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Extreme rainfall events in the Limpopo Basin

Extreme rainfall events in the Limpopo Basin R Rapolaki (PhD student), R Blamey ( postdoc )., CJC Reason. 4 countries, SA, Botswana , Zimbabwe, Mozambique ~1750 km, 415000 km 2 Semi-arid except over southern Mozambique

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Extreme rainfall events in the Limpopo Basin

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  1. Extreme rainfall events in the Limpopo Basin R Rapolaki (PhD student), R Blamey (postdoc)., CJC Reason • 4 countries, SA, Botswana , Zimbabwe, Mozambique ~1750 km, 415000 km2 • Semi-arid except over southern Mozambique • Dec-Mar rainy season, virtually no rain in the other 8 months 20S 25S Radar at Irene

  2. Ongoing PhD research • Rapolaki, R. and C.J.C. Reason, 2018: Tropical storm Chedza and associated floods over south-eastern Africa, Nat. Haz., 93, 189-217, doi.org/10.1007/s11069-018-3295-y • Rapolaki, R., R. Blamey, J. Hermes and C.J.C. Reason, 2019: A classification of synoptic weather patterns linked to extreme rainfall over the Limpopo River basin in southern Africa. Climate Dyn., doi.org/10.1007/s00382-019-04829-7

  3. Highly variable climate on intraseasonal, interannual and interdecadal scales. Summer rainfall strongly impacted by ENSO and regional SST modes. Well-known 18-20 year signal in summer rainfall • Severe droughts during some El Ninos especially 82/3, 91/92, 15/16 • Occasionally impacted by landfalling tropical cyclones which led to floods in 2000, 2017. MCCs, tropical lows and tropical-extratropical cloud bands bring most of the summer rainfall. Intense cloud band produced 2013 floods Lower Limpopo near Chokwe – 25 Jan 2013 “Average” summer – 11 Feb 2005 ASTER TERRA NASA Earth Observatory

  4. Jan 2013 floods: cloud band preceded by tropical low

  5. Important circulation systems in summer Blamey et al 2018; Driver and Reason (2019)

  6. Blamey et al 2018

  7. Data Rainfall data • SAWS daily rainfall station data available from 1979 to 2014. • CHIRPS (0.05°) available from 1981- near present (Funk et al.,2015). Reanalysis data • 6- hourly CFSR (0.5° x 0.5°) data available since 1979-2010 (Saha et al., 2010) • 6- hourly CFSv2 (0.5° x 0.5°) A map showing rainfall stations over Limpopo

  8. Spatial distribution of rainfall: Station rainfall (left) VS CHIRPS

  9. Methods : Events identification

  10. Daily rainfall (shaded; units are in mm) of the top 20 ranked extreme rainfall events over the Limpopo River Basin. The contours in the panels represent the local topography, while for clarity a zoomed topography map is inserted in the bottom right. Symbols CB, TL, and MC in the bottom left corner of each panel denote the different rainfall mechanism associated with each event (CB- cloud band, TL- tropical low, MC- mesoscale convective system). The maximum rainfall during each event is given in the bottom right-hand corner – Rapolaki et al (2019).

  11. 1981-2016 total number of events in the top 200 for(a) October-December (early summer, in blue) and the standardized rainfall anomalies in red, (b) same as subplot a, but for January-April (late summer). Dashed lines are +/- 1 STD. The solid line in panels a and b denotes the average number of extreme events.

  12. (a) standardized rainfall anomalies over the basin (October-April), dashed lines are +/- 1STD; (b) number of days where rainfall amounts exceeded 10 (25) mm for at least 10% grid points within the LRB in grey (black) bars with the mean showed as the dashed (solid) lines; (c) the occurrence of the top 200 events for the entire basin, during the period 1981-2016, average number of events shown as dashed line.

  13. Correlations between event frequency and ENSO or SIOD in early summer are ~-0.25 and not significant

  14. Correlations in late summer between event frequency and ENSO are -0.47 and p < -0.05 Correlations with SIOD and SAM are weak

  15. Moisture flux climatology during late summers Moisture flux anomalies during late neutral summers with above average numbers of heavy rain events

  16. Moisture flux anomalies during La Nina late summers Moisture flux anomalies during late neutral summers with above average numbers of heavy rain events

  17. Vertical velocity anomalies at 500 hPa during late neutral summers with above average numbers of heavy rain events

  18. Summary • Most extreme rainfall events occur in late summer (JFM) and are due to cloud bands or tropical lows (including landfalling ex-tropical cyclones (Eline in 2000, Dineo in 2017) • Cut-off lows make important contributions in October and March on average • JFM events are significantly correlated with ENSO • Moisture flux anomalies during neutral ENSO summers with anomalously large numbers of extreme events show enhanced westerly moisture fluxes from tropical SE Atlantic and westerly anomalies across southern Africa that oppose incoming Indian Ocean easterly fluxes => convergence over Limpopo River basin • If time blocked Limpopo valley, MCT

  19. Why is the Limpopo region so dry ? WRF expts (Barimalala et al 2018) Madagascan topo results in a strong trough in the Channel that diverts moisture away What if the Limpopo valley didn’t exist?

  20. Enhanced rainfall over and downstream of the blocked Limpopo valley due to cyclonic moisture flux anomaly and increased uplift there Mountains in Madagascar and lack of mountains in Limpopo valley lead to semi-arid climate

  21. Monsoons MJO BNino floods L SCTR L ABFZ ITCZ TC S Atl HIGH COLs MOC S Ind Ocn HIGH Agulhas C & retroflection L

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