Ocean margin exchange—water flux estimates

Hydrographic data define source regions for deep, intermediate and upper-ocean water types found near the northwest Iberian margin; these water massesʹ structure and geostrophic flow are described. Labrador Sea Water (LSW) moves south through the region; above this at 1000 m, particularly near the coast, Mediterranean Sea Outflow Water (MSOW) moves north. Near-surface waters are more seasonal, and we distinguish summer and winter regimes. Summer: above 600 m, poleward flow over the slope is at a minimum in spring and summer, and a maximum in the autumn/early winter. Overlying this is a general southward flow linked to wind stress. Summer upwelling along the Iberian shelf has associated offshore filaments stretching westward, and appears most intense around Finisterre (starting earlier, more persistent). The vertical upwelling flux is τ/ρf but lateral exchange in filaments may be more. Winter: it is proposed that upper layers of the water column offshore are fed from the north. Potential vorticity calculations suggest enhanced diapycnic mixing. However, other data indicate an east–west front down to about 300 m at 39–40°N, i.e., north–south convergence. This front bends to the north near the shelf edge, in accord with a warm flow along the upper slope, 0.15 m/s or more. Surface temperatures often show a much broader (hundreds of kilometers) northward extension of warm water, probably affected by winds. The thermocline from the previous summer can persist to January at least. Coastal waters show cooler river runoff. Budgeting of water fluxes is discussed in relation to direct estimates (from water mass properties and current measurements) and in relation to possible process contributions to fluxes. On/off-shelf exchange has a relatively short time scale, estimated at about 12 days. Upwelling (mostly) and mixing may supply the nutrient content of about 200 m water depth for new production over the shelf.