4.1 Regional moisture supply
a. Seasonal mean
Moisture is a necessary component in rain-producing systems. Insufficient supply of moisture can lead to weakening of synoptic rain-producing systems thereby affecting total seasonal rainfall amounts as well as dry spell frequency and duration (Cook et al., 2004). Thus, to determine mechanisms through which the El Niño type impacts rainfall characteristics, moisture transport analysis becomes critical.
Figure 10 shows mean moisture flow and 850 hPa geopotential height for the bi-monthly summer periods. In ON, strong easterly flow of moisture into central east and northeastern regions of southern Africa (located between 8°S and 24°S) is apparent (Fig. 10a). This flow is aided by the high-pressure system (Mascarene High) in the southwest Indian Ocean. For the latter part of summer, this south Indian anticyclone tends to ridge over Madagascar and southeastern region; its core contracts relative to early summer, as a result the inflow of moisture into the region is more southeasterly adjacent to Mozambique Channel and northeasterly in the northeastern region. It around this time that the Mozambique Channel Trough (MCT) becomes more defined (Barimalala et al., 2018).
During DJ, a low-pressure system develops over southern Angola, centered at [17°S, 18°E], and is associated with low-level cyclonic moisture flow (Fig. 10b). The region with cyclonic flow is linked to the presence of the Angola Low. The synoptic expression of the Angola Low during this time of the season is primarily influenced by the tendency of moist tropical lows to linger longer in the region, thereby deepening the system (Howard & Washington, 2018). An unusually intense Angola Low often results in above-normal rainfall conditions over southern Africa, while a weak Angola Low typically leads to below-normal rainfall (Munday & Washington, 2017). At the same time, the mean flow over the Mozambique Channel indicates the presence of a weaker MCT. A weaker trough tends to feed more moisture inland and into the Angola Low region, which can be attributed to the increased frequency of rainy days over southern Africa during this time (as shown in Fig. 3c-d). This assertion was documented in more detail by Barimalala et al., (2018). The other distinctive low-level flow feature that contributes to the Angola Low is the northwesterly monsoon consisting of recurved flow in the eastern flank of the South Atlantic High (SAH) and flow from the tropical Atlantic Ocean (Silvério & Grimm, 2022). Moreover, this northwesterly flow converges with the northeasterly monsoon hailing from the Indian subcontinent to form a Congo Air Boundary (CAB) that is seen extended southwards. The CAB is typically an indicator of the location of the African rain belt (Howard & Washington, 2019). In FM, these northeasterly and northwesterly monsoonal flows appear to weaken (Fig. 10c).
b. Changes in moisture supply linked to EP El Niño
Figures 11a-c shows composite anomaly of moisture flux and geopotential heights at 850 hPa related to EP El Niño event over the summer period. In ON, during EP El Niño event, the pressure drops off the southeast coast of southern Africa and as a result, the climatological easterly moist-laden flow from the southwest Indian Ocean (depicted in Fig. 10a) into central east and northeastern regions of southern Africa is reduced. Furthermore, continental moisture influx from the tropical regions into southern Africa tends to be reduced as well and the low-level geopotential heights tends increase over the subcontinent.
During DJ, this anomalous rise in geopotential heights over the southern African mainland appears to intensify, suggesting an increased rate of subsidence. The analysis of moisture flux anomaly during this period reveals that both northwestern and northeastern monsoonal flows fail to penetrate the Angola Low domain. This reduction in moisture within this crucial rain-bearing region could have significant impact on rainfall pattern over southern Africa. Additionally, there is clear presence of an anticyclonic moisture flow in the Mozambique channel, indicating a weakened MCT that appears to have significant reduced moisture flux in its south Madagascan branch. This suggests that changes in the MH might affect the dynamics of a weaker MCT. Proceeding into FM, conditions persist with general subsidence, limited moisture inflow from tropical region and monsoonal flows, and reduced southeasterly flow. Consequently, conditions conducive for below normal rainfall conditions are prevalent.
c. Changes in moisture supply linked to CP El Niño
Figures 11d-f depicts the composite anomaly of the moisture supply over southern Africa related to CP El Niño. During ON and FM periods, the moisture flow anomaly in the northern domain of the region tends to be mostly westerly and the climatological anticyclonic flow in the central interior of the region is significantly reduced. Moving into DJ, a cyclonic flow anomaly occurs in the Mozambique channel indicating a possible intensification of MCT. Analyzing individual CP events, this cyclonic anomalous flow was found to be present during the DJ periods of 2002/03, 2006/07, 2014/15 and 2018/19. In the northern flank of this anomalous feature, it becomes evident that moist-laden air carried by northeasterly monsoon is drawn out of the tropical region without effectively penetrating the subtropical domain. Pascale et al. (2019) showed that a strong MCT tends to limit penetration of moisture into southern Africa and is often associated with the presence of weaker Angola Low, consequently below-normal rainfall conditions. Furthermore, in the southwest Indian Ocean, there are anomalously positive geopotential heights located diagonally to this anticyclonic flow in the Mozambique channel. The flow induced by the difference in height anomalies between these two domains tends to be southerly, implying a substantial reduction in climatological moist-laden transport in subtropical southern Africa by southeasterly trade winds during the CP El Niño.
4.2 Large-scale atmospheric circulations
a. Atmospheric circulation response to EP El Niño
Figure 12 shows composite anomalies of mid-troposphere geopotential heights in the southern hemisphere during EP El Niño events. In ON, a pronounced pressure decrease in the mid-troposphere is seen over southwest Indian Ocean. The geopotential height drop, ranging from 1 to 5m, connects this region to the areas of noticeable geopotential height decrease over Antarctica, mid-latitude regions across South Pacific Ocean, and extending towards the Greenwich line over the South Atlantic Ocean (see Fig 12a). These regions of negative height anomalies tend to correspond to positive anomalies of the upper westerlies winds (not shown). In DJ, the subtropics experience positive geopotential heights anomalies, with the most significant increase observed over Botswana in southern African subcontinent. This pronounced positive height anomaly suggest that, during an EP El Niño event, a stronger Botswana High is more likely to be occur. When this state manifests, the Botswana High is associated with considerable subsidence in the mid- to upper troposphere. This subsidence suppresses atmospheric convection, resulting in increased air temperatures and low-level moisture divergence in the subtropical southern Africa (Driver & Reason, 2017; Blamey et al., 2018; Reason, 2019; Chikoore & Jury, 2021). All these conditions are conducive to below normal rainfall. From DJ to FM, negative height anomalies tend to be mostly confined in the midlatitudes. During this time, the subtropics experiences mid-level subsidence indicated by positive geopotential height anomalies.
b. Atmosphere circulations in response to CP El Niño
Figure 13 illustrates the changes in the mid-level atmospheric circulation related to CP El Niño in the Southern Hemisphere to help understand possible mechanisms through which this phenomenon drives the rainfall conditions in that differs from those related EP El Niño, as presented in the previous sections. Over the summer period, the analysis of mid-troposphere geopotential heightindicates that there tends to be a general subsidence over the subcontinent. However, unlike during the EP El Niño events, the Botswana High is not as prominent. In ON, areas of pronounced negative mid-tropospheric geopotential heights appear to be situated southeast of Australia and in the vicinity of the Drake Passage. Meanwhile, positive geopotential height anomalies are observed in the higher latitudes between these two regions. Strong upper westerly anomalies are associated with the areas of negative height anomalies. As summer progresses, upper westerly winds seem to be less intense. Compared to during EP El Niño, these upper westerlies are weaker. This has implications on the dynamics of Rossby waves which are thought to be the key pathway of the teleconnection link between SST fluctuations in the tropical Pacific and interannual climate variability (e.g., Karoly, 1989; Alexander et al., 2002; Alizadeh-Choobari, 2017). Overall, these results showing weaker large-scale atmospheric circulation in the Southern Hemisphere during CP El Niño corroborate with previous studies (e.g., Ratnam et., 2014).
c. The jetstreams and El Niño types
Demonstrated in Figs. 14 a-c are the climatology of vertical cross-sections at 10°E of zonal wind between 20°S and 60°S for ON, DJ and FM periods. It is evident that throughout summer, intense upper westerly flow tends to occur between 40°S and 50°S. These are often referred to as jet streams. In southern Africa, jet streams tend to support the development of cloud bands, thereby playing a significant role in rainfall variability over the region (Hart et al., 2010; Macron et al., 2014; Hart et al., 2018). Summers associated with anomalous displacement of the jet towards the equator (pole) tend bring below (above) normal rainfall conditions over southern Africa, coinciding with El Niño (La Niña) (Tyson & Preston-Whyte, 2000; Hart et al., 2018). To determine whether different El Niño types lead to differing characteristics of jet streams, composite anomaly analysis of EP and CP El Niño events was conducted, and the results thereof are displayed in Figs. 14 d-f and Figs. 14 g-i, respectively.
During EP El Niño, in ON and DJ, positive anomalies in the upper atmosphere tend to shift equatorward relative to climatology. Although the anomalous displacement upper westerlies are similar during these summer periods, the intensity varies, with DJ exhibiting more enhanced winds extending deeper into the lower troposphere. In FM, the vertical profile of winds anomalies show that intense upper westerly winds tend to be located farther northward. The negative anomalies located near this region of the jet does not necessarily represent a flow in the opposite direction but rather indicate that the magnitude of the jet is relatively weaker compared to the climatology.
During CP El Niño, In ON, positive wind anomalies appear to have shifted towards the equator and are only confined between mid and upper atmosphere, indicating occurrence of a shallower jet. In DJ, the jet maintains the equatorward shift, more intense and extends deeper. In FM, the core of positive wind anomalies in the upper atmosphere appears to be displaced slightly south of 50°S.
These nuanced differences in the displacement and intensity of the jet during the two El Niño types could help demystify possible the mechanisms through which these events affect lead to different rainfall pattern throughout summer period as presented in the earlier sections.