Interannual to interdecadal variability of winter and summer southern African rainfall, and their teleconnections

Abstract

Original citation: Dieppois, B. , Pohl, B. , Rouault, M. , New, M. , Lawler, D. and Keenlyside, N. (2016) Interannual to Interdecadal variability of winter and summer southern African rainfall, and their teleconnections. Abstract This study examines for the first time the changing characteristics of summer and winter southern African rainfall and their teleconnections with large-scale climate through the dominant time scales of variability. As determined by wavelet analysis, the austral summer and winter rainfall indices exhibit three significant time scales of variability over the twentieth century: interdecadal (15-28 years), quasi-decadal (8-13 years), and interannual (2-8 years). Teleconnections with global sea surface temperature and atmospheric circulation anomalies are established here but are different for each time scale. Tropical/subtropical teleconnections emerge as the main driver of austral summer rainfall variability. Thus, shifts in the Walker circulation are linked to the El Niño-Southern Oscillation (ENSO) and, at decadal time scales, to decadal ENSO-like patterns related to the Pacific Decadal Oscillation and the Interdecadal Pacific Oscillation. These global changes in the upper zonal circulation interact with asymmetric ocean-atmospheric conditions between the South Atlantic and South Indian Oceans; together, these lead to a shift in the South Indian Convergence Zone and a modulation of the development of convective rain-bearing systems over southern Africa in summer. Such regional changes, embedded in quasi-annular geopotential patterns, consist of easterly moisture fluxes from the South Indian High, which dominate southerly moisture fluxes from the South Atlantic High. Austral winter rainfall variability is more influenced by midlatitude atmospheric variability, in particular the Southern Annular Mode. The rainfall changes in the southwestern regions of southern Africa are determined by asymmetrical changes in the midlatitude westerlies between the Atlantic and Indian Oceans.

Anthropogenic influence on the drivers of the Western Cape drought 2015–2017

“…The small number of stations contributing to the CRU data estimate, combining both weak and strong drought signals because of the complex topography of the region is one reason for the uncertainty in the observational analysis. Further, the length of the station data is likely also important: the analysed region has a quasi 40 years component (Dieppois et al 2016) that underlies the previous severe droughts recorded in 1930s and 1970s, and the records' length is bound to reflect this multidecadal variability in rainfall.…”

Correlations Between Extreme Atmospheric Hazards and Global Teleconnections: Implications for Multihazard Resilience

“…Positive (negative) ENSO events are correlated with high (low) pressure over Southern Africa that produce anomalous midtropospheric descent (ascent) and moisture divergence (convergence) that suppresses (enhances) convection, resulting in lower (higher) than average precipitation (Crétat et al, ; Hoell et al, ; Pohl et al, ; Ratnam et al, ). The correlation coefficient between precipitation and December–March (Austral summer) Pacific sea surface temperature anomalies are estimated to be −0.30 to ≥−0.4 (≥90% level (Dieppois et al, ; Hoell et al, ; Saji & Yamagata, ), but Hoell et al (, among others) also show that this is strongly modulated by the concurrent activity of the IOD. Depending on the phase of the subtropical IOD, it can either complement or disrupt the precipitation response to ENSO.…”

“…When ENSO and the subtropical IOD were in phase, Southern Africa precipitation is only marginally reduced during El Niño, and precipitation is only marginally enhanced during La Niña. Other authors have suggested that the Pacific Decadal Oscillation may also have a role in modulating ENSO‐driven precipitation anomalies (e.g., Dieppois et al, ; S. Wang et al, ).…”

Variability in the Botswana High and its relationships with rainfall and temperature characteristics over southern Africa

“…Southern Africa is a region of pronounced climate variability on a range of scales (Tyson et al, 1975;Mason, 1990Mason, , 1995Lindesay, 1988;Reason, 1998;Reason et al, 2000;Richard et al, 2001;Reason and Rouault, 2002;Rouault et al, 2002;Colberg et al, 2004;Cook et al, 2004;Washington and Preston, 2006;Singleton and Reason, 2007;Pohl et al, 2009Pohl et al, , 2010Crétat et al, 2012;Malherbe et al, 2014;Grimm and Reason, 2015;Dieppois et al, 2016) whose relationships with regional circulation systems are still not well understood. One such prominent circulation system in the mid-levels of the troposphere over southern Africa that occurs during austral spring, summer and early autumn is the Botswana High.…”