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

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-Efficiency) considered with 4 to 15-month time-lag. An analysis over six SA sub-regions, corresponding to the six major land-cover types of the area reveals two distinct memory effects. A “negative” memory effect (with both rainfall and vegetation) is detected at 7 to 10-month time-lag while a “positive” memory effect (with vegetation only) is observed at 12 to 14-month time-lag. These results suggest that interannual memory effects in early summer vegetation dynamics of semi-arid South Africa may preferably be driven by biological rather than hydrological mechanisms

Southern Ocean phytoplankton dynamics and carbon export: insights from a seasonal cycle approach

Quantifying the strength and efficiency of the Southern Ocean biological carbon pump (BCP) and its response to predicted changes in the Earth's climate is fundamental to our ability to predict long-term changes in the global carbon cycle and, by extension, the impact of continued anthropogenic perturbation of atmospheric CO2. There is little agreement, however, in climate model projections of the sensitivity of the Southern Ocean BCP to climate change, with a lack of consensus in even the direction of predicted change, highlighting a gap in our understanding of a major planetary carbon flux. In this review, we summarize relevant research that highlights the important role of fine-scale dynamics (both temporal and spatial) that link physical forcing mechanisms to biogeochemical responses that impact the characteristics of the seasonal cycle of phytoplankton and by extension the BCP. This approach highlights the potential for integrating autonomous and remote sensing observations of fine scale dynamics to derive regionally optimized biogeochemical parameterizations for Southern Ocean models. Ongoing development in both the observational and modelling fields will generate new insights into Southern Ocean ecosystem function for improved predictions of the sensitivity of the Southern Ocean BCP to climate change. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'

Transport, bilan et cycle de l'eau atmosphérique en Afrique australe: mise en relation avec la variabilité de la température de surface de mer dans l'hémisphère sud

Thèse de doctorat, Université de Bourgogn

The Southern Annular Mode seen through weather regimes

International audienceThis article investigates the prominent features of the Southern Hemisphere (south of 20°S) atmospheric circulation when extracted using EOF analysis and a k-means clustering algorithm. The focus is on the Southern Annular Mode (SAM), the nature of its recent trend and the zonal symmetry of associated spatial patterns. We use the NCEP2 reanalyses (period 1979-2009), to obtain robust patterns over the recent years, and the Twentieth Century Reanalyses (period 1871-2008), to document decadal changes. We also present a comparison of these signals against station-based reconstruction of the SAM index and a gridded interpolated dataset (HadSLP2). Over their common period, both reanalyses are in fair agreement, both in terms of spatial patterns and temporal variability. In particular, both datasets show weather regimes that can be interpreted as the opposite phases of the SAM. At the decacal timescale, we show that the trend towards the positive SAM phase (as inferred from usual index) is related more to an increase in the frequency of clusters corresponding to the positive phase, with little changes in the frequency of the negative SAM events. Similarly, the long-term tropospheric warming trend already discussed in the literature is shown to be related more to a decrease in the number of abnormally cold days, with little changes in the number of abnormally warm days. The cluster analysis allows then to complement descriptions based on simple indexes or EOF decompositions, highlighting the non-linear nature of the decadal changes in the Southern Hemisphere atmospheric circulation and temperature

Utilisation d'ensembles de simulations climatiques sur Modèles de Circulation Générale de l'Atmosphère: concepts, méthodes et applications à la climatologie de l'espace Sud-Africain

Les séries (ensembles) de simulations longues sur modèles numériques de Circulation Générale de l'Atmosphère (MCGA) - réalisées par la communauté des physiciens de l'atmosphère – offrent au géographe climatologue plusieurs informations relatives à la variabilité du climat complétant l'analyse des séries observées. L'étude présentée ici se base sur un ensemble de 8 simulations longues (de 1948 à 1997) réalisées sur le MCGA français ARPEGE Climat - Version 3 (Centre National de Recherches Météorologiques). Ces simulations utilisent comme conditions aux limites l'énergie solaire et les champs de Températures de Surface de la Mer. Ces dernières varient en fonction des observations mensuelles et sont donc la principale source de la variabilité interannuelle du forçage exercé sur l'atmosphère. Les principaux concepts (variabilité forcée de l'atmosphère versus variabilité interne ou « chaotique », prévisibilité de l'atmosphère) sont d'abord exposés. Les méthodes statistiques utilisées (Analyse de Variance, Analyse en Composantes Principales étendue) sont ensuite rapidement présentées. L'exemple donné concerne l'étude de la variabilité et de la prévisibilité des précipitations en Afrique Australe

Extratropical impacts of the Madden-Julian oscillation over New Zealand from a weather regime perspective.

15 pagesInternational audienceThe Madden–Julian oscillation (MJO) signal in the Southern Hemisphere (SH) extratropics during the austral summer (November–March) is investigated over the New Zealand (NZ) sector, using the paradigm of atmospheric weather regimes (WRs), following a classification initially established by Kidson. The MJO is first demonstrated to have significant impacts on daily rainfall anomalies in NZ. It is suggested that orographic effects arising from the interaction between regional atmospheric circulation anomalies and NZ’s topography can explain the spatially heterogeneous precipitation anomalies that are related to MJO activity. These local impacts and circulation anomalies are shown to be better understood as resulting from changes in the occupation statistics of regional WRs (the Kidson types) through the MJO life cycle, although both constructive and destructive effects are demonstrated. The hypothesis of a significant forcing of the MJO over the NZ sector is further supported by lagged composite analyses, which reveal timing characteristics of the delayed regional circulation response compatible with the average propagation speed of the MJO. While the southern annular mode (SAM) has been previously shown to be statistically related to the MJO and is known to be a significant driver of NZ’s climate, no evidence is found that the impact of the MJO over the NZ sector is mediated by the SAM. It is therefore suggested that the MJO directly impacts regional circulation and climate in the NZ region, potentially through extratropical Rossby wave response to tropical diabatic heating. These findings suggest a new potential for predictability for some aspects of NZ’s weather and climate deriving from the MJO beyond the meteorological time scales

Water vapour transport from the Tropical Atlantic and summer rainfall in Tropical Southern Africa

Rainfall over Southern Africa is highly variable and present different space and time scales. The occurrence of prolonged wet and dry spells remains a crucial threat to society with dramatic implications for the local ecosystems and for populations. Sea surface temperatures (SST) fluctuations in the neighboring oceanic basins are known to influence substantially the local atmospheric circulation. Even if the tropical Atlantic Ocean is regarded as a secondary source of moisture for Southern Africa during summer, it is important to better understand this flux, especially for the bordering countries. In that respect, we applied an EOF analysis to the onshore flow of moisture along the west coast of southern Africa using NCEP-DOE AMIP II Re-analyses. It reveals two dominant modes of variability that are linked to (a) the latitudinal movement of the South Atlantic anticyclone (b) the intensity of the flow that penetrates from the tropical Atlantic. The second mode, referred as the Equatorial Westerly mode, contributes the most to moisture input from the Atlantic onto the subcontinent at tropical latitudes. Substantial correlations in austral summer between the Atlantic moisture flux and rainfall over the upper lands surrounding the Congo basin suggest the potential role played by this zonal mode of water vapour transport. Composite for austral summer months when this Equatorial Westerly mode had a particularly strong expression, shows an enhanced moisture input at tropical latitudes that feeds in deep convection mechanisms over the Congo basin. A sustained meridional energy flux results in above normal rainfall to the east and south of the Congo belt. For years of reduced equatorial westerly moisture flux, a deficit of available humidity occurs in the southern tropics. A concomitant eastward shift of the Walker circulation ascending branch to the southwest Indian ocean and southeastern Africa, reduces convection processes and leads to below normal rainfall over the uplands surrounding the Congo basin. We present here an attempt in assessing the role of water vapour from the tropical Atlantic onto southern African tropics summer rainfall. The study of such mechanisms must contribute to improve predictability of extreme climatic events linked with the conditioning of the neighboring oceans.Pages: 1219-122

Moisture transport between the South Atlantic Ocean and southern Africa: relationships with summer rainfall and associated dynamics

International audienceMoisture exchange between the South Atlantic and southern Africa is examined in this study through zonal moisture transport. Along the west coast of southern Africa, a multivariate analysis of the zonal flow of moisture computed from NCEP-DOE AMIP II Re-analyses reveals a primary mode of variability typical of variations in intensity and of the latitudinal migration of the circulation associated with the midlatitude westerlies and the South Atlantic anticyclone. In austral summer (January–February), this mode, referred to as the South Atlantic midlatitude mode, is found to be well correlated with rainfall over southern Africa (i.e. to the south of the upper lands surrounding the Congo basin). Its positive/negative phases are found to correspond with surface pressures changes over the South Atlantic region in austral summer when the South Atlantic anticyclone is shifted northward/southward respectively. Such changes are accompanied by dipole-like SST anomalies in the midlatitude South Atlantic Ocean, while simultaneous SST anomalies with a similar structure are also found over South Indian Ocean regions. In January–February, positive/negative events linked to the South Atlantic midlatitude mode are marked by meridional shifts (northward/southward) and weakening/strengthening of the ITCZ over the southern tropics, together with modulations in intensity (weakened/sustained) of the Angola low, which could act as a tropical source of moisture for Tropical Temperate Troughs (TTTs). In association with a strengthened/weakened zonal component of the southern extension of the African Easterly Jet (AEJ), this could modulate the meridional transfer of moisture south of 15°S to the advantage/detriment of Angolan coastal regions, where above/below rainfall are expected. Variations in the latitudinal position (northward/southward) of the South Atlantic anticyclone, and thus of the midlatitude westerlies, are also found to reduce/favour moisture advection towards southern Africa subtropics allowing the southern Indian trades to penetrate less/more over the subcontinent south of 25°S. This would create a situation where convection processes are inhibited/supported within the SICZ/TTTs region resulting in drier/wetter conditions locally for positive/negative events respectively