Temporal and spatial patterning of sea surface temperature in the northern Benguela upwelling system: Possible environmental indicators of clupeoid production

The physical dynamics of the northern Benguela upwelling system between July 1981 and August 1987 were investigated by applying standardized Principal Components Analysis to a time-series of 235 mean, weekly sea surface temperature satellite images of the region. The first three principal components accounted for 87% of the total variance in the standardized input images and were the only components retained for interpretation. Principal Component I (PC I) represents the mean spatial structure of the system in terms of relative SST gradients, PC II represents the balance in dominance between inshore-offshore v, longshore SST gradients and PC III highlights patterns associated with the warming of the central Namibian region (19-23 degreesS) in conjunction with the maintenance of cooler conditions to the north and/or south. These results are interpreted within the context of previous findings on the behaviour of the system, and their relevance to clupeoid recruitment is discussed. Strong negative loadings on PC II indicated conditions which would promote the longshore "retention" of clupeoid eggs and larvae and "concentration" of planktonic food across thermoclines and thermal fronts, whereas strong positive loadings on PC III indicated conditions which would promote onshore retention and concentration. Conditions promoting onshore retention would, on occasion, have also been reflected by positive loadings on PC II whenever there was a uniform contraction of the offshore upwelling front, as a result of reduced, but similar, levels of upwelling along the entire coast.