Papers by Theodore Shepherd
We have implemented a new stratospheric ozone model in the European Centre for Medium-Range Weath... more We have implemented a new stratospheric ozone model in the European Centre for Medium-Range Weather Forecasts (ECMWF) system and tested its performance for different timescales to assess the impact of stratospheric ozone on meteorological fields. We have used the new ozone model to provide prognostic ozone in medium-range and long-range (seasonal) experiments, showing the feasibility of this ozone scheme for a seamless numerical weather prediction (NWP) modelling approach. We find that the stratospheric ozone distribution provided by the new scheme in ECMWF forecast experiments is in very good agreement with observations, even for unusual meteorological conditions such as Arctic stratospheric sudden warmings (SSWs) and Antarctic polar vortex events like the vortex split of year 2002. To assess the impact it has on meteorological variables, we have performed experiments in which the prognostic ozone is interactive with radiation. The new scheme provides a realistic ozone field able to improve the description of the stratosphere in the ECMWF system, as we find clear reductions of biases in the stratospheric forecast temperature. The seasonality of the Southern Hemisphere polar vortex is also significantly improved when using the new ozone model. In medium-range simulations we also find improvements in high-latitude tropospheric winds during the SSW event considered in this study. In long-range simulations, the use of the new ozone model leads to an increase in the correlation of the winter North Atlantic Oscillation (NAO) index with respect to ERA-Interim and an increase in the signal-to-noise ratio over the North Atlantic sector. In our study we show that by improving the description of the stratospheric ozone in the ECMWF system, the stratosphere-troposphere coupling improves. This highlights the potential benefits of this new ozone model to exploit stratospheric sources of predictability and improve weather predictions over Europe on a range of timescales.
Current climate change uncertainty communication guidelines encourage risk taking
Normal and Anomalous Self-Similarity of Decaying Two-Dimensional Turbulence
1 Ozone and Climate : A Review of Interconnections Coordinating Lead Authors
EXECUTIVE SUMMARY 85 1.1 Introduction 87 1.1.1 Purpose and scope of this chapter 87 1.1.2 Ozone i... more EXECUTIVE SUMMARY 85 1.1 Introduction 87 1.1.1 Purpose and scope of this chapter 87 1.1.2 Ozone in the atmosphere and its role in climate 87 1.1.3 Chapter outline 93 1.2 Observed changes in the stratosphere 93 1.2.1 Observed changes in stratospheric ozone 93 1.2.2 Observed changes in ODSs 96 1.2.3 Observed changes in stratospheric aerosols, water vapour, methane and nitrous oxide 96 1.2.4 Observed temperature changes in the stratosphere 98 1.3 Stratospheric ozone and climate feedback processes 99 1.3.1 Impact of ODSs on stratospheric ozone 101 1.3.2 Impact of temperature changes on ozone chemistry 102 1.3.3 Impact of methane and water vapour changes on ozone chemistry 104 1.3.4 The role of transport for ozone changes 105 1.3.5 Stratosphere-troposphere dynamical coupling 107 1.3.6 Possible dynamical feedbacks of ozone changes 108 1.4 Past and future stratospheric ozone changes (attribution and prediction) 110 1.4.1 Current understanding of past ozone changes 110 1.4.2 The Montreal Pr...
In order to study the non-stationary dynamics of atmospheric circulation regimes, the use of mode... more In order to study the non-stationary dynamics of atmospheric circulation regimes, the use of model ensembles is often necessary. However, the regime representation within models exhibits substantial variability, making it difficult to detect robust signals. To this end we employ a regularised k-means clustering algorithm to prevent overfitting. The approach allows for the identification of six robust regimes and helps filter out noise in the transition probabilities and frequency of occurrence of the regimes. This leads to more pronounced regime dynamics, compared to results without regularisation, for the wintertime Euro-Atlantic sector. We find that sub-seasonal variability in the regime occurrence rates is mainly explained by the seasonal cycle of the mean climatology. On interannual timescales, 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 circulation response to ENSO compared to the common use of four regimes. In particular, whilst with four regimes El Niño has been identified with a more frequent negative phase of the North Atlantic Oscillation (NAO), with six regimes it is also associated with more frequent occurrence of a regime constituting a negative geopotential height anomaly over the Norwegian Sea and Scandinavia. Predictable interannual signals in occurrence rate are found for the two zonal flow regimes, namely the just-described Scandinavian regime and the positive phase of the NAO. The signal strength for these regimes is comparable for observations and model, in contrast to the NAO-index where the signal strength in the model is underestimated by a factor of two compared to observations. 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.
Adam A. ScaifeThomas SpangehlDavid R. FeredayUlrich CubaschUlrike Langematz • Hideharu AkiyoshiSlimane BekkiPeter BraesickeNeal ButchartMartyn P. Chipperfield • Andrew GettelmanSteven C. HardimanMartine MichouEugene RozanovTheodore G. Shepherd
Clim Dynam, 2011
Climate change is expected to increase winter rainfall and flooding in many extratropical regions... more Climate change is expected to increase winter rainfall and flooding in many extratropical regions as evaporation and precipitation rates increase, storms become more intense and storm tracks move polewards. Here, we show how changes in stratospheric circulation could play a significant role in future climate change in the extratropics through an additional shift in the tropospheric circulation. This shift in the circulation alters climate change in regional winter rainfall by an amount large enough to significantly alter regional climate change pro- jections. The changes are consistent with changes in stratospheric winds inducing a change in the baroclinic eddy growth rate across the depth of the troposphere. A change in mean wind structure and an equatorward shift of the tropospheric storm tracks relative to models with poor stratospheric resolution allows coupling with surface cli- mate. Using the Atlantic storm track as an example, we show how this can double the predicted increase in extreme winter rainfall over Western and Central Europe compared to other current climate projections.
Journal of Geophysical Research, 2003
Correlations between various chemical species simulated by the Canadian Middle Atmosphere Model, ... more Correlations between various chemical species simulated by the Canadian Middle Atmosphere Model, a general circulation model with fully interactive chemistry, are considered in order to investigate the general conditions under which compact correlations can be expected to form. At the same time, the analysis serves to validate the model. The results are compared to previous work on this subject, both from theoretical studies and from atmospheric measurements made from space and from aircraft. The results highlight the importance of having a data set with good spatial coverage when working with correlations and provide a background against which the compactness of correlations obtained from atmospheric measurements can be confirmed. It is shown that for long-lived species, distinct correlations are found in the model in the tropics, the extratropics, and the Antarctic winter vortex. Under these conditions, sparse sampling such as arises from occultation instruments is nevertheless suitable to define a chemical correlation within each region even from a single day of measurements, provided a sufficient range of mixing ratio values is sampled. In practice, this means a large vertical extent, though the requirements are less stringent at more poleward latitudes.
PNAS Nexus, 2022
There is a widely accepted gap between the production and use of climate information. It is also ... more There is a widely accepted gap between the production and use of climate information. It is also widely accepted that at least part of the reason for this situation lies in the challenge of bridging between what may be characterized as ‘‘top-down’’ approaches to climate information on the global scale, and local decision contexts, which necessarily take a ‘‘bottom-up’’ perspective, in which climate change is just one factor among many to consider. We here reflect on the insights provided in a different context—that of economics—by E.F. Schumacher in his celebrated book Small is Beautiful (1973), to see what light they might shed on this challenge, with a focus on climate-change science for adaptation. Schumacher asked how economics might look if it was structured “as if people mattered”. We ask the same question of climate-change science, and find many parallels. One is the need to grapple with the complexity of local situations, which can be addressed by expressing climate knowledg...
Current and future risks of unprecedented UK droughts
<p>The UK has experienced recurring periods of hydrological droughts in the... more <p>The UK has experienced recurring periods of hydrological droughts in the past and their frequency and severity are predicted to increase with climate change. However, quantifying the risks of extreme droughts is challenging given the short observational record, the multivariate nature of droughts and large internal variability of the climate system. We use EC-Earth time-slice large ensembles, which consist of 2000 years of data each for present day, 2°C and 3°C conditions, to drive the GR6J hydrological model at UK river catchments to obtain a large set of plausible droughts. Applying the UNSEEN (UNprecedented Simulation of Extreme Events using ENsembles) approach show an increasing chance of unprecedented dry summers with future warming and highlight the chance of an unprecedented drought with characteristics exceeding that of past severe droughts.</p><p>This study also aims to bridge the probabilistic UNSEEN approach with “bottom-up” storyline approaches. Physical climate storylines of preconditioned compound drought events are created by searching within the large ensemble for events resembling specific conditions that have led to past severe droughts and are relevant for water resources planning. This includes conditions such as 1) dry autumns followed by dry winters, 2) consecutive dry winters (both of which are relevant for slow-responding catchments), and 3) dry springs followed by dry summers (relevant for fast-responding catchments). The storylines can be used to understand the conditions leading to unprecedented droughts and the impacts of future droughts triggered by the same conditions. Unprecedented drought sequences and synthetic experiments conditioned on these storylines can be used to stress-test hydrological systems and inform decision-making.</p>
Geophysical Research Letters, 2021
Ozone depletion led to a positive trend in the summertime Southern Annular Mode (SAM) during the ... more 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.
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2019
Climate science seeks to make statements of confidence about what has happened, and what will hap... more Climate science seeks to make statements of confidence about what has happened, and what will happen (conditional on scenario). The approach is effective for the global, thermodynamic aspects of climate change, but is ineffective when it comes to aspects of climate change related to atmospheric circulation, which are highly uncertain. Yet, atmospheric circulation strongly mediates climate impacts at the regional scale. In this way, the confidence framework, which focuses on avoiding type 1 errors (false alarms), raises the prospect of committing type 2 errors (missed warnings). This has ethical implications. At the regional scale, however, where information on climate change has to be combined with many other factors affecting vulnerability and exposure—most of which are highly uncertain—the societally relevant question is not ‘What will happen?’ but rather ‘What is the impact of particular actions under an uncertain regional climate change?’ This reframing of the question can cut t...
Nature Climate Change, 2017
Biases in climate model simulations introduce biases in subsequent impact simulations. Therefore,... more Biases in climate model simulations introduce biases in subsequent impact simulations. Therefore, bias correction methods are operationally used to postprocess regional climate projections. However many problems have been identified, and some researchers question the very basis of the approach. Here we demonstrate that a typical cross-validation is unable to identify improper use of bias correction. Several examples show the limited ability of bias correction to correct and to downscale variability, and demonstrate that bias correction can cause implausible climate change signals. Bias correction cannot overcome major model errors, and naive application might result in ill-informed adaptation decisions. We conclude with a list of recommendations and suggestions for future research to reduce, post-process, and cope with climate model biases.
How do major SSWs develop in CCMVal and CMIP5 models?
Journal of the Atmospheric Sciences, 2006
The spatial structure and phase velocity of tropopause disturbances localized around the subpolar... more The spatial structure and phase velocity of tropopause disturbances localized around the subpolar jet in the Southern Hemisphere are investigated using 6-hourly European Centre for Medium-Range Weather Forecasts reanalysis data covering 15 yr (1979–93). The phase velocity and phase structure of the tropopause disturbances are in good agreement with those of an edge wave vertically trapped at the tropopause. However, the vertical distribution of the ratio of potential to kinetic energy exhibits maxima above and below the tropopause and a minimum around the tropopause, in contradiction to edge wave theory for which the ratio is unity throughout the troposphere and stratosphere. This difference in vertical structure between the observed tropopause disturbances and edge wave theory is attributed to the effects of a finite-depth tropopause together with the next-order corrections in Rossby number to quasigeostrophic dynamics.
Journal of Geophysical Research, 2006
The extended Canadian Middle Atmosphere Model is used to investigate the large-scale dynamics of ... more The extended Canadian Middle Atmosphere Model is used to investigate the large-scale dynamics of the mesosphere and lower thermosphere (MLT). It is shown that the 4-day wave is substantially amplified in southern polar winter in the presence of instabilities arising from strong vertical shears in the MLT zonal mean zonal winds brought about by parameterized nonorographic gravity wave drag. A weaker 4-day wave in northern polar winter is attributed to the weaker wind shears that result from weaker parameterized wave drag. The 2-day wave also exhibits a strong dependence on zonal wind shears, in agreement with previous modeling studies. In the equatorial upper mesosphere, the migrating diurnal tide provides most of the resolved westward wave forcing, which varies semiannually in conjunction with the tide itself; resolved forcing by eastward traveling disturbances is dominated by smaller scales. Nonmigrating tides and other planetary-scale waves play only a minor role in the zonal mean zonal momentum budget in the tropics at these heights. Resolved waves are shown to play a significant role in the zonal mean meridional momentum budget in the MLT, impacting significantly on gradient wind balance. Balance fails at low latitudes as a result of a strong Reynolds stress associated with the migrating diurnal tide, an effect which is most pronounced at equinox when the tide is strongest. Resolved and parameterized waves account for most of the imbalance at higher latitudes in summer. This results in the gradient wind underestimating the actual eastward wind reversal by up to 40%.
Journal of Geophysical Research: Atmospheres, 2002
This paper describes the energetics and zonal-mean state of the upward extension of the Canadian ... more This paper describes the energetics and zonal-mean state of the upward extension of the Canadian Middle Atmosphere Model, which extends from the ground to $210 km. The model includes realistic parameterizations of the major physical processes from the ground up to the lower thermosphere and exhibits a broad spectrum of geophysical variability. The rationale for the extended model is to examine the nature of the physical and dynamical processes in the mesosphere/lower thermosphere (MLT) region without the artificial effects of an imposed sponge layer which can modify the circulation in an unrealistic manner. The zonal-mean distributions of temperature and zonal wind are found to be in reasonable agreement with observations in most parts of the model domain below $150 km. Analysis of the global-average energy and momentum budgets reveals a balance between solar extreme ultraviolet heating and molecular diffusion and a thermally direct viscous meridional circulation above 130 km, with the viscosity coming from molecular diffusion and ion drag. Below 70 km, radiative equilibrium prevails in the global mean. In the MLT region between $70 and 120 km, many processes contribute to the global energy budget. At solstice, there is a thermally indirect meridional circulation driven mainly by parameterized nonorographic gravity-wave drag. This circulation provides a net global cooling of up to 25 K d À1 .
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Papers by Theodore Shepherd