Winter geostrophic currents and eddies in the western Iberia coastal transition zone

Abstract

The results of a fine-resolution hydrological survey conducted in the western Iberia coastal transition zone in February 2000 are used to describe the vertical structure of the density field and geostrophic currents in the Central Water (CW) layer. The vertical density distribution was characterized by the presence of three pycnocline layers: (i) at the surface driven by the buoyancy input of a low-salinity plume, (ii) at about 200 m ( σ θ =27.0), below the surface mixed layer and, (iii) in the transition layer between the Central and Mediterranean Water at depths close to 600 m ( σ θ =27.3). It is shown that the density distribution at the depth of the deeper pycnocline controls much of the geopotential anomaly field when the reference level is below the lower Mediterranean Water core (≃1200 m) , leading to a high sensitivity in the geostrophic velocity profiles to the choice of the reference level. The use of a reference level above 600 m artificially reduces the flow in the lower CW layer (27.0< σ θ <27.3) where significant volume transports are found when a reference level at 1400 dbar is used. In particular, the presence of an equatorward flow at about 400 m in the near-slope region at 41°N was found to be responsible for the volume transport of 0.3 Sv in the direction opposite to the same volume transport in the upper CW layer ( σ θ <27.0), a feature not reported in previous studies based on field data. In the northernmost sections, the flow in the lower CW layer was directed in the same poleward direction as the upper layer in the near-slope region, leading to a deep signature in the poleward current with a maximum volume transport increase from 0.5 Sv in the upper CW layer to 1.1 Sv in both the layers. Schematic views of the non-divergent circulation in two isopycnal CW layers are used to present the main flow paths and volume transports associated with the mesoscale eddy field.