Xulu et al., (2020) present a review analysis of the impact of the Mascarene High (MH) pressure system on weather and climate over southern Africa (SAF).
The MH is a subtropical semi-permanent anticyclone, located over the South Indian Ocean (called also "South Indian Ocean High") (Fig. 1) and is associated with the descending branch of the Hadley cell (Nguyen et al., 2013). Its importance for weather and climate over SAF is basically related to the transport of moisture via the trade winds and its role on "piloting" tropical weather systems that develop over the SW Indian Ocean and move towards SAF.
An important contributor to the MH variability is related to the Sea Surface Temperatures (SSTs) of the Indian Ocean. SSTs are largely affected by the atmosphere-ocean coupling via the Bjerknes feedback (Bjerknes, 1969). An additional effect of the atmosphere-ocean coupling is related to a feedback involving SSTs, evaporation and wind over SE tropical Indian Ocean. In summary, the MH plays an important role in modulating the Indian Ocean Dipole (IOD), with IOD events exerting a severe impact on drought or flood events over SAF. Drought events over SAF are related to IOD+ (positive) phase, while IOD- (negative) phase is related to flood events. Extreme drought events are usually observed when IOD+ and El Nino events co-occurr (2015/16). In this point we note that IOD events should not be mixed with Indian Ocean Subtropical Dipole (IOSD) events!
Apart from its impact on IOD, the MH controls the transport of moisture from the Indian Ocean towards SAF. Towards this end, the MH "collaborates" with the Angola Low (AL) pressure system (Munday and Washington, 2017). When the AL is deep (very low pressure at its center) the pressure gradient between the AL and the MH becomes steep and moisture transport towards SAF is enhanced. The MH also projects as the Botswana High pressure system which is identified in the mid-troposphere over the region of Botswana (Reason 2019), due to the fact that the MH displays a tilt towards SAF with increasing height. When the Botswana High exhibits higher than average pressure values, drought conditions over SAF are favored. Additionally, the MH is often involved in atmospheric blocking events that further affect the track of cut-off lows.
Figure 1: The climatological location of the Mascarene High, according to Xulu et al., 2020.