Surface temperature, chlorophyll and advection patterns during a summer upwelling event off central Portugal

Abstract

Satellite-derived sea surface temperature (SST) and chlorophyll ([Chl]) concentration maps are used together with numerical models to study the advection patterns observed during a summer upwelling event off central Portugal, a region characterized by a complex coastline and bathymetry. It is shown that the model solutions realistically reproduce the main patterns of spatial and temporal SST variability, namely the 2 – 2.5 ∘ C decrease in the active upwelling areas during the wind intensification phase, the 1-day lag between the peak of the northerlies and minimum SST, and the small SST variability in the sheltered embayments. The expected circulation features are reproduced in the model solutions: the strong along-slope flows, the development of filaments and separated jets, shadow areas downstream of main capes, frontal-scale instabilities and the rapid onset of coastal counter currents along the inshore zone during relaxation. The evolution of the oceanographic conditions, specifically the mixed layer depth and horizontal velocity, and the advection patterns obtained from a Lagrangian model are compared with the observed SST and [Chl] variability. The results imply that the potential phytoplankton accumulation/growth (PPAG) areas are characterized by surface temperatures lower than 16.5 ∘ C , mixed layer depth and horizontal velocity less than 30 m and 0.3 m/s, respectively. It is shown that remotely-sensed [Chl] concentration patterns, namely its asymmetric distribution relative to SST during the intense wind phase, is primarily related to the mixed layer depth and secondly to the horizontal velocity. The trajectories obtained with the Lagrangian model confirmed earlier suggestions of the presence of recirculation cells, downstream of the capes where the coastal jet separates from the coast, and revealed that significant vertical displacements occur in these cells.