Interaction of the Marine Atmosphere, Coastal Topography and Sea Surface Temperature on Marine Fog Distribution Along the West Coast of North America

Abstract:

ICOADS ship observations for 1950-2007 were used to examine the distribution of marine fog along the West Coast of North America between 20°N and 50°N. Quarterly (June-August, JJA; Sept-Nov, SON; Dec-Feb, DJF; March-May, MAM) long-term means were obtained and gridded using a 1°x 1° degree-arc mesh. The ship observation includes a subjective assessment of the weather that is codified into 100 categories. The most important and relevant are deep fog at the time of observation with the visibility less than or equal to 1 km. The most frequent deep fog is during JJA, with the highest values in the grid points closest to shore. There is a general fog frequency maximum along the coast between 34°N – 44°N that decreases offshore. More frequent occurrences tend to occur at major capes. The maximum JJA occurrences are at Cape Blanco and Pt Arena. In contrast, deep fog is infrequent along Baja California. In SON, deep fog occurrences are lowest with a narrower coastal maximum. In DJF, there is a broad, uneven fog maximum along the coast between 35°N-39°N. The frequency of deep fog occurrence is intermediate in MAM, with a weak, broad area north of 35°N. The distribution of the occurrence of deep fog in JJA is determined by a three way interaction between the atmosphere, the land and the ocean. The southbound marine layer is channeled by the coastal mountains and interacts with land topography via hydraulic dynamics. The marine layer flow accelerates on the downwind side of major capes, forces greater wind driven upwelling and colder sea surface temperatures along the coast of Southern Oregon and Northern California. However, the fast southbound winds are largely free of marine fog. Marine fog tends to occur along the inner coast when the southbound winds decay or reverse direction for 1-3 days, with the densest fog downwind of a major cape where the sea surface temperatures are lowest.