Moisture sources associated with heavy rainfall over the Limpopo River Basin, southern Africa
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Moisture sources associated with heavy rainfall over the Limpopo River Basin, southern Africa R. S. Rapolaki1,2 · R. C. Blamey1 · J. C. Hermes1,2 · C. J. C. Reason1 Received: 30 January 2020 / Accepted: 12 June 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Moisture sources and pathways over the Limpopo River Basin (LRB) in southern Africa were identified using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model applied to NCEP II (2.5° × 2.5°) reanalysis data for 1981–2016. The 10-day air parcel backward trajectories were produced for the extended wet season (October–April) as well as the early and late summer. Analysis of a 36-year climatology of air parcel trajectories indicated seven moisture source regions for the LRB; namely, local continental, tropical southeast Atlantic Ocean, midlatitude South Atlantic Ocean, tropical northwest Indian Ocean, tropical southwest Indian Ocean, subtropical southwest Indian Ocean, and the Agulhas Current. The results have shown that important differences in moisture source regions and pathways exist between early (October– December) and late (January–April) summers, with the tropical northwestern Indian Ocean and the northern Agulhas Current sources more prominent during JFMA than OND. On interannual time scales, there are notable differences in moisture source regions between anomalously wet and dry summers, with the South Indian Ocean moisture contribution and transport over the LRB smaller during dry summers than wet summers. Changes in specific humidity for trajectory linked to heavy daily rainfall events (defined from CHIRPS data) showed that subtropical South Indian Ocean source is more extensive for the heavy rainfall than for the moderate case. Generally, moisture source regions and transport pathways for LRB tend to be influenced by both the regional summer season circulation and the synoptic setting. Keywords Moisture-transport · Southern Africa · Limpopo River Basin · Rainfall · HYSPLIT
1 Introduction The Limpopo River Basin (LRB; Fig. 1) in southern Africa is a semi-arid to arid region prone to frequent droughts and occasional devastating flooding events whose causes are still not well understood. It contains a relatively poor rural population many of whom depend directly on rain-fed sources for farming and drinking water. Although there is some largescale irrigated agriculture, subsistence farming is common in the region. In addition, there are important national park and conservation areas in various parts of the LRB. While drought is common (Mulenga et al. 2003; Reason et al. * C. J. C. Reason [email protected] 1
Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, Cape Town 7701, South Africa
SAEON Egagasini Node, Private Bag X2, Roggebaai, Cape Town 8012, South Africa
2
2005), the LRB sometimes experiences flooding events during the summer rainy season (October–April), due to various synoptic weather systems (Rapolaki et al. 2019). These systems include cloud ba
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