Coastal upwelling and tropical warm water intrusions are key drivers of interannual fog variability along the southwestern African coast

Abstract. Fog and low clouds (FLCs) are a key source of moisture for ecosystems in the Namib Desert, yet their variability and underlying mechanisms remain poorly quantified. We investigate monthly FLC cover in two fog hotspots: the Angolan Namib (15–17° S) and the Central Namib (22–24° S), using satellite-based observations from 2004 to 2019. Assuming that most fog originates from advected marine low clouds, we apply a cloud-controlling factor framework in which FLC anomalies are modeled as a linear function of spatial anomaly fields in estimated inversion strength (EIS), relative humidity at 700 hPa (R700), sea surface temperature (SST), and the eastward and northward components of the 10 m wind (U10 and V10). Sensitivities of FLCs to these drivers are quantified using a statistical model. Results indicate positive sensitivities to coastal EIS, negative sensitivities to R700, localized negative sensitivities to SST, and a strong influence of onshore circulation, consistent with an advective origin of fog in the Namib region. The statistical models are then used to reconstruct historical FLC anomalies for 1982–2019 using reanalysis data. The reconstructions reveal near-zero trends, resulting from two opposing influences: enhanced atmospheric stability increases FLCs, while SST warming reduces them. Finally, the reconstructions are used to assess interannual variability. ENSO slightly enhances FLC occurrence. However, variability is more closely linked to coastal upwelling and Benguela Niño events, SST warming episodes associated with tropical water intrusions, which explain up to half of the interannual FLC variability in the Angolan Namib.