Winter dynamics on the northern Portuguese shelf. Part 1: physical processes

Results of a program of observations carried out on the northern Portuguese shelf and slope, between July 1996 and June 1999 are used to characterise the main physical processes of the winter regime. The data set includes time series of waves and currents at mid-shelf (86 m depth) and of winds, atmospheric pressure and sea level at the coast, as well as hydrographic and suspended particle matter (SPM) observations carried during three multidisciplinary surveys. The wave regime was found to be characterised by highly energetic conditions during the winter, with storm events leading to waves of significant heights exceeding 5 m with mean periods of over 10 s. Prevailing southerly winds force a downwelling regime over the shelf with poleward flow at mid-shelf (~10–20 cm/s) and offshore transport in the bottom Ekman layer. Episodic strong upwelling events can occur during this period, leading to important changes in the near bottom stratification at mid-shelf. Tidal currents at mid-shelf are dominated by the contribution of the M2 tide (~3 cm/s) and are polarised in the cross-shelf direction, reflecting the guiding effect of the Porto Canyon.