Publicação
Atmospheric circulation over Southern Africa and its relationship with regional rainfall
| Resumo: | Throughout the last decades, southern Africa has become increasingly vulnerable to weather variability, which affects moisture advection and convergence in the lower levels of the atmosphere and impact local rainfall, essential for rudimentary rainfed agricultural systems, in which most of countries’ economies rely on. Thus, understanding atmospheric circulation dynamics is crucial, as well as the identification of humidity sources and advection mechanisms that control precipitation across the region. Using ERA5 reanalysis (1980-2020) and 10 weather stations’ data (2014-2020), synoptical and climatological analysis over the region were performed, namely regarding atmospheric moisture circulation and geopotential height and rainfall anomalies. Recent literature identifies the Angola Low (AL) and the Mozambique Channel Trough (MCT) as two main summertime low-pressure systems responsible for converting humidity into rainfall. The Zambezi and Limpopo river-valley’ associated low-level jets (ZRV LLJ and LRV LLJ) have been identified as major gateways transporting Indian Ocean moisture into central and SW Africa. Both LLJs are controlled by MCT and AL intensities, often conjugated within the same phase (both weak or both strong). Results showed these conjugations of MCT and AL intensities had a higher control on ZRV LLJ than on LRV LLJ, with weaker (stronger) MCT and AL being associated with a stronger (weaker) ZRV LLJ, thus promoting increases (decreases) of rainfalls over SW Africa. At a larger scale, it was shown that El-Niño (La-Niña) phases of ENSO were linked with northward (southward) shifts of the AL system and with decreases (increases) of precipitations across the subcontinent, regardless of SIOD (Subtropical Indian Ocean Dipole) phase. SW Africa region lies in a border-zone of weak and non-significant rainfall anomalies. An analysis of two exceptional rainy seasons showed that during the analysed events, large-scale variability and unusual circulation features could have overlapped its influence over both the AL and MCT systems. |
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| Autores principais: | Pereira, Carlos Afonso Jacob Carlota da Rosa |
| Assunto: | Jato de níveis baixos advecção de humidade sul de África Depressão de Angola Sistema ciclónico do Canal de Moçambique Teses de mestrado - 2022 |
| Ano: | 2022 |
| País: | Portugal |
| Tipo de documento: | dissertação de mestrado |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Lisboa |
| Idioma: | inglês |
| Origem: | Repositório da Universidade de Lisboa |
| Resumo: | Throughout the last decades, southern Africa has become increasingly vulnerable to weather variability, which affects moisture advection and convergence in the lower levels of the atmosphere and impact local rainfall, essential for rudimentary rainfed agricultural systems, in which most of countries’ economies rely on. Thus, understanding atmospheric circulation dynamics is crucial, as well as the identification of humidity sources and advection mechanisms that control precipitation across the region. Using ERA5 reanalysis (1980-2020) and 10 weather stations’ data (2014-2020), synoptical and climatological analysis over the region were performed, namely regarding atmospheric moisture circulation and geopotential height and rainfall anomalies. Recent literature identifies the Angola Low (AL) and the Mozambique Channel Trough (MCT) as two main summertime low-pressure systems responsible for converting humidity into rainfall. The Zambezi and Limpopo river-valley’ associated low-level jets (ZRV LLJ and LRV LLJ) have been identified as major gateways transporting Indian Ocean moisture into central and SW Africa. Both LLJs are controlled by MCT and AL intensities, often conjugated within the same phase (both weak or both strong). Results showed these conjugations of MCT and AL intensities had a higher control on ZRV LLJ than on LRV LLJ, with weaker (stronger) MCT and AL being associated with a stronger (weaker) ZRV LLJ, thus promoting increases (decreases) of rainfalls over SW Africa. At a larger scale, it was shown that El-Niño (La-Niña) phases of ENSO were linked with northward (southward) shifts of the AL system and with decreases (increases) of precipitations across the subcontinent, regardless of SIOD (Subtropical Indian Ocean Dipole) phase. SW Africa region lies in a border-zone of weak and non-significant rainfall anomalies. An analysis of two exceptional rainy seasons showed that during the analysed events, large-scale variability and unusual circulation features could have overlapped its influence over both the AL and MCT systems. |
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