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The physical climate storyline (PCS) approach is increasingly recognized by the physical climate research community as a tool to produce and communicate decision-relevant climate risk information. While PCS is generally understood as a single concept, different varieties of the approach are applied according to the aims and purposes of the PCS and the scientists that build them. To unpack this diversity of detail, this article gives an overview of key practices and assumptions of the PCS approach as developed by physical climate scientists, as well as their ties to similar approaches developed by the broader climate risk and adaptation research community. We first examine varieties of PCSs according to the length of the causal chain they explore, and the type of evidence used. We then describe how they incorporate counterfactual elements and the temporal perspective. Finally, we examine how value judgments are implicitly or explicitly included in the aims and construction of PCSs. We conclude the discussion by suggesting that the PCS approach can further mature in the way it incorporates the narrative element, in the way it incorporates value judgments, and in the way that the evidence chosen to build PCSs constrains what is considered plausible. [ABSTRACT FROM AUTHOR]

We apply a causal inference‐based framework to test and quantify previously suggested causal relationships between Northern Hemisphere blocking, upward wave‐activity fluxes and stratospheric polar vortex (SPV) variability using reanalysis data. We show that the influence of blocking on the polar vortex is entirely mediated by upward wave‐activity fluxes, as the classical view would suggest. However, the causal pathway is not completely straightforward. In contrast to the vortex‐weakening effect of European blocking, the vortex‐strengthening effect of west Pacific blocking on lower stratospheric wave‐activity fluxes is only partially mediated by upper tropospheric wave‐activity fluxes. In addition, only two‐thirds of the effect of upper tropospheric wave‐activity fluxes on polar vortex variability is mediated by lower stratospheric wave‐activity fluxes. We also show that sudden stratospheric warmings are not entirely explainable in terms of upward wave‐activity fluxes. These findings help clarify the pathways of influence between blocking and SPV variability. Plain Language Summary: Blocking has been thought to influence the wintertime stratospheric polar vortex (SPV) via its effect on upward propagating planetary waves. Yet there has been debate about the exact pathways of influence. In this study, we use reanalysis data to examine and quantify previously suggested causal relationships between Northern Hemisphere blocking, upper tropospheric wave‐activity fluxes, lower stratospheric wave‐activity fluxes, SPV variability, and sudden stratospheric warmings. By making the causal reasoning explicit in the statistical analysis we can quantify indirect (mediated) and direct pathways of influence. We confirm the classical view that upward wave‐activity fluxes determine the influence of blocking on the polar vortex. However, we show that the pathway is not the simple causal chain described above. For example, only two‐thirds of the effect of upper tropospheric wave‐activity fluxes on polar vortex variability is mediated by lower stratospheric wave‐activity fluxes. These non‐mediated pathways suggest that additional causal factors play a role in how blocking affects polar vortex variability and sudden stratospheric warmings. Key Points: The pathways of influence between European and west Pacific blocking and polar vortex variability are quantified using causal inferenceBoth upper tropospheric and lower stratospheric wave‐activity fluxes mediate these pathways, and both are needed for a full descriptionOur results suggest additional causal factors play a role in how blocking affects polar vortex variability and sudden stratospheric warmings [ABSTRACT FROM AUTHOR]

To study the forced variability of atmospheric circulation regimes, the use of model ensembles is often necessary for identifying statistically significant signals as the observed data constitute a small sample and are thus strongly affected by the noise associated with sampling uncertainty. However, the regime representation is itself affected by noise within the atmosphere, which can make it difficult to detect robust signals. To this end we employ a regularised k‐means clustering algorithm to better identify the signal in a model ensemble. The approach allows for the identification of six regimes for the wintertime Euro‐Atlantic sector and leads to more pronounced regime dynamics, compared to results without regularisation, both overall and on sub‐seasonal and interannual time‐scales. We find that sub‐seasonal variability in the regime occurrence rates is mainly explained by changes in the seasonal cycle of the mean climatology. On interannual time‐scales relations between the occurrence rates of the regimes and the El Niño Southern Oscillation (ENSO) are identified. The use of six regimes captures a more detailed response of the circulation to ENSO compared to the common use of four regimes. Predictable signals in occurrence rate on interannual time‐scales are found for the two zonal flow regimes, namely a regime consisting of a negative geopotential height anomaly over the Norwegian Sea and Scandinavia, and the positive phase of the NAO. The signal strength for these regimes is comparable between observations and model, in contrast to that of the NAO‐index where the signal strength in the observations is underestimated by a factor of 2 in the model. Our regime analysis suggests that this signal‐to‐noise problem for the NAO‐index is primarily related to those atmospheric flow patterns associated with the negative NAO‐index as we find poor predictability for the corresponding NAO− regime. [ABSTRACT FROM AUTHOR]

The philosophy of forecast verification is rather different between deterministic and probabilistic verification metrics: generally speaking, deterministic metrics measure differences, whereas probabilistic metrics assess reliability and sharpness of predictive distributions. This article considers the root‐mean‐square error (RMSE), which can be seen as a deterministic metric, and the probabilistic metric Continuous Ranked Probability Score (CRPS), and demonstrates that under certain conditions, the CRPS can be mathematically expressed in terms of the RMSE when these metrics are aggregated. One of the required conditions is the normality of distributions. The other condition is that, while the forecast ensemble need not be calibrated, any bias or over/underdispersion cannot depend on the forecast distribution itself. Under these conditions, the CRPS is a fraction of the RMSE, and this fraction depends only on the heteroscedasticity of the ensemble spread and the measures of calibration. The derived CRPS–RMSE relationship for the case of perfect ensemble reliability is tested on simulations of idealised two‐dimensional barotropic turbulence. Results suggest that the relationship holds approximately despite the normality condition not being met. [ABSTRACT FROM AUTHOR]

Atmospheric science relies on numerical models to simulate the complex multiscale nature of atmospheric variability, but our confidence in weather and climate predictions relies on theory and simplified models that describe scale interactions at a mechanistic level and can provide causal accounts of atmospheric behaviour. Global simulations at kilometre‐scale resolution are now feasible and offer new opportunities to the atmospheric science community for testing and expanding our understanding of climate variability and change. Taking full advantage of this new tool requires smart strategies for evaluating and analysing the output, especially as kilometre‐scale climate modelling will be limited to relatively short simulations with a rather small number of realizations. We here review some of the available tools for diagnosing and studying the dynamics of waves, coherent flows, and the interactions between them in terms of their ability to provide causal accounts of the behaviour seen in observations and in comprehensive simulation models. We describe their successes but also some of their limitations. The limitations are seen to be especially pronounced in the Tropics, where clouds, convection and atmospheric circulation are inextricably linked. The lack of a natural spatial truncation scale in the Tropics has given rise to many theoretical challenges, but it is for precisely this reason that the Tropics are where we might expect the largest gain from global kilometre‐scale models. [ABSTRACT FROM AUTHOR]

Ozone depletion led to a positive trend in the summertime Southern Annular Mode (SAM) during the last decades of the 20th century. During the present century, global warming (GW) is expected to contribute to a positive SAM trend while ozone recovery is expected to act in the opposite direction. Here, Southern Hemisphere (SH) circulation and regional precipitation change are studied with a methodology that separates the effects from GW and ozone depletion/recovery. Our results show that a "tug‐of‐war" between ozone and GW occurs in the summertime stratosphere, propagating to the troposphere where it is manifest in the SAM. However, at the regional scale this "tug‐of‐war" is not as relevant as the combined effects of other remote drivers of circulation change, which force different kinds of precipitation changes in the SH. For regional precipitation changes, the uncertainty in future circulation change is as important as the uncertainty in the GW level. Plain Language Summary: In the Southern Hemisphere, both ozone depletion and global warming have influenced climate in the past decades. Ozone recovery is expected to reverse the influence of ozone depletion, but the influence of global warming is expected to grow. Here, we investigate the combined effects of these anthropogenic forcings through the second half of the 20th century and the entire 21st century, quantifying their influence on stratospheric and tropospheric circulation and on regional precipitation changes. We find that the summertime stratosphere is affected by a "tug‐of‐war" between these two forcings, and that its effect propagates downward to the tropospheric circulation. On the other hand, summer precipitation in the land areas of the SH that are expected to experience large effects of climate change are not strongly affected by this "tug‐of‐war," but are instead influenced by the effects of both stratospheric and tropical tropospheric changes. Key Points: Long‐term changes in the delay of the breakdown of the Southern Hemisphere stratospheric polar vortex can be largely explained by a linear response to ozone‐depleting substances and to global warmingThe tug‐of‐war between ozone recovery and global warming manifests itself in the stratospheric vortex breakdown delay and propagates to the troposphereThe uncertainty in future changes in regional precipitation in the Southern Hemisphere is subject to the combined effects of the uncertainty in tropical warming and in the vortex breakdown delay [ABSTRACT FROM AUTHOR]

The future climate projections in the IPCC reports are visually communicated via maps showing the mean response of climate models to alternative scenarios of socio‐economic development. The presence of large changes is highlighted by stippling the maps where the mean climate response (the signal) is large compared to internal variability (the noise) and the response is robust, that is, consistent in sign, across the individual models. In addition, hatching is used to mark the regions with a small multi‐model mean change. This approach may fail to recognize the risk of large changes in regions where the uncertainty is large and the response is not robust. Here, we present a more informative diagnostic to support risk assessment that is obtained by quantifying the mean forced signal‐to‐noise ratio of the individual model responses, rather than the signal‐to‐noise ratio of the mean response. This enables us to identify regions where a large future change compared to year‐to‐year variability is plausible, regardless of whether the signal is robust across the ensemble. For mean precipitation changes, we find that the majority (58% in surface area) of the unmarked regions and a sizeable portion (19%) of the hatched regions from the AR5 projections hid climate change responses to the RCP8.5 scenario that are on average large compared to the year‐to‐year variability. Based on the newer CMIP6 ensemble, a considerable potential for large annual‐mean precipitation changes, despite the lack of a robust multi‐model projection, exists over 22% of the surface land area, particularly in Central America, northern South America (including the Amazon), Central and West Africa (including parts of the Sahel), and the Maritime Continent. [ABSTRACT FROM AUTHOR]

The simultaneous occurrence of extremely wet winters at multiple locations in the same region can contribute to widespread flooding and associated socio‐economic losses. However, the spatial extent of precipitation extremes (i.e., the area in which nearby locations experience precipitation extremes simultaneously) and its future changes are largely overlooked in climate assessments. Employing new multi‐thousand‐year climate model simulations, we show that under both 2.0 °C and 1.5 °C warming scenarios, wintertime total precipitation extreme extents would increase over about 80%–90% of the Northern Hemisphere extratropics (i.e., of the latitude band 28°–78°N). Stabilizing at 1.5 °C rather than 2.0 °C would reduce the average magnitude of the increase by 1.7–2 times. According to the climate model, the increased extents are caused by increases in precipitation intensity rather than changes in the spatial organization of the events. Relatively small percentage increases in precipitation intensities (e.g., by 4%) can drive disproportionately larger, by 1–2 orders of magnitude, growth in the spatial extents (by 93%). Plain Language Summary: One of the most impact‐relevant and studied effects of global warming is the intensification of precipitation extremes. When extremely wet winters occur simultaneously at multiple locations within the same region, their cumulative impacts can be particularly high and enhanced as a result of limited resources available to cope with simultaneous damages. Despite the impacts caused by widespread extremes, climate change studies have typically disregarded the spatial extension of the extremes. Here, based on new multi‐thousand‐year climate model simulations, we show that—over most of the Northern Hemisphere extratropics, that is, most of the latitude band 28°–78°N—the spatial footprint of total wintertime precipitation extremes is projected to largely widen in the future. This widening results from a warmer, and therefore moister, atmosphere that will intensify precipitation. Holding global warming to 1.5 °C rather than 2.0 °C, in line with the Paris Agreement, would be beneficial to society as it could limit the average increase in the extension over the Northern Hemisphere extratropics by up to a factor of 2. To develop better preparedness for such extreme events, stakeholders should consider that a small increase in precipitation intensities (for example, by 4%) could result in large (by 93%) increases in spatial extent. Key Points: The spatial extent of wintertime total precipitation extremes is projected to increase over most of the Northern Hemisphere in the futureChanges in the spatial organization, that is, dependence, of precipitation extremes only marginally affect the spatial extentsIncreased precipitation magnitudes can cause a disproportionate (even 20–90 times larger) increase in the precipitation extreme extents [ABSTRACT FROM AUTHOR]

The climate science community is challenged to adopt an actionable risk perspective, which is difficult to align with the traditional focus on model‐based probabilistic climate change projections. Event‐based storylines can provide a way out of this conundrum by putting emphasis on plausibility rather than probability. This links directly to common practices in disaster risk management using "stress‐testing" for emergency preparedness based on events that are conditional on specific and plausible assumptions. Event‐based storylines allow for conditional explanations, without full attribution of every causal factor, which is crucial when some aspects of the latter are complex and highly uncertain. Plain Language Summary: One of today's major challenges is how to use insights and information from climate sciences to inform decision‐making regarding managing risks from climate change, where weather and climate extremes represent a major component of climate‐related risk. So far, climate science has taken a probabilistic approach producing large model ensembles and exploring likely ranges, thereby neglecting low‐likelihood but potentially high‐impact events that pose significant risks to society. Event‐based storylines are emerging as an alternative way to explore future high‐impact events while taking into account aspects of vulnerability and exposure of the considered system with an emphasis on plausibility rather than probability. This concept links directly to common practices in disaster risk management using "stress‐testing" for emergency preparedness based on events that are conditional on specific, but plausible assumptions. When co‐developed by climate scientists and stakeholders, event‐based storylines can be informed by physical climate and impact modeling and can provide a useful way of communicating and assessing climate‐related risk in a specific decision‐making context. Key Points: Event‐based storylines are a way to communicate and assess climate risk taking into account aspects of vulnerability and exposureEvent‐based storylines focus on plausibility rather than probability when looking at high‐impact eventsEvent‐based storylines can provide climate information that feeds directly into a particular decision‐making context [ABSTRACT FROM AUTHOR]

Atmospheric circulation is often clustered in so‐called circulation regimes, which are persistent and recurrent patterns. For the Euro‐Atlantic sector in winter, most studies identify four regimes: the Atlantic Ridge, Scandinavian Blocking and the two phases of the North Atlantic Oscillation. These results are obtained by applying k‐means clustering to the first several empirical orthogonal functions (EOFs) of geopotential height data. Studying the observed circulation in reanalysis data, it is found that when the full field data are used for the k‐means cluster analysis instead of the EOFs, the optimal number of clusters is no longer four but six. The two extra regimes that are found are the opposites of the Atlantic Ridge and Scandinavian Blocking, meaning they have a low‐pressure area roughly where the original regimes have a high‐pressure area. This introduces an appealing symmetry in the clustering result. Incorporating a weak persistence constraint in the clustering procedure is found to lead to a longer duration of regimes, extending beyond the synoptic time‐scale, without changing their occurrence rates. This is in contrast to the commonly used application of a time‐filter to the data before the clustering is executed, which, while increasing the persistence, changes the occurrence rates of the regimes. We conclude that applying a persistence constraint within the clustering procedure is a better way of stabilizing the clustering results than low‐pass filtering the data. [ABSTRACT FROM AUTHOR]

In our discussion of environmental and ecological catastrophes or disasters resulting from extreme weather events, we unite disparate literatures, the biological and the physical. Our goal is to tie together biological understandings of extreme environmental events with physical understandings of extreme weather events into joint causal accounts. This requires fine‐grained descriptions, in both space and time, of the ecological, evolutionary, and biological moving parts of a system together with fine‐grained descriptions, also in both space and time, of the extreme weather events. We find that both the "storyline" approach to extreme event attribution and the probabilistic "risk‐based" approach have uses in such descriptions. However, the storyline approach is more readily aligned with the forensic approach to evidence that is prevalent in the ecological literature, which cultivates expert‐based rules of thumb, that is, heuristics, and detailed methods for analyzing causes and mechanisms. We introduce below a number of preliminary examples of such studies as instances of what could be pursued in the future in much more detail. [ABSTRACT FROM AUTHOR]

It is well known from modelling studies that surface topography influences the large‐scale atmospheric circulation and that several model biases are associated with incorrect representation of topography. The textbook explanation of topographic effects on large‐scale circulation appeals to the theoretical relationship between surface forcing and vortex stretching along trajectories in single‐layer models. The goal of this study is to design and use a simple diagnostic of the large‐scale forcing on the atmosphere when air is passing over topography, directly from atmospheric fields, based on this theoretical relationship. The study examines the interaction of the atmosphere with the North American Cordillera and samples the flow by means of trajectories during Northern Hemisphere winter. We detect a signal of topographic forcing in the atmospheric dataset, which, although much less distinct than in the theoretical relationship, nevertheless exhibits a number of expected properties. Namely, the signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive Pacific–North American index (PNA). Furthermore, a connection is found between an enhanced signal of topographic forcing downslope of the North American Cordillera and periods of more frequent downstream European blocking. [ABSTRACT FROM AUTHOR]

Barotropic variability plays an important role in a variety of extratropical atmospheric processes, such as annular modes, teleconnections, and baroclinic life cycles, which occur on a wide range of time‐scales. Extratropical dynamics is dominated by high‐frequency (periods shorter than 10 days) transient waves, which drive barotropic variability through baroclinic life cycle events. However, other types of waves (e.g. low‐frequency, with periods longer than 10 days, and stationary waves) also play an important role in shaping extratropical dynamics on various time‐scales. This study uses reanalysis data in the context of the zonal momentum budget to address the relative importance of stationary, low‐frequency and high‐frequency waves in driving barotropic variability at high (synoptic) and low (subseasonal‐to‐seasonal) frequencies both locally in storm‐track regions and in the zonal mean in both hemispheres. The analysis reveals that the eddy forcing of barotropic variability on synoptic time‐scales is dominated by the interaction between low‐frequency (and stationary) and high‐frequency waves, and not by high‐frequency self‐interactions. On longer (subseasonal‐to‐seasonal) time‐scales the picture is more complex, with increased importance of low‐frequency self‐interactions but still largely negligible high‐frequency self‐interactions. A better understanding of the mechanisms driving barotropic variability on subseasonal‐to‐seasonal time‐scales may help advancing predictability on these time‐scales. [ABSTRACT FROM AUTHOR]

The Southern Hemisphere zonal circulation manifests a downward influence of the stratosphere on the troposphere from late spring to early summer. However, the strength and timescale of the connection, given the stratospheric state, have not been explicitly quantified. Here, SH zonal wind reanalysis time series are analyzed with a methodology designed to detect the minimal set of statistical predictors of multiple interacting variables via conditional independence tests. Our results confirm from data that the variability of the stratospheric polar vortex is a predictor of the tropospheric eddy‐driven jet between September and January. The vortex variability explains about 40% of monthly mean jet variability at a lead time of 1 month and can entirely account for the observed jet persistence. Our statistical model can quantitatively connect the multidecadal trends observed in the vortex and jet during the satellite era. This shows how short‐term variability can help understand statistical links in long‐term changes. Plain Language Summary: In the Southern Hemisphere, the onset of summertime weather at midlatitudes is closely related to the location of the midlatitude jet stream. The springtime breakdown of the stratospheric polar vortex, a strong current of winds circling the pole higher up in the stratosphere, is known to influence the jet and its seasonal meridional migration. In this work we quantify this stratosphere‐troposphere connection using reanalysis data. We adopt a statistical technique that isolates direct connections between variables from the many indirect ones that would be detected using standard correlations. Our analysis reveals the confounding effect of vortex memory: it biases the understanding of the timescale and strength of the coupling studied with cross correlations. We quantify the direct influence of the vortex variability on the jet, which builds up in the days preceding the vortex breakdown. The resulting statistical model can explain the observed high persistence of jet anomalies during this time of year. It also can quantitatively link the multidecadal trends in vortex and jet strength, which are known to have been induced by ozone depletion in the late 20th century. This approach could help understanding forced changes from relationships of the internal variability. Key Points: Autocorrelation of stratospheric polar vortex variability inflates persistence of cross correlations with the tropospheric eddy‐driven jetA conditional analysis identifies the vortex as statistical predictor for 40% of monthly jet variability in spring/summerThe inferred statistical model explains the enhanced jet persistence and connects the vortex and jet trends observed in the satellite era [ABSTRACT FROM AUTHOR]

Several recent studies have suggested that the stratosphere can be a source of subseasonal‐to‐seasonal predictability of Southern Hemisphere circulation during the austral spring and early summer seasons, through its influence on the zonal‐mean eddy‐driven jet. We exploit the large sample size afforded by the hindcasts from the European Centre for Medium‐Range Weather Forecasts Integrated Forecast System to address a number of unanswered questions. It is shown that the picture of coherent seasonal variability of the coupled stratosphere‐troposphere system apparent from the reanalysis record during the spring/early summer period is robust to sampling uncertainty and that there is evidence of nonlinearity in the case of the most extreme variations. The effect of El Niño–Southern Oscillation on the eddy‐driven jet during this time of year is found to occur via the stratosphere, with no evidence of a direct tropospheric pathway. A simple two‐state statistical model of the stratospheric vortex is introduced to estimate the subseasonal‐to‐seasonal predictability associated with shifts of the seasonal cycle in the Southern Hemisphere extratropical atmosphere. This simple model, along with a more general model, is subsequently used to interpret skill scores associated with hindcasts made using the full seasonal forecast system. Together, the results provide evidence of tropospheric predictability on subseasonal‐to‐seasonal timescales during this time of year from at least as early as 1 August and show no evidence of a "signal‐to‐noise paradox" between the hindcasts and the reanalysis. Key Points: Southern Hemisphere midlatitude jet variations in spring and early summer are predictable several months aheadThis subseasonal‐to‐seasonal predictability of the Southern Hemisphere jet comes via the stratospheric polar vortexThe observed influence of ENSO on the jet during this time is via this stratospheric pathway [ABSTRACT FROM AUTHOR]

Future shifts of the austral midlatitude jet are subject to large uncertainties in climate model projections. Here we show that, in addition to other previously identified sources of intermodel uncertainty, changes in the timing of the stratospheric polar vortex breakdown modulate the austral jet response to greenhouse gas forcing during summertime (December–February). The relationship is such that a larger delay in vortex breakdown favors a more poleward jet shift, with an estimated 0.7–0.8° increase in jet shift per 10‐day delay in vortex breakdown. The causality of the link between the timing of the vortex breakdown and the tropospheric jet response is demonstrated through climate modeling experiments with imposed changes in the seasonality of the stratospheric polar vortex. The vortex response is estimated to account for about 30% of the intermodel variance in the shift of the summertime austral jet and about 45% of the mean jet shift. Key Points: Response of austral stratospheric polar vortex to CO2 forcing is highly uncertain in CMIP5Larger delay in vortex breakdown favors larger poleward midlatitude jet shift in austral summerVortex response accounts for about 30% of intermodel variance in austral summer jet shift and about 45% of the mean jet shift [ABSTRACT FROM AUTHOR]

Abstract: The impact of parametrized nonorographic gravity wave drag (NOGWD) on the stratosphere‐troposphere dynamical coupling in atmospheric models is relatively unexplored. Using the European Centre for Medium‐Range Weather Forecasts Integrated Forecast System, we find that changes in NOGWD strength have a substantial impact on the tropospheric eddy‐driven jet (EDJ) in both hemispheres, but the sense of the impact is opposite in the two hemispheres. In the Northern Hemisphere the impact occurs via changes in the amplitude and persistence of stratospheric anomalies. In the Southern Hemisphere it occurs instead via differences in the sensitivity of the EDJ to a given stratospheric anomaly, arising from changes in the seasonal cycle leading up to the polar vortex breakdown. Increasing NOGWD eliminates the springtime phase of the Southern Hemisphere tropospheric semiannual oscillation, resulting in a more equatorward annual‐mean EDJ and showing that the semiannual oscillation cannot be explained entirely from tropospheric mechanisms. [ABSTRACT FROM AUTHOR]

The relative contributions of resolved and parametrized surface drag towards balancing the atmospheric angular momentum flux convergence (AMFC) and their sensitivity to horizontal resolution and parametrization are investigated in an atmospheric model. This sensitivity can be difficult to elucidate in free-running climate models, in which the AMFC varies with changing climatologies and, as a result, the relative contributions of surface terms balancing the AMFC also vary. While the sensitivity question has previously been addressed using short-range forecasts, we demonstrate that a nudging framework is an effective method for constraining the AMFC. The Met Office Unified Model is integrated at three horizontal resolutions ranging from 130 ( N96) to 25 km ( N512), while relaxing the model's wind and temperature fields towards the ERA-Interim reanalysis within the altitude regions of maximum AMFC. This method is validated against short-range forecasts and good agreement is found. These experiments are then used to assess the fidelity of the exchange between parametrized and resolved orographic torques with changes in horizontal resolution. Although the parametrized orographic torque reduces substantially with increasing horizontal resolution, there is little change in resolved orographic torque over 20-50°N. The tendencies produced by the nudging routine indicate that the additional drag at lower horizontal resolution is excessive. When parametrized orographic blocking is removed at the coarsest of these resolutions, there is a lack of compensation, and even compensation of the opposite sense, by the boundary layer and resolved torques, which is particularly pronounced over 20-50°N. This study demonstrates that there is strong sensitivity in the behaviour of the resolved and parametrized surface drag over this region. [ABSTRACT FROM AUTHOR]

A recent intercomparison exercise proposed by the Working Group for Numerical Experimentation (WGNE) revealed that the parameterized, or unresolved, surface stress in weather forecast models is highly model-dependent, especially over orography. Models of comparable resolution differ over land by as much as 20% in zonal mean total subgrid surface stress ( τtot). The way τtot is partitioned between the different parameterizations is also model-dependent. In this study, we simulated in a particular model an increase in τtot comparable with the spread found in the WGNE intercomparison. This increase was simulated in two ways, namely by increasing independently the contributions to τtot of the turbulent orographic form drag scheme (TOFD) and of the orographic low-level blocking scheme (BLOCK). Increasing the parameterized orographic drag leads to significant changes in surface pressure, zonal wind and temperature in the Northern Hemisphere during winter both in 10 day weather forecasts and in seasonal integrations. However, the magnitude of these changes in circulation strongly depends on which scheme is modified. In 10 day forecasts, stronger changes are found when the TOFD stress is increased, while on seasonal time scales the effects are of comparable magnitude, although different in detail. At these time scales, the BLOCK scheme affects the lower stratosphere winds through changes in the resolved planetary waves which are associated with surface impacts, while the TOFD effects are mostly limited to the lower troposphere. The partitioning of τtot between the two schemes appears to play an important role at all time scales. [ABSTRACT FROM AUTHOR]

Abstract: The interannual variability of the stratospheric winter polar vortex is correlated with the phase of the quasi‐biennial oscillation (QBO) of tropical stratospheric winds. This dynamical coupling between high and low latitudes, often referred to as the Holton–Tan effect, has been the subject of numerous observational and modelling studies, yet important questions regarding its mechanism remain unanswered. In particular it remains unclear which vertical levels of the QBO exert the strongest influence on the winter polar vortex, and how QBO–vortex coupling interacts with the effects of other sources of atmospheric interannual variability such as the 11‐year solar cycle or the El Niño Southern Oscillation. As stratosphere‐resolving general circulation models begin to resolve the QBO and represent its teleconnections with other parts of the climate system, it seems timely to summarize what is currently known about the QBO's high‐latitude influence. In this review article, we offer a synthesis of the modelling and observational analyses of QBO–vortex coupling that have appeared in the literature, and update the observational record. [ABSTRACT FROM AUTHOR]