detailed routine monitoring of forecasts, with improved tools, to identify regional climate extremes and hence utilise windows of opportunity to issue trustworthy and actionable early warnings.
The Yangtze region of South East China has experienced several extreme hot summer months in recent years. Such events can have devastating socio–economic impacts. We use a large ensemble of ...initialised climate simulations to assess the current chance of unprecedented hot summer months in the Yangtze River region. We find a 10% chance of an unprecedented hot summer month each year. Our simulations suggest that monthly mean temperatures up to 3 °C hotter than the current record are possible. The dynamics of these unprecedented extremes highlights the occurrence of a stationary atmospheric wave, the Silk Road Pattern, in a significant number of extreme hot events. We present evidence that this atmospheric wave is driven by variability in the Indian summer monsoon. Other extreme events are associated with a westward shift in the western North Pacific subtropical high. The most extreme simulated events exhibit combined characteristics of both the Silk Road Pattern and the shifted western North Pacific subtropical high.
Historical Simulations With HadGEM3‐GC3.1 for CMIP6 Andrews, Martin B.; Ridley, Jeff K.; Wood, Richard A. ...
Journal of advances in modeling earth systems,
June 2020, 2020-06-00, 20200601, 2020-06-01, Letnik:
12, Številka:
6
Journal Article
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We describe and evaluate historical simulations which use the third Hadley Centre Global Environment Model in the Global Coupled configuration 3.1 (HadGEM3‐GC3.1) model and which form part of the ...UK's contribution to the sixth Coupled Model Intercomparison Project, CMIP6. These simulations, run at two resolutions, respond to historically evolving forcings such as greenhouse gases, aerosols, solar irradiance, volcanic aerosols, land use, and ozone concentrations. We assess the response of the simulations to these historical forcings and compare against the observational record. This includes the evolution of global mean surface temperature, ocean heat content, sea ice extent, ice sheet mass balance, permafrost extent, snow cover, North Atlantic sea surface temperature and circulation, and decadal precipitation. We find that the simulated time evolution of global mean surface temperature broadly follows the observed record but with important quantitative differences which we find are most likely attributable to strong effective radiative forcing from anthropogenic aerosols and a weak pattern of sea surface temperature response in the low to middle latitudes to volcanic eruptions. We also find evidence that anthropogenic aerosol forcings play a role in driving the Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation, which are key features of the North Atlantic ocean. Overall, the model historical simulations show many features in common with the observed record over the period 1850–2014 and so provide a basis for future in‐depth study of recent climate change.
Plain Language Summary
Historical simulations, which successfully reproduce features of the observed climate from the end of the preindustrial period to the near‐present day, contribute to our understanding of the underlying mechanisms that drive or influence climate variability and climate change. The historical simulations described in this paper use the third Hadley Centre Global Environment Model in the Global Coupled configuration 3.1 model. These simulations form part of the UK's contribution to the sixth Coupled Model Intercomparison Project. We assess various aspects of the historical climate system in our simulations against observations. This includes the evolution of global mean surface temperature, ocean heat content, sea ice extent, ice sheet mass balance, permafrost extent, snow cover, North Atlantic sea surface temperature and circulation, and decadal precipitation. The key findings include (a) that the model global mean surface temperature broadly follows the observed record, with a few quantitative differences, and (b) that the ocean circulation and sea surface temperatures of the North Atlantic are likely influenced by historical forcings. In general, the simulations respond to historically evolving influences in a similar manner to the observed world. Therefore, these simulations contribute, as part of the wider CMIP6 multimodel effort, to the understanding of the causes of observed climate change since 1850.
Key Points
We describe and evaluate the UK's CMIP6 historical simulations
We identify drivers of the modeled global temperature evolution and compare with the observed record
We find that anthropogenic aerosol forcings influence the simulation of North Atlantic ocean variability
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