Régionaliser le climat pour en évaluer les impacts : l'apport des modèles numériques

National audience; La simulation de l'évolution future du climat repose sur l'utilisation de modèles numériques complexes. Ces modèles doivent prendre en compte les diverses composantes du système climatique (atmosphère, biosphère, cryosphère et lithosphère) et leurs temps de réponse respectifs pour simuler les réactions de ce système à différents scénarios de perturbations d'origine naturelle (activités solaire et volcanique...) ou humaine (émissions de gaz à effet de serre et d'aérosols). Afin d'étudier l'impact du climat sur une région donnée, il faut disposer d'une résolution spatiale suffisamment fine (quelques kilomètres) dont ne disposent pas les modèles de circulation générale plané…

Interannual memory effects for spring NDVI in semi-arid South Africa

[1] Almost 20 years of Normalized Difference Vegetative Index (NDVI) and precipitation (PPT) data are analysed to better understand the interannual memory effects on vegetation dynamics observed at regional scales in Southern Africa (SA). The study focuses on a semi-arid region (25°S–31°S; 21°E–26°E) during the austral early summer (September–December). The memory effects are examined using simple statistical approaches (linear correlations and regressions) which require the definition of an early summer vegetation predictand (December NDVI minus September NDVI) and a consistent set of potential predictors (rainfall amount, number of rainy days, rainfall intensity, NDVI and Rain-Use-Efficie…

Current and emerging developments in subseasonal to decadal prediction

Weather and climate variations on subseasonal to decadal time scales can have enormous social, economic, and environmental impacts, making skillful predictions on these time scales a valuable tool for decision-makers. As such, there is a growing interest in the scientific, operational, and applications communities in developing forecasts to improve our foreknowledge of extreme events. On subseasonal to seasonal (S2S) time scales, these include high-impact meteorological events such as tropical cyclones, extratropical storms, floods, droughts, and heat and cold waves. On seasonal to decadal (S2D) time scales, while the focus broadly remains similar (e.g., on precipitation, surface and upper-…

On the Role of Water Vapour Transport from the Tropical Atlantic and Summer Rainfall in Tropical Southern Africa

An original way to evaluate daily rainfall variability simulated by a regional climate model: the case of South African austral summer rainfall

We discuss the value of a clustering approach as a tool for evaluating daily rainfall output from climate models. Ascendant hierarchical clustering is used to evaluate how well South African recurrent daily rainfall patterns are simulated during the austral summer (December to February 1970–1971 to 1998–1999). A set of 35-km regional climate simulations, run with the WRF model and driven by the ERA40 reanalysis, is chosen as a case study. Six recurrent patterns are identified and compared to the observed clusters obtained by applying the same methodology to 5352 daily rain gauge records. Two of the WRF clusters describe either a persistent and widespread dryness (65% of the days) or pattern…

Variations in intensity of the westerly monsoon-like flow from the tropical Atlantic and summer rainfall over equatorial and tropical southern Africa

International audience; An empirical orthogonal function (EOF) of the onshore flow of moisture along the west coast of southern Africa using NCEP-DOE AMIP-2 re-analyses suggests two dominant modes of variability that are linked to: (a) variations within the circulation linked to the mid-latitude westerlies and the South Atlantic anticyclone, (b) the intensity of the westerly flow from the tropical Atlantic. The second mode, referred to as the Equatorial Westerly mode, contributes the most to moisture input from the Atlantic onto the subconti-nent at tropical latitudes. This mode appears to be associated with large-scale rainfall anomalies over the upper lands surrounding the Congo basin in …