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

=== 10.6.2 Cape Town Drought ===

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==== 10.6.2.1 Motivation and Regional Context ====

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Cape Town’s ‘Day Zero’ water crisis in 2018 threatened a shut-down of water supply to 3.4 million inhabitants of the city and resulted in domestic water use restriction of 50 litres per person per day lasting for nine months (pre-drought unconstrained water use was about 170 litres per person per day, [[#DWA--2013|DWA, 2013]] ), punitive water tariffs, and temporary closure of irrigation systems. Problems with water supply in many large cities in developing countries are endemic and rarely reported internationally. The water crisis in Cape Town attracted considerable international attention to a city with functional government structures, well-developed services (compared to other urban centres in Africa), a centre of international tourism, and an economic hub with GDP of 22 billion USD (about 7,500 USD per capita, [[#Gallie--2018|Gallie et al., 2018]] ). Economic and social impacts of the crisis were significant. Loss of revenue for companies of all sizes resulted not only from the scaling down of water-dependent activities, but also from the need to invest in water-efficient technologies and processes. Tourism was affected through reduced arrivals and bookings, although only temporarily ( [[#CTT--2018|CTT, 2018]] ). In the agricultural sector, 30,000 people were laid-off and production dropped by 20% ( [[#Piennaar--2018|Piennaar and Boonzaaier, 2018]] ). The crisis initially polarized society, with conflict emerging between various water users and erosion of trust in the government, but eventually social cohesion and an acute awareness of limited water resources emerged ( [[#Robins--2019|Robins, 2019]] ).

Cape Town’s crisis resulted from a combination of a strong, rare multi-year meteorological drought (Figure 10.18), estimated at 1 in 300 years ( [[#Wolski--2018|Wolski, 2018]] ), and factors related to the nature of the water supply system, operational water management and water resource policies. Cape Town was very successful in implementing water-saving actions after the previous drought of 2000–2003, reducing water losses from over 22% to 15% ( [[#Frame--2007|Frame and Killick, 2007]] ; [[#DWA--2013|DWA, 2013]] ), breaking the previous coupling of growth in water demand with growth in population. As a consequence, Cape Town won a Water Smart City award from the C40 Cities program only three years prior to the crisis. However, the water-saving actions, together with changing priorities in water resource provision from infrastructure-oriented towards resource and demand management, may well have led to delays in implementation of the expansion of water supply infrastructure ( [[#Muller--2018|Muller, 2018]] ). The expansion plan, formulated a decade prior to the crisis, included an expectation of long-term climate-change drying in the region ( [[#DWAF--2007|DWAF, 2007]] ). The crisis also exposed structural deficiencies of water management and inadequacy of a policy process in which decisions about local water resources are taken at a national level, particularly in a situation of political tension ( [[#Visser--2018|Visser, 2018]] ). The crisis was widely seen as a harbinger of future problems to be faced by the city, and a highlight of vulnerability of many cities in the world resulting from the interplay of three factors: (i) the fast urban-population growth, (ii) the economic, policy, infrastructural and water resource paradigms and constraints, and (iii) anthropogenic climate change.

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'''Figure 10.1''' '''8 |''' '''Historical and projected rainfall and Southern Annular Mode (SAM) over the Cape Town region. (a)''' Yearly accumulation of rainfall (in mm) obtained by summing monthly totals between January and December, with the drought years 2015 (orange), 2016 (red), and 2017 (purple) highlighted in colour. '''(b)''' Monthly rainfall for the drought years (in colour) compared with the 1981–2014 climatology (grey line). Rainfall in (a) and (b) is the average of 20 quality controlled and gap-filled series from stations within the Cape Town region (31°S–35°S, 18°W–20.5°W). '''(c)''' Time series of the SAM index and of historical and projected rainfall anomalies (%, baseline 1980–2010) over the Cape Town region. Observed data presented as 30-year running means of relative total annual rainfall over the Cape Town region for station-based data (black line, average of 20 stations as in (a) and (b), and gridded data (average of all gridcells falling within 31°S–35°S, 18°W–20.5°W), GPCC (green line) and CRU TS (olive line). Model ensemble results presented as the 90th-percentile range of relative 30-year running means of rainfall and the SAM index from 35 CMIP5 (blue shading) and 35 CMIP6 (red shading) simulations, 6 CORDEX simulations driven by 1 to 10 GCMs (cyan shading), 6 CCAM (purple shading) simulations from individual ensemble members, and 50 members from the MIROC6 SMILE simulations (orange shading). The light blue, dark red and yellow lines correspond to NCEP/NCAR, ERA20C and 20CR, respectively. The SAM index is calculated from sea level pressure reanalysis and GCM data as per [[#Gong--1999|Gong and Wang (1999)]] and averaged over the aforementioned bounding box. CMIP5, CORDEX and CCAM projections use RCP8.5, and CMIP6 and MIROC6 SMILE projections use SSP5-8.5. '''(d)''' Historical and projected trends in rainfall over the Cape Town region and in the SAM index. Observations and gridded data processed as in (c). Trends calculated as Theil-Sen trend with block-bootstrap confidence interval estimate. Markers show median trend, bars 95% confidence interval. Global models in each CMIP group were ordered according to the magnitude of trend in rainfall, and the same order is maintained in panels showing trends in the SAM. Further details on data sources and processing are available in the chapter data table (Table 10.SM.11).

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==== 10.6.2.2 The Region’s Climate ====

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An evaluation of the relative role of rainfall and temperature signal in the 2015–2017 hydrological drought gives a strong indication that lack of rainfall was the primary driver ( [[#Otto--2018|Otto et al., 2018]] ) leading to the 2018 water crisis. Thus, the remainder of this section focuses on rainfall. [[IPCC:Wg1:Chapter:Chapter-11#11.6|Section 11.6]] offers a discussion of African drought over broader areas, including mechanisms relevant to them.

Cape Town is located at the south-western tip of Africa, within an approximately 100 km × 300 km region that receives 80% of its rainfall during the austral winter (March to October), with the largest portion in June to August. In the vicinity of Cape Town, rainfall is strongly heterogeneous, ranging from about 300 mm/year on coastal plains to &gt;2,000 mm/year in mountain ranges. The Cape Town water supply relies on surface water reservoirs located in a few small mountain catchments (about 800 km <sup>2</sup> in total). The Cape Town region receives 85% of its rainfall from a series of cold fronts forming within mid-latitude cyclones. The remainder is brought in by infrequent cut-off lows that occur throughout the year ( [[#Favre--2013|Favre et al., 2013]] ). This creates a very strong water resource dependency on a single rainfall delivery mechanism that may be strongly affected by anthropogenic climate change ( [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] and [[#10.6.2.6|Section 10.6.2.6]] ).

The 2015–2017 drought had strong low-rainfall anomalies in shoulder seasons (March to May and September to November, though weaker in the latter), and average rainfall in June and July ( [[#Sousa--2018a|Sousa et al., 2018a]] ; [[#Mahlalela--2019|Mahlalela et al., 2019]] ). The anomaly resulted from fewer rainfall events and lower average intensity of events. The anomaly was strongest in the mountainous region where the water supply system’s catchments are located ( [[#Wolski--2021|Wolski et al., 2021]] ).

Although the 2015–2017 drought was unprecedented in the historical record, the Cape Town region has experienced other droughts of substantial magnitude, notably in the 1930s, 1970s and more recently in 2000–2003. Long-term (&gt;90 years) rainfall trends are mixed in sign, location-dependent, and weak ( [[#Kruger--2017|Kruger and Nxumalo, 2017]] ; [[#Wolski--2021|Wolski et al., 2021]] ); mid-term (about 50 years) trends are similarly mixed in sign ( [[#MacKellar--2014|MacKellar et al., 2014]] ). In the south-western part of the region, rainfall is mostly decreasing in the post 1981 period, particularly in December–January–February and March–April–May, although there is no trend or a weak wetting in June–July–August ( [[#Sousa--2018a|Sousa et al., 2018a]] ; [[#Wolski--2021|Wolski et al., 2021]] ). Rainfall trends of similar magnitude and duration to the post-1981 trend accompanied previous strong droughts in the region ( [[#Wolski--2021|Wolski et al., 2021]] ).

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==== 10.6.2.3 Observational Issues ====

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South Africa and the Cape Town region have good instrumental weather data. Records start in the late 1800s, with in excess of 10 gauges reporting since the 1920s, expanding to about 80 gauges in the 1980s, but the number of stations has declined since. The mountains have only a few stations, which receive more than 1000 mm per year. In view of the strong heterogeneity of rainfall, changes in the number of stations contributing to datasets such as Climatic Research Unit (CRU) and Global Precipitation Climatology Project results in a lack of consistency between them, which limits their reliability in the region ( [[#10.2|Section 10.2]] ; [[#Wolski--2021|Wolski et al., 2021]] ).

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

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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]] ).

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==== 10.6.2.5 Model Simulation and Attribution Over the Historical Period ====

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Due to the small scale of the Cape Town region, robust comparison of CMIP simulations to observations is difficult. However, in general, CMIP5 models capture the seasonality well, such as the dominance of austral winter rains, although they overestimate the peak and underestimate the shoulder season rainfall ( [[#Mahlalela--2019|Mahlalela et al., 2019]] ). Trends in rainfall are particularly difficult to assess as they are generally weak and depend strongly on the time period and dataset adopted for the analyses ( [[#10.6.2.3|Section 10.6.2.3]] ). A multi-method attribution study ( [[#Otto--2018|Otto et al., 2018]] ) estimates the probability of the 2015–2017 drought to have increased by a factor of three since pre-industrial times (with a wide 95% confidence interval of 1.5 to 6). However, throughout the 20th century, a substantial portion of the global models (about 36% of CMIP5 and 44% of CMIP6 models, as well as many of the MIROC SMILE members) simulate a statistically significant (95% level) decline in total annual rainfall, while there is no robust long-term trend in observations (Figure 10.18). [[#10.4|Section 10.4]] offers a more detailed assessment of attribution challenges.

Global models capture the overall behaviour of the observed main hemispherical processes, such as the expansion of the tropics, a positive trend in SAM and the poleward shift of the westerly jet. However, they fail to capture details of their observed climatology and variability ( [[#Simpson--2016|Simpson and Polvani, 2016]] ), and the magnitudes of simulated trends vary, though the models typically underestimate observed trends in these processes ( [[#Purich--2013|Purich et al., 2013]] ; [[#Staten--2018|Staten et al., 2018]] ). In general, CMIP5 models do capture the SAM-regional rainfall association, although not consistently across all seasons ( [[#Purich--2013|Purich et al., 2013]] ; E.-P. [[#Lim--2016|]] [[#Lim--2016|Lim et al., 2016]] ).

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==== 10.6.2.6 Future Climate Information from Global Simulations ====

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Global models show strong consistency in a drying signal for the Cape Town region, with the reduction in total annual rainfall of up to 20% by the end of the 21st century in CMIP5 RCP8.5 and CMIP6 SSP5-8.5 simulations (Figure 10.18; [[#Almazroui--2020c|Almazroui et al., 2020c]] ). The consistency across the models is a robust signal compared to the rest of southern Africa, where the climate change signal varies spatially: stronger drying in the west and moderate drying or weak wetting in the east ( [[#DEA--2013|DEA, 2013]] , 2018; see Atlas.4.4 for further discussion of southern Africa precipitation projections). Rainfall changes projected for the Cape Town region are consistent with projected changes in hemispheric-scale processes and regional-scale dynamics that point toward reduced frequency of frontal systems affecting that region. These changes include robust signals in CMIP5 models for the Southern Hemisphere for a poleward expansion of the tropics ( [[#Hu--2013b|Hu et al., 2013b]] ), poleward displacement of mid-latitude storm tracks ( [[#Chang--2012|Chang et al., 2012]] ), increased strength and poleward shift of the westerly winds ( [[#Bracegirdle--2018|Bracegirdle et al., 2018]] ) and subtropical jet-streams ( [[#Chenoli--2017|Chenoli et al., 2017]] ), and a shift toward a more positive phase of the SAM (E.-P. [[#Lim--2016|]] [[#Lim--2016|Lim et al., 2016]] ). However, despite the consistency in circulation changes, the emergence of anthropogenic rainfall change above unforced variability in West Southern Africa remains uncertain for annual rainfall throughout most of the 21st century, even under SSP5-8.5 (Figure 10.15).

There is also a substantial increase in the frequency of conditions supporting atmospheric rivers and water vapour transport towards the south-west coast of southern Africa in the projected climate ( [[#Espinoza--2018|Espinoza et al., 2018]] ). This behaviour has strong implications for the region, as most topographically high locations receive rainfall from persistent atmospheric rivers ( [[#Blamey--2018|Blamey et al., 2018]] ). A thorough understanding of the role of atmospheric rivers in the Cape Town region under a changing climate is missing.

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==== 10.6.2.7 Future Climate Information from Regional Downscaling ====

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Dynamical downscaling studies implemented with a stretched-grid model ( [[#Engelbrecht--2009|Engelbrecht et al., 2009]] ) revealed a signal compatible with the driving CMIP5 ensemble, that is, consistent drying throughout the region, amplifying in time, irrespective of the considered emissions scenario and the generation of global models ( [[#DEA--2013|DEA, 2013]] , 2018). A multi-model CORDEX ensemble indicates a robust signal of reduction of total annual rainfall in the future, although there is less agreement on how changes in rainfall occurrence may evolve in the region, such as through fewer consecutive rain days or longer dry spells ( [[#Abiodun--2017|Abiodun et al., 2017]] ; [[#Maúre--2018|Maúre et al., 2018]] ). For the end of the century under RCP8.5, [[#Dosio--2019|Dosio et al. (2019)]] also found drying. Moreover, in their analysis, the drying is associated with an increase in the number of consecutive dry days and a reduction in number of rainy days. Their results are consistent with the driving global models for all the precipitation indices, and they are robust independent of the choice of the regional climate model (RCM) or global model. However, collectively, these analyses indicate that uncertainty remains in the characteristics of the precipitation decrease.

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==== 10.6.2.8 Storyline Approaches ====

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There is a consistency in rainfall projections with the projections of rainfall drivers and with the general understanding of the influence of global warming on the circulation dynamics and rainfall patterns in the region. Thus, the expansion of the South Atlantic high-pressure system, related to widespread warming of the tropics and poleward shift of the subsiding limb of the Hadley cell, is associated with the southward displacement of the subtropical jet, and southward migration of mid-latitude westerlies and storm tracks, in addition to changes in the SAM ( [[#10.6.2.4|Section 10.6.2.4]] ). These effects are also relatively consistent with recent (post-1980s) declines in rainfall in the Cape Town region. The storyline of an extended drought is thus a set of events that can yield reduced rainfall in the Cape Town region: a poleward shift of the downward branch of the Hadley cell that produces a sustained southward shift in mid-latitude westerlies and storm tracks. The behaviour is potentially reinforced by changes in the SAM.

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==== 10.6.2.9 Climate Information Distilled From Multiple Lines of Evidence ====

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There is ''high agreement'' among observational data and reanalyses that the recent (post-1979) downward trend in the Cape Town region’s rainfall leading to the 2015–2017 drought is related to the hemispheric processes of poleward shift in the westerlies and expansion of the Hadley circulation. However, there is less support for the precipitation–circulation relationship in historical CMIP5 and CMIP6 simulations. As a consequence, there is only ''medium confidence'' that these process changes produced the 2015–2017 drought leading to the 2018 water crisis.

For the water-resource planner who has to deal with potential drought like the 2015–2017 event, several lines of evidence indicate future drying: the projected precipitation by global models and RCMs of different spatial resolutions, and the observed and projected changes of circulation patterns consistent with drier conditions, the paleoclimatic evidence confirming a millennial-scale circulation–rainfall link. However, the distillation is limited by a lack of information about whether or not a relationship between Cape Town precipitation and large-scale circulation processes adequately explains droughts in the twentieth century prior to 1979.

Thus, although a clear association appears in observations from 1979 onward between increasing GHG concentrations, drying in the Cape Town region and behaviour of a key circulation process, the SAM, further analysis suggests caution. Not all global models show the historical post-1979 association among these factors, and when the observational record is extended back further to times when the anthropogenic greenhouse forcing was weaker, there is no strong association between the SAM and Cape Town drought. Thus, there is only ''medium confidence'' in the expectation of a future drier climate for Cape Town.

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