A classification of synoptic weather patterns linked to extreme rainfall over the Limpopo River Basin in southern Africa
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In the last few decades, the Limpopo River Basin (LRB) has experienced a number of extreme rainfall events which were responsible for considerable socio-economic and environmental impacts. Most of the population here is poor and dependent on rain-fed agriculture. In order to better understand these events over the LRB, CHIRPS, 0.05° gridded rainfall data are used to identify the daily extreme events, analyse their interannual variability and examine relationships with large scale climate modes over the 1981–2016 period. Analysis of the top 20 events suggests a pattern with rainfall generally decreasing from the eastern to western parts of the basin. Typically, the highest rainfall amounts occur over the regions where there are steep topographical gradients between the mountainous regions of northeastern South Africa and the Mozambican floodplains. Almost half of the top 200 extreme events are associated with tropical extra-tropical cloud bands (48%), with tropical low-pressure systems (28%), Mesoscale Convective Systems (14%), and cut-off lows (10%) in the mid-upper atmosphere, also making sizeable contributions. The monthly distribution of the events showed that most of the events occurred during the late summer months (January–March) when tropical lows and cloud bands are more common. On interannual time-scales, most of the summers with above average number of events coincide with La Niña conditions and, to lesser extent, a positive subtropical South Indian Ocean Dipole.
KeywordsExtreme rainfall events Limpopo River Basin Cloud bands Tropical low-pressure ENSO
The authors thank the anonymous reviewer for helping improve the original manuscript. Ramontsheng Rapolaki was funded by the National Research Foundation (NRF) through the South African Environmental Observation Network (SAEON; Grant number: 101035) and the University of Cape Town (UCT) Doctoral Research Scholarship. Ross Blamey and Chris Reason were supported by the Natural Environment Research Council (NERC) Future Climate For Africa (FCFA) regional consortium project ‘UMFULA’ (NE/M020223/1) and the EU Water JPI funded IMDROFLOOD project. The authors thank Neil Hart for providing the cloud bands dates. Daily CHIRPS data have been downloaded from the Climate Hazards Group data website (http://chg.geog.ucsb.edu/data/). The 3-hourly GridSat-B1 and daily IBTrACS data were obtained from the National Centers for Environmental Information (NOAA) website https://www.ncdc.noaa.gov/. South African Weather Service (SAWS) provided the synoptic charts. The monthly Niño 3.4 index, SIOD index, and the SAM index were downloaded from the KNMI Climate Explorer (https://climexp.knmi.nl/, https://climexp.knmi.nl/). We acknowledge the use of NCEP/NCAR reanalyses dataset. Opinions expressed and conclusions arrived at are those of the author and are not necessarily to be attributed to SAEON.
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