The North Atlantic Waveguide and Downstream Impact Experiment

The North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) explored the impact of diabatic processes on disturbances of the jet stream and their influence on downstream high-impact weather through the deployment of four research aircraft, each with a sophisticated set of remote sensing and in situ instruments, and coordinated with a suite of ground-based measurements. A total of 49 research flights were performed, including, for the first time, coordinated flights of the four aircraft: the German High Altitude and Long Range Research Aircraft (HALO), the Deutsches Zentrum für Luft- und Raumfahrt (DLR) Dassault Falcon 20, the French Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE) Falcon 20, and the British Facility for Airborne Atmospheric Measurements (FAAM) BAe 146. The observation period from 17 September to 22 October 2016 with frequently occurring extratropical and tropical cyclones was ideal for investigating midlatitude weather over the North Atlantic. NAWDEX featured three sequences of upstream triggers of waveguide disturbances, as well as their dynamic interaction with the jet stream, subsequent development, and eventual downstream weather impact on Europe. Examples are presented to highlight the wealth of phenomena that were sampled, the comprehensive coverage, and the multifaceted nature of the measurements. This unique dataset forms the basis for future case studies and detailed evaluations of weather and climate predictions to improve our understanding of diabatic influences on Rossby waves and the downstream impacts of weather systems affecting Europe

Barotropic Regeneration of upper-level synoptic disturbances in different configurations of the zonal weather regime

International audienceBarotropic dynamics of upper-tropospheric midlatitude disturbances evolving in different configurations of the zonal weather regime (i.e., in different zonal-like large-scale flows) were studied using observational analyses and barotropic model experiments. The contraction stage of upper-level disturbances that follows their elongation stage leads to an increase of eddy kinetic energy that is called the barotropic regeneration process in this text. This barotropic mechanism is studied through notions of barotropic critical regions (BtCRs) and effective deformation that have been introduced in a previous paper. The effective deformation field is equal to the difference between the square of the large-scale deformation magnitude and the square of the large-scale vorticity. Regions where the effective deformation is positive correspond to regions where the large-scale flow tends to strongly stretch synoptic disturbances. A BtCR is an area separating two large-scale regions of positive effective deformation, one located upstream and on the south side of the jet and the other downstream and on the north side. Such a region presents a discontinuity in the orientation of the dilatation axes and is a potential area where the barotropic regeneration process may occur. Winter days presenting a zonal weather regime in the 40-yr ECMWF Re-Analysis dataset are decomposed, via a partitioning algorithm, into different configurations of the effective deformation field at 300 hPa. A six-cluster partition is obtained. Composite maps of the barotropic generation rate for each cluster exhibit a succession of negative and positive values on both sides of the BtCRs. It confirms statistically that the barotropic regeneration mechanism occurs preferentially about BtCRs. Numerical experiments using a forced barotropic model on the sphere are performed. Each experiment consists of adding a synoptic-scale perturbation to one of the zonal-like jet configurations found in observations, which is kept fixed with time. The combined effects of the effective deformation and nonlinearities are shown to be crucial to reproduce the barotropic regeneration process about BtCRs

Understanding the contrasting North Atlantic Oscillation anomalies of the winters of 2010 and 2014

International audienceThe present study investigates the two contrasting winters of 2010 and 2014 during which the North Atlantic Oscillation (NAO) was mainly negative and positive, respectively. In the North Pacific, contrasting anomalies were also present, with a straight zonal Pacific jet in 2010 and a strong poleward deviation of the Pacific jet in its exit region in 2014. Using reanalysis data sets and adopting a nonlinear initial-value approach with a quasi-geostrophic model, we show that the Pacific-North American anomalies are responsible for shaping synoptic wave trains propagating across North America. This in turn largely determines the nature of wave breaking and the synoptic eddy feedback onto the mean flow in the North Atlantic and finally the NAO phase. In such a proposed mechanism, synoptic wave activity forms the cornerstone of the dynamical relationship between the North Pacific and North Atlantic large-scale anomalies during the contrasting winters of 2010 and 2014

On the Efficiency of Baroclinic Eddy Growth and How It Reduces the North Pacific Storm-Track Intensity in Midwinter

International audienceThis study investigates the efficiency of baroclinic eddy growth in an effort to better understand the suppression of the North Pacific storm-track intensity in winter. The efficiency of baroclinic eddy growth depends on the magnitude and orientation of the vertical tilt of the eddy geopotential isolines. The eddy efficiency is maximized if the orientation of the vertical tilt creates an eddy heat flux that aligns with the mean baroclinicity (defined as minus the temperature gradient divided by a stratification parameter) and if the magnitude of the vertical tilt is neither too strong nor too weak. The eddy efficiency is, in contrast to most other eddy measures, independent of the eddy amplitude and thus useful for improving our mechanistic understanding of the effective eddy growth. During the midwinter suppression, the eddy efficiency is reduced north of 408N over a region upstream of the main storm track, and baroclinic growth is reduced despite a maximum in baroclinicity. Eulerian diagnostics and feature tracking suggest that the reduction in eddy efficiency is due to a stronger poleward tilt with height of eddies entering the Pacific through the northern seeding branch, which results in a more eastward-oriented eddy heat flux and a reduced alignment with the baroclinicity. The stronger poleward tilt with height is constrained by the eddy propagation direction, which is more equatorward when the subtropical jet moves equatorward in winter. In addition, the westward tilt with height is too strong. South of 408N, the eddy efficiency increases during midwinter but in a region far away from the main storm track

Tempêtes hivernales des latitudes tempérées : théories et prévisions

International audienceLes dépressions et anticyclones qui se succèdent aux latitudes tempérées sont des perturbations atmosphériques dont les périodes sont inférieures à la semaine et les échelles spatiales de l'ordre de quelques milliers de kilomètres. Ces perturbations sont plus intenses en hiver et responsables de la formation de phénomènes météo rologiques extrêmes comme les tempêtes. Elles jouent également un rôle déterminant dans la circulation générale de l'atmosphère et le climat. Leur dynamique a été étudiée de manière intensive depuis des décennies mais certains aspects sont encore mal connus, notamment leur évolution dans le cadre du changement climatique. Enfin, la prévision météorologique de ces perturba tions et, plus particulièrement, des tempêtes hivernales constitue plus que jamais un défi majeur

The Effect of the Madden–Julian Oscillation on the North Atlantic Oscillation Using Idealized Numerical Experiments

International audienceThe aim of the paper is to investigate the influence of the Madden-Julian oscillation (MJO) on the North Atlantic Oscillation (NAO) using a quasigeostrophic model on the sphere. A simplified forcing based on potential vorticity anomalies in the tropics is used to mimic the MJO. The idealized nature of our setup allows us to determine the distinct roles played by stationary and synoptic waves. This is done by means of several series of almost 10 000 short runs of 30 days. Ensemble averages and a streamfunction budget analysis are used to study the modifications of the flow induced by the MJO. We find that a stationary Rossby wave is excited in the tropics during MJO phase 3. The western part of the Pacific jet is displaced poleward, which modifies the transient eddy activity in that basin. These changes create a ridge south of Alaska, which favors equatorward propagation of synoptic waves and larger poleward eddy momentum fluxes from the eastern Pacific toward the Atlantic, increasing the frequency of occurrence of the positive NAO events. The situation is essentially reversed following phase 6 of the MJO and conducive to the negative phase of the NAO. For a realistic MJO forcing amplitude, we find increases in both NAO phases to be around 30%, in reasonable agreement with the observations given the model simplicity. Finally, we present a series of experiments to assess the relative importance of linear versus nonlinear effects

Storm track response to uniform global warming downstream of an idealized sea surface temperature front

International audienceThe future evolution of storm tracks, their intensity, shape, and location, is an important driver of regional precipitation changes, cyclone-associated weather extremes, and regional climate patterns. For the North Atlantic storm track, Coupled Model Intercomparison Project (CMIP) data indicate a tripole pattern of change under the RCP8.5 scenario. In this study, the tripole pattern is qualitatively reproduced by simulating the change of a storm track generated downstream of an idealized sea surface temperature (SST) front under uniform warming on an aquaplanet. The simulated tripole pattern consists of reduced eddy kinetic energy (EKE) upstream and equatorward of the SST front, extended and poleward shifted enhanced EKE downstream of the SST front, and a regionally reduced EKE increase at polar latitudes. In the absence of the idealized SST front, in contrast, the storm track exhibits a poleward shift but no tripole pattern. A detailed analysis of the EKE and eddy available potential energy (EAPE) sources and sinks reveals that the changes are locally driven by changes in baroclinic conversion rather than diabatic processes. However, globally the change in baroclinic conversion averages to zero; thus the observed global EAPE increase results from diabatic generation. In particular, resolved-scale condensation plus parameterized cloud physics dominate the global EAPE increase followed by longwave radiation. Amplified stationary waves affect EKE and EAPE advection, which contributes to the local EKE and EAPE minimum at polar latitudes. Feature-based tracking provides further insight into cyclone life cycle changes downstream of the SST front. Moderately deepening cyclones deepen less in a warmer climate, while strongly deepening cyclones deepen more. Similarly, the average cyclone becomes less intense in a warmer climate, while the extremely intense cyclones become more intense. Both results hold true for cyclones with genesis in the vicinity of the SST front and elsewhere. The mean cyclone lifetime decreases, while it increases for those cyclones downstream of the SST front. The mean poleward displacement between genesis and maximum intensity increases for the most intense cyclones, while averaged over all cyclones there is a mild reduction and the result depends on the definition of the displacement. Finally, the number of cyclones decreases by approximately 15 %. Aquaplanet simulations with a localized SST front thus provide an enriched picture of storm track dynamics and associated changes with warming

Effect of upper- and lower-level baroclinicity on the persistence of the leading mode of midlatitude jet variability

The sensitivity of the variability of an eddy-driven jet to the upper- and lower-level baroclinicity of the mean state is analyzed using a three-level quasi-geostrophic model on the sphere. The model is forced by a relaxation in temperature to a steady, zonally symmetric profile with varying latitude and intensity of the maximum baroclinicity. The leading EOF of the zonally- and vertically-averaged zonal wind is characterized by a meridional shift of the eddy-driven jet. While changes in the upper-level baroclinicity have no significant impact on the persistence of this leading EOF, an increase in lower-level baroclinicity leads to a reduced persistence. For small lower-level baroclinicity, the leading EOF follows a classical zonal index regime, for which the meridional excursions of the zonal wind anomalies are maintained by a strong positive eddy feedback. For strong lower-level baroclinicity, the jet enters a poleward propagation regime, for which the eddy forcing continuously acts to push the jet poleward and prevents its maintenance at a fixed latitude. The enhanced poleward propagation when the lower-level baroclinicity increases is interpreted as resulting from the broader and weaker potential vorticity gradient which enables the waves to propagate equatorward and facilitates the poleward migration of the critical latitude. Finally, the decrease in EOF1 persistence as the lower-level baroclinicity increases is shown not to result from the impact of changes in the mean climatological jet latitude