The influence of ENSO events on the stratospheric QBO in a multi-model ensemble

AbstractThe Quasi-Biennial Oscillation (QBO) and the El Niño–Southern Oscillation (ENSO) are two dominant modes of climate variability at the Equator. There exist observational evidences of mutual interactions between these two phenomena, but this possibility has not been widely studied using climate model simulations. In this work we assess how current models represent the ENSO/QBO relationship, in terms of the response of the amplitude and descent rate of stratospheric wind regimes, by analyzing atmosphere-only and ocean–atmosphere coupled simulations from a large multi-model ensemble. The annual cycle of the QBO descent rate is well represented in both coupled and uncoupled models. Previous results regarding the phase alignment of the QBO after the 1997/98 strong warm ENSO event are confirmed in a larger ensemble of uncoupled experiments. However, in general we find that a relatively high horizontal resolution is necessary to reproduce the observed modulation of the QBO descent rate under strong ENSO events, while the amplitude response is generally weak at any horizontal resolution. We argue that biases in the mean state and over-dependence on parameterized wave forcing undermine the realism of the simulated coupling between the ocean and the stratosphere in the tropics in current climate models. The modulation of the QBO by the ENSO in a high emission scenario consistently differs from that in the historical period, suggesting that this relationship is sensitive to changes in the large-scale circulation

Bidimensional Diagnostics, Variability, and Trends of Northern Hemisphere Blocking

AbstractIn this paper, Northern Hemisphere winter blocking is analyzed through the introduction of a set of new bidimensional diagnostics based on geopotential height that provide information about the occurrence, the duration, the intensity, and the wave breaking associated with the blocking. This analysis is performed with different reanalysis datasets in order to evaluate the sensitivity of the index and the diagnostics adopted. In this way, the authors are able to define a new category of blocking placed at low latitudes that is similar to midlatitude blocking in terms of the introduced diagnostics but is unable to divert or block the flow. Furthermore, over the Euro-Atlantic sector it is shown that it is possible to phenomenologically distinguish between high-latitude blocking occurring over Greenland, north of the jet stream and dominated by cyclonic wave breaking, and the traditional midlatitude blocking localized over Europe and driven by anticyclonic wave breaking. These latter events are uniformly present in a band ranging from the Azores up to Scandinavia. Interestingly, a similar distinction cannot be pointed out over the Pacific basin where the blocking activity is dominated by high-latitude blocking occurring over eastern Siberia. Finally, considering the large impact that blocking may have on the Northern Hemisphere, an analysis of the variability and the trend is carried out. This shows a significant increase of Atlantic low-latitude blocking frequency and an eastward displacement of the strongest blocking events over both the Atlantic and Pacific Oceans

Diagnostics of the tropical tropopause layer from in-situ observations and CCM data

A suite of diagnostics is applied to in-situ aircraft measurements and one Chemistry-Climate Model (CCM) data to characterize the vertical structure of the Tropical Tropopause Layer (TTL). The diagnostics are based on vertical tracer profiles and relative vertical tracer gradients, using tropopause-referenced coordinates, and tracer-tracer relationships in the tropical Upper Troposphere/Lower Stratosphere (UT/LS). Observations were obtained during four tropical campaigns performed from 1999 to 2006 with the research aircraft Geophysica and have been compared to the output of the ECHAM5/MESSy CCM. The model vertical resolution in the TTL (~500 m) allows for appropriate comparison with high-resolution aircraft observations and the diagnostics used highlight common TTL features between the model and the observational data. The analysis of the vertical profiles of water vapour, ozone, and nitrous oxide, in both the observations and the model, shows that concentration mixing ratios exhibit a strong gradient change across the tropical tropopause, due to the role of this latter as a transport barrier and that transition between the tropospheric and stratospheric regimes occurs within a finite layer. The use of relative vertical ozone and carbon monoxide gradients, in addition to the vertical profiles, helps to highlight the region where this transition occurs and allows to give an estimate of its thickness. The analysis of the CO-O3 and H2O-O3 scatter plots and of the Probability Distribution Function (PDF) of the H2O-O3 pair completes this picture as it allows to better distinguish tropospheric and stratospheric regimes that can be identified by their different chemical composition. The joint analysis and comparison of observed and modelled data allows to state that the model can represent the background TTL structure and its seasonal variability rather accurately. The model estimate of the thickness of the interface region between tropospheric and stratospheric regimes agrees well with average values inferred from observations. On the other hand, the measurements can be influenced by regional scale variability, local transport processes as well as deep convection, that can not be captured by the model

Diagnostics of the tropical tropopause layer from in-situ observations and CCM data

A suite of diagnostics is applied to in-situ aircraft measurements and one Chemistry-Climate Model (CCM) data to characterize the vertical structure of the Tropical Tropopause Layer (TTL). The diagnostics are based on the vertical tracers profiles, relative vertical tracers gradients, and tracer-tracer relationships in the tropical Upper Troposphere/Lower Stratosphere (UT/LS), using tropopause coordinates. Observations come from the four tropical campaigns performed from 1998 to 2006 with the research aircraft Geophysica and have been directly compared to the output of the ECHAM5/MESSy CCM. The model vertical resolution in the TTL allows for appropriate comparison with high-resolution aircraft observations and the diagnostics used highlight common TTL features between the model and the observational data. The analysis of the vertical profiles of water vapour, ozone, and nitrous oxide, in both the observations and the model, shows that concentration mixing ratios exhibit a strong gradient change across the tropical tropopause, due to the role of this latter as a transport barrier and that transition between the tropospheric and stratospheric regimes occurs within a finite layer. The use of relative vertical ozone gradients, in addition to the vertical profiles, helps to highlight the region where this transition occurs and allows to give an estimate of its thickness. The analysis of the CO-O3 and H2O-O3 scatter plots and of the Probability Distribution Function (PDF) of the H2O-O3 pair completes this picture as it allows to better distinguish tropospheric and stratospheric regimes that can be identified, first, by their differing chemical composition. The joint analysis and comparison of observed and modelled data allows us to evaluate the capability of the model in reproducing the observed vertical structure of the TTL and its variability, and also to assess whether observations from particular regions on a monthly timescale can be representative of the fine scale mean structure of the Tropical Tropopause Layer

Impact of deep convection in the tropical tropopause layer in West Africa: in-situ observations and mesoscale modelling

We present the analysis of the impact of convection on the composition of the tropical tropopause layer region (TTL) in West-Africa during the AMMA-SCOUT campaign. Geophysica M55 aircraft observations of water vapor, ozone, aerosol and CO2 show perturbed values at altitudes ranging from 14 km to 17 km (above the main convective outflow) and satellite data indicates that air detrainment is likely originated from convective cloud east of the flight. Simulations of the BOLAM mesoscale model, nudged with infrared radiance temperatures, are used to estimate the convective impact in the upper troposphere and to assess the fraction of air processed by convection. The analysis shows that BOLAM correctly reproduces the location and the vertical structure of convective outflow. Model-aided analysis indicates that in the outflow of a large convective system, deep convection can largely modify chemical composition and aerosol distribution up to the tropical tropopause. Model analysis also shows that, on average, deep convection occurring in the entire Sahelian transect (up to 2000 km E of the measurement area) has a non negligible role in determining TTL composition

Comparison of Antarctic polar stratospheric cloud observations by ground-based and space-borne lidar and relevance for chemistry–climate models

Abstract. A comparison of polar stratospheric cloud (PSC) occurrence from 2006 to 2010 is presented, as observed from the ground-based lidar station at McMurdo (Antarctica) and by the satellite-borne CALIOP lidar (Cloud-Aerosol Lidar with Orthogonal Polarization) measuring over McMurdo. McMurdo (Antarctica) is one of the primary lidar stations for aerosol measurements of the NDACC (Network for Detection of Atmospheric Climate Change). The ground-based observations have been classified with an algorithm derived from the recent v2 detection and classification scheme, used to classify PSCs observed by CALIOP. A statistical approach has been used to compare ground-based and satellite-based observations, since point-to-point comparison is often troublesome due to the intrinsic differences in the observation geometries and the imperfect overlap of the observed areas. A comparison of space-borne lidar observations and a selection of simulations obtained from chemistry–climate models (CCMs) has been made by using a series of quantitative diagnostics based on the statistical occurrence of different PSC types. The distribution of PSCs over Antarctica, calculated by several CCMVal-2 and CCMI chemistry–climate models has been compared with the PSC coverage observed by the satellite-borne CALIOP lidar. The use of several diagnostic tools, including the temperature dependence of the PSC occurrences, evidences the merits and flaws of the different models. The diagnostic methods have been defined to overcome (at least partially) the possible differences due to the resolution of the models and to identify differences due to microphysics (e.g., the dependence of PSC occurrence on T−TNAT). A significant temperature bias of most models has been observed, as well as a limited ability to reproduce the longitudinal variations in PSC occurrences observed by CALIOP. In particular, a strong temperature bias has been observed in CCMVal-2 models with a strong impact on PSC formation. The WACCM-CCMI (Whole Atmosphere Community Climate Model – Chemistry-Climate Model Initiative) model compares rather well with the CALIOP observations, although a temperature bias is still present

Teleconnections of the Quasi‐Biennial Oscillation in a multi‐model ensemble of QBO‐resolving models

The Quasi-biennial Oscillation (QBO) dominates the interannual variability of the tropical stratosphere and influences other regions of the atmosphere. The high predictability of the QBO implies that its teleconnections could lead to increased skill of seasonal and decadal forecasts provided the relevant mechanisms are accurately represented in models. Here modelling and sampling uncertainties of QBO teleconnections are examined using a multi-model ensemble of QBO-resolving atmospheric general circulation models that have carried out a set of coordinated experiments as part of the Stratosphere-troposphere Processes And their Role in Climate (SPARC) QBO initiative (QBOi). During Northern Hemisphere winter, the stratospheric polar vortex in most of these models strengthens when the QBO near 50 hPa is westerly and weakens when it is easterly, consistent with, but weaker than, the observed response. These weak responses are likely due to model errors, such as systematically weak QBO amplitudes near 50 hPa, affecting the teleconnection. The teleconnection to the North Atlantic Oscillation is less well captured overall, but of similar strength to the observed signal in the few models that do show it. The models do not show clear evidence of a QBO teleconnection to the Northern Hemisphere Pacific-sector subtropical jet

Analyse des facteurs de variabilité de la température dans la stratosphère.

The stratospheric processes play an important role in the climate system and addressing the issue of the impact of the stratosphere on climate is of fundamental importance to the scientific community. Observed long term changes in the stratosphere include increase of GHG (greenhouse gases), of lower stratospheric water vapour, decrease of stratospheric ozone and a systematic cooling of the stratosphere during the last two decades (1980-2000). This research was dedicated to the estimation of thermal and dynamical long term changes of the stratosphere and to the attribution of the causes, in particular the role of the ozone in the observed changes. The work was designed to cover the recent past (1980 to 2000), for which good data coverage exists; data analysis has then been coupled with GCM transient simulations to identify the role of the ozone decrease on the observed changes. Three datasets based on monthly-mean satellite and radio sounding product analysis have been considered: the TOVS/3I dataset gives a high resolution picture of the lower stratosphere on a short timescale, the FUB one has a lower resolution but is available for a longer period for the northern hemisphere; finally, the SSU/MSU dataset provides the entire vertical thermal structure of the stratosphere with a coarse vertical resolution (SSU/MSU). Stratospheric temperature trends for the period 1980-2000 have been determined using a multiple linear regression model (AMOUNTS) to separate the effect of the major sources of atmospheric temperature variability from a long-term linear trend. First, a detailed analysis of the impact of the factors of temperature variability is given: the stratospheric Quasi-Biennial Oscillation (QBO), the troposphere pattern of variability described by the El Nino Southern Oscillation (ENSO), the external solar variability and the low-frequency extra tropical mode of variability defined as the Arctic Oscillation. It has been found that the amplitude of the response of temperature to some of these forcing are of the same order of magnitude as the calculated trends. Temperature trends are then described as a function of altitude, latitude and season, showing an unambiguous general cooling of the stratosphere, with a maximum global-mean annual-mean cooling observed in the higher stratosphere northern hemisphere (of the order of 3 K/decade) which decreases in the middle stratosphere and increases again in the lower stratosphere (reaching 1 K/decade in global mean but strongly latitude-dependent). The analysis of temperatures permits also an indirect estimation of a weakening of the stratospheric mean circulation. To investigate the role of ozone change on stratospheric temperature and dynamical fields, two ensemble of GCM transient simulations have been considered (Unified Model, Met office and University of Reading); the first one, representing conditions prior to ozone depletion and the second one, including zonal and monthly mean ozone trends as input of the simulations (ozone run). They have been compared with observed trends. Results confirm the role of the ozone decreases in contributing to the temperature trends in the upper stratosphere (up to 60%) and lower stratosphere (up to 30%). While a general good agreement between observations and ozone run is found in the middle and upper stratosphere, in the lower stratosphere trends are however still underestimated by the ozone run very likely due to the effect of water vapour changes, not taken into account in the simulations. The spring Arctic cooling in the lower stratosphere is captured by the ozone run, but not entirely reproduced. Changes in the wave activity entering the lower stratosphere, estimated through the vertical component of the EP-flux, are also observed in the simulations.On sait aujourd'hui que les processus stratosphériques jouent un rôle important dans le système climatique et leur étude est donc de grande importance pour la communauté scientifique. Les changements à long terme observés dans la stratosphère comprennent l'augmentation des gaz à effet de serre, de la vapeur d'eau, la diminution de l'ozone et un refroidissement systématique de cette région de l'atmosphère pendant les deux dernières décennies (1980-2000). Cette recherche est dédiée à la quantification des changements à long terme de la structure thermique et dynamique de la stratosphère et à l'attribution des causes des changements observés, et en particulier l'effet de la diminution de l'ozone stratosphérique. Le travail a été mené pour les vingt dernières années, où des mesures globales de la stratosphère existent; les analyses effectuées ont été ensuite couplées avec des simulations GCM pour quantifier le rôle de la diminution de l'ozone sur les changements à long terme observés. Trois bases de données (températures mensuelles) résultant d'analyses de données satellitaires et/ou de radiosondages ont été considérées : la base TOVS/3I fournit une description de la basse stratosphère à haute résolution spatiale pour une période de 8 ans; la base FUB a une résolution spatiale moins bonne mais est disponible pour une période plus longue; enfin, la base SSU/MSU couvre les vingt dernières années et surtout la totalité de la stratosphère, mais avec une résolution verticale plus faible. Un modèle de régression linéaire multiparamétrique, qui permet de séparer l'effet de la variabilité naturelle de la tendance à long terme, a été utilisé. Dans un premier temps, une analyse détaillée de l'impact des différents facteurs de variabilité de la température stratosphérique été réalisée. Les forçages considérés sont: l'Oscillation Quasi-Biennale (QBO), l'ENSO, la variabilité de 11 ans associée au cycle solaire, et le mode de variabilité extratropicale connu sous le nom d'Oscillation Arctique (AO). Nous montrons tout d'abord que l'amplitude de la réponse de la température à ces forçages peut être du même ordre de grandeur que les tendances calculées. Ensuite, les tendances de la température sont décrites en fonction de l'altitude, de la latitude et de la saison; elles montrent un refroidissement général de la stratosphère, avec une amplitude maximale en moyenne globale dans la haute stratosphère de l'hémisphère nord (de l'ordre de 3 K/décennie); le refroidissement se réduit dans la moyenne stratosphère pour atteindre 1 K/décennie en moyenne globale dans la basse stratosphère (mais avec une structure fortement dépendante de la latitude). L'analyse des températures nous permet aussi de mettre en évidence, de façon indirecte, un affaiblissement de la circulation moyenne de la stratosphère. Pour étudier le rôle de la diminution d'ozone stratosphérique sur les champs dynamiques et thermiques, nous avons enfin considéré deux ensembles de simulations GCM de type « transitoire »; en entrée des simulations, le premier ensemble considère les conditions prévalant avant la diminution de l'ozone, alors que le deuxième inclut les tendances d'ozone observées en moyenne zonale et mensuelle (dite « simulation ozone »). Les tendances de température simulées ont été comparées entre elles. Les résultats indiquent que la diminution de l'ozone stratosphérique est responsable du refroidissement observé à hauteur de 60% dans la haute stratosphère et de 30% dans la basse stratosphère. Si le refroidissement de la haute et moyenne stratosphère est bien reproduit par les simulations ozone, dans la basse stratosphère il est sous-estimé, très probablement à cause de l'effet des tendances de vapeur d'eau, qui n'ont pas été prises en compte dans les simulations. Le refroidissement pendant le printemps Arctique dans la basse stratosphère est observé, mais non complètement reproduit. Enfin, les simulations ozone indiquent une modification dans l'activité des ondes qui se propagent dans la basse stratosphère, observée à travers l'étude de la composante verticale du flux d'Eliassen-Palm