Future trends in atmospheric circulation patterns over Africa south of the equator

The climate system is sensitive to forcing from increased greenhouse gas emissions (Zeebe 2013). With increasing atmospheric greenhouse gas concentrations, the atmospheric composition is altered, and the resulting changes in the flow of energy imply an increase in global mean near-surface temperature (Arnell et al. 2019); hydroclimatic extremes (Giorgi et al. 2018); and changes in large-scale atmospheric circulation patterns (e.g., Zappa 2019), which is the focus of this study.

Atmospheric transport both at the global and regional scale is largely governed by the distinct patterns of atmospheric circulations. The responses of large-scale circulation patterns to climate change have been reported; for example, the poleward expansion of the descent region of the Hadley circulation (Byrne & Schneider 2016); the expansion of the subtropics (Lau & Kim 2015); the narrowing of the width of the Inter-Tropical Convergence Zone (ITCZ) (Hu & Fu 2007), among others. Furthermore, while the response of the El Niño Southern Oscillation (ENSO) to radiative heating is uncertain (e.g., Latif & Keenlyside 2009; Endris et al. 2019), Müller & Roeckner (2006) found that changing the state of ENSO with increased variability can significantly impact northern hemispheric mid-latitude circulation in future climate. The Southern Annular Mode (SAM), which is a major mode of variability that impacts synoptic circulations in southern Africa, is projected to undergo a shift toward positive polarity (Gillett & Thompson 2003), implying more frequent higher sea level pressure (SLP) over the mid-latitudes, especially during the austral summer season (Ding et al. 2012). These changes in large-scale circulation patterns in response to climate change have consequences on regional weather conditions and air quality (e.g., Ibebuchi & Paeth 2021). Given the vulnerability of the southern African region to climate change (Archer et al. 2018), such as an increase in drought in the subtropical domains of southern Africa (e.g., Lazenby et al. 2018; Maúre et al. 2018; Sousa et al. 2018; Spinonia et al. 2019) and changes in the track and intensity of tropical cyclones in the southwest Indian Ocean (e.g., Malherbe et al. 2013; Fitchett et al. 2016; Muthige et al. 2018), this study is dedicated to advance the understanding of how future climate change will affect the frequency of occurrence of circulation patterns in Africa south of the equator.

The climate of southern Africa is significantly modulated by atmospheric transport and sea surface temperature anomalies in the adjacent oceans – i.e., the South Indian Ocean, the South Atlantic Ocean, and the Southern Ocean. The anticyclonic circulations at the western branch of the Mascarene high drive southeasterly moisture fluxes from the southwest Indian Ocean into the southern African mainland (Cook 2000). Conversely, the anticyclonic circulation at the South Atlantic Ocean high-pressure drives moisture from the South Atlantic east coast offshore (Reason & Smart 2015). Both high-pressure systems are semi-permanent and migrate southward (northward) during austral summer (winter), in line with the seasonal migration of the ITCZ, and consequently impact sea surface temperature anomalies (Vigaud et al. 2009). Angola low through its cyclonic circulation advects moisture (from the Angola warm current and the tropical Indian Ocean) toward the eastern subtropical regions of southern Africa (Reason & Smart 2015). The convergence of southeasterly moisture fluxes driven by the Mascarene high and the Angola low is the main mechanism that forms the South Indian Ocean Convergence Zone, which is the major synoptic system that modulates the (austral summer) hydroclimate of southern Africa (Cook 2000; Lazenby et al. 2016). Furthermore, the band of westerlies associated with the mid-latitude cyclone, during its northward track allows cold fronts to sweep across the southwestern parts of South Africa (i.e., the regions with the Mediterranean type of climate) leading to rainfall formation over the regions (Reason & Rouault 2005; Ibebuchi 2021a) and pollutant dispersion (Ibebuchi & Paeth 2021). Thus, alterations in the synoptic circulations over southern Africa can have a significant impact on the weather and climate outlook of the region, as well as atmospheric pollutant concentration.

The modes of atmospheric circulations over the African domain, in addition to their reproducibility in climate models and linkages to surface variables, have been characterized by several studies (e.g., Moron et al. 1998, 2004, 2008a, 2008b, 2015; Pohl et al. 2005; Reason & Rouault 2005). It is documented in the aforementioned works of literature that the relationship between atmospheric circulations and the local climates of the African domains is expected and appropriate climate models can faithfully simulate the observed circulation patterns over the African domain. Over the East African domain, Souverijns et al. (2016) reported that the future changes in the frequency of occurrence of circulation patterns contribute to about 23% of the total change in precipitation. However, to the author's knowledge, no study has applied an ensemble of climate models to address the projected changes in the spatial structure and frequency of occurrence of circulation types (CTs) in Africa south of the equator under future climate change scenarios. Therefore, this study applies an ensemble of 11 CMIP6 global climate models (GCMs) to study the impact of climate change on the CTs, in terms of their (i) mean shape/reproducibility (ii) and frequency of occurrence, using a higher and lower future warming scenario (i.e., SSP5-8.5 and SSP2-4.5 emission scenarios). Hence, this study adds to improving the understanding of how climate change can impact the major synoptic circulations that modulate the climate of southern Africa and beyond.

Large parts of southern Africa receive the highest rainfall during the austral summer season when the southwest Indian Ocean is warmer. Hence, due to enhanced moisture uptake in the southwest Indian Ocean, the western branch of the Mascarene high through its anticyclonic circulation transports more moisture from the southwest Indian Ocean, which converges with moisture from the South Atlantic Ocean and the tropical Indian Ocean, over the southern African landmass. The mid-latitude cyclones impact rainfall variability in the regions with the Mediterranean type of climate (e.g., Western Cape). The ITCZ that relates well with rainfall variability in the tropical regions modulates the Angola low (Reason & Smart 2015). During the austral winter seasons, the subtropical anticyclone is located more northward over the landmasses, leading to large-scale subsidence and suppressed rainfall (Dedekind et al. 2016).

Studies have used multiple variables that are related to atmospheric circulations in classifying circulation patterns over different regions (e.g., Moron et al. 2007, 2015; Saenz & Duran-Quesada 2015). In this study, due to the correlation between the climate variables, SLP was found sufficient in classifying CTs in the study domain. Moreover, according to Kidson (1997), SLP provides a good representation of synoptic-scale systems and explains the relationship between topography and low-level flow. Daily SLP data sets are obtained from ERA5 reanalysis (Hersbach et al. 2020) from 1979 to 2014 when there is an overlap with CMIP6 (Eyring et al. 2016) historical data sets.

The CTs from the ERA5 reanalysis data that are more consistent with the observed climate are used as a reference in evaluating the simulated CTs from the CMIP6 GCMs in the historical experiment. For analysis of the changes in the CTs under future climate change, SLP data sets are obtained from the SSP2-4.5 and SSP5-8.5 emission scenarios.

The ERA5 data are obtained at a horizontal resolution of 0.25° longitude and latitude. All the available CMIP6 GCMs are examined and ranked based on their applicability for circulation typing in the study region. The ranking of the GCMs is based on their capability to replicate the circulation patterns and their characteristics as obtained from ERA5 reanalysis under the historical, SSP2-4.5 and SSP5-8.5, experiments. GCMs that replicate the circulation patterns from ERA5 are pre-selected to be suitable for this particular study. Overall, 11 CMIP6 GCMs, outlined in Table 1, passed this test and thus are used in studying the impact of future climate change on the CTs. The GCMs were obtained at a common realization (i.e., r1i1p1f1).

The absolute value of the loadings represents an important signal and thresholds within the range of 0.2–0.35 can be sufficient to separate noise (i.e., loadings that are close to zero) from the signal (Richman & Gong 1999; Ibebuchi & Richman 2022). Following its efficacy, in previous studies (e.g., Ibebuchi 2021a), ±0.2 is used in this study to assign a day to CTs. Each retained component gives two classes (i.e., clusters above or below the ±0.2 threshold) and the mean SLP of the days in a given class designates the spatial map of the CT. Since atmospheric circulation is a continuum (Compagnucci & Richman 2008), more than one CT occurring at a given time is realistic. This implies that as the simple structure simplification of the loadings permits, a day can have its loading magnitude greater than |0.2| under more than one retained component, and thus, more than one CT can be considered on a given day.

The decision that CTs from different data sets are truly counterparts is based on (i) a high congruence coefficient (>0.93) between the component scores and between the SLP composite anomaly patterns of the CTs; (ii) visual inspection of the SLP composite patterns; and (iii) replication of the frequency distribution of the corresponding CTs (from reanalysis). These criteria have to be in perfect agreement to conclude that two CTs from different data sets are counterparts. Also, the classification is applied to each of the SLP data sets from ERA5, and the individual selected GCMs, under each of the experiments.

The impact of future climate change is investigated on (i) the reproducibility and shapes of the SLP composite patterns; (ii) the annual cycle of the CTs; and (iii) the annual frequency of occurrence of the CTs.

Using the criterion of matching the PC loadings to the correlation patterns, an optimal number of four components are retained at a Promax solution of m = 4 (i.e., the power at which the Varimax solution is raised). Table 1 shows the output of the congruence match between the rotated PC loadings vectors and the correlation vectors, applied to the ERA5 classification and the classification from the individual GCMs. For some of the GCMs, the fourth component has congruence matches below 0.92. However, the ERA5 classification was used as the reference in deciding the optimal number of components to keep.

For the winter dominant CTs, CT1+, CT2−, and CT3− have distinct synoptic structures but are similar because of the enhanced westerly fluxes over the mid-latitudes (Supplementary Material, Figs. A2 and A3). This is more expressed under CT3−: the mid-latitude cyclone is strongest, displacing the high-pressure over South Africa, and westerly winds dominate over the subtropical and mid-latitude regions. Furthermore, for all winter CTs, cyclonic activity is lowest in the southwest Indian Ocean (i.e., less convection). CT4+ is also a winter BL pattern but with the subtropical anticyclone more situated over the subtropical landmasses relative to the summer BL CTs. With the enhanced cyclonic activity in the southwest Indian Ocean, coupled with the convergence of moisture fluxes, the summer dominant CTs bring wetter conditions, and with enhanced subsidence and weaker moisture advection from the southwest Indian Ocean, the winter CTs bring drier conditions over southern Africa (Ibebuchi 2021a).

As highlighted in Ibebuchi (2021b), when the classification method is applied to the individual (appropriate) GCMs, the circulation patterns are replicated as obtained from reanalysis data. Given that GCMs are characterized by systematic biases and also have different responses to future climate change, in analyzing future changes in the CTs, the ensemble of GCMs listed in Table 1 is preferred over a single GCM in achieving a better and more robust projection. The representation of the ERA5 CTs and the characteristics of the CTs, in the historical experiments, are examined for the individual GCMs, and the ensemble median of the CTs as simulated by the individual GCMs. In the hindcast evaluation, the performance of the ensemble median is compared to using the individual GCMs. Table 2 shows that the selected GCMs replicate the SLP composite patterns of the CTs, with a congruence match generally greater than 0.92. The performances vary among the GCMs and the CTs. For example, CT4+ is the least represented under ACCESS GCM. On average, CESM2 best represents the SLP composite patterns of the CTs obtained from ERA5. The added value of the ensemble median is notable in Table 2, as the congruence match is 0.99 in all cases (i.e., an excellent match). Thus, from the hindcast evaluation, there is a better representation of the CTs in the ensemble median over any of the ensemble members. From Figure 2, though there are small differences in the structure of the isopleths, a cursory investigation also validates that the pattern configuration of the simulated CTs from the ensemble median is quite comparable to the CTs from ERA5.

An increase in the anti-circulation at the western branch of the Mascarene high is also evident in Figure 5 under CT3+. Under CT1−, CT4−, and CT2−, increased blocking adjacent to the south of South Africa is evident and the same goes for CT3−, associated with enhanced cyclonic activity over the mid-latitudes (cf. Figure 1 and Supplementary Material, Fig. A3). An increase in cyclonic activity adjacent to South Africa is only projected under CT2+, which can be a result of enhanced deformation of the cyclonic system by the stronger Mascarene high (cf. Figure 1). Under CT4+, which is a dominant winter BL CT, subsidence (i.e., positive SLP anomaly) is projected to increase under the future emission scenarios. In mostly all CTs, a negative SLP anomaly is equally projected in the tropical oceans. Moreover, under CT1− and CT4−, the projected increase in cyclonic circulation (i.e., negative SLP) is also evident over the southwest Indian Ocean. Again, these changes are more robust under the higher emission scenario.

Under the future emission scenarios, there are changes in the relative frequency of occurrence of the CTs at specific months, but the structure of the annual cycle (i.e., the dominant periods of the CTs) is the same and barely altered by future climate change. For example, under CT1− that is a summer BL CT, the relative frequency of occurrence under the emission scenarios increased from December to May (i.e., austral summer to autumn seasons). The months for the projected increase in CT3+, which is also a summer BL CT, are from May to November when the southwest Indian Ocean is relatively cooler. During the warmer months, CT4− associated with the enhanced cyclonic activity in the southwest Indian Ocean is projected to be more frequent (Figure 6), which can be a possible reason why the Mascarene high under CT3+ might experience periods with low occurrence during summer seasons. Also with the increase of CT3+ during the colder months, CT3− associated with the enhanced cyclonic activity adjacent to South Africa is projected to be less frequent during the colder months – under the emission scenarios – relative to the historical climate.

Furthermore, using the modified Mann–Kendall test (Mann 1945; Kendall 1975; Yue & Wang 2004) that is both non-parametric and applicable to data that are serially correlated, Table 4 shows the test of statistical significance of linear trends in the annual frequency of occurrence of the CTs under all the considered experiments. There is a statistically significant decrease in the frequency of occurrence of CT1+ in the historical climate (both from ERA5 and the GCM ensemble). Also, toward the end of the 21st century under the SSP5-8.5 scenario, the frequency of occurrence of CT1+ is projected to decrease (Table 4). From Supplementary Material, Table A1, six of the ensemble members agree with ERA5 on the significant decreasing trend in the frequency of occurrence of CT1+ under the historical climate. Also, from Supplementary Material, Table A2, eight of the ensemble members agree on the significant projected decrease in the frequency of occurrence of CT1+. CT1− is a summer BL CT. Table 4 shows that the increase in the frequency of occurrence of CT1− is statistically significant both under the historical and SSP5-8.5 experiments. From Supplementary Material, Table A1, six of the ensemble members agree with ERA5 on the significant increased frequency of occurrence of CT1−; and eight of the ensemble members agree on the significant projected increase in the frequency of occurrence of CT1− (Supplementary Material, Table A2).

Though some ensemble members predict significant changes in CT2+ and CT2− (Supplementary Material, Tables A1 and A2), based on the median statistic and the ERA5 CTs, no significant change was found in the frequency of occurrence of CT2+ and CT2− in all experiments considered here. The asymmetric CTs from PC3, i.e., CT3+/CT3−, associated with suppressed/enhanced westerly moisture fluxes in the Southern Hemisphere mid-latitudes (Supplementary Material, Fig. A2), were reported in Ibebuchi (2021a) to be significantly modulated by the anomalies of the SAM. CT3+(CT3−) is related to positive (negative) SAM. In the historical experiment, the ensemble median of the GCMs simulates a significant increase in the frequency of occurrence of CT3+ (Table 4), and nine of the ensemble members agree on the significant increase (Supplementary Material, Table A1). Under the SSP5-8.5 scenario, the frequency of occurrence of CT3+ (i.e., summer dominant BL CT) is projected to increase toward the end of the 21st century (Supplementary Material, Fig. A4), and nine of the ensemble members agree on the significant increase (Supplementary Material, Table A2). Conversely, the frequency of occurrence of CT3− is projected to significantly decrease under the SSP5-8.5 emission scenario, and nine of the ensemble members agree on the significant decrease (Supplementary Material, Table A2).

The frequency of occurrence of CT4− that is an austral summer CT associated with the enhanced cyclonic activity in the southwest Indian Ocean has a positive trend under the historical experiment – both from the GCM ensemble and ERA5 (Table 4). Five of the ensemble members agree with ERA5 on the significant increase in the frequency of CT4− (Supplementary Material, Table A1). Toward the end of the 21st century, the frequency of occurrence of CT4− is projected to significantly increase under the SSP5-8.5 scenario, and 10 of the ensemble members agree on the significant increase (Supplementary Material, Table A2). CT4+ is a winter BL CT. While from Figure 5, the magnitude of subsidence is projected to increase under CT4+, in the historical climate, the frequency of CT4+ has a negative significant trend both from ERA5 and the GCM ensemble (of which, five of the ensemble members agree with ERA5). The frequency of occurrence of CT4+ is projected to decrease under both the SSP5-8.5 scenario and the SSP2-4.5 scenario. All the ensemble members agree on the significant decrease in the frequency of occurrence of CT4+ under the SSP5-8.5 scenario (Supplementary Material, Table A2), and six of the ensemble members agree on the significant decrease under the SPP2-4.5 scenario (Supplementary Material, Table A3).

Several studies have indicated that future climate change can be expected to impact regional patterns of atmospheric circulations (Müller & Roeckner 2006; Hu & Fu 2007). This can have a wide range of consequences ranging from weather extremes to the modification of pollutant transport (Schlef et al. 2019; Marshall et al. 2020; Ibebuchi & Paeth 2021). Changes in the frequency distribution of regional atmospheric circulation patterns can be a function of variations in the leading modes of climate variability (e.g., ENSO and SAM) that can have a remote influence on the regional circulation patterns (Müller & Roeckner 2006). Studies have reported that anthropogenic climate change might alter the amplitude and frequency of ENSO and the SAM (Collins 2000; Wang & Cai 2013; Cai et al. 2021). Consequently, it can be expected that through the impact of anthropogenic climate change on ENSO and SAM, the related regional modes of atmospheric circulation will be impacted through a series of feedbacks.

Within the regional context of Africa, south of the equator, this study showed that while large-scale atmospheric circulation patterns are stationary, based on their existence regardless of the analyzed climate, their spatial structure and frequency of occurrence can be impacted by future climate change. Under the SSP5-8.5 emission scenario, the significant increase in the frequency of occurrence of summer BL CTs implies that the ensemble of the climate models projects a possible increase in the frequency of atmospheric blocking of the Southern Hemisphere mid-latitude cyclones, adjacent to South Africa. The shift of the SAM toward its positive phase during the austral summer season (Ding et al. 2012) might be implicated as a driver of the projected frequent increase in SLP, adjacent to South Africa. This is because positive SAM that correlates with the summer BL CTs (e.g., Ibebuchi 2021a) is associated with increased SLP in the mid-latitudes. Increased SLP variability at the Southern Hemisphere mid-latitudes can have a direct implication on the hydroclimate of the Western Cape (Reason & Rouault 2005) and the climatology of the Mascarene high (Morioka et al. 2015). Since rainfall formation and pollutant dispersion at parts of the Western Cape are associated with the band of westerlies linked to the northward track of the mid-latitude cyclones, it is thus projected that under the higher warming scenario, parts of the Western Cape become drier, coupled with higher pollutant concentrations (e.g., Reason & Rouault 2005; Ibebuchi 2021a; Ibebuchi & Paeth 2021).

Furthermore, increased anticyclonic circulation at the western branch of the Mascarene high was projected under CT3+ (Figure 5), implying an increase in southeasterly moisture fluxes advected into the southern African mainland. Nonetheless, the climate models also projected an increase in cyclonic activity in the southwest Indian Ocean and a weaker circulation at the western branch of the Mascarene high under CT4− (Figure 5), which was also projected to be more frequent under future climate change. Given that the CTs are continuous and their distinct signals can interact at a given time, the combined changes in CT3+ and CT4− suggest that under future climate change, the climatology of the western branch of the Mascarene high might be subjected to alternating periods of being anomalously stronger due to a more positive SAM that increases SLP at the Mascarene high – as reported by Morioka et al. (2015) – and periods of being anomalously weaker due to warmer southwest Indian Ocean.

Already, there is high confidence that projected precipitation decreases in the western parts of southern Africa, coupled with an increase in the frequency of drought (Engelbrecht et al. 2015; Liu et al. 2018). Also, annual mean rainfall in the austral summer rainfall region is projected to decrease by 10–20%, coupled with an increase in the occurrence of consecutive dry days during the wet season under high emission scenarios (Kusangaya et al. 2014; Engelbrecht et al. 2015). The projected weaker Angola low under all the CTs (Figure 5) suggests that the rate at which northwesterly moisture fluxes are transported to the subtropical regions of southern Africa might be weakened. Also under CT4+, during the winter seasons, the magnitude of subsidence is projected to increase over the study region (Figure 5), which can result in drier conditions. Weakening of the Angola low, increase in subsidence, atmospheric blocking of the mid-latitude storms that bring rainfall to the southwestern regions, and (occasional) weakening of the Mascarene high can result in drier conditions over the Southern Africa region. Furthermore, while heavy precipitation in the southwestern parts of southern Africa is projected to decrease (e.g., Donat et al. 2019), it is also projected that heavy precipitation increases in the eastern parts of southern Africa under future warming scenarios (e.g., Li et al. 2021a, 2021b). The projected increase in (the magnitude and frequency of) convective/cyclonic activity in the southwest Indian Ocean under CT4− can result in heavy rainfall in the eastern regions when atmospheric conditions are favorable, due to enhanced moisture availability in the adjacent oceans of the eastern coastal regions, during the summer months.

This study examined the impact of future climate change on the atmospheric circulation patterns in Africa, south of the equator. The major conclusions of this study are:

• The classified CTs are stationary based on their existence and reproducibility under future emission scenarios. However, the spatial structure of the CTs and so the strength of atmospheric circulation associated with the synoptic geographical features captured in the CTs can be impacted by climate change.

• Under future climate change, there were marked changes in the monthly relative frequency of occurrence of the CTs; nonetheless, the dominant period of the CTs was barely affected. The annual frequency of occurrence of some of the CTs has started changing in the historical climate. Under future emission scenarios, the frequency of occurrence of some of the CTs is projected to change with different magnitudes dependent on the warming scenario. Specifically, the annual frequency of occurrence of summer blocking CTs is projected to increase under higher warming scenarios. Also, the frequency of the CT associated with the enhanced cyclonic activity in the southwest Indian Ocean and a weaker western branch of the Mascarene high (Figure 1) is projected to increase under the higher warming scenario.

The wider implication of this study is that under future climate change, (i) the poleward expansion of the subtropics over and adjacent to South Africa can be expected due to increased blocking of the mid-latitude cyclones by the subtropical high-pressure systems; (ii) southeast moisture fluxes driven by the western branch of the Mascarene high can be subjected to periods of being anomalous stronger (weaker) possibly due to a more positive SAM (warmer southwest Indian Ocean); (iii) the Angola low can be expected to weaken.

Finally, the major limitation of this study is that depending on the analyzed GCM, the simulated circulation patterns have different responses to future climate change (cf. Supplementary Material, Tables A1, A2, and A3), which is a source of uncertainty in the projected changes. In future work, an ensemble of high-resolution regional climate models will be used to study how changes in the atmospheric circulations (e.g., projected changes in moisture fluxes) under the classified CTs affect the spatiotemporal variability of a range of other climate variables in southern Africa.

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.