Cross-shelf flow forced by daily variable winds in the presence of a cape

Tese de doutoramento, Ciências Geofísicas e da Geoinformação (Oceanografia), Universidade de Lisboa, Faculdade de Ciências, 2018This thesis intends to contribute to the knowledge of the nearshore circulation in the vicinity of a cape under daily variable wind conditions. Throughout this study, several different tools were used to understand the diurnal variability of the circulation, with special emphasis on the cross-shelf transport in the leeside of the cape. In-situ data collected near Cape Sines, on the southwestern coast of Portugal, during three summers, indicated that the wind was consistently favourable to upwelling with recurrent sea breeze-like diurnal variability. Current data collected by an ADCP in the leeside of the cape during events of strong diurnal variability of the wind showed that the average day of the cross-shelf circulation consisted of three distinct periods: a morning period with a 3-layer structure with onshore velocities at mid-depth, a mid-day period where the flow is reversed and has a 2-layer structure with onshore velocities at the surface and offshore flow below, and, lastly, in the evening, a 2-layer flow with intensified offshore velocities at the surface and onshore flow at the bottom. A simplified 2D numerical model hinted at the fact that the cross shelf flow reversal which occurred at mid-day was forced by the local acceleration of the along-shelf velocity. A numerical modelling experiment exclusively forced by winds simulated successfully most of the circulation at a location consistent with the ADCP, especially the mid-day reversal and the evening’s upwelling-type structure. This supported the hypothesis that the observed crossshelf circulation at diurnal timescales was dominantly wind-driven. Analysing a set of modelling experiments with simplified topography and wind forcing helped to clarify the dynamics behind the ocean circulation around the cape, indicating that the cross-shelf flow reversal resulted as a response to the rapid change of the wind magnitude at mid-day. The fact that the wind diurnally undergoes relaxation and intensification strongly affects the cross-shelf circulation, promoting surface onshore transport in the leeside of the cape