The Galician shelf response to the varying seasonal forcing has been characterised through a set of observations including satellite, cruise and mooring data encompassing the period 1997-2001. The typical spatial variability of wind and its effect on the shelf circulation has been investigated. The seasonality of the large scale wind during July 1999- May 2001 was masked by upwelling/downwelling patterns throughout the year. However, upwelling winds were found to be more consistent during summer and the system showed clear seasonality with upwelling during summer and downwelling during winter mediated by a meridional density gradient. Despite its spatial complexity, the mesoscale wind variability was reduced to a finite number
of spatial patterns. These favoured upwelling north or south of Cape Finisterre or
downwelling along the west coast of Galicia. Their relative abundance influenced the interannual variability during both upwelling and downwelling regimes.
Slope poleward flow characteristic of the downwelling regime was measured at
all times, suggesting it is a permanent feature in the Galician CTZ system. Its
winter variability was related to unusual periods of upwelling winds. It has a surface signature in winter, autumn and during spring. The shelf was effectively isolated from the ocean due to a shelf-edge front , as indicated by the lagrangian horizontal diffusivity although some exchange still occurred via a bottom Ekman layer. In the summer it formed a subsurface poleward flow.
The weakening of the meridional density gradient initiated decay of the poleward
slope current. The warm tongue broadened, and the organised flow of the poleward current broke down as eddies were generated and the flow branched into separate streams, one along t he shelf break and one along the outer slope.
During persistent upwelling on the west coast, the upper column structure becomes dominated by coastal upwelling and filaments while the poleward flow is restricted to subsurface layers. A detailed survey of a filament showed limited export capabilities, although the lagrangian diffusivity was enhanced during the summer upwelling regime. Interannual variability during the upwelling regime is characterised by either presence/absence of filaments or whether the main filament is at Cape Finisterre or at 42°N latitude.
Towards the end of the upwelling regime, when upwelling winds weaken, the
poleward flow returns to the upper slope. However, it is not until the meridional
density strengthens that the poleward flow surfaces again. If filaments are still present, the slope poleward flow cuts their water supply and they slowly mix away.