Investigating changes in rainfall seasonality across South Africa: 1987–2016

Abstract

Changes in rainfall seasonality across South Africa may have detrimental impacts on crop yields and surface water supplies. Despite this, these have rarely been investigated across the South African summer-, winter-and yearround rainfall zones (SRZ, WRZ and YRZ respectively) using consistent methods. Rainfall seasonality changes are explored here using daily rainfall and temperature records for 1987-2016 from 46 meteorological stations spanning a grid across South Africa. Focusing on comparable methods that statistically define the SRZ, WRZ, YRZ, we apply a ratio of monthly rainfall: temperature and a percentile-based wet-season start-and end-date metric, which each provide annual outputs of the degree of seasonality, and the timing, duration, magnitude and intensity of wet-season rainfall. Although relatively few statistically significant trends are detected, some regional spatial patterns are evident. Trends for most locations across the southwestern Cape and western coast are indicative of a decrease in the strength of seasonality with a reduction in winter wet-season rainfall, and are broadly consistent with observed and projected poleward dynamics of the southern hemisphere subtropical high-pressure belt and mid-latitude westerlies. Trends towards later wet-season start-dates together with a reduction in summer wet-season rainfall amount are prominent across most of the interior and northeastern locations, and may be linked to Hadley cell expansion recorded in recent decades. Along the southern coast, trends towards an increase in the degree of seasonality together with a shorter wet-season, concentrated around the winter months, are evident. If sustained, the trends quantified present a concerning outlook for continued crop production and water resource management, and thus warrant further investigations on rainfall seasonality shifts and the drivers thereof across southern Africa.

Assessment of smallholder farmers’ perception and adaptation response to climate change in the Olifants catchment, South Africa

“…In the past decades, early rainfall in the catchment usually commences at the beginning of spring month (September), which prompted the starting of planting season; however, this has now shifted to October-November due to climate change (Shoko et al 2019). Similar findings were also reported in Roffe et al (2020), indicating the delay in wet season start date along with a reduction in summer wet season rainfall.…”

Multi-Decadal Variability and Future Changes in Precipitation over Southern Africa

“…Thus, SA is regarded as a climate change hotspot [27]. Several studies (e.g., [4,25,28,29]) have pointed to exacerbating situations, with an overall decrease in SA seasonal precipitation over the last century. However, SA's large zonal and meridional extent leads to significant differences in local precipitation characteristics.…”

South African winter rainfall zone shifts: A comparison of seasonality metrics for Cape Town from 1841–1899 and 1933–2020

“…Notably, the SQMKT results for the scores and wetseason length appear to broadly track each other in direction throughout 1841-2020 (Fig. 3a, d), thus highlighting good agreement between the two methods (Roffe et al 2021a). The U(t) line re ects cyclic patterns, with periods of stronger and weaker seasonality (Fig.…”

“…Using this metric, summer-, winter-and year-round rainfall regimes are distinguished primarily through the wet-season timing and duration, where WRZ conditions are classi ed when the wet-season is shorter than nine months (~ 270 days) and includes the winter solstice, but not the summer solstice (Roffe et al 2020). For the wet-season, we quanti ed annual start-and end-dates, length, total rainfall, number of rain days and daily rainfall rate (Roffe et al 2020(Roffe et al , 2021a. As wet-season start-and end-dates broadly equate to dry-season end-and start-dates, respectively, and can be used to infer the dry-season length, only the dry-season total rainfall, number of rain days and daily rainfall rate were calculated.…”