Climate trends in the Cape Town area, South Africa

COPYRIGHT © The Author(s) Published under a Creative Commons Attribution 4.0 International Licence (CC BY 4.0) Climatic conditions near Cape Town, South Africa (34°S, 19°E) are analysed for historical trends in station measurements in the 20th century and in modern satellite-blended datasets. Despite the variety of datasets and record lengths, all show a steady drying trend. Faster rates of warming, 0.1°C∙yr-1, are found in land surface temperatures during the period 2000–2017. Drying trends are most acute to the northwest of the Hottentots Holland mountains. Hydrology station measurements in the Upper Berg River catchment since 1956 reveal a decline in streamflow of −0.012 m3∙s-1∙month-1, and an upward slope in potential evaporation of +0.020 W∙m-2 ∙month-1. Rainfall has declined most in May and September, indicating a shorter winter wet season. Features supporting the drying trend include an increase of easterly winds and low-level subsidence during summer. The clockwise circulation trend around Cape Town entrains dry air from the Karoo interior and the south coast upwelling zone, leading to negative sensible heat flux, a capping inversion and diminished orographic rainfall. Climate trends in the Cape Town area, South Africa


INTRODUCTION
The southwestern (SW) Cape of South Africa (34°S, 19°E) lies at the transition between the subtropical easterly and mid-latitude westerly wind regimes, and has a semi-arid, rainy-winter climate. The adjacent interior (Karoo) has sparse vegetation fronted by coastal mountains over 1 000 m. Past research on climate change at country-scale has found a ~0.02°C•yr -1 increase of temperature (Kruger and Shongwe, 2004;Morishima and Akasaka, 2010;Kruger and Nxumalo, 2016) consistent with global averages, and mixed trends in rainfall (Tadross et al., 2005;MacKellar et al., 2014). Yet, water resources within reach of Cape Town's 4 million people (west of 20°E) dwindled to <30% of storage capacity according to the Department of Water and Sanitation (DWS), at the time of conducting this research in 2018. The scarcity of water supplies initiated numerous engineering projects (Muller, 2017), and rationing of 50 L 1 •day -1 •person -1 . Here an analysis of meteorological factors underlying the climate trends contributing to water deficit was conducted to promote awareness and strategic planning. Since 1980, global greenhouse gases have risen 2 ppm•yr -1 , South Africa's industrial emissions exceeded 600 Mt•yr -1 (Bekker et al., 2008;DEAT, 2009), and continental agricultural emissions reached 1 350 Mt•yr -1 (Semazzi and Song, 2001;Sinha et al., 2003). This study focuses on the mesoscale structure of the drying trend, with equal focus on winter rain and summer desiccation.
The DWS hydrology analyses are conducted by averaging streamflow and (A-pan) potential evaporation data at the three stations in the Upper Berg River catchment (1956-2017), then filtering with an 18-month low-pass filter to remove seasonal cycles. The DWS time-series are then comparable with the filtered CRU4 PDSI precipitation minus potential evaporation anomalies. To achieve a mesoscale perspective on climate change, high-resolution products of limited record length are utilized. However multi-year variability may affect the results and requires evaluation. This is done by wavelet spectral analysis, and by cropping the PDSI time series at various start dates, and recalculating the linear regression slope and explained (r 2 ) variance.
Ranking the satellite land surface temperature, a hot dry spell of 1-8 January 2011 was analysed using MERRA reanalysis fields and South African Weather Service hourly observations and radiosonde profiles from Cape Town airport (CPT). The case study helps link the Cape Town regional hydrology with climate change from rising greenhouse gases, amid natural multi-year fluctuations (Poccard et al., 2000).
The method for detecting trends, using linear regression and its r 2 fit, is a simple approach that contributes to robust interpretations. One issue is the constraint of record length that depends on technological advances: (i) station rainfall since 1900, (ii) streamflow since 1950, (iii) satellite data since 1980, and (iv) highresolution data since 2000. This issue is resolved by comparison of the longest record (PDSI) in those same eras. SST trends (Fig. 1c) are positive offshore but negative nearshore (cf. Rouault et al., 2010), and point to the intensification of coastal upwelling by south-easterly winds from 1981-2017. The offshore Ekman transport and cyclonic wind shear lifts cool water over the shelf and strengthens the Benguela Current, with a desiccating effect as seen below.    , 2004). Such a high rate of warming is related with construction of the dam followed by water receding around its perimeter, and subsequent heating of bare soil during periods of high solar radiation (Appendix, Fig. A1; cf. Earth observatory 2018). In conjunction, the vegetation colour fraction has declined in the mountain catchments (Fig. 2d), around the water reservoirs (−0.02•yr -1 ), and over the Cape Flats urban area. A lower vegetation fraction gives off less transpired moisture. In contrast, 'greening' is noted in the MODIS era (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) to the east of the Hottentots Holland > 19.1°E (+0.01•yr -1 ), where CHIRPS2 rainfall trends  are weakly positive (Fig. 2e) due to up-slope easterly winds (cf. Fig. 4e).

Cape Town regional and spatial trends
The main hydrological concern is the diminishing rainfall northwest of the Hottentots Holland, across the populated Cape Flats. The annual rain trend is −0.5 mm•month -1 •yr -1 in the vicinity of Wellington (33.64°S, 19.00°E). The histogram of NOAA infrared vegetation temperature, comparing the 1982-1988 and 2010-2016 era (Fig. 2f), illustrates shifts in both cold and warm seasons. In the 12.5°C range typical of winter, there was a reduction from 18 to 12 months. In the 30-32.5°C range typical of summer, there was an increase from 3 to 24 months. The histogram indicates a rapidly warming summer climate, which is placed in context below. The reader is referred to Fig. A2 (Appendix) for a comparison of PDSI trends over different record lengths.

Temporal characteristics
In this section, a temporal analysis is presented for station observations in the study area (cf. Fig. 2a,b). Ensemble rainfall trends per month since 1901 (Fig. 3a) are downward in most months, especially in May and September, at the beginning and end of winter. Filtered streamflow and potential evaporation anomalies in the Upper Berg River catchment since 1956 (Fig. 3b) reveal significant drying trends, with r 2 variance of 31% and 41%, respectively. The downward slope of filtered streamflow anomalies is −0.012 m 3 •s -1 •month -1 ; while the upward slope of filtered potential evaporation anomalies is +0.020 W•m -2 •month -1 . Streamflows nearly ceased by 2017 (end of record).
The filtered CRU4 PDSI anomalies since 1901 (Fig. 3c) reveal a persistent downward trend through the 20 th century, consistent with Wolski, (2018). The 1970s saw a prolonged dry spell that recurred in the most recent decade (2010+). Apart from the 1950s and 1980s, the surface water balance has shown deficit conditions: potential evaporation exceeded precipitation. The downward trend in PDSI is 17% of total variance (1901-2017).
Multi-year variability is evaluated by wavelet spectral analysis in Fig. 3d. A persistent ~11-year period is found from 1930 to 1980. This cycle fades out and is replaced by a ~5-year oscillation. Sousa et al. (2018) show how the Southern Hemisphere circulation affects moisture flows, producing multi-year variability around the drying trend.
The filtered PDSI record is cropped at various start dates to assess the influence of multi-year variability on linear trends in Fig. A2 (Appendix). Starting in 1956: the slope is −0.0023 and r 2 variance 12%; cropped from 1980: the slope is −0.0082 and variance is 55%; and cropped from 2000 the slope is −0.0116 and variance is 41%.
Both linear slope and variance intensify, suggesting that multiyear variability is gradually overshadowed by trends attributable to climate change.
The above-mentioned station-based results are supported by satellite net OLR data averaged over the study area (Table 2) segregated by month. Positive trends that refer to reduced cloud cover are most significant in the December to April months . A similar analysis of MERRA zonal winds (not shown) found significantly increased surface easterly winds in December, January and April.

Regional features associated with the drying trend
Trends are mapped in vertical sections along 34°S from 15.5°E to 24.5°E in Fig. 4a-d. A surprising result is that subsident trends within the easterlies are strongest south of the Langeberg mountains (20-24°E). Air temperature trends are relatively weak at 0.01°C•yr -1 in the 1 000-900 hPa layer. Warming increases westward, consistent with a change from moistening 21-24°E to drying 16-19°E. Botai et al. (2017) found a similar east-west pattern in their trend analyses.
Turning attention to the shelf oceanography, it is evident that upwelling has intensified (−0.02°C•yr -1 ) from surface to 40 m depth to the east of 19°E, despite a warming trend elsewhere in the period 1980-2016. The warm air/cool sea trend causes sensible heat flux (Qh) to decline at −0.2 W•m -2 •yr -1 . Hence the lower atmosphere is stabilized along the South Cape coast. As a secondary consequence, the subsidence inversion strengthens and caps the surface easterly flow, as outlined in Jury and Reason (1989). The Froude number, calculated from F = U∕NH, offers a way to determine whether the airflow will ascend the SW Cape mountains (F ≥ 1) or go around them (F < 1). With a westerly wind of U = 10 m•s -1 , a mountain height of H = 10 3 m, and a winter-time unstable (dθ∕dz) lapse rate of N = ((g∕θ o )(dθ∕dz)) 0.5 ~10 -2 s -1 , the airflow lifts over the mountains causing stratiform rainfall. Under the summer-time easterly wind regime, the U and H values remain; but SST cooling and low-level warming (eg. −Qh) induce a stable lapse rate of N ~2•10 -2 s -1 , so F = 0.5. The marine airflow is diverted around the mountains instead of going over the Hottentots Holland; leaving them sunny and prone to desiccation.
As the sub-tropical ridge shifts poleward, there is a broad zone of intensifying easterly flow from 32-40°S and 0-40°E (Fig. 4e), in agreement with Sousa et al. (2018). Evidence of Venturi acceleration at the southern tip of Africa (35°S, 20°E) emerges in zonal wind trends of −0.05 m•s -1 •yr -1 . The associated easterly shear spins-up a wind rotor over the SW Cape (cf. Fig. 1b) with desiccating consequences.

Dry spell case study
Ranking the MODIS daytime land surface temperature record in the SW Cape (33.5-34.5°S, 18.5-19.5°E), the period 1-8 January 2011 is the hottest case (42°C) in the period 2000-2017. The map of surface temperatures (Fig. 5a) reveals a narrow strip of cool conditions along the windward coastal promontories such as Cape Point (20°C). However, across the Cape Flats and interior valleys (Berg, Breede) the daytime land surface temperatures exceed 50°C! The airflow at 850 hPa (Fig. 5b) shows cyclonic curvature and > 5 m•s -1 easterlies over the ocean to the south. The vegetation colour fraction (Fig. 5c) diminished by 0.1 from the end of December to mid-January 2011 in the Upper Berg River catchment, west of the Hottentots Holland mountains. A vertical section analysis shows a strong easterly 'jet' capped by an inversion (Fig. 5d) in the period 1-8 January 2011. In the offshore zone (35°S), the near-surface winds were 10 m•s -1 and temperatures were < 20°C. During this dry spell, offshore ocean transport and coastal upwelling caused significant cooling over the shelf, evidenced by SST < 15°C (Fig. 5e). A consequence of warm air overlying cool sea is thermal stability, which inhibits the inland penetration of moisture (cf. Fig. 5b). The CPT weather station reported desiccating weather conditions for 3-6 January ( Fig. 5f), characterized by Tmax = 34°C and Tdew = 16°C. Southerly winds strengthened for 3-4 January to 35 km•h -1 , and then abated following passage of the South Atlantic high pressure cell. CPT radiosonde profiles on 6 January 2011 (Table A1, Appendix) describe characteristics in the 400-700 m subsidence inversion: wind direction/speed 110°/14 kn, temperature 32.6°C, dewpoint −14.6°C, and specific humidity 1.36 g•kg -1 , due to entrainment of dry air from aloft (cf. 500 hPa sinking motions in Fig. 5e). Many DWS stations near Wellington recorded potential evaporation > 12 mm•day -1 ! SUMMARY Hydro-climate trends near Cape Town, South Africa, were studied using historical observations since 1901, and gridded reanalysis fields and satellite observations since 1979. Trends (slope) were determined by linear regression and its r 2 fit. This simple approach led to objective interpretations of a drying trend, regardless of record length. Hydrology station measurements in the Upper Berg River catchment since 1956 reveal a downward slope in filtered streamflow anomalies of −0.012 m 3 •s -1 •month -1 , and an upward slope in filtered potential evaporation anomalies of +0.020 W•m -2 •month -1 . Rainfall has declined most in May and September, indicating a shortening of the winter wet season.
Multi-year variability has little effect on filtered PDSI trends in different periods: slopes are consistently downward (−0.0015 to −0.0116•month -1 ) and explained variance remains significant (12-55%; cf. Fig. A2, Appendix). Despite the steadiness of drying, the variety of datasets and record lengths poses limitations.
The drying trend near Cape Town was associated with a poleward shift of the sub-tropical ridge, more frequent easterly winds, and a longer summer dry season. The easterly winds have secondary consequences: increased south coast upwelling and low-level subsidence over the Hottentots Holland (cf. Fig. 4a, c). The wind shear induces a cyclonic rotor around Cape Town which entrains dry air from the Karoo interior. Precipitation has declined particularly northwest of the Hottentots Holland mountains and many water reservoirs show diminishing inflows, increased surface temperature (+.1°C•yr -1 ) and browning of nearby vegetation since 2000. The easterly winds derive from a zone of negative sensible heat flux over coastal upwelling, so the capping inversion (cf. Fig. 5d, Table A1, Appendix) inhibits orographic cloud development.
The regime shift from mid-latitude westerly winds and winter rainfall, to sub-tropical easterly winds and summer dry spells, has depleted water resources (Otto et al., 2018). Hence recycling, desalination, importation, and aquifer extraction projects are underway (Patel, 2018), and water conservation has become rooted in popular thinking and community awareness. The above results contribute to this educational initiative, and suggest that water scarcity in Cape Town, South Africa, will require ongoing vigilance.