130 research outputs found
SENSIBILITE D'UN MODELE A AIRE LIMITEE A SA PARAMETRISATION PHYSIQUE : APPLICATION EN AFRIQUE AUSTRALE
Cette Ă©tude examine pour la premiĂšre fois en Afrique australe les incertitudes d'un modĂšle Ă aire limitĂ©e (Advanced Weather Research Forecast (WRF-ARW V3011)) liĂ©es Ă sa paramĂ©trisation physique. Les incertitudes sont analysĂ©es au pas de temps saisonnier en dĂ©terminant les principaux points communs et diffĂ©rences de 27 expĂ©riences numĂ©riques, avec un focus sur le champ pluviomĂ©trique. Ces 27 expĂ©riences documentent le trimestre DĂ©cembre-Janvier-FĂ©vrier 1993-94, coeur de la saison des pluies de l'Afrique du Sud Ă rĂ©gime pluvial tropical, et correspondent Ă toutes les combinaisons possibles entre 3 schĂ©mas de couche limite, 3 schĂ©mas de convection et 3 schĂ©mas de microphysique. Quelle que soit la paramĂ©trisation testĂ©e, la distribution spatiale des pluies est similaire et relativement proche des estimations de pluies. WRF sous-estime la pluviomĂ©trie des deux zones de convergence de la rĂ©gion (ZCIT et ZCSI) et la surestime en Afrique subtropicale, surtout sur les reliefs. Les principales diffĂ©rences inter-membres concernent l'amplitude des cumuls saisonniers et les processus pluviogĂšnes qui dĂ©pendent principalement des schĂ©mas de convection. Grell simule des quantitĂ©s comparables aux observations in situ alors que Kain-Fritsch et Betts-Miller-Janjic les surestiment nettement, ce qui peut rĂ©sulter d'une sous-estimation (surestimation) de l'humiditĂ© spĂ©cifique en moyenne et basse couche observĂ©e avec Grell (Kain-Fritsch et Betts-Miller-Janjic). Grell et Kain-Fritsch simulent essentiellement des pluies convectives, ce qui semble cohĂ©rent avec l'influence de la circulation tropicale sur cette rĂ©gion. ComparĂ©e aux rĂ©analyses ERA40 utilisĂ©es pour le forçage latĂ©ral, la convergence d'humiditĂ© associĂ©e Ă la majoritĂ© des expĂ©riences est renforcĂ©e sur le subcontinent, de mĂȘme que la vitesse verticale de l'air en moyenne atmosphĂšre. C'est la raison pour laquelle WRF corrige gĂ©nĂ©ralement les biais secs d'ERA40. Les diffĂ©rences inter-membres des champs thermo-dynamiques sont fonction des schĂ©mas de convection Ă ce pas de temps, mais aussi d'alliances non systĂ©matiques entre les trois types de schĂ©mas testĂ©s
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Fewer rainy days and more extreme rainfall by the end of the century in Southern Africa
Future changes in the structure of daily rainfall, especially the number of rainy days and the intensity of extreme events, are likely to induce major impacts on rain-fed agriculture in the tropics. In Africa this issue is of primary importance, but the agreement between climate models to simulate such descriptors of rainfall is generally poor. Here, we show that the climate models used for the fifth assessment report of IPCC simulate a marked decrease in the number of rainy days, together with a strong increase in the rainfall amounts during the 1% wettest days, by the end of the 21st century over Southern Africa. These combined changes lead to an apparent stability of seasonal totals, but are likely to alter the quality of the rainy season. These evolutions are due to the superposition of slowly-changing moisture fluxes, mainly supported by increased hygrometric capacity associated with global warming, and unchanged short-term atmospheric configurations in which extreme events are embedded. This could cause enhanced floods or droughts, stronger soil erosion and nutriment loss, questioning the sustainability of food security for the 300 million people currently living in Africa south of the Equator
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Orbitally forced and internal changes in West African rainfall interannual-to-decadal variability for the last 6000Â years
Recent variability in West African monsoon rainfall (WAMR) has been shown to be influenced by multiple ocean-atmosphere modes, including the El Niño Southern Oscillation, Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation. How these modes will change in response to long term forcing is less well understood. Here we use four transient simulations driven by changes in orbital forcing and greenhouse gas concentrations over the past 6000 years to examine the relationship between West African monsoon rainfall multiscale variability and changes in the modes associated with this variability. All four models show a near linear decline in monsoon rainfall over the past 6000 years in response to the gradual weakening of the interhemispheric gradient in sea surface temperatures. The only indices that show a long-term trend are those associated with the strengthening of the El Niño Southern Oscillation from the mid-Holocene onwards. At the interannual-to-decadal timescale, WAMR variability is largely influenced by Pacific-Atlantic - Mediterranean Sea teleconnections in all simulations; the exact configurations are model sensitive. The WAMR interannual-to-decadal variability depicts marked multi-centennial oscillations, with La Niña/negative Pacific Decadal Oscillation and a weakening and/or poleward shift of subtropical high-pressure systems over the Atlantic favoring wet WAMR anomalies. The WAMR interannual-to-decadal variability also depicts an overall decreasing trend throughout the Holocene that is consistent among the simulations. This decreasing trend relates to changes in the North Atlantic and Gulf of Guinea Sea Surface Temperature variability. Supplementary information The online version contains supplementary material available at 10.1007/s00382-023-07023-y
The impact of ENSO on Southern African rainfall in CMIP5 ocean atmosphere coupled climate models
We study the ability of 24 ocean atmosphere global coupled models from the Coupled Model Intercomparison Project 5 (CMIP5) to reproduce the teleconnections between El Niño Southern Oscillation (ENSO) and Southern African rainfall in austral summer using historical forced simulations, with a focus on the atmospheric dynamic associated with El Niño. Overestimations of summer rainfall occur over Southern Africa in all CMIP5 models. Abnormal westward extensions of ENSO patterns are a common feature of all CMIP5 models, while the warming of the Indian Ocean that happens during El Niño is not correctly reproduced. This could impact the teleconnection between ENSO and Southern African rainfall which is represented with mixed success in CMIP5 models. Large-scale anomalies of suppressed deep-convection over the tropical maritime continent and enhanced convection from the central to eastern Pacific are correctly simulated. However, regional biases occur above Africa and the Indian Ocean, particularly in the position of the deep convection anomalies associated with El Niño, which can lead to the wrong sign in rainfall anomalies in the northwest part of South Africa. From the near-surface to mid-troposphere, CMIP5 models underestimate the observed anomalous pattern of pressure occurring over Southern Africa that leads to dry conditions during El Niño years
From synoptic to interdecadal variability in southern African rainfall: towards a unified view across timescales
International audienceDuring the austral summer season (NovemberâFebruary), southern African rainfall, south of 20°S, has been shown to vary over a range of time scales, from synoptic variability (3â7 days, mostly tropical temperate troughs) to interannual variability (2â8 years, reflecting the regional effects of El NiñoâSouthern Oscillation). There is also evidence for variability at quasi-decadal (8â13 years) and interdecadal (15â28 years) time scales, linked to the interdecadal Pacific oscillation and the Pacific decadal oscillation, respectively. This study aims to provide an overview of these ranges of variability and their influence on regional climate and large-scale atmospheric convection and quantify uncertainties associated with each time scale. We do this by applying k-means clustering onto long-term (1901â2011) daily outgoing longwave radiation anomalies derived from the 56 individual members of the Twentieth Century Reanalysis. Eight large-scale convective regimes are identified. Results show that 1) the seasonal occurrence of the regimes significantly varies at the low-frequency time scales mentioned above; 2) these modulations account for a significant fraction of seasonal rainfall variability over the region; 3) significant associations are found between some of the regimes and the aforementioned modes of climate variability; and 4) associated uncertainties in the regime occurrence and convection anomalies strongly decrease with time, especially the phasing of transient variability. The short-lived synoptic anomalies and the low-frequency anomalies are shown to be approximately additive, but even if they combine their respective influence at both scales, the magnitude of short-lived perturbations remains much larger
Interannual to Interdecadal variability of winter and summer southern African rainfall, and their teleconnections.
25 pagesInternational audienceThis study examines for the first time the changing characteristics of summer and winter southern African rainfall and their teleconnections with large-scale climate through the dominant time scales of variability. As determined by wavelet analysis, the austral summer and winter rainfall indices exhibit three significant time scales of variability over the twentieth century: interdecadal (15â28âyears), quasi-decadal (8â13âyears), and interannual (2â8âyears). Teleconnections with global sea surface temperature and atmospheric circulation anomalies are established here but are different for each time scale. Tropical/subtropical teleconnections emerge as the main driver of austral summer rainfall variability. Thus, shifts in the Walker circulation are linked to the El NiñoâSouthern Oscillation (ENSO) and, at decadal time scales, to decadal ENSO-like patterns related to the Pacific Decadal Oscillation and the Interdecadal Pacific Oscillation. These global changes in the upper zonal circulation interact with asymmetric ocean-atmospheric conditions between the South Atlantic and South Indian Oceans; together, these lead to a shift in the South Indian Convergence Zone and a modulation of the development of convective rain-bearing systems over southern Africa in summer. Such regional changes, embedded in quasi-annular geopotential patterns, consist of easterly moisture fluxes from the South Indian High, which dominate southerly moisture fluxes from the South Atlantic High. Austral winter rainfall variability is more influenced by midlatitude atmospheric variability, in particular the Southern Annular Mode. The rainfall changes in the southwestern regions of southern Africa are determined by asymmetrical changes in the midlatitude westerlies between the Atlantic and Indian Oceans
Predicting the seasonal evolution of southern African summer precipitation in the DePreSys3 prediction system
We assess the ability of the DePreSys3 prediction system to predict austral summer precipitation (DJF) over southern Africa, defined as the African continent south of 15°S. DePresys3 is a high resolution prediction system (at a horizontal resolution of ~â60 km in the atmosphere in mid-latitudes and of the quarter degree in the Ocean) and spans the long period 1959â2016. We find skill in predicting interannual precipitation variability, relative to a long-term trend; the anomaly correlation skill score over southern Africa is greater than 0.45 for the first summer (i.e. lead month 2â4), and 0.37 over Mozambique, Zimbabwe and Zambia for the second summer (i.e. lead month 14â16). The skill is related to the successful prediction of the El-Nino Southern Oscillation (ENSO), and the successful simulation of ENSO teleconnections to southern Africa. However, overall skill is sensitive to the inclusion of strong La-Nina events and also appears to change with forecast epoch. For example, the skill in predicting precipitation over Mozambique is significantly larger for the first summer in the 1990â2016 period, compared to the 1959â1985 period. The difference in skill in predicting interannual precipitation variability over southern Africa in different epochs is consistent with a change in the strength of the observed teleconnections of ENSO. After 1990, and consistent with the increased skill, the observed impact of ENSO appears to strengthen over west Mozambique, in association with changes in ENSO related atmospheric convergence anomalies. However, these apparent changes in teleconnections are not captured by the ensemble-mean predictions using DePreSys3. The changes in the ENSO teleconnection are consistent with a warming over the Indian Ocean and modulation of ENSO properties between the different epochs, but may also be associated with unpredictable atmospheric variability
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Tracing future spring and summer drying in southern Africa to tropical lows and the Congo Air Boundary
In southern Africa, models from the 5th Coupled Model Intercomparison Project (CMIP5) predict robust future drying associated with a delayed rainy season onset in the Austral spring and a range of wetting and drying patterns in the Austral summer. This paper relates these rainfall changes to dynamical shifts in two classes of weather systems: the Congo Air Boundary (CAB) and tropical lows. Objective algorithms are used to track these features in CMIP5 model output. It is then established that the climatological locations and frequencies of these systems are reasonably well represented in the CMIP5 models. RCP8.5 end of 21st century projections are compared with historical end of 20th century simulations. Future projections in tropical low locations and frequencies diverge, but indicate an overall average decrease of 15% and in some cases a northward shift. The projected spatial change in the tropical low frequency distribution is weakly positively correlated to the projected spatial change in the Austral summer rainfall distribution. Meanwhile, future projections indicate a 13% increase in CAB frequency from October to December. This is associated with the gradual climatological CAB breakdown occurring half a month later on average in end of 21st century RCP8.5 projections. A delay in the gradual seasonal decline of the CAB prevents rainfall to the south of the CABâs mean position, most of which is shown to occur on CAB breakdown days, hence creating the Austral spring drying signal and delayed wet season onset. Inter-model variability in the magnitude of CAB frequency increase is able to explain inter-model variability in the projected drying
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