Climatological annual cycle of Benguela upwelling system using different upwelling indices

Several long-established upwelling indices derived from the observed wind fields, Chlorophyll-a concentration, sea surface temperature (SST) are used to investigate the climatology annual cycle of Benguela Upwelling System (BUS). Chlorophyll-a concentration is taken as an indicator of ocean primary production. In addition, we analyze a multi-decadal simulation of a state-of-the-art eddy resolving ocean model which was forced by observed atmospheric heat and momentum fluxes. We take the vertically averaged of simulated vertical velocity in water column as a direct measure of upwelling strength.

The Ekman offshore transport tends to have two distinctive upwelling cells near the coast of Lüderitz (26.3°S) and Cape Frio (17°S) with large seasonal cycles. The former peaks between September and December. The latter features a biannual cycle with two peaks over April-June and September-December, which is concurrent with meridional migration of Angola-Benguela SST front. The offshore (30-200 km) vertical velocity, primarily induced by Ekman transport divergence, depicts a similar annual cycle, but with smaller magnitude. It becomes broader from south to north with four distinctive upwelling cells located near the coast of Cape Columbine (33°S), Orange River (28°S), Walvis Bay (23°S) and northern part of Cape Frio (16°S). The spatial and temporal variation of Ekman pumping and Chlorophyll-a, as measures of upwelling, show a clear correlation. However, such a correlation is not evident when Ekman coastal transport is taken. SST-based index depicts a very similar spatial pattern. However, the seasonal cycle does not match with other observational and simulated indices. Our finding suggests that the local SST anomalies are strongly influenced by horizontal heat advection and surface heat flux anomaly which can dominate over the anomalies associated with the upwelling; meaning that SST-index alone may not give a realistic estimate of upwelling strength over the region.