Cloud bands over southern Africa: seasonality, contribution to rainfall variability and modulation by the MJO

Hart, Neil C. G.; Reason, C. J. C.; Fauchereau, Nicolas

doi:10.1007/s00382-012-1589-4

Cited by 129 publications

(143 citation statements)

References 22 publications

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“…Its occurrence is related to the interaction between the Indian Ocean sub-tropical high pressure system, bringing in moisture to the subcontinent by tropical easterlies, and a weak continental thermal low (Tyson and Preston-White 2000). This basic mechanism can be enhanced through conditions facilitating development of moisture fronts, or so-called tropical temperate troughs (TTTs), where disturbances in the tropical easterlies interact with those in the subtropical westerlies creating linear NW-SE convective cloud band systems (Harrison 1984;Hart et al 2013). The conditions facilitating TTTs commonly occur when the easterly tropical low over the interior connects with the westerly wave or a cut-off low to the south of the continent.…”

Section: Domain Selectionmentioning

confidence: 99%

Interannual rainfall variability and SOM-based circulation classification

et al. 2017

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synoptic variables over a particular location; even though they are used in the development of the SOM their influence, however, diminishes with the size of the SOM spatial domain. The influence of the SOM domain size, the choice of SOM atmospheric variables and grid-point explanatory variables on the levels of explained variance, is consistent with the general understanding of the dominant processes and atmospheric variables that affect rainfall variability at a particular location. Keywords SOM · Synoptic circulation · Rainfall variability · Southern AfricaThe relationship between synoptic circulation and local expressions of climate is the foundation of synoptic climatology and traditionally underpins the process of interpretative weather forecasting. The basic premise of synoptic climatology is that the large-scale atmospheric circulation exerts some control over weather and related environmental phenomena at the Earth's surface, thereby providing a level of deterministic forcing which influences the local weather. This directly leads to the use of atmospheric circulation patterns as a means to express drivers of weather (and climate) variability. Self-Organizing Maps (SOM, Kohonen 2001) is a pattern clustering method that is used as one of the methods to derive synoptic circulation types in this and other contexts, including statistical downscaling (e.g. Hewitson and Crane 2006;Yin et al. 2011;Ohba et al. 2016) and process-based validation of GCMs (e.g. Brown et al. 2010;Finnis et al. 2009;Higgins and Cassano 2010).In the application of SOM to study climate variability, the evolution of synoptic drivers of weather is typically represented by progression through classes of daily synoptic states identified as SOM nodes. That progression Abstract Self-Organizing Maps (SOM) based classifications of synoptic circulation patterns are increasingly being used to interpret large-scale drivers of local climate variability, and as part of statistical downscaling methodologies. These applications rely on a basic premise of synoptic climatology, i.e. that local weather is conditioned by the large-scale circulation. While it is clear that this relationship holds in principle, the implications of its implementation through SOM-based classification, particularly at interannual and longer time scales, are not well recognized. Here we use a SOM to understand the interannual synoptic drivers of climate variability at two locations in the winter and summer rainfall regimes of South Africa. We quantify the portion of variance in seasonal rainfall totals that is explained by year to year differences in the synoptic circulation, as schematized by a SOM. We furthermore test how different spatial domain sizes and synoptic variables affect the ability of the SOM to capture the dominant synoptic drivers of interannual rainfall variability. Additionally, we identify systematic synoptic forcing that is not captured by the SOM classification. The results indicate that the frequency of synoptic states, as schematized by a relatively disa...

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Section: Domain Selectionmentioning

confidence: 99%

Interannual rainfall variability and SOM-based circulation classification

et al. 2017

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“…Indeed, COLs may occur in conjunction with a number of different low-level circulation patterns-most commonly in association with a strong ridge of high pressure in the low-levels polewards of the upper COL (e.g. Taljaard 1985;Tennant and van Heerden 1994;Katzfey and McInnes 1996;Favre et al 2012), in combination with tropical-temperate troughs (Hart et al 2013), and further, as a COL system evolves, in association with the evolving high pressure system (ridging progressively from the southwest to southeast of South Africa). The typical low-level circulation associated with tropical-temperate troughs-a meridional trough that links the Angola low and a mid-latitude wave/cyclone, is illustrated in Tennant (2003) and Hart et al (2010Hart et al ( , 2012).…”

Section: Identification Of Cut-off Lowsmentioning

confidence: 99%

“…The spatial rainfall distribution over southern Africa is employed to aid in the identification of nodes representative of tropical-temperate troughs-characterized by the presence of a cloud band originating from tropical Africa extending south or southeastwards over South Africa (e.g. Hart et al 2013). As the station data used is limited to the study region, the FEWS rainfall estimate was employed to represent the spatial rainfall distribution associated with each node over southern Africa (Fig.…”

Section: Relative Contribution Of Synoptic Types To Rainfall Over Submentioning

confidence: 99%

“…Tropical-temperate troughs cause on the average about 39 % of rainfall over the summer rainfall region (Crimp et al 1997). The maximum frequency of occurrence of tropical-temperate troughs is during November (Hart et al 2013), when they are also responsible for 30-60 % of rainfall occurring along the Cape south coast (Hart et al 2013). Their relative contribution for December remains high (30-50 %) (Hart et al 2013).…”

Section: Introductionmentioning

confidence: 99%

“…When rainfall over the region is of a tropical nature, it is usually from tropical-temperate trough cloud bands (e.g. Taljaard 1996;Hart et al 2013). Of the different rain bearing systems, it is mostly COLs that are associated with high-impact events over the region.…”

Section: Introductionmentioning

confidence: 99%

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A synoptic decomposition of rainfall over the Cape south coast of South Africa

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Interannual to interdecadal variability of winter and summer southern African rainfall, and their teleconnections

et al. 2016

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Original citation: Dieppois, B. , Pohl, B. , Rouault, M. , New, M. , Lawler, D. and Keenlyside, N. (2016) Interannual to Interdecadal variability of winter and summer southern African rainfall, and their teleconnections. Abstract This 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 (15-28 years), quasi-decadal (8-13 years), and interannual (2-8 years). 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 Niño-Southern 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.

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Cloud bands over southern Africa: seasonality, contribution to rainfall variability and modulation by the MJO

Cited by 129 publications

References 22 publications

Interannual rainfall variability and SOM-based circulation classification

Interannual rainfall variability and SOM-based circulation classification

A synoptic decomposition of rainfall over the Cape south coast of South Africa

Interannual to interdecadal variability of winter and summer southern African rainfall, and their teleconnections

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