| ||
 |
The Portugal current as represented by the Mariano Global Surface Velocity Analysis (MGSVA). The average flow is towards the south and feeds the Canary Current. Highlighted in red is the area where the coastal current can flow northward, depending on the winds. Click here for example plots of seasonal averages. | |
1) The Portugal Current, which is a broad, slow, generally southward-flowing current that extends from about 10°W to about 24°W longitude;
2) The Portugal Coastal Countercurrent (PCCC), a southward flowing surface current along the coast during downwelling season, mainly over the narrow continental shelf to about 10-11°W longitude and flow from about 41-44°N; and
3) the Portugal Coastal Current (PCC), a generally poleward current that dominates over the PCCC during times of upwelling and like the PCCC, extends to about 10-11°W from shore, also present mainly from 41-44°N, where flow is 13.5 ± 5.7 cm s-1 (Perez et al., 2001; Martins et al., 2002).
The Portugal Current system is supplied mainly by the intergyre zone in the Atlantic, a region of weak circulation bounded to the north by the North Atlantic Current and to the south by the Azores Current (Perez et al., 2001). This current system should be envisaged within a very broad perspective of the dynamics of eastern oceanic boundary layers as outflow from the Mediterranean Sea has a significant influence on the underlying water masses that are part of the PC system (Ambar & Fuiza, 1994). The PC system is also influenced by the more dominant neighboring Canary and Azores Currents (Perez et al., 2001). The literature cites several inconsistencies in the seasonal patterns of this current system. As noted by Maze and others (1997), "these apparently contradictory views reveal the need for a better definition of the Portugal Current in terms of lateral extent and possible temporal variability and should be addressed by repeated surveys in the periods of well-established winter and summer regimes."
Analysis of CTD measurements, drifter data, and satellite images from the past several decades taken from the Portugal Current system itself has revealed the hydrology of the region and proven its geostrophic circulation to include a complex and not completely understood interaction between underlying water masses as well as meteorological conditions. Other significant influences on the Portugal Current System are seasonal winds, freshwater runoff from the Iberian Peninsula, the bottom topography found along the continental shelf break and slope, and the three main underlying water masses that are found below the seasonally variable surface layer. (Perez et al., 2001; Ambar & Fuiza, 1994; Huthnance et al., 2002; Martins et al., 2002). Meddies (eddies comprised of Mediterranean water) are also present, particularly in the region of the Tagus Abyssal Plain (about 11-13°W; 37-39°N) and along the shelf break, which are thought to be controlled mainly by the topography of the seafloor (Bower et al., 2002; Cuelho et al., 2002; Cherubin, 2000; Huthnance et al., 2002).
Generally, the mean flow on the surface is southward, however seasonal winds in the region can result in both northward and southward flows, which were observed with drifters equipped with holey sock drogues centered at about 15 m over a period of 14.4 years (Martins et al., 2002). During the summer, predominant trade winds from the north cause wind-driven and persistent upwelling along the coast of the Iberian Peninsula. Cooler water from depths of 100-300 m is upwelled (Smyth et al., 2001). These events usually begin around and are particularly intense off of Cape Finisterre and Cabo da Roca, often forming filaments that can reach as far as 100 km westward (Cuelho et al., 2002; Hutnance et al., 2002) and their velocities, tracked by thermal features in satellite images, can reach up to 0.28 m s -1 (Smyth et al., 2001).
The Portugal Current roughly marks the northern extent of the Canary Current and is estimated, on average, to extend about 300 km beyond the shelf, transporting about 2.0 Sv ± 1.2 Sv (Maze et al. 1997) at an average of 1.6 cm s-1 with maximum speeds reaching up to 5.7 cm s-1 (Martins et al., 2002). The surface current can range between 0.3 - 12 Sv region-wide (10Sv at 37.2°N, 12Sv at 43°N and in the narrow strip between 10.5°W and 1000m water depths, 9Sv at 37.2°N and 4Sv at 43°N) (Huthnance et al., 2002; Martins et al., 2002). According to Huthnance and others (2002), the mean flow of the upper-most water column (<600m) fluctuates according to season and varies more with increasing proximity to the coast: northwards in autumn and winter, west and southwards in spring and summer. However, significant on- and off-shelf transport of about 2 Sv can be measured throughout the year (Perez et al., 2001; Martins et al., 2002).
The salinity in the upper 100 meters varies between 35.8 and 36.0 and between 14-19°C, depending on whether upwelling (cooler water) or downwelling (warmer waters) dominates. Sea-surface temperatures on the shelf have been observed by Ambar et al. (2002) to reach up to 24°C in the summer and remain between 18°C to 10°C in the winter. The mixed surface layer is about 100 m in thickness (Ambar & Fiuza, 1994; Ambar et al., 2002), although during heavy convergence events can extend down to about 150 m (Fuiza et al., 1998). Below the surface mixed layer, the salinity decreases to about 35.6 and ~11°C at depths of 400-500m (McCave & Hall, 2002; Smyth et al., 2001). Current measurements of horizontal surface velocities vary throughout the year, however were measured between 0.1-1.0 m/s for the underlying Central and Mediterranean Water (MW) masses (Maze et al., 1997).
In the case of the Portugal Current system, underlying water masses are not to be ignored as these directly impinge on the seasonal surface variability, ventilation of underlying water masses, and mean flow (Perez et al., 2001). Beneath this seasonal variable layer, is a layer of Eastern North Atlantic Central Water (ENACW) of subtropical origins from about 200-300 m moving north and from about 300-400 m, a component of ENACW of subpolar origin moving south (Peliz & Fuiza, 1999; Fuiza et al., 1998; Ambar & Fiuza, 1994). From depths of 400m to about 1300 m, Mediterranean outflow Water (MW) dominates, signified by its warm temps (12°C) and high salinities (36.3 USP). It is the MW interacting with slope sediments that causes the relatively high turbidity found along the shelf and shelf slope, particularly in the depth range of between 500m-800m, where the density gradient between the ENACW and MW is at its highest. Below this, flows are much slower than average and therefore turbidity is also low (McCave & Hall, 2002).
Beneath the MW lies North Atlantic Deep Water, characterized by its very low temperatures and salinities. The influence of both the subtropical component of the ENACW and the MW decreases northwards (Ambar & Fiuza, 1994). In fact, the Mediterranean water plume that spreads westward, descending the continental slope, mixes intensely with the overlying North Atlantic Water while still in the Gulf of Cadiz, losing much of its density anomaly, although it still retains a salinity of 1 part per thousand above the western North Atlantic water of similar density (Cherubin et al., 2003; Bower et al., 2002; Baringer & Price, 1999). Major Portuguese capes, Portimao Canyon, Cape St. Vincent, Estremadura Promontory and Cape Finesterre, appear to be regular sources of Meddies (Cherubin et al., 2000; Cerubin, 2000), although some appear to come from the slope near 41°N. About four or five Meddies may exist at the same time in the Iberian Margin region (Huthnance et al., 2002). Bower et al. (2002) estimated a formation rate of 15-20 Meddies per year. At all intermediate levels, the continental slope in the Bay of Biscay seems to be a focal point for intermediate water mass modification by diapycnal mixing (Van Aken, 2000).
During January and Febrary, particularly, dominant wind stress along the Iberian margin is northward-contrary to the annual mean-and suppresses upwelling conditions while supporting a poleward slope current (Van Aken, 2000; Frouin et al., 1990). In the fall, winter and the beginning of spring, the Portugal Coastal Countercurrent, the eastern-most flow included in the Portugal Current System, generally flows poleward off the western and northern portions of the Iberian Peninsula and vanishes during the upwelling season (Perez et al., 2001). Off the slope, the poleward-moving water mass extends to about 600 m in depth, moving at about 0.03-.10 m s-1 (Huthance et al., 2002). Conditions usually favor downwelling during this time, however because winds are highly variable along this coast, especially throughout the winter, upwelling is often favored at some time during any given month. Yet it is worthwhile to note that a poleward current has been observed to persist through the summer at about 280 m, only reversing briefly during the most intense upwelling events (Huthnance et al., 2002).
During the spring and summer months, surface current flow is mainly towards the south at rates of between 0.05 and 0.15 m s-1 (Cuelho et al., 2002; Huthnance et al., 2002). The ocean surface layer, comprising the upper ocean mixed layer and seasonal thermocline, highly varies in thickness both spatially (north to south) and from summer to winter as a consequence of the seasonality of air-sea fluxes and land runoff, and can range anywhere from 200 m to 700 m (Fuiza et al., 1998). The mechanism behind seasonality along the Iberian Peninsula is most likely the migration of the semi-permanent subtropical high-pressure system known as the Azores High, which migrates meridionally between 27°N (March) and 33°N (August), also weakening significantly in the fall and winter months (Cuehlo et al., 2002; Maze et al., 1997).