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Home , Azores Current, Canary Current, North Atlantic Current, Portugal Current

380 CANARY AND PORTUGAL CURRENTS

CANARY AND PORTUGAL CURRENTS

E. D. Barton, University of Wales, Bangor, UK Current and further south into the . The latter separates from the African coast at Copyright ^ 2001 Academic Press around 203N to become the North Equatorial doi:10.1006/rwos.2001.0360 Current, which eventually feeds into the and back to the . Introduction Using all available hydrographic data, the long- term average geostrophic Sow for the region has The Canary and Portugal Currents form the eastern been calculated assuming a level of no motion near limb of the North Atlantic Subtropical Gyre. De- 1200 m. Scarcity of data limits the resolution of the tailed knowledge of the currents is still surprisingly analysis to a grid of 33;33; near the coast, where sparse in some respects and is largely based on deep data are even fewer, results are more uncer- indirect methods of estimating the Sow. The entire tain. Where the eastward-Sowing eastern boundary of the gyre is affected by the turns south as it nears the eastern boundary, two process of coastal , driven by the sea- branches of the Canary Current are formed (Figure sonally varying Trade Winds. Upwelling is intimate- 2A) separated by Madeira. West of Iberia only weak ly related to the currents on the , near-surface Sow toward the eastern boundary is and varies on timescales from several days upwards. indicated, while nearer to shore the Portugal Cur- This phenomenon has been studied intensively at rent carries about 2;106 m3 s\1 equatorward in the different places and times and long-term sampling layers above 200 m depth. Only some of this con- has only begun in recent years. tinues southward into the Canary Current, while the rest apparently enters the Mediterranean in a shal- Large-scale Circulation low surface layer. The total amount of water carried equatorward above 200 m in the Canary Current, The low to midlatitudes of the North Atlantic including input from the , was esti- are occupied by the clockwise rotating sub- mated at about 4;106 m3 s\1 between 353W and tropical gyre (Figure 1). The western boundary of the African coast. Near 203N the Canary Current this system is made up by the Gulf Stream, which breaks away from the African coast to turn west- feeds into the and the ward as the near 153N. Azores Current. The latter Sows eastward to supply South of this separation point, a recirculation cell the eastern subtropical boundary region. Branches around the ‘Guinea Dome’ lies east of the Cape of the Azores Current loop gently into the Portugal Verde Islands between the coast and the equator- ward Sow. The Canary Current varies seasonally by slight . 90˚N nd C changes in position but not greatly in transport la C. reen ian st G eg Ea rw (Figure 3). Areas of larger uncertainty, shaded in the No 60˚N Labrador Subpolar Gyre . R c C gure, correspond to few or no hydrographic sam- nti tla A rth ples in that study. The near-shore branch of the No . C C l . Azores C. a current migrates seasonally across the Canary

tug r o Islands, closer to in summer and farther of- P

t Gulf StreamSubtropical n fshore in winter. As it does so, the Azores Current e 30˚N r r Gyre u oscillates south in summer and north in winter, so C

y Latitude r

a n that the eastern part of the gyre has an annual a North C Equatorial ‘wobble.’ Some streamlines intersect the African Caribbean C. Current coast in the northern half of the area and leave in

G the south, suggestive of a narrow, intense equator- uy ana ward Sow in the under-sampled coastal band be- Eq C uatoria l Counter C. 0˚ tween 203 and 303N, particularly in spring and 90˚W 60˚W 30˚W 0˚ 30˚E summer. Recirculation south of 203N is clearest in Longitude winter and spring, but indications of northward Figure 1 Sketch of the general near-surface circulation of the Sow are also seen there during other seasons. In North . autumn, near-shore northward Sow extends as far CANARY AND PORTUGAL CURRENTS 381

41°N 41°N

Azores 38°N 38°N

35°N 35°N

33°N Madeira 32°N

29°N 29°N

Canaries 26°N 26°N

Latitude 23°N 23°N AFRICA Latitude

20°N 20°N

17°N 17°N Cape Verde _1 Islands 1 cm s 14°N 14°N

11°N 11°N

8°N 8°N 35°W 32°W 29°W 26°W 23°W 20°W 17°W 14°W 11°W 8°W 35°W 32°W 29°W 26°W 23°W 20°W 17°W 14°W 11°W 8°W (A) Longitude (B) Longitude Figure 2 (A) Total transport of volume calculated from the long-term mean density field with the geostrophic assumption and summed from 200 m depth to the sea surface. Between any pair of streamlines the volume transport is 0.5;106 m3 s\1. The calculations have less uncertainty inside the dashed line. (B) vectors (thin arrows) at 200 m depth calculated from the long-term mean density field and mean observed currents (thick arrows) near 200 m at the sites marked by dots. The circled dot indicates the longest record, Kiel276.

as the Canaries, although again in the coastal Canary and Portugal Currents as well, although no under-sampled band. long-term current monitoring has yet been achieved The few available long-term moored observations there. are mainly well away from shore and the shallowest Several year-long moorings, deployed recently be- records are at about 200 m depth. Most lasted 1}2 tween the and the African shelf, years, but one (Kiel276) is continuous since showed highly variable Sow, with maximum 1980 on the southern edge of the Azores Current velocities near 0.3}0.4 m s\1, but seasonal mean (333N, 223W). In general, Suctuations were more currents 10 times smaller. In mid-channel the upper than 5 times more energetic than the mean Sow and 200 m layer Sowed southward with variable so most records do not provide a reliable statistical strength. The bulk of the upper 600 m Sowed main- estimate of the average. Nevertheless, the measured ly southward in winter and spring, but northward in mean currents agree broadly with the large-scale summer and autumn. Despite increased Trade geostrophic Sow (Figure 2B) but indicate that the Winds in summer the Canary Current decreased east transport may be '50% more than indicated by of the Canary Islands, at the same time as the Sow the geostrophic calculations. No subsurface record through the western islands increased. These obser- indicated signiRcant seasonal variability. However, vations are compatible with the suggested seasonal the Kiel276 record demonstrated the dominance of variation in the gyre and the extension of near- meanders and eddies in the Azores Current and surface poleward Sow to the latitude of the Cana- variability on the scale of a decade. Since the Azores ries, although the timing is out of phase with the Current feeds into the eastern boundary system, long-term seasonal average results. Of course, any similar long-term variability likely occurs in the single year is not necessarily representative of the 382 CANARY AND PORTUGAL CURRENTS

40°N

30°N Latitude

20°N

10°N

(A) 30°W 20°W 10°W (B) 30°W 20°W 10°W

40°N

30°N Latitude

20°N

10°N 30°W 20°W 10°W 30°W 20°W 10°W (C) Longitude(D) Longitude Figure 3 Seasonal variation of the geostrophic volume transport above 200 m depth in the eastern boundary region, calculated as before for (A) spring, (B) summer, (C) autumn, and (D) winter. Areas of larger uncertainty are shaded. Flow between any pair of streamlines is 0.5;106 m3 s\1.

long-term mean. There was no evidence of strong Eastern boundary with a strong alongshore com- equatorward Sow along the in ponent that produces offshore in spring and summer, though measurements did not the surface layers and therefore upwelling at the extend over the upper slope and shelf. coast. The strength of upwelling is conventionally expressed in terms of the upwelling (or Bakun) in- " Coastal Upwelling dex, which is simply the Ekman transport T# (q/of ) where q is the component of For some part of the year the north-east Trade parallel to shore, o is the density of sea water, and Winds blow along every part of the subtropical f is the Coriolis parameter. The index is routinely CANARY AND PORTUGAL CURRENTS 383 calculated from coastal surface wind data on a daily Iberian and African coasts north of 203N. Off the or monthly basis. It represents the rate at which West Coast of Iberia, upwelling generally starts in water is removed from shore in the surface layer May or June and lasts only until September. The and, in a two-dimensional system, the amount of southern, Algarve, coast of Portugal and north coast water upwelled to replace it. of (33}373N) are oriented at a large angle The annual cycle of upwelling for the region (Fig- to the Trade Winds and so upwelling there is inter- ure 4) shows the coastal temperature anomaly with mittent and short-lived. In winter the Trades shift respect to central ocean alongside the monthly mean southward to provoke upwelling between 303 and upwelling index. Summer Trade Winds affect the 123N. South of 203N upwelling starts in December

∆T (°C) Ekman transport (m 3 s _ 1 m _ 1) 531_ 1 _ 3 _ 5 _1 _ 0.5 _ 0.3 _ 0.1 0.1 0.3 0.5 0.7 1.0 1.5 2

40°

38°

36°

34°

32°

30°

28°

26°

24°

22°

20°

18°

16°

14°

12°

10°

8°

6°

4° Winter Spring Summer Autumn Winter Spring Summer Autumn (A) (B)

Figure 4 Annual cycle of upwelling represented by (A) temperature difference between the coast and midocean determined by satellite estimates, and (B) the upwelling index calculated from surface atmospheric pressure analyses. 384 CANARY AND PORTUGAL CURRENTS and lasts until April or May. Between 203 and 303N On short timescales the Trade Winds typically the coast is subject to year round upwelling that remain nearly constant over periods of 7}10 days peaks in July and August. and then relax to near zero or weakly northward for Wind forcing and strength of upwelling, as repre- several days. Observations over the continental shelf sented by coastal temperature anomaly, show of NW Africa have shown how the system responds variations up to a factor of 2 between years and (Figure 5). Favorable winds drive offshore Ekman decades. The 1960s were typiRed by upwelling of transport above about 30 m depth and raise the about half of the average intensity off West Africa. pycnocline near shore so that within one day colder, Upwelling increased through the 1970s, only to less salty subsurface water breaches the sea surface. weaken again in the 1980s and increase in the The upwelled water, which can originate from as 1990s. An intriguing Rnding is that the strength of deep as 200 m, is denser, and there is a slight down- upwelling appears to be increasing in the long term ward slope of the sea surface toward the coast } a trend common to all the upwelling regions of because of the offshore transport. These give rise to the world over the last 40 years. This may be linked an equatorward geostrophic Sow that weakens with to global warming, because increased summer depth over the shelf. The jet is strongest where the heating deepens the continental low-pressure sys- pycnocline intersects the surface as a boundary be- tems so increasing the atmospheric pressure differ- tween the denser upwelled waters and the less dense ence with the oceanic highs to intensify the Trade oceanic waters, where it reaches velocities of up to Winds. 0.8 m s\1 in the example shown. As the wind varies,

0 25.7 36.4 26.0 36.6 26.5 100 > 36.6 26.7

26.8 200 36.2

Depth (m) 300 36.0 27.0 35.8 400

35.6 Bojador 26˚N 27.2 500 Salinity (PSU) Density anomaly (kg m_3)

0 21 80 _ 60 18 _ 40 100 _ 20 16 200 _ 10

Depth (m) 300 14

0 400 12

20 km > 0 500 _ Temperature (˚C) Alongshore velocity (cm s1 )

Figure 5 Sections of temperature, salinity, density, and alongshore current off north-west Africa during favorable winds in August. Contours are elevated from &200 m to the sea surface. The current field is based on a combination of moored current meter (solid dots), profiling current meter (solid vertical lines), and geostrophic estimates (dashed vertical lines). CANARY AND PORTUGAL CURRENTS 385 so do the slopes of the sea surface and the pycno- Iberian and African coasts appear to develop from cline, the strength of upwelling, and the speed of the instabilities of the along-frontal Sow, often triggered alongshore jet. The scale of the upwelling region, by coastal or topographic irregularities such as i.e., the distance within which the isosurfaces are capes or ridges. Some appear associated with off- uplifted is given by the Rossby radius of deforma- shore eddies, which may themselves be topographi- tion j"((g*o/oh)/f, where g is the gravitational cally anchored. Filaments tend to recur in the same acceleration, h is the undisturbed depth of the sur- positions year after year, as, for example, the one face layer above the pycnocline, o is the density of off north-west Spain at 423NinFigure 7. They are the deep water, and *o is the density contrast be- associated with localized areas of net offshore Sow tween the surface and deep layers. The characteristic (therefore local imbalance in the cross-shelf trans- upwelling velocity, w, is given by the upward trans- port) that take the form of a narrow jet where the port divided by the width of the upwelling zone alongshore Sow is diverted seaward. The Rgure w"T/j. For a typical situation j&10}20 km, and shows a surface drifter following the Rlament of- w&10 m d\1. fshore at a mean rate of 0.3 m s\1. This offshore The onshore}offshore Sow observed off NW transport can help effect exchange of water proper- Africa conforms to the classic picture of upwelling ties between shelf and deep ocean because of mixing (Figure 6) where the surface layer offshore Sow is along the Rlament boundary and interaction with compensated by deeper onshore Sow. When the the deeper waters beneath. As upwelled waters wind relaxes, the whole on the shelf move offshore in the Rlaments, they gradually warm moves shoreward, carrying with it warm oceanic and become indistinguishable from surrounding waters. Detailed comparisons of the time-varying waters so that any return Sow to the coast is not offshore and onshore transports with the Ekman easily identiRed in satellite images. index have shown good agreement between offshore S ow and the index, in this and other upwelling Poleward Undercurrent regions. Poorer agreement is found between offshore and onshore transport, which often appear to be Beneath the near-surface equatorward Sow of the locally out of balance. Canary Current, a subsurface current Sows pole- Satellite imagery of sea surface temperature Relds ward, counter to the general circulation and tightly shows that the boundary between upwelled waters bound to the continental slope. It has been and open ocean is often strongly contorted into long documented along the entire continental margin be- Rlaments of cooler water stretching hundreds of tween the Gulf of Guinea and north-west Spain, and kilometers out to sea. Filaments identiRed off the is a common feature of all eastern boundaries.

0

20

40 Depth (m)

60

27 March 30 March 80 _ 20 _ 10 0 10 20 _ 20 _ 10 0 10 20 _ _ (A) E← Flow (cm s1 )→ W (B) E← Flow (cm s1 )→ W

Figure 6 Profiles of cross-shelf flow measured by current meters (solid dots) in 76 m of water on the African continental shelf during (A) near-zero winds and (B) upwelling favorable wind. 386 CANARY AND PORTUGAL CURRENTS

Temperature (°C)

15 16 17 18 19 20 43°N

42.5°N

42°N Latitude

41.5°N

41°N 12°W 11.5°W 11°W 10.5°W 10°W 9.5°W 9°W 8.5°W Longitude

Figure 7 Satellite sea surface temperature image in August showing an upwelling filament extending 200 m offshore. Clouds obscure the image near to shore. A surface layer drift buoy traces the current along the filament. The dots mark daily positions starting on 14 August near shore. White curves mark the 50, 100, 200, 500, and 1000 m isobaths.

Despite many direct and indirect observations of the Islands (283N) before mixing with the surrounding Sow, there are remarkably few systematic observa- cooler and saltier North Atlantic Central Water tions. The structure of the undercurrent is shown by dilutes it beyond recognition. The seasonal analysis measurements made close to 203N(Figure 8). Its showed that during autumn the poleward Sow may maximum speed is close to 0.1 m s\1 at about 150 m occur at the sea surface, again reaching the Canary below the surface. The core extends about Islands, and the few available direct observations 300}400m vertically and apparently less than appear to corroborate this. 50 km horizontally (although its offshore limit was Off Iberia, most of the water column Sows pole- not directly observed). Above the undercurrent, ward, although the surface layer is Sowing equator- shallow equatorward Sow predominates, while in ward above 200 m depth in the long-term mean. layers deeper than 500 m, Antarctic Intermediate During winter, all of the water column moves Water is carried northward at depths around 900 m. northward over the continental slope, but in sum- Note also the weak undercurrent in Figure 5. mer, when the equatorward Trade Winds are pres- Along most of the eastern subtropical gyre the ent, the currents in the upper few hundred meters poleward Sow is restricted to the subsurface layers, are driven equatorward. The undercurrent is known though it may surface when the Trade Winds to extend deeply off Iberia, where it carries Mediter- weaken or turn northward. Off Iberia and south of ranean Intermediate Water at levels between 600 203N it appears to extend to the sea surface for and 1500 m depth. This is water that has escaped more of the annual cycle. In the latter area it forms through the Strait of Gibraltar and is constrained by the inshore loop of the cyclonic recirculation. Where the Earth’s rotation to Sow northward, hugging the it meets the Canary Current separating from the continental slope. As it travels northward it tends to coast, some of the poleward Sow continues north- separate intermittently from the coast in various ward as the undercurrent, carrying with it the typi- locations to form subsurface eddies known as Med- cally warmer, fresher, and higher nutrient content dies. Three preferred paths are reported to carry the South Atlantic Central Water of this region. The Mediterranean Water away from the Iberian conti- anomalous water can be traced as far as the Canary nental slope: northward where the undercurrent CANARY AND PORTUGAL CURRENTS 387

0 highly variable in both strength and direction. _ _30 10 _20 Figure 9 shows a near-synoptic view of the currents, _ _ 10 derived from the combined TOPEX and ERS-1 al- 0 5 0 timetry on 14 August 1993. The small sea surface +2 height slope anomalies measured by satellites have 100 been added to the mean summer surface elevations calculated with respect to a 400 m reference level +10 > +8 from to compensate in part for the lack of the mean signal in the altimetry. The Azores 200 Current meanders along latitude 323N, gradually +8 turning southward into the Canary Current. Little Sow seems to come from the weak Portugal Current + 6 southwards. Near the African coast from 303N the 300 + 6 S Depth (m) ow is southward as far as 203N, where it turns abruptly offshore on meeting poleward Sow from _ farther south. A large number of eddies are seen + 4 V _ throughout the region, especially associated with the 400 +4 V Azores Current and farther south. The resolution of the altimetry is limited by the ground track separ- ation (&50 km or more) and the repeat interval +2 ('15 days). A survey of currents just south of the Canary 500 Islands made near the same time (10}18 August 1993) shows patterns of Sow on shorter scales as complex as in the satellite view (Figure 10). One 30 20 10 0 3020 10 0 Distance (km) Distance (km) might ask where the Canary Current is in this com- plexity. It lies, of course, in the average Sow over Figure 8 Two sections of alongshore flow measured by cur- the area of the survey, about 0.05 m s\1 toward the rent meters (solid dots) near 203N off north-west Africa, showing south west as expected. However, the instantaneous structure of the poleward undercurrent. Speeds are given in current in any location can have almost any direc- cm s\1, northward positive. tion and speed. The Sow Reld is composed of nar- row, strong jets of current, which may meander extends beyond Cape Finisterre, north-westward through the area changing their position with time, west of the Galicia Bank, and south-westward off and circulations that drift with the background the Gorringe Bank. In each case the topographic Sow. Here the alongshore, equatorward Sow ap- feature seems to trigger the formation of the Med- pears diverted around a 100 km diameter cyclonic dies, which then migrate away from the boundary eddy generated in the trough of bottom topography and can maintain their identity for up to 4 years. between Gran Canaria, Fuerteventura, and Africa. One unresolved question is whether the poleward This eddy was only just resolved in the satellite Sow off NW Africa is in any sense continuous with analysis. This level of detail of Reld observation has that off Iberia. There are few observations of the only become available in the last two decades with undercurrent off northern Morocco and none that the introduction of acoustic Doppler methods of might indicate how the Sow interacts with the determining upper-level currents from a moving re- deepening Mediterranean Intermediate Water. The search vessel. In this way rapid surveys can be made latter is dense and sinks from shallow levels on to reveal the intricate patterns of ocean currents, leaving the Strait of Gibraltar to its equilibrium level though still in an area limited in size by ship speed. around 1200 m. It therefore must pass through any undercurrent continuing north from Morocco to the Numerical Models Iberian slope in the Gulf of Cadiz. Numerical modeling techniques for ocean circula- Spatial Variability tion are improving rapidly as computer power allows more detailed calculations on Rner grids. Textbook pictures like Figure 1 tend to show broad Recent results for the Canary Current region, rep- currents of weak unidirectional Sows. However, resenting the near-surface Sow in early September, measurements almost always indicate currents are shown in Figure 11. The model realistically 388 CANARY AND PORTUGAL CURRENTS

40°N

36°N

32°N

28°N Latitude

24°N

20°N

16°N

32°W 28°W 24°W 20°W 16°W 12°W 8°W

Longitude 1 m s_1 Sea surface height (cm)

20 30 40 50 60 70 80

Figure 9 Surface geostrophic currents superimposed on sea surface height fields from TOPEX/ERS-1 altimeter (14 August 1993). The August mean sea surface height calculated from density fields has been added to compensate for lack of altimetric mean.

27°N Latitude

_ 50 cm s 1

26°N _16°W _15°W _14°W _13°W Longitude

Figure 10 Field of current observed by acoustic Doppler current profiler near the time of Figure 9. Sea surface height contours are shown. Note the southward shelf flow turning offshore to describe a cyclonic eddy &100 km diameter. CANARY AND PORTUGAL CURRENTS 389

42°N weak Portugal Current. To north and south of the Canary Current proper, regions of predominantly northward Sow persist through most of the year. These are apparently connected by a narrow under- current trapped to the continental slope near 300 m 35°N depth. Variability of the system is dominated by the Trade Wind, which varies on timescales of weeks, seasons, decades, and longer. The Trades directly

Latitude force coastal upwelling and continental shelf cur- 30°N rents throughout the region. Unresolved questions include the reality of the intense equatorward Sow _ 20 cm s 1 alongshore between 303 and 203N suggested by the seasonal analyses, and the continuity of the under- 25°N current along the continental margin. 25°W 20°W 15°W 10°W 5.5°W Longitude See also Figure 11 Numerical model results of calculated surface currents for September. (Johnson J and Stevens I 2000 Deep . Canary and Portugal Currents. Sea Research I, 47(5): 875}900.) California and Alaska Currents. Ekman Transport and Pumping. Fisheries and Climate. Mesoscale Eddies. Meddies and Sub-surface Eddies. Re- depicts a meandering Azores Current at 353N, gional and Shelf Sea Models. Satellite Altimetry. which separates into branches entering the Gulf of Satellite Remote Sensing of Sea Surface Temper- Cadiz and Sowing south-east into the Canary Cur- atures. Upwelling Ecosystems. Water Types and rent. Note the southward Sow near-shore off NW Water Masses. Wind Driven Circulation. Africa, typical of late summer upwelling conditions, and the prevalence of meanders and eddies through- Further Reading out the Reld. Also noteworthy is the near-shore Barton ED (1989) The poleward undercurrent on the poleward current off Iberia, similar to conditions eastern boundary of the Subtropical North Atlantic. observed there in September with the cessation of In: Neshyba S, Smith RL and Mooers CNK (eds) upwelling and onset of the winter season. Poleward Flows Along Eastern Ocean Boundaries, This ‘state of the art’ numerical model is run pp. 82}95. (Springer Lecture Note Series). Berlin: using climatic winds, i.e., monthly averaged winds Springer-Verlag. that vary smoothly through the year, and has a lim- Barton ED (1998) Eastern boundary of the North Atlan- ited number of layers in the vertical. Nevertheless, tic: Northwest Africa and Iberia. In: Brink KH and the Rne horizontal resolution allows the realistic Robinson AR (eds) The Sea, vol. 11: The Global reproduction of oceanic features often only partially Coastal Ocean: Regional Studies and Syntheses, sampled because of ship time and equipment con- ch. 22. New York: Wiley. straints. Even this model is on a coarse scale com- Brink KH (1997) Wind driven currents over the continen- tal shelf. In: Brink KH and Robinson AR (eds) The pared to the size of many important features like Sea, vol. 10: The Global Coastal Ocean: Processes and islands or coastal capes. Entire islands are represent- Methods, ch. 1. New York: Wiley. ed by one or two model grid points so the level of Johnson J and Stevens I (2000) A Rne resolution model detail does not yet reproduce features on the scales of the eastern North Atlantic between the Azores, seen in Figure 10. In the not too distant future, the Canary Islands and the Gibraltar Strait. Deep-Sea models driven by actual or forecast winds and Research I 47(5): 875}900. incorporating from monitoring Krauss W (ed) (1996) The Warmwatersphere of the North networks will provide ocean forecasts that will rival Atlantic Ocean, chs 10}12. Berlin: Borntraeger. in accuracy present-day meteorological models. Mann KH and Lazier JRN (1991) Dynamics of Marine Ecosystems. Boston: Blackwell. Mittelstaedt E (1983) The upwelling area off Northwest Conclusions Africa } a description of phenomena related to coastal upwelling. Progress in 12: 307}331. The overall features of the Canary and Portugal Stramma L and Siedler G (1988) Seasonal changes in the currents, such as the mean pattern and seasonal North Atlantic Subtropical Gyre. Journal of Geophysi- variation, are well established despite relatively few cal Research 93(C7): 8111}8118. systematic observations. The Canary Current is fed Tomczak M and Godfrey JS (1994) Regional Oceano- from the Azores Current to a lesser extent from the graphy: An Introduction. Oxford: Elsevier.