Spatial scales in CZCS‐chlorophyll imagery of the southeastern U.S. continental shelf1

Abstract

A 1‐yr time series of Coastal Zone Color Scanner (CZCS) imagery of continental shelf waters off the southeastern U.S. was statistically analyzed to determine dominant scale lengths in the range from 2 to 100 km for near‐surface chlorophyll (Chl) variability. For each image, we extracted two‐dimensional matrices of CZCS‐Chl concentrations from each of 10 study areas. Variograms (structure functions) were calculated for both the along‐ and across‐shelf dimension of each data matrix, although we designed the study areas to focus on along‐shelf variability. The variograms were used to determine whether the images contained nonrandom structure and, if so, to estimate its scale lengths. Outer shelf CZCS‐Chl distributions lacked apparent spatial structure in about 50% of the data matrices vs. <10% for the inner shelf matrices. In general, scale length increased with distance offshore and did not change with season. For example, the mean scale length (n = 30 images) for along‐shelf CZCS‐Chl variability on the inner shelf off Georgia was 32 km vs. 84 km (n = 18 images) for the outer shelf. We propose that Gulf Stream frontal eddies are the principal determinants of outer shelf scale length and that topographic effects on the position of a coastal salinity front determine scale length on the inner shelf off Georgia and South Carolina.