Analyse du mécanisme de la mise en place de la mousson Africaine: dynamique régionale ou forçage de grande échelle?

The rainy season in West Africa is crucial for local population. The onset of the monsoon (WAM) occurs in late June and corresponds to the weakening of convection over the entire region and to the sudden transition of rainfall from the Guinean coast to the Sahel. The mechanism which provokes this monsoon "jump" is investigated in this thesis through modeling experiments. The ability of the regional WRF model to reproduce the movement of MAO in 2006 was first evaluated. Different parameterizations of convection and boundary layer have been tested, providing an analysis of their impact on the simulations. Sensitivity tests were then performed in order to evaluate the role of the Saharan heat low and SST (mechanisms proposed in other studies) to provoke the WAM onset. The results show that the transitional phase depends on large scale dynamics rather than on regional climate. More specifically, the Indian monsoon onset releases a westward propagating Rossby wave which passes over north Africa favoring dry air intrusion over west Africa inhibiting convection. In parallel, the meridional gradient of surface pressure is strengthened, accelerating the WAM and therefor enhancing humidity advection over the Sahel. Once the wave is passed, convective activity gets reorganized above the Sahel where thermodynamic conditions are more favorable. The use of simulations by the LMDz global model confirmed the role of the Indian monsoon over the period 1989-2008, however with years more or less marked.La saison des pluies en Afrique de l'Ouest est primordiale pour les populations locales. La mise en place de la mousson (MAO) se produit fin juin et correspond à un affaiblissement global de la convection sur la région puis une transition brutale des précipitations de la côte de Guinée vers le Sahel. C'est le mécanisme responsable de ce " saut " qui est étudié ici à partir d'expériences numériques. La capacité du modèle à aire limitée WRF à reproduire la circulation de la MAO en 2006 a d'abord été évaluée. Différentes paramétrisations de la convection et de la couche limite ont été testées et leur impact sur les simulations analysé. Des tests de sensibilité ont ensuite été effectués pour évaluer le rôle de la dépression thermique saharienne et de la SST (mécanismes proposés dans d'autres études) sur la mise en place de la MAO. Les résultats montrent que la phase de transition dépend plus fortement de la dynamique de grande échelle que des éléments régionaux. Plus précisément, la mise en place de la mousson Indienne libère une onde de Rossby qui se propage vers l'ouest, arrive au dessus de l'Afrique du Nord en favorisant les intrusions de masses d'air sec au dessus de l'Afrique de l'Ouest qui inhibent la convection. En parallèle, le gradient méridien de pression de surface est renforcé et la MAO s'intensifie en advectant de l'humidité au dessus du Sahel. Une fois l'onde évacuée, la convection se réorganise au dessus du Sahel où les conditions thermodynamiques sont favorables. L'utilisation de simulations globales avec LMDz a confirmé le rôle de la mousson indienne sur toute la période 1989-2008 avec cependant des années plus ou moins marquées

Inferring change points and nonlinear trends in multivariate time series: Application to West African monsoon onset timings estimation

International audienceTime series in statistical climatology are classically represented by additive models. For example, a seasonal part and a linear trend are often included as components of the sum. Less frequently, hidden elements (e.g., to represent the impact of volcanic forcing on temperatures) can be integrated. Depending on the complexity and the interactions among the different components, the statistical inference challenge can quickly become difficult, especially in a multivariate context where the timings and contributions of hidden signals are unknown. In this article we focus on the statistical problem of decomposing multivariate time series that may contain both nonlinear trends and change points (discontinuities), the change points being assumed to occur simultaneously in time for all variables in the multivariate analysis. The motivation for such a study comes from the statistical analysis of the West African monsoon (WAM) phenomenon for which unknown preonset and onset dates occur each year. The impacts of such onsets can be statistically viewed as yearly change points that affect, almost synchronously, trends in observed time series such as daily Outgoing Longwave Radiation and the Intertropical Discontinuity. Our proposed model corresponds to a multivariate additive model with nonlinear trends and possible yearly discontinuities, modeling the onsets. An inference scheme based on a nonlinear Kalman filtering approach is proposed. It enables to identify the different parts hidden in the original multivariate vector. Our inference strategy is tested on simulated data and applied to the analysis of the WAM phenomenon during the period 1979-2008. Our extracted onset dates are then compared to the ones obtained from past studies

Heavy rainfall in Mediterranean cyclones. Part I: contribution of deep convection and warm conveyor belt

In this study, we provide an insight to the role of deep convection (DC) and the warm conveyor belt (WCB) as leading processes to Mediterranean cyclones’ heavy rainfall. To this end, we use reanalysis da-ta, lighting and satellite observations to quantify the relative contribution of DC and the WCB to cyclone rainfall, as well as to analyse the spatial and temporal variability of these processes with respect to the cy-clone centre and life cycle. Results for the period 2005-2015 show that the relationship between cyclone rainfall and intensity has high variability and demonstrate that even intense cyclones may produce low rainfall amounts. However, when considering rainfall averages for cyclone intensity bins, a linear relationship was found. We focus on the 500 most intense tracked cyclones (responsible for about 40-50% of the total 11-year Mediterrane-an rainfall) and distinguish between the ones producing high and low rainfall amounts. DC and the WCB are found to be the main cause of rainfall for the former (producing up to 70% of cyclone rainfall), while, for the latter, DC and the WCB play a secondary role (producing up to 50% of rainfall). Further analysis showed that rainfall due to DC tends to occur close to the cyclones’ centre and to their eastern sides, while the WCBs tend to produce rainfall towards the northeast. In fact, about 30% of rainfall produced by DC overlaps with rainfall produced by WCBs but this represents only about 8% of rainfall produced by WCBs. This suggests that a considerable percentage of DC is associated with embedded convection in WCBs. Finally, DC was found to be able to produce higher rain rates than WCBs, exceeding 50 mm in 3-hourly accumulated rainfall compared to a maximum of the order of 40 mm for WCBs. Our results demonstrate in a climatological framework the relationship between cyclone intensity and pro-cesses that lead to heavy rainfall, one of the most prominent environmental risks in the Mediterranean. Therefore, we set perspectives for a deeper analysis of the favourable atmospheric conditions that yield high impact weather

Future projections of Mediterranean cyclone characteristics using the Med-CORDEX ensemble of coupled regional climate system models

Here, we analyze future projections of cyclone activity in the Mediterranean region at the end of the twenty-first century based on an ensemble of state-of-the-art fully-coupled Regional Climate System Models (RCSMs) from the Med-CORDEX initiative under the Representative Concentration Pathway (RCP) 8.5. Despite some noticeable biases, all the RCSMs capture spatial patterns and cyclone activity key characteristics in the region and thus all of them can be considered as plausible representations of the future evolution of Mediterranean cyclones. In general, the RCSMs show at the end of the twenty-first century a decrease in the number and an overall weakening of cyclones moving across the Mediterranean. Five out of seven RCSMs simulate also a decrease of the mean size of the systems. Moreover, in agreement with what already observed in CMIP5 projections for the area, the models suggest an increase in the Central part of the Mediterranean region and a decrease in the South-eastern part of the region in the cyclone-related wind speed and precipitation rate. These rather two opposite tendencies observed in the precipitation should compensate and amplify, respectively, the effect of the overall reduction of the frequency of cyclones on the water budget over the Central and South-eastern part of the region. A pronounced inter-model spread among the RCSMs emerges for the projected changes in the cyclone adjusted deepening rate, seasonal cycle occurrence and associated precipitation and wind patterns over some areas of the basin such as Ionian Sea and Iberian Peninsula. The differences observed appear to be determined by the driving Global Circulation Model (GCM) and influenced by the RCSM physics and internal variability. These results point to the importance of (1) better characterizing the range of plausible futures by relying on ensembles of models that explore well the existing diversity of GCMs and RCSMs as well as the climate natural variability and (2) better understanding the driving mechanisms of the future evolution of Mediterranean cyclones properties

A dynamical link between deep Atlantic extratropical cyclones and intense Mediterranean cyclones

Breaking of atmospheric Rossby waves has been previously shown to lead to intense Mediterranean cyclones, one of the most prominent environmental risks in the region. Wave breaking may be enhanced by warm conveyor belts (WCBs) associated with extratropical cyclones developing over the Atlantic Ocean. More precisely, WCBs supply the upper troposphere with air masses of low potential vorticity that, in turn, amplify ridges and thus favor Rossby wave breaking. This study identifies the mechanism that connects Atlantic cyclones and intense mature Mediterranean cyclones through ridge amplification by WCBs, and validates its climatological relevance. Using European Centre for Medium‐Range Weather Forecasts (ECMWF) ERA‐Interim reanalyses and a feature‐based approach, we analyze the 200 most intense Mediterranean cyclones for the years 1989–2008 and show that their majority (181 cases) is indeed associated with this mechanism upstream. Results show that multiple Atlantic cyclones are associated with each case of intense Mediterranean cyclone downstream. Moreover, the associated Atlantic cyclones are particularly deeply intensifying compared with climatology

Regional climate modelling of the 2006 West African monsoon: sensitivity to convection and planetary boundary layer parameterisation using WRF

International audienceRegional climate model (RCM) is a valuable scientific tool to address the mechanisms of regional atmospheric systems such as the West African monsoon (WAM). This study aims to improve our understanding of the impact of some physical schemes of RCM on the WAM representation. The weather research and forecasting model has been used by performing six simulations of the 2006 summer WAM season. These simulations use all combinations of three convective parameterization schemes (CPSs) and two planetary boundary layer schemes (PBLSs). By comparing the simulations to a large set of observations and analysis products, we have evaluated the ability of these RCM parameterizations to reproduce different aspects of the regional atmospheric circulation of the WAM. This study focuses in particular on the WAM onset and the rainfall variability simulated over this domain. According to the different parameterizations tested, the PBLSs seem to have the strongest effect on temperature, humidity vertical distribution and rainfall amount. On the other hand, dynamics and precipitation variability are strongly influenced by CPSs. In particular, the Mellor–Yamada–Janjic PBLS attributes more realistic values of humidity and temperature. Combined with the Kain–Fritsch CPS, the WAM onset is well represented. The different schemes combination tested also reveal the role of different regional climate features on WAM dynamics, namely the low level circulation, the land–atmosphere interactions and the meridional temperature gradient between the Guinean coast and the Sahel

Evaluation of the impact of surface albedo, sea surface temperature and orography in the simulation of the monsoon onset in 2006

The aim of this study is to improve our understanding on the role of the different forcings (oceans, continental surface, large scale activity) on the West African Monsoon (WAM) onset. We thus use a regional model to simulate the WAM circulation of year 2006 so that we can evaluate the model's performance by comparison with observations collected during the Special Observation Period of the AMMA experiment. In particular we bear our attention on the timing of the WAM onset (abrupt displacement of the ITCZ from 5°N to 10°N approximately) and the evaluation of the volume and the location of precipitations. The criterium used to detect the onset is the date when a significant decrease of convection lasting a few days is observed, followed by a northerly displacement of the ITCZ and the associated precipitations. For this purpose, the Weather Research and Forecasting model (WRF) is used to represent the atmospheric circulation over western Africa -with a resolution of 40km, a domain that covers the West and North Africa, the Guinean Gulf, part of the Atlantic ocean (western limit is located at 28.3°W) and of the Mediterranean sea (up to 39°N)-, for a period of seven months (from March to September). In the work presented here, we performed four simulations with a similar configuration of the WRF model but with different albedo, SST and orography values in order to investigate the impact of these elements in the model. Results show that although, the changes done for the sensitivity tests have a striking impact on the WAM dynamics, such as the depth and the seasonal cycle of the Sahelian Heat Low and the Intertropical Discontinuity, the onset date is not subject to any change. The precipitation regime and the ITCZ location are different from one simulation to another but the phase of decreased convection show no temporal flexibility. So, according to this study, albedo and orography are not proved to be key elements to the onset mechanisms. Effects of high SST values in the Guinean Gulf are more important by deranging the ITCZ localization and intensity and furthermore showing a constant rain band over the coastal area at 5°N

The upstream-downstream connection of North Atlantic and Mediterranean cyclones in semi-idealized simulations

Cyclogenesis in the Mediterranean is typically triggered by the intrusion of a potential vorticity (PV) streamer over the Mediterranean. The intrusion of the PV streamer results from a preceding Rossby wave breaking (RWB) upstream over the North Atlantic. The ridge leading to the RWB is typically amplified by the presence of warm conveyor belts (WCBs) in at least one North Atlantic cyclone about 4 d prior to Mediterranean cyclogenesis. Thus, the sequence of these four main events (namely a North Atlantic cyclone, WCBs, RWB, and the resulting PV streamers) forms an archetypal scenario leading to Mediterranean cyclogenesis. However, they rarely occur in a spatially consistent, fully repetitive pattern for real cyclone cases. To more systematically study this connection between upstream North Atlantic cyclones and Mediterranean cyclogenesis, we perform a set of semi-idealized simulations over the Euro-Atlantic domain. For these simulations, we prescribe a constant climatological atmospheric state in the initial and boundary conditions. To trigger the downstream Mediterranean cyclogenesis scenario, we perturb the climatological polar jet through the inversion of a positive upper-level PV anomaly. The amplitude of this perturbation determines the intensity of the triggered North Atlantic cyclone. This cyclone provokes RWB, the intrusion of a PV streamer over the Mediterranean, and thereby the formation of a Mediterranean cyclone. Therefore, our results show a direct connection between the presence of a North Atlantic cyclone and the downstream intrusion of a PV streamer into the Mediterranean, which causes cyclogenesis about 4 d after perturbing the polar jet. We refer to this as the upstream-downstream connection of North Atlantic and Mediterranean cyclones. To investigate the sensitivity of this connection, we vary the position and amplitude of the upper-level PV anomaly. In all simulations, cyclogenesis occurs in the Mediterranean. Nevertheless, the tracks and intensity of the Mediterranean cyclones may vary by up to 20° and 10 hPa (at the time of the mature stage), respectively. This indicates that the Mediterranean cyclone dynamics are sensitive to the dynamical structure and amplitude of the intruding PV streamer, which itself is sensitive to the interaction of the upstream cyclone and the RW(B). By applying different seasonal climatological atmospheric states as initial conditions we show that cyclogenesis occurs in distinct regions in different seasons. Thus, the seasonal cycle of Mediterranean cyclogenesis might be partly determined by the large-scale atmospheric circulation, i.e., the seasonal location of the polar jet. Furthermore, we show that the Mediterranean cyclones in these semi-idealized simulations show characteristics that agree with the observed climatology of Mediterranean cyclones in the respective season.ISSN:2698-4016ISSN:2698-400