Regional Winter Westerly Wave Variability and Cape Town's Day Zero Drought
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 in the WRZ and the relationship between the frequency of such states, 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 for 1979-2017. A self-organising map (SOM) analysis is performed on 500-hPa geopotential height. Nodes with strong troughs or ridges in the Westerly Wave are identified using rainfall or dynamic (divergence and vorticity) criteria. The two approaches produce similar results; around 80% of rainfall occurs on days mapped to trough nodes (35% of days), which we subjected to a multi-dimensional SOM analysis to identify the causes of variations in 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 500-hPa and their contribution to rainfall is assessed separately. COLs are detected for approximately 3% of all winter time steps, contributing between 10 and 15% 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 much 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 long-term increase in blocking high frequency.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A31N3127C
- Keywords:
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- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSESDE: 3319 General circulation;
- ATMOSPHERIC PROCESSESDE: 3337 Global climate models;
- ATMOSPHERIC PROCESSESDE: 4313 Extreme events;
- NATURAL HAZARDS