Evaluating upwelling estimates off the west coasts of North and South America

Both low- and high-resolution studies of wind and upwelling were conducted off the west coasts of North and South America between August 1999 and December 2001. For the low-resolution study, done at 25 regional sites spanning both coasts, weekly mean winds were calculated from satellite measurements, from geostrophic estimates, and from an operational 100-km-resolution global atmospheric model. The satellite-measured winds, used as the reference, showed that the coastal regions of both North and South America were divided into fairly uniform climatic domains located at higher, mid, and lower latitudes. All three weekly-mean wind data sets were compared at each of the 25 sites, and then used to estimate upwelling based on the method employed by NOAA's Pacific Fisheries Environmental Laboratory. Within each of the wind domains, model-derived wind and upwelling estimates agreed with satellite-derived values better than those from geostrophic-derived estimates. To investigate variability between the regional sites, a 9-km-resolution atmospheric model was run for an area off California which spanned four of the regional sites. This high-resolution model, verified with satellite measurements, revealed jets of wind curving and intensifying around coastal promontories. These near-shore wind jets, undetected in the global model, resulted in strong upwelling bands on the order of 50 km alongshore by 20 km offshore. These bands are critical for calculating local upwelling. Our results indicate that regional upwelling estimates for fisheries research can be improved by replacing geostrophic estimates of winds with those from a global atmospheric model. For localized coastal upwelling estimates, however, models with resolution an order of magnitude higher are required.