Impact of Zonal Configuration of the South Atlantic and South Indian Ocean Subtropical Highs on March-May Surface Air Temperature Variability over Southern Africa

Abstract

The zonal configuration of the South Atlantic (SASH) and South Indian (SISH) subtropical highs significantly influences southern Africa’s climate variability from daily to decadal scales. This study examines the impact of their zonal positioning on March-May (MAM) surface air temperature (SAT) from 1940-2024. The Empirical Orthogonal Function (EOF) method was used to analyze monthly sea level pressure (SLP) to identify different zonal displacement phases. Composite analysis was also used on net shortwave radiation (SSR), surface energy fluxes, and total cloud cover to diagnose physical mechanisms associated with SAT variability. Joint westward displacement of the highs triggers SAT cooling over the central plateaus and Mozambique Channel, while joint eastward displacement is associated with SAT cooling over East Africa and parts of the southern and central plateaus. Westward SASH and eastward SISH displacement results in slight SAT cooling over East Africa, the central plateaus, western Madagascar, and the western coast. Conversely, eastward SASH and westward SISH displacement cool the eastern region extending to the central interior, while warming the remaining parts. These configurations strengthen the subtropical ridge, increasing subsidence and reducing cloud cover, which increases net SSR and surface sensible heat flux (SSH) and reduces outgoing longwave radiation (OLR) and surface latent heat (SLH) flux, leading to surface warming. Away displacement triggers ascent and increased cloud cover, leading to less SSR and SSH and more OLR and SLH flux, contributing to observed SAT cooling. These findings demonstrate that the zonal configurations of subtropical high structures modify the net energy pattern in opposing ways depending on the phase configuration, highlighting their importance for improved seasonal SAT predictability.