Meteorology of 'ridging high' rainfall over the KwaZulu-Natal coastal plains

The meteorology of ridging highs on the coast of KwaZulu-Natal (KZN) South Africa was studied using Principal Component (pc) analysis of daily ERA5 sea level air pressure (SLP) fields 1980-2021. The second pc mode time score representing ridging highs, was associated with rain-showers moving northeast-ward along the KZN coast. Temporal analysis revealed an Oct-Dec peak occurrence and 12-day pulse intervals induced by the eastward migration of atmospheric Rossby waves in early summer. From a ranking of SLP pc2 and associated daily time series, a case study emerged: 14-18 Nov. 2009. This ridging high weather scenario had cold southerly winds subsiding over the warm Agulhas Current. Surface moisture fluxes >350 W/m(2) create a buoyant airmass that slows and lifts over the coastal plains, inducing shallow clouds that reduce net solar radiation and potential evaporation. Interannual variability of ridging highs was studied by filtering the daily SLP pc2 time score to retain seasonal fluctuations and correlating with regional fields of SST. This revealed warming in the SW Indian Ocean associated with positive phase Indian Ocean Dipole (IOD). The atmospheric circulation forms a NW trough over Madagascar which deforms eastward moving mid-latitude anticyclones. Thus, ridging highs that bring beneficial Oct-Dec rains to the coast of KZN depend on downstream climate coupling with the IOD. Long-term trends from a coupled model simulation 1980-2100 suggest a poleward retreat of the anticyclonic ridge, consistent with declining Durban-area river flow in early summer.