Representation of Coastal Upwelling and Environmental Interactions in the Southern Benguela in Satellite Era Reanalysis

Abstract

The ability of ocean and atmosphere reanalysis products to represent coastal upwelling in the southern Benguela (28°-36°S, 14°-20°E) is studied in the period 1980-2008. Mean maps, depth sections and seasonal cycle are analyzed for ocean color, sea surface temperature (SST), vertical velocity, winds and ocean currents in the zone. The paper first evaluates coastal winds and cross-shelf gradients of temperature in a variety of products; then provides an analysis of inter-relationships among variables averaged in the St Helena Bay sub-area. The 30km Coupled Forecast System reanalysis resolves the cool strip of SST with a 1℃~2℃ warm bias, while subsurface zonal overturning and a longshore current jet are featured in the 50km Simple Ocean Data Assimilation reanalysis. Mean coastal upwelling rates of 1-2 m/day are consistent with observations. The expected links are found between the annual cycle of meridional wind stress and longshore currents, and between uplift, temperature and salinity. A new finding is a connection between the zonal wind stress and chlorophyll anomalies (-35.86 mg m -3 /N m -2 ). When the subtropical anticyclone is located south-west of Africa, offshore winds prevail and phytoplankton blooms in the southern Benguela.