Weather types organize fog and low-cloud variability along the South-Western African coastline

Abstract. The hyperarid Namib Desert relies on fog and low clouds (FLCs) as a critical water source, a role likely to grow in importance under a warmer, drier climate. While the climatological mean conditions favoring FLC formation are well understood, its day-to-day drivers are less clear. FLCs have been linked to transient synoptic-scale disturbances, but previous studies relied on two-state (e.g. fog versus clear-sky) composites, which mixed different synoptic states and could not resolve a disturbance's evolution across the coast. To fill this gap, we extend an existing weather-type (WT) classification of the south-eastern Atlantic atmospheric circulation to the full annual cycle, and combine it with 22 years of satellite FLC observations, ERA5 reanalysis, and more than a decade of station observations. We find that in the Namib, FLC occurrence is strongly tied to the life cycle of eastward-propagating mid-latitude disturbances, that drive long-range moisture transport ahead of mid-latitude troughs, and favor free-tropospheric drying and boundary-layer deepening on their rear flanks. This response persists for around three days and is state-dependent, varying with season and latitude. Along the south coast where radiation and advection-radiation fog rather than stratus-interception fog dominate, FLC occurrence instead responds oppositely to the same weather types, pointing to a systematic link between dominant fog type and synoptic sensitivity. These results establish transient mid-latitude disturbances as a unifying, multi-scale control on Namib fog variability and a physical basis for anticipating its response to future circulation change.