Circulation pattern controls of wet days and dry days in Free State, South Africa

Chibuike Chiedozie Ibebuchi

doi:10.1007/s00703-021-00822-0

Abstract

Atmospheric circulation is a vital process in the transport of heat, moisture, and pollutants around the globe. The variability of rainfall depends to some extent on the atmospheric circulation. This paper investigates synoptic situations in southern Africa that can be associated with wet days and dry days in Free State, South Africa, in addition to the underlying dynamics. Principal component analysis was applied to the T-mode matrix (variable is time series and observation is grid points at which the field was observed) of daily mean sea level pressure field from 1979 to 2018 in classifying the circulation patterns in southern Africa. 18 circulation types (CTs) were classified in the study region. From the linkage of the CTs to the observed rainfall data, from 11 stations in Free State, it was found that dominant austral winter and late austral autumn CTs have a higher probability of being associated with dry days in Free State. Dominant austral summer and late austral spring CTs were found to have a higher probability of being associated with wet days in Free State. Cyclonic/anti-cyclonic activity over the southwest Indian Ocean, explained to a good extent, the inter-seasonal variability of rainfall in Free State. The synoptic state associated with a stronger anti-cyclonic circulation at the western branch of the South Indian Ocean high-pressure, during austral summer, leading to enhanced low-level moisture transport by southeast winds was found to have the highest probability of being associated with above-average rainfall in most regions in Free State. On the other hand, the synoptic state associated with enhanced transport of cold dry air, by the extratropical westerlies, was found to have the highest probability of being associated with (winter) dryness in Free State.

Highlights

Parts of southern Africa have already started experiencing extreme conditions (Sousa et al 2018; Archer et al 2019), and drought conditions are expected to increase in southern Africa under greenhouse-induced climate change (IPCC 2013)
In as much as it is common for the circulation types (CTs) to occur at any time of the year, the CTs can be further classified with respect to their dominance in either austral winter/autumn season or austral summer/ spring season
Wet days in Free State can be associated with (i) enhanced low-level transport of moisture by strong anticyclonic circulation at the western branch of the South Indian Ocean high-pressure coupled with the moistening of the Benguela region, (ii) and enhanced convective activity at the Agulhas current coupled with the formation of continental tropical lows at the western regions of southern Africa

Summary

Introduction

Parts of southern Africa have already started experiencing extreme conditions (e.g. drought) (Sousa et al 2018; Archer et al 2019), and drought conditions are expected to increase in southern Africa under greenhouse-induced climate change (IPCC 2013). The hydroclimate of South Africa which is located at the subtropical high-pressure cell is relatively more vulnerable to a changing climate making it necessary for enhanced predictability of extremes in the region, in addition to understanding the underlying mechanism associated with extreme events at specific regions. Free State is centrally located in South Africa, and its economy is largely dependent on maize agriculture. Zuma-Netshiukhwi et al (2013), most farmers in parts of the Free State are unfamiliar with the application of weather forecasts and still depend on their experience and traditional knowledge for farming decision-making.

Results

Discussion

Conclusion