Editing IPCC:AR6/WGI/Chapter-10 (section)

==== 10.6.2.4 Relevant Anthropogenic and Natural Drivers ====

<div id="h3-58-siblings" class="h3-siblings"></div>

Because the primary rainfall mechanism is frontal rain, the most relevant large-scale drivers are those that affect cyclogenesis, frontogenesis and the mid-latitude westerlies’ latitudinal position and moisture supply. These drivers and, thus, the region’s rainfall are linked to the Antarctic Oscillation (AAO; [[#Reason--2005|Reason and Rouault, 2005]] ) or Southern Annular Mode (SAM), the dominant monthly and interannual mode of Southern Hemisphere atmospheric variability, and a measure of the pressure gradient between mid- and high latitudes. (See Sections 3.3, 3.7, 4.3 and Annex IV.2.2 for more general discussion of the SAM.) While in the post-1930 period, the SAM displays a long-term positive trend, the Cape Town region’s rainfall does not, and only the post-1979 trends of rainfall and SAM are conceptually consistent. For example, a positive trend in the SAM is associated with a negative trend in rainfall ( [[#10.6.2.5|Section 10.6.2.5]] and Figure 10.18). There is also good agreement between the seasonality of the SAM and rainfall trends in the post-1979 period: a drying trend appears strongly in December to February and March to May, but not in June to August and September to November ( [[#Wolski--2021|Wolski et al., 2021]] ), and trends in the SAM have similar seasonal dependence (E.-P. [[#Lim--2016|]] [[#Lim--2016|Lim et al., 2016]] ; [[IPCC:Wg1:Chapter:Chapter-3#3.7.2|Section 3.7.2]] ). Additionally, there is a similar seasonal pattern in the post-1979 trends in indices capturing the southern edge of the Hadley circulation ( [[#Grise--2018|Grise et al., 2018]] ).

In the longer-term, Cape Town regional rainfall is characterized by a multi-decadal scale quasi-periodicity (Figure 10.18; [[#Dieppois--2019|Dieppois et al., 2019]] ; [[#Wolski--2021|Wolski et al., 2021]] ), with the 2015–2017 drought and previous strong droughts (1930s and 1970s) occurring during the rainfall’s periodic low phases. However, the studies linking the Cape Town 2015–2017 drought to the hemispheric processes expressed by the SAM ( [[#Sousa--2018a|Sousa et al., 2018a]] ; [[#Burls--2019|Burls et al., 2019]] ; [[#Mahlalela--2019|Mahlalela et al., 2019]] ) focused almost exclusively on the post-1979 period, when global reanalyses are available. Detailed understanding of the drivers of previous (1930s and 1970s) Cape Town region droughts and the role of hemispheric processes expressed by the SAM in the pre-1979 period is missing.

The Cape Town regional rainfall is also potentially linked to other hemispheric phenomena, such as the expansion of the tropics and, specifically, the South Atlantic high-pressure system and the position of the subtropical jet, which share some variability with the SAM. The relationships between these phenomena and Cape Town rainfall have not been thoroughly investigated outside of the context of the 2015–2017 drought, but the drought itself was associated with poleward expansion of the subtropical anticyclones in the South Atlantic and South Indian oceans and (a resulting) poleward displacement of the moisture corridor across the South Atlantic ( [[#Sousa--2018a|Sousa et al., 2018a]] ), as well as a weaker subtropical jet ( [[#Mahlalela--2019|Mahlalela et al., 2019]] ). [[#Burls--2019|Burls et al. (2019)]] also link the decline in the number of rainy days to the increase in sea level pressure along the poleward flank of the South Atlantic high-pressure system and the intensity of the post-frontal ridging high. Additionally, there is a possible linkage between Cape Town rainfall and near-shore cold sea surface temperature (SST) anomalies arising from Ekman upwelling due to reduced westerly and increased south-easterly winds. These might lead to suppression of convection and reduction of rainfall over land ( [[#Rouault--2010|Rouault et al., 2010]] ). All these phenomena are conceptually consistent with the poleward migration of the westerlies and expansion of the tropics.

Rainfall in the Cape Town region also responds to SST anomalies in the south-east Atlantic, including the Agulhas Current retroflection region, which may drive intensification of low-pressure systems, leading to the trailing front strengthening as it makes landfall over the Cape Town region ( [[#Reason--2005|Reason and Jagadheesha, 2005]] ). There are also linkages at the seasonal time scale between the Cape Town regional rainfall and Antarctic sea ice ( [[#Blamey--2007|Blamey and Reason, 2007]] ).

In addition to mid-latitude controls, subtropical processes also play a role in the Cape Town region’s rainfall variability. The 10°S–30°S region of the subtropical Atlantic, parts of the South American continent and even parts of the African continent north of Cape Town are sources of moisture for atmospheric river events contributing to frontal rainfall ( [[#Blamey--2018|Blamey et al., 2018]] ; [[#Ramos--2019|Ramos et al., 2019]] ), with implications for the 2015–2017 drought ( [[#Sousa--2018a|Sousa et al., 2018a]] ). Also, the second major rainfall contributing system, cut-off-lows, is conditional on moisture supply from the subtropics ( [[#Abba%20Omar--2020|Abba Omar and Abiodun, 2020]] ).

Although El Niño–Southern Oscillation (ENSO) influences climate in southern Africa, any relationship between ENSO and Cape Town’s rainfall is weak and inconsistent, showing the strongest impact in May to June ( [[#Philippon--2012|Philippon et al., 2012]] ). ENSO, however, does influence large-scale processes and phenomena relevant to the drought, though the relationship between ENSO and the SAM is complex, with each ENSO event influencing the SAM differently in different seasons ( [[#Ding--2012|Ding et al., 2012]] ). Similarly, ENSO affects meridional circulation and thus the subtropical anticyclone as well as the polar and subtropical jets ( [[#Seager--2019|Seager et al., 2019]] ), but only modifying, not controlling, their role in Cape Town’s rainfall.

Paleoclimate studies reveal that long-term variability in the winter rainfall region of South Africa (including Cape Town) is consistent with a general framework of warming/cooling-induced latitudinal migration of the westerlies and transformation of the subtropical high-pressure belt and associated hemispherical processes (see section 10.2.3.2 for assessment of paleoclimate analysis). The synchronicity of winter rainfall with Antarctic ice-core-derived polar temperature anomalies is consistently revealed in studies using different paleoclimate proxies and time scales of 1400 years ( [[#Stager--2012|Stager et al., 2012]] ), about 3000 years ( [[#Hahn--2016|Hahn et al., 2016]] ) and 12,000 years ( [[#Weldeab--2013|Weldeab et al., 2013]] ). Changes in rainfall regimes at shorter (decadal) time scales appear to reflect influence of local processes such as the Agulhas current’s interaction with the Atlantic, resulting in changes in SST and coastal upwelling, as well as modification of the wind tracks by topography ( [[#Stager--2012|Stager et al., 2012]] ).

<div id="10.6.2.5" class="h3-container"></div>

<span id="model-simulation-and-attribution-over-the-historical-period"></span>