Abstract
Southern east Africa is prone to some extreme weather events and interannual variability of the hydrological cycle, including tropical cyclones and heavy rainfall events. Most of these events occur during austral summer and are linked to shifts in the intertropical convergence zone, changes in El Niño Southern Oscillation signatures, sea surface temperature and sea level pressure. A typical example include mesoscale convective systems (MCSs) that occur between October and March along the eastern part, adjacent to the warm waters of Mozambique Channel and Agulhas Current. In this study we discuss a heavy rainfall event over southern Africa, focusing particularly on the period 15–20 January 2013, the period during which MCSs were significant over the subcontinent. This event recorded one of the historic rainfalls due to extreme flooding and overflows, loss of lives and destruction of economic and social infrastructure. An active South Indian Convergence Zone was associated with the rainfall event sustained by a low-level trough linked to a Southern Hemisphere planetary wave pattern and an upper-level ridge over land. In addition, also noteworthy is a seemingly strong connection to the strength of the African Easterly Jet stream. Using rainfall data, satellite imagery and re-analysis (model processed data combined with observations) data, our analysis indicates that there was a substantial relation between rainfall totals recorded/observed and the presence of MCSs. The low-level trough and upper-level ridge contributed to moisture convergence, particularly from tropical South East Atlantic Ocean, which in turn contributed to the prolonged life span of the rainfall event. Positive temperature anomalies favored the substantial contribution of moisture fluxes from the Atlantic Ocean. This study provides a contextual assessment of rainfall processes and insight into the physical control mechanisms and feedback of large-scale convective interactions over tropical southern Africa.
Highlights
Southern Africa experiences considerable spatial and temporal variability in climate [1,2,3].The subcontinent is susceptible to a wide range of weather phenomena including cut-off lows, thunderstorms, warm fronts, mesoscale convective systems (MCSs), subtropical lows, mid- to upper-tropospheric troughs, cloud bands and tropical cyclones [4,5]
Satellite imagery and re-analysis data, our analysis indicates that there was a substantial relation between rainfall totals recorded/observed and the presence of MCSs
This study provides a contextual assessment of rainfall processes and insight into the physical control mechanisms and feedback of large-scale convective interactions over tropical southern Africa
Summary
The subcontinent is susceptible to a wide range of weather phenomena including cut-off lows, thunderstorms, warm fronts (narrow transition boundaries between air masses), mesoscale convective systems (MCSs), subtropical lows, mid- to upper-tropospheric troughs, cloud bands and tropical cyclones [4,5]. The subcontinent is generally characterized by two distinct seasons: a wet season running from October to March and a dry season running from April to September. Maximum precipitable water content occurs between December and March when the low-level winds are more easterly, corresponding to the Climate 2019, 7, 73; doi:10.3390/cli7060073 www.mdpi.com/journal/climate. Climate 2019, 7, 73 southwards displacement of the intertropical convergence zone (ITCZ) (maximum displacement occurs in January). A dry season occurs when the ITCZ retreats northward (maximum displacement occurs in July). The most influential climate factors include ocean currents, ITCZ and the quasi-stationary high-pressure systems
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