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The coastal mountains of semiarid Chile are punctuated by patches of fog- dependent evergreen forests. Fog episodes often occur when the prominent coastal topography intercepts a well developed deck of stratocumulus (Sc) off north central Chile. A 22-year record of ground-based fog observations at Fray Jorge Biosphere Reserve (FJBR, 30 degrees S), atmospheric reanalysis and satellite derived low cloud amount are used to document the annual cycle and interannual variability of fog frequency there. The number of foggy days minimizes during austral winter and then increases rapidly to reach a maximum in spring (the growing season of FJBR trees). The mean annual cycle of the fog- frequency follows closely the annual cycle of the nearby marine Sc amount and lower tropospheric stability (LTS). The springtime fog frequency, nearby marine cloud amount and LTS are also well correlated at interannual timescales. Colder than normal sea surface temperatures and warmer than normal air temperatures aloft near 30 degrees S strengthen the temperature inversion and lead to a more persistent cloud deck and higher than normal fog frequency at FJBR. La Nina years produce temperature anomalies very similar to the pattern described before and consequently they are associated with higher than normal springtime fog frequency at FJBR. Conversely, El Nino years are associated with less foggy conditions at FJBR. Interestingly, El Nino-Southern Oscillation (ENSO) related rainfall anomalies in north central Chile are opposite to ENSO-related anomalies in fog-frequency. We discuss the overall impact of ENSO in FJBR ecosystems as well as the prospects of FJBR in future climate scenarios driven by increased greenhouse gases.