Increase California-Oregon Coastal Summer Sea Level Fog from 1950 to 2007

Abstract:

An analysis is presented of the marine fog distribution based upon the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) ship observations taken during 1950–2007. Deep fog occurrence is reported in routine weather reports that are encoded in an ICOADS ship observation. Occurrence is estimated by the number of deep fog observations divided by the total present weather observations in a one-degree area centered on latitude and longitude grid point intersections. The mean fog occurrence for the summer (June-July-August) 1950-2007 was computed for each one degree point. There is a long term, deep fog occurrence maximum on the California-Oregon coast with its highest value of 16.6 % at 38° N 123° W. This fog maximum is coincident with coldest June-July-August sea surface temperatures (SST) along the coast. To compute annual averages of the maximum, a block average was based on the 19 over water grid points with the deep fog occurrences generally greater than 0.6 times the highest long term maximum value that extended along the California-Oregon coast from 37° N to 44° N. The June-July-August block averaged, annual value computed for each of the 58 summers for the period 1950-2007 has a distinct positive trend. A line fitted to the data has a deep fog percent occurrence increase of +7.4 % from 1950 through 2007 or a trend of +0.13 % per year. The Mann-Kendall test was applied and the trend is significant at the 0.05 level. The increase in long term coastal fog is coincident with a decrease in the California-Oregon coastal SST. The SST decrease is consistent with interior California land temperatures increasing, increasing the cross shore sea level pressure gradient, and increasing the along coast winds creating a positive feedback that causes more upwelling and lower SST.