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Regional SH Westerly Wave Variability and Cape Town's "Day Zero'' Drought
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  • Willem Stefaan Conradie,
  • Bruce Hewitson,
  • Piotr Wolski,
  • Sabina Abba Omar
Willem Stefaan Conradie
Climate System Analysis Group, University of Cape Town

Corresponding Author:[email protected]

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Bruce Hewitson
Climate System Analysis Group, University of Cape Town
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Piotr Wolski
Climate System Analysis Group, University of Cape Town
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Sabina Abba Omar
Climate System Analysis Group, University of Cape Town
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Abstract

The recent severe “Day Zero” drought (2015-2017) over the winter rainfall zone (WRZ) of South Africa has highlighted low-frequency winter climate variability, possible trends and our generally poor understanding of their mechanisms. We investigate the contribution of dynamic conditions and moisture transport to daily station rainfall events in the WRZ and the relationship between the frequency of such states, seasonal to inter-decadal rainfall variability and hemispheric modes of Westerly Wave variability. Dynamic conditions are assessed using reanalysis data over the South-East Atlantic during the austral winter half-year (AMJJASO) from 1979-2017. A self-organising map (SOM) analysis is performed on 500-hPa geopotential height. Nodes indicating strong troughs and ridges in the Westerly Wave are identified using either rainfall or dynamic (divergence and vorticity) criteria. The two approaches produce similar results; most rainfall (around 80%) occurs on days mapped to trough nodes (35% of days). The trough nodes are subjected to a multi-dimensional SOM analysis to identify conditions leading to variations of rainfall within the original SOM nodes. Nodes showing intense (high divergence and vorticity west of South Africa) troughs extending equatorward of the WRZ (14% of all winter days) account for around 60% of trough rainfall. Cut-off lows (COLs) are independently identified as closed, cold-cored lows at the 500-hPa-level and their contribution to precipitation is assessed separately. COLs are detected on approximately 3% of all winter time steps, contributing only about 11% of the total rainfall, although they account for almost all heavy rainfall events not associated with intense troughs. During the Day Zero drought, the frequency of almost all trough nodes decreased, especially in the shoulder seasons, while ridge nodes occurred 1.5-2 times more frequently and persisted for longer, especially in late autumn. Average rainfall per trough node was lower and COL frequency reduced. The only SOM nodes showing significant trend over 1979-2017 are ridge nodes associated with large anticyclonic vorticity anomalies south of the WRZ. Correlation between the Southern Annular Mode and ridge/trough nodes is weak. We conclude that the Day Zero drought resulted from fewer mobile troughs passing the WRZ in the shoulder seasons, possibly linked to a multi-decadal increase in blocking high frequency.