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The possibility that Arctic sea ice loss weakens mid-latitude westerlies, promoting more severe cold winters, has sparked more than a decade of scientific debate, with apparent support from observations but inconclusive modelling evidence. Here we show that sixteen models contributing to the Polar Amplification Model Intercomparison Project simulate a weakening of mid-latitude westerlies in response to projected Arctic sea ice loss. We develop an emergent constraint based on eddy feedback, which is 1.2 to 3 times too weak in the models, suggesting that the real-world weakening lies towards the higher end of the model simulations. Still, the modelled response to Arctic sea ice loss is weak: the North Atlantic Oscillation response is similar in magnitude and offsets the projected response to increased greenhouse gases, but would only account for around 10% of variations in individual years. We further find that relationships between Arctic sea ice and atmospheric circulation have weakened recently in observations and are no longer inconsistent with those in models. The degree to which Arctic sea ice decline influences the mid-latitude atmospheric circulation is widely debated. Here, the authors use a coordinated multi-model experiment to show that Arctic sea ice loss causes a weakening of the mid-latitude westerly winds, but the effect is overall small.

The most recent version of the Max Planck Institute for Meteorology atmospheric general circulation model, ECHAM5, is used to study the impact of changes in horizontal and vertical resolution on seasonal mean climate. In a series of Atmospheric Model Intercomparison Project (AMIP)-style experiments with resolutions ranging between T21L19 and T159L31, the systematic errors and convergence properties are assessed for two vertical resolutions. At low vertical resolution (L19) there is no evidence for convergence to a more realistic climate state for horizontal resolutions higher than T42. At higher vertical resolution (L31), on the other hand, the root-mean-square errors decrease monotonically with increasing horizontal resolution. Furthermore, except for T42, the L31 versions are superior to their L19 counterparts, and the improvements become more evident at increasingly higher horizontal resolutions. This applies, in particular, to the zonal mean climate state and to the stationary wave patterns in boreal winter. As in previous studies, increasing horizontal resolution leads to a warming of the troposphere, most prominently at midlatitudes, and to a poleward shift and intensification of the midlatitude westerlies. Increasing the vertical resolution has the opposite effect, almost independent of horizontal resolution. Whereas the atmosphere is colder at low and middle latitudes, it is warmer at high latitudes and close to the surface. In addition, increased vertical resolution results in a pronounced warming in the polar upper troposphere and lower stratosphere, where the cold bias is reduced by up to 50% compared to L19 simulations. Consistent with these temperature changes is a decrease and equatorward shift of the midlatitude westerlies. The substantial benefits in refining both horizontal and vertical resolution give some support to scaling arguments deduced from quasigeostrophic theory implying that horizontal and vertical resolution ought to be chosen consistently.

ICON‐A is the new icosahedral nonhydrostatic (ICON) atmospheric general circulation model in a configuration using the Max Planck Institute physics package, which originates from the ECHAM6 general circulation model, and has been adapted to account for the changed dynamical core framework. The coupling scheme between dynamics and physics employs a sequential updating by dynamics and physics, and a fixed sequence of the physical processes similar to ECHAM6. To allow a meaningful initial comparison between ICON‐A and the established ECHAM6‐LR model, a setup with similar, low resolution in terms of number of grid points and levels is chosen. The ICON‐A model is tuned on the base of the Atmospheric Model Intercomparison Project (AMIP) experiment aiming primarily at a well balanced top‐of atmosphere energy budget to make the model suitable for coupled climate and Earth system modeling. The tuning addresses first the moisture and cloud distribution to achieve the top‐of‐atmosphere energy balance, followed by the tuning of the parameterized dynamic drag aiming at reduced wind errors in the troposphere. The resulting version of ICON‐A has overall biases, which are comparable to those of ECHAM6. Problematic specific biases remain in the vertical distribution of clouds and in the stratospheric circulation, where the winter vortices are too weak. Biases in precipitable water and tropospheric temperature are, however, reduced compared to the ECHAM6. ICON‐A will serve as the basis of further development and as the atmosphere component to the coupled model, ICON‐Earth system model (ESM). Plain Language Summary ICON‐A is a new atmospheric model as needed for research on the general circulation of the atmosphere, or as atmospheric component in an Earth system model, as used in climate research. This article describes the construction of the atmospheric model, in particular how two major parts are coupled to each other: “dynamics” and “physics.” Dynamics is the part that solves the equations for the atmospheric motion, temperature, density, and concentrations of water vapor, cloud water, and cloud ice. Physics is the part that computes the changes in these fields related to processes like radiation, cloud condensation, or turbulence. These physical changes depend on the state of the atmosphere as computed by the dynamics, and the changes computed by physics force change in the dynamics. The article documents the details of this construction. Further, the article describes how the physics is tuned to obtain a good representation of the general circulation of the period 1979 to 1988 in comparison to observations. A more detailed evaluation of such simulations is presented in a companion article by Crueger et al. (2018, https://doi.org/10.1029/2017MS001233). Key Points Physics package for climate modeling is coupled to a nonhydrostatic dynamical core Tuning in five steps to obtain a balanced net radiation at top of atmosphere Overall biases of ICON‐A are comparable to ECHAM6.3, but circulation biases remain due to problems with parameterized drag

The Northern Hemisphere (NH) stratospheric signals of eastern Pacific (EP) and central Pacific (CP) El Niño events are investigated in stratosphere-resolving historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), together with the role of the stratosphere in driving tropospheric El Niño teleconnections in NH climate. The large number of events in each composite addresses some of the previously reported concerns related to the short observational record. The results shown here highlight the importance of the seasonal evolution of the NH stratospheric signals for understanding the EP and CP surface impacts. CMIP5 models show a significantly warmer and weaker polar vortex during EP El Niño. No significant polar stratospheric response is found during CP El Niño. This is a result of differences in the timing of the intensification of the climatological wavenumber 1 through constructive interference, which occurs earlier in EP than CP events, related to the anomalous enhancement and earlier development of the Pacific–North American pattern in EP events. The northward extension of the Aleutian low and the stronger and eastward location of the high over eastern Canada during EP events are key in explaining the differences in upward wave propagation between the two types of El Niño. The influence of the polar stratosphere in driving tropospheric anomalies in the North Atlantic European region is clearly shown during EP El Niño events, facilitated by the occurrence of stratospheric summer warmings, the frequency of which is significantly higher in this case. In contrast, CMIP5 results do not support a stratospheric pathway for a remote influence of CP events on NH teleconnections.

The effect of climate change on the Brewer-Dobson circulation and, in particular, the large-scale seasonal-mean transport between the troposphere and stratosphere is compared in a number of middle atmosphere general circulation models. All the models reproduce the observed upwelling across the tropical tropopause balanced by downwelling in the extra tropics, though the seasonal cycle in upwelling in some models is more semi-annual than annual. All the models also consistently predict an increase in the mass exchange rate in response to growing greenhouse gas concentrations, irrespective of whether or not the model includes interactive ozone chemistry. The mean trend is 11 kt s^sup -1^ year^sup -1^ or about 2% per decade but varies considerably between models. In all but one of the models the increase in mass exchange occurs throughout the year though, generally, the trend is larger during the boreal winter. On average, more than 60% of the mean mass fluxes can be explained by the EP-flux divergence using the downward control principle. Trends in the annual mean mass fluxes derived from the EP-flux divergence also explain about 60% of the trend in the troposphere-to-stratosphere mass exchange rate when averaged over all the models. Apart from two models the interannual variability in the downward control derived and actual mass fluxes were generally well correlated, for the annual mean.[PUBLICATION ABSTRACT]

Paraneoplastic syndromes (PS) are a heterogeneous group of diseases related to a neoplasm, indirectly dependent on it. Diagnosis and the treatment are often a challenge for clinicians, not least because the pathogenetic mechanisms are highly complex and not entirely known. Nonetheless, in most cases, PS precede the diagnosis of malignancies, thus their identification is particularly important in addressing physicians’ diagnostic work-up with regard to early cancer diagnosis. Among paraneoplastic syndromes, those of rheumatologic interest represent a large component. In this paper, we review the main rheumatic PS.

The quasi-biennial oscillation (QBO) in the equatorial zonal wind is an outstanding phenomenon of the atmosphere. The QBO is driven by a broad spectrum of waves excited in the tropical troposphere and modulates transport and mixing of chemical compounds in the whole middle atmosphere. Therefore, the simulation of the QBO in general circulation models and chemistry climate models is an important issue. Here, aspects of the climatology and forcing of a spontaneously occurring QBO in a middle-atmosphere model are evaluated, and its influence on the climate and variability of the tropical middle atmosphere is investigated. Westerly and easterly phases are considered separately, and 40-yr ECMWF Re-Analysis (ERA-40) data are used as a reference where appropriate. It is found that the simulated QBO is realistic in many details. Resolved large-scale waves are particularly important for the westerly phase, while parameterized gravity wave drag is more important for the easterly phase. Advective zonal wind tendencies are important for asymmetries between westerly and easterly phases, as found for the suppression of the easterly phase downward propagation. The simulation of the QBO improves the tropical upwelling and the atmospheric tape recorder compared to a model without a QBO. The semiannual oscillation is simulated realistically only if the QBO is represented. In sensitivity tests, it is found that the simulated QBO is strongly sensitive to changes in the gravity wave sources. The sensitivity to the tested range of horizontal resolutions is small. The stratospheric vertical resolution must be better than 1 km to simulate a realistic QBO.

The role of interannual variations in sea surface temperatures (SSTs) on the Northern Hemisphere winter polar stratospheric circulation is addressed by means of an ensemble of nine simulations performed with the middle atmosphere configuration of the ECHAM5 model forced with observed SSTs during the 20-yr period from 1980 to 1999. Results are compared to the 40-yr ECMWF Re-Analysis (ERA-40). Three aspects have been considered: the influence of the interannual SST variations on the climatological mean state, the response to El Niño–Southern Oscillation (ENSO) events, and the influence on systematic temperature changes. The strongest influence of SST variations has been found for the warm ENSO events considered. Namely, it has been found that the large-scale pattern associated with the extratropical tropospheric response to the ENSO phenomenon during northern winter enhances the forcing and the vertical propagation into the stratosphere of the quasi-stationary planetary waves emerging from the troposphere. This enhanced planetary wave disturbance thereafter results in a polar warming of a few degrees in the lower stratosphere in late winter and early spring. Consequently, the polar vortex is weakened, and the warm ENSO influence clearly emerges also in the zonal-mean flow. In contrast, the cold ENSO events considered do not appear to have an influence distinguishable from that of internal variability. It is also not straightforward to deduce the influence of the SSTs on the climatological mean state from the simulations performed, because the simulated internal variability of the stratosphere is large, a realistic feature. Moreover, the results of the ensemble of simulations provide weak to negligible evidence for the possibility that SST variations during the two decades considered are substantially contributing to changes in the polar temperature in the winter lower stratosphere.

This paper introduces the three-dimensional Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), which treats atmospheric dynamics, radiation, and chemistry interactively for the height range from the earth’s surface to the thermosphere (approximately 250 km). It is based on the latest version of the ECHAM atmospheric general circulation model of the Max Planck Institute for Meteorology in Hamburg, Germany, which is extended to include important radiative and dynamical processes of the upper atmosphere and is coupled to a chemistry module containing 48 compounds. The model is applied to study the effects of natural and anthropogenic climate forcing on the atmosphere, represented, on the one hand, by the 11-yr solar cycle and, on the other hand, by a doubling of the present-day concentration of carbon dioxide. The numerical experiments are analyzed with the focus on the effects on temperature and chemical composition in the mesopause region. Results include a temperature response to the solar cycle by 2 to 10 K in the mesopause region with the largest values occurring slightly above the summer mesopause. Ozone in the secondary maximum increases by up to 20% for solar maximum conditions. Changes in winds are in general small. In the case of a doubling of carbon dioxide the simulation indicates a cooling of the atmosphere everywhere above the tropopause but by the smallest values around the mesopause. It is shown that the temperature response up to the mesopause is strongly influenced by changes in dynamics. During Northern Hemisphere summer, dynamical processes alone would lead to an almost global warming of up to 3 K in the uppermost mesosphere.

The general circulation of the middle atmosphere, particularly of the mesosphere, is strongly dependent on the forcing arising from gravity wave processes. Their sources in the troposphere are both orographic and nonorographic, the latter being strongly intermittent. In climate models, the effects of gravity waves need to be parameterized, often assuming that their properties are constant. In this work we focus on the changes of the middle atmosphere due to the introduction of intermittency in a parameterization of nonorographic gravity waves, using a stochastic version of the Hines scheme. The stochastic approach is tailored to the diagnosed sensitivity of the model to this forcing, and peculiar changes emerge even if a relatively small amount of intermittency is prescribed. We analyze in detail the changes of the stratospheric dynamics in the tropical region and the global circulation of the middle atmosphere, when the stochastic parameterization is employed in place of the deterministic one. The mean state and variability of the model, realistic also in the default version, are preserved when stochasticity is added. Significant changes are observed in the mesosphere due to an enhanced poleward transport, leading to warming in the winter season. While there are some improvements of the mean state, the interannual variability is not significantly affected in the extratropics. The impacts on the simulated equatorial stratosphere are evident, as the stochasticity reduces the overall period of the quasi‐biennial oscillation but also leads to a net reduction of the variability between cycles. Plain Language Summary Present‐day climate models are not able to simulate small‐scale phenomena, which are represented by means of simplified descriptions. Among these processes, atmospheric gravity waves are important for the upper atmosphere, as they influence its structure, interacting with the mean flow. These small waves are often associated to convection, but their intermittent nature is not always present in climate models. Here we describe how this property can be included in the representation of gravity waves and which are the effects on the simulated upper atmosphere. Key Points Description of a stochastic modification of a spectral nonorographic gravity wave parameterization in a climate model The mean state of the upper stratosphere and mesosphere is modified by a stronger residual meridional circulation with the stochastic scheme The properties of the quasi-biennial oscillation change under a stochastic forcing, with a shorter period and reduced variability