Worsening temperature extremes are among the most severe impacts of human-induced climate change. These extremes are often defined as rare events that exceed a specific percentile threshold within ...the distribution of daily maximum temperature. The percentile-based approach is chosen to follow regional and seasonal temperature variations so that extremes can occur globally and in all seasons, and frequently uses a running seasonal window to increase the sample size for the threshold calculation. Here, we show that running seasonal windows as used in many studies in recent years introduce a time-, region-, and dataset-depended bias that can lead to a striking underestimation of the expected extreme frequency. We reveal that this bias arises from artificially mixing the mean seasonal cycle into the extreme threshold and propose a simple solution that essentially eliminates it. We then use the corrected extreme frequency as reference to show that the bias also leads to an overestimation of future heatwave changes by as much as 30% in some regions. Based on these results we stress that running seasonal windows should not be used without correction for estimating extremes and their impacts.
Equilibrium climate sensitivity (ECS) is a widely accepted measure of Earth's susceptibility to radiative forcing. While ECS is often assumed to be constant to a first order of approximation, recent ...studies suggested that ECS might depend on the climate state. Here it is shown that the latest generation of climate models consistently exhibits an increasing ECS in warmer climates due to a strengthening of the water‐vapor feedback with increasing surface temperatures. The increasing ECS is replicated by a one‐dimensional radiative‐convective equilibrium model, which further shows that the enhanced water‐vapor feedback follows from the rising of the tropopause in a warming climate. This mechanism is potentially important for understanding both warm climates of Earth's past and projections of future high‐emission scenarios.
Key Points
Climate sensitivity to CO2 forcing is explored in warm climates
Equilibrium climate sensitivity systematically rises
The main cause is enhanced water vapor feedback
Thermodynamic control of anvil cloud amount Bony, Sandrine; Stevens, Bjorn; Coppin, David ...
Proceedings of the National Academy of Sciences - PNAS,
08/2016, Letnik:
113, Številka:
32
Journal Article
Recenzirano
Odprti dostop
General circulation models show that as the surface temperature increases, the convective anvil clouds shrink. By analyzing radiative–convective equilibrium simulations, we show that this behavior is ...rooted in basic energetic and thermodynamic properties of the atmosphere: As the climate warms, the clouds rise and remain at nearly the same temperature, but find themselves in a more stable atmosphere; this enhanced stability reduces the convective outflow in the upper troposphere and decreases the anvil cloud fraction. By warming the troposphere and increasing the upper-tropospheric stability, the clustering of deep convection also reduces the convective outflow and the anvil cloud fraction. When clouds are radiatively active, this robust coupling between temperature, high clouds, and circulation exerts a positive feedback on convective aggregation and favors the maintenance of strongly aggregated atmospheric states at high temperatures. This stability iris mechanism likely contributes to the narrowing of rainy areas as the climate warms. Whether or not it influences climate sensitivity requires further investigation.
Previous work showed that the poleward expansion of the annual-mean zonal-mean atmospheric circulation in response to global warming is strongly modulated by changes in clouds and their radiative ...heating of the surface and atmosphere. Here, a hierarchy and an ensemble of global climate models are used to study the circulation impact of changes in atmospheric cloud-radiative heating in the absence of changes in sea surface temperature (SST), which is referred to as the atmospheric pathway of the cloud-radiative impact. For the MPI-ESM model, the atmospheric pathway is responsible for about half of the total cloud-radiative impact, and in fact half of the total circulation response. Changes in atmospheric cloud-radiative heating are substantial in both the lower and upper troposphere. However, because SST is prescribed the atmospheric pathway is dominated by changes in upper-tropospheric cloud-radiative heating, which in large part results from the upward shift of high-level clouds. The poleward circulation expansion via the atmospheric pathway and changes in upper-tropospheric cloud-radiative heating are qualitatively robust across three global models, yet their magnitudes vary by a factor of 3. A substantial part of these magnitude differences are related to the upper-tropospheric radiative heating by high-level clouds in the present-day climate. A comparison with observations highlights the model deficits in representing the radiative heating by high-level clouds and indicates that reducing these deficits can contribute to improved predictions of regional climate change.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present baroclinic life‐cycle simulations with two versions of the atmosphere model ICON to understand how cloud‐radiative heating and cooling affect an idealized midlatitude cyclone. Both ...versions simulate the same cyclone when run without radiation, but disagree when cloud‐radiation‐interaction is taken into account. The radiative effects of clouds weaken the cyclone in ICON2.1 but strengthen it in ICON2.6. We attribute the disagreement to low‐level clouds, which in ICON2.1 are more abundant and show stronger radiative cooling of the boundary layer. We argue that radiative cooling from low‐level cloud tops weakens the cyclone by increasing boundary‐layer static stability, and that radiative cooling from high‐level cloud tops strengthens the cyclone by decreasing static stability in the upper troposphere and sharpening the tropopause. Our results indicate that clouds and the vertical distribution of their radiative heating and cooling can influence the dynamics of midlatitude cyclones.
Plain Language Summary
The interaction of tiny cloud particles with even smaller photons leads to cooling and heating of the atmosphere. We use computer simulations to show that this cloud‐radiative cooling and heating changes the intensity of a midlatitude low‐pressure system. Clouds near the surface lead to a less intense low‐pressure system, while clouds in the upper troposphere, about 10 km above the surface, strengthen the low‐pressure system.
Key Points
Radiative heating from high clouds leads to a stronger cyclone, radiative heating from low clouds leads to a weaker cyclone
Because of this tug‐of‐war, the overall effect of cloud‐radiative heating can be a stronger or weaker cyclone
The radiative impact of clouds can be understood from the effect on static stability
Climate models robustly project that global warming will lead to a poleward shift of the annual-mean zonal-mean extratropical jet streams. The magnitude of such shifts remains uncertain, however, and ...recent work has indicated a potentially important role of cloud radiative interactions. The model spread found in realistic simulations with interactive sea surface temperatures (SSTs) is captured in aquaplanet simulations with prescribed SSTs, because of which the latter setup is adapted here to study the impact of regional atmospheric cloud radiative changes on the jet position. Simulations with two CMIP5 models and prescribed regional cloud changes show that the rise of tropical high-level clouds and the upward and poleward movement of midlatitude high-level clouds lead to poleward jet shifts. High-latitude low-level cloud changes shift the jet poleward in one model but not in the other. The impact of clouds on the jet operates via the atmospheric radiative forcing that is created by the cloud changes and is qualitatively reproduced in a dry model, although the latter is too sensitive because of its simplified treatment of diabatic processes. The 10-model CMIP5 aquaplanet ensemble of global warming exhibits correlations between jet shifts, regional temperature changes, and regional cloud changes that are consistent with the prescribed cloud simulations. This provides evidence that the atmospheric radiative forcing from tropical and midlatitude high-level cloud changes contributes to model uncertainty in future jet shifts, in addition to the surface radiative forcing from extratropical cloud changes highlighted by previous studies.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We use the coupled atmosphere-ocean general circulation model ECHAM5/MPI-OM to investigate the transition from the present-day climate to a modern Snowball Earth, defined as the Earth in modern ...geography with complete sea-ice cover. Starting from the present-day climate and applying an abrupt decrease of total solar irradiance (TSI) we find that the critical TSI marking the Snowball Earth bifurcation point is between 91 and 94% of the present-day TSI. The Snowball Earth bifurcation point as well as the transition times are well reproduced by a zero-dimensional energy balance model of the mean ocean potential temperature. During the transition, the asymmetric distribution of continents between the Northern and Southern Hemisphere causes heat transports toward the more water-covered Southern Hemisphere. This is accompanied by an intensification of the southern Hadley cell and the wind-driven subtropical ocean cells by a factor of 4. If we set back TSI to 100% shortly before the transition to a modern Snowball Earth is completed, a narrow band of open equatorial water is sufficient for rapid melting. This implies that for 100% TSI the point of unstoppable glaciation separating partial from complete sea-ice cover is much closer to complete sea-ice cover than in classical energy balance models. Stable states can have no greater than 56.6% sea-ice cover implying that ECHAM5/MPI-OM does not exhibit stable states with near-complete sea-ice cover but open equatorial waters.
Geological and geochemical evidence can be interpreted as indicating strong hysteresis in global climate during the Neoproterozoic glacial events (∼630 Ma and ∼715 Ma). Standard climate theory only ...allows such strong hysteresis if global climate enters a fully‐glaciated “Snowball” state. However, the survival of photosynthetic, eukaryotic, marine species through these glaciations may indicate that there were large areas of open ocean. A previously‐proposed “Slushball” model for Neoproterozoic glaciations could easily explain the survival of these organisms because it has open ocean throughout the tropics, but there is only a small amount of hysteresis associated with the Slushball state. In this paper a new state of global climate, the “Jormungand” state, is proposed. In this state the ocean is very nearly globally ice‐covered, but a very small strip of the tropical ocean remains ice‐free. The low ice latitude of the Jormungand state is a consequence of the low albedo of snow‐free (bare) sea ice. If the ice latitude propagates into the subtropical desert zone, it can stabilize without collapsing to the equator because subtropical ice‐covered regions have a relatively low top‐of‐atmosphere albedo as a result of the exposure of bare sea ice and relatively lower cloud cover. Moreover, there is strong hysteresis associated with the Jormungand state as greenhouse gas levels are varied because of the high albedo contrast between regions of bare and snow covered sea ice. The Jormungand state is illustrated here in two different atmospheric global climate models and in the Budyko‐Sellers model. By offering a scenario that could explain both strong hysteresis in global climate and the survival of life, the Jormungand state represents a potential model for Neoproterozoic glaciations, although further study of this issue is needed.
Key Points
The Jormungand global climate state has a very narrow strip of open ocean
This state could serve as a model for Neoproterozoic glaciations
We show that this state exists in AGCMs and the Budyko‐Sellers model
In this review, we highlight the complementary relationship between simple and comprehensive models in addressing key scientific questions to describe Earth's atmospheric circulation. The systematic ...representation of models in steps, or hierarchies, connects our understanding from idealized systems to comprehensive models and ultimately the observed atmosphere. We define three interconnected principles that can be used to characterize the model hierarchies of the atmosphere. We explore the rich diversity within the governing equations in the dynamical hierarchy, the ability to isolate and understand atmospheric processes in the process hierarchy, and the importance of the physical domain and resolution in the hierarchy of scale. We center our discussion on the large‐scale circulation of the atmosphere and its interaction with clouds and convection, focusing on areas where simple models have had a significant impact. Our confidence in climate model projections of the future is based on our efforts to ground the climate predictions in fundamental physical understanding. This understanding is, in part, possible due to the hierarchies of idealized models that afford the simplicity required for understanding complex systems.
Plain Language Summary
Model hierarchies are fundamental to our understanding of the large‐scale circulation of Earth's atmosphere. They have played a critical role in forming and testing our ability to simulate and predict the natural variability of the atmosphere, such as the variations of the extratropical jet streams, and for exploring how the climate will respond to external forcing, such as increased carbon dioxide. In this review we discuss simple models that form the basis of our understanding of the atmosphere and how they connect to the comprehensive models used for climate prediction through the model hierarchies. We describe three principles that help organize the model hierarchies and discuss benchmark models that have been influential in understanding the large‐scale circulation in the midlatitudes, middle atmosphere, and tropics.
Key Points
Model hierarchies help address open research questions; we focus on how they have improved our understanding of atmospheric circulation
Model hierarchies are commonly referred to but remain poorly defined; we identify three principles to organize models into hierarchies
Key benchmark models of the atmospheric circulation are identified and connected to comprehensive models through model hierarchies
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