Abstract

Precipitation levels in southern Africa exhibit a marked east-west gradient, and are characterised by strong seasonality and high interannual variability. Much of the mainland south of 15°S exhibits a semi-arid to dry subhumid climate. More than 66% of rainfall in the extreme southwest of the subcontinent occurs between April and September. Rainfall in this region - termed the "winter rainfall zone" (WRZ) - is most commonly associated with the passage of mid-latitude frontal systems embedded in the austral westerlies. In contrast, more than 66% of mean annual precipitation over much of the remainder of the subcontinent falls between October and March. Climates in this "summer rainfall zone" (SRZ) are dictated by the seasonal interplay between subtropical high-pressure systems and the migration of easterly flows associated with the Intertropical Convergence Zone. Fluctuations in both SRZ and WRZ rainfall are linked to variability of sea surface temperatures in the oceans surrounding southern Africa, and are modulated by the interplay of large scale modes of climate variability, including the El Niño-Southern Oscillation (ENSO), Southern Indian Ocean Dipole and Southern Annular Mode. Ideas about the nature of long-term rainfall variability in southern Africa have shifted over time. During the early- to mid-19th century, the prevailing narrative was that the climate was progressively desiccating. By the late-19th to early-20th century, when gauged precipitation data became more readily available, debate shifted towards the identification of cyclical rainfall variation. The integration of gauge data, evidence from historical documents, and information from natural proxies such as tree-rings during the late-20th and early 21st centuries has allowed the nature of precipitation variability since ~1800 to be more fully explored. Drought episodes affecting large areas of the SRZ occurred during the first decade of the 19th century, in the early and late 1820s, late 1850s-mid 1860s, mid-late 1870s, early-mid 1880s and mid-late 1890s. Of these, the drought during the early 1860s was the most severe of the 19th century, with the droughts of the 1820s and 1890s the most protracted. Many of these droughts correspond with more extreme ENSO warm phases. Widespread wetter conditions are less easy to identify. The year 1816 appears to have been relatively wet across the Kalahari and other areas of south central Africa. Other wetter episodes were centred on the late 1830s-early 1840s, 1855, 1870 and 1890. In the WRZ, drier conditions occurred during the first decade of the 19th century, for much of the mid-late 1830s through to the mid-1840s, during the late 1850s and early 1860s, and in the early-mid 1880s and mid-late 1890s. As for the SRZ, markedly wetter years are less easily identified, although the periods around 1815, the early 1830s, the mid 1840s, mid-late 1870s and early 1890s saw enhanced rainfall. Reconstructed rainfall anomalies for the SRZ suggest that, on average, the region was significantly wetter during the 19th century than the 20th, and that there appears to have been a drying trend during the 20th century that has continued into the early 21st. In the WRZ, average annual rainfall levels appear to have been relatively consistent between the 19th and 20th centuries, although rainfall variability increased during the 20th century compared to the 19th.
Original languageEnglish
PublisherOxford University Press
Place of PublicationOxford Research Encyclopedia of Climate Science
DOIs
Publication statusPublished - 1 Feb 2017

Fingerprint

rainfall
drought
summer
Southern Africa
winter
Southern Oscillation
climate
high pressure system
intertropical convergence zone
twenty first century
tree ring
seasonality
gauge
sea surface temperature
anomaly

Bibliographical note

Changes in precipitation over southern Africa during recent centuries by David Nash, 2017, reproduced by permission of Oxford University Press http://climatescience.oxfordre.com/view/10.1093/acrefore/9780190228620.001.0001/acrefore-9780190228620-e-539

Keywords

  • southern Africa
  • rainfall variability
  • 19th century
  • precipitation reconstruction
  • documentary sources

Cite this

Nash, D. (2017, Feb 1). Changes in precipitation over southern Africa during recent centuries. Oxford Research Encyclopedia of Climate Science: Oxford University Press. https://doi.org/10.1093/acrefore/9780190228620.013.539
Nash, David. / Changes in precipitation over southern Africa during recent centuries. 2017. Oxford Research Encyclopedia of Climate Science : Oxford University Press.
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Changes in precipitation over southern Africa during recent centuries. / Nash, David.

Oxford Research Encyclopedia of Climate Science : Oxford University Press. 2017, .

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N2 - Precipitation levels in southern Africa exhibit a marked east-west gradient, and are characterised by strong seasonality and high interannual variability. Much of the mainland south of 15°S exhibits a semi-arid to dry subhumid climate. More than 66% of rainfall in the extreme southwest of the subcontinent occurs between April and September. Rainfall in this region - termed the "winter rainfall zone" (WRZ) - is most commonly associated with the passage of mid-latitude frontal systems embedded in the austral westerlies. In contrast, more than 66% of mean annual precipitation over much of the remainder of the subcontinent falls between October and March. Climates in this "summer rainfall zone" (SRZ) are dictated by the seasonal interplay between subtropical high-pressure systems and the migration of easterly flows associated with the Intertropical Convergence Zone. Fluctuations in both SRZ and WRZ rainfall are linked to variability of sea surface temperatures in the oceans surrounding southern Africa, and are modulated by the interplay of large scale modes of climate variability, including the El Niño-Southern Oscillation (ENSO), Southern Indian Ocean Dipole and Southern Annular Mode. Ideas about the nature of long-term rainfall variability in southern Africa have shifted over time. During the early- to mid-19th century, the prevailing narrative was that the climate was progressively desiccating. By the late-19th to early-20th century, when gauged precipitation data became more readily available, debate shifted towards the identification of cyclical rainfall variation. The integration of gauge data, evidence from historical documents, and information from natural proxies such as tree-rings during the late-20th and early 21st centuries has allowed the nature of precipitation variability since ~1800 to be more fully explored. Drought episodes affecting large areas of the SRZ occurred during the first decade of the 19th century, in the early and late 1820s, late 1850s-mid 1860s, mid-late 1870s, early-mid 1880s and mid-late 1890s. Of these, the drought during the early 1860s was the most severe of the 19th century, with the droughts of the 1820s and 1890s the most protracted. Many of these droughts correspond with more extreme ENSO warm phases. Widespread wetter conditions are less easy to identify. The year 1816 appears to have been relatively wet across the Kalahari and other areas of south central Africa. Other wetter episodes were centred on the late 1830s-early 1840s, 1855, 1870 and 1890. In the WRZ, drier conditions occurred during the first decade of the 19th century, for much of the mid-late 1830s through to the mid-1840s, during the late 1850s and early 1860s, and in the early-mid 1880s and mid-late 1890s. As for the SRZ, markedly wetter years are less easily identified, although the periods around 1815, the early 1830s, the mid 1840s, mid-late 1870s and early 1890s saw enhanced rainfall. Reconstructed rainfall anomalies for the SRZ suggest that, on average, the region was significantly wetter during the 19th century than the 20th, and that there appears to have been a drying trend during the 20th century that has continued into the early 21st. In the WRZ, average annual rainfall levels appear to have been relatively consistent between the 19th and 20th centuries, although rainfall variability increased during the 20th century compared to the 19th.

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