Distinct Trends of Arctic Sea Fog Frequency Depend on Sea Ice Concentration: Observations and Model Simulations

Sea fog over the Arctic Ocean influences sea ice evolution by modulating downward longwave radiation and affects trans‐Arctic shipping by reducing visibility. Under anthropogenic warming, its long‐term trend is governed by two compensating mechanisms: a more humidified boundary layer which favors fog, and rising surface temperatures which suppress it by reducing boundary layer stability. Here, we show that the dominant mechanism depends on sea ice concentration (SIC), from the perspective of relative humidity (RH) (the ratio of vapor pressure, e , and saturation vapor pressure es ; e / es ). Using in situ observations and model simulations of monthly fog frequency, we find that fog increases where and when SIC exceeds ∼50%. In these areas and time periods, global warming enhances atmospheric moisture capacity, while sea ice melting maintains a stable boundary layer that traps moisture. Consequently, e increases at a rate comparable to es , promoting higher RH and more fog frequency. Conversely, where SIC is below ∼50%, fog will decrease. The heat absorbed by melting sea ice inadequately offsets atmospheric warming over the expanding water areas. This process weakens atmospheric stability, elevates boundary layer height, and enhances vertical mixing. Under these conditions, although es increases more rapidly with temperature, the increase in e is limited, causing RH and fog frequency to decrease with warming. This study reveals distinct spatiotemporal trends of sea fog in ice‐covered regions, and highlights that such patterns result from the competing responses of fog to warming, with a transition occurring near 50% SIC.