Canary Current

The Canary Current is a wind-driven surface ocean current that flows southward along the northwestern coast of Africa as the eastern boundary component of the North Atlantic Gyre, branching from the North Atlantic Current and extending from approximately 43°N near northwest Iberia to 10°N off Senegal.^[1] It is driven primarily by persistent trade winds, which generate Ekman transport and coastal upwelling, resulting in cool, nutrient-rich surface waters that support one of the world's four major eastern boundary upwelling systems.^[2][3] This current is relatively weak and broad compared to western boundary currents like the Gulf Stream, with speeds typically ranging from 0.1 to 0.5 m/s, and it detaches from the coast near Cape Blanc (21°N), turning westward toward the Cape Verde Islands before merging into the North Equatorial Current.^[1] The Canary Current Large Marine Ecosystem (CCLME), spanning about 1.1 million km², encompasses the coastal waters and exclusive economic zones of Morocco, Western Sahara, Mauritania, Senegal, Gambia, Guinea-Bissau, and Spain's Canary Islands, with influences extending to Cape Verde and Guinea.^[4] Upwelling is seasonal and intensifies during summer due to northerly winds, bringing deep, nutrient-laden waters to the surface and fostering high primary productivity exceeding 300 g C m⁻² y⁻¹, classifying it as a Class I highly productive ecosystem.^[3][4] Ecologically, the Canary Current sustains diverse marine biodiversity, including small pelagic fish such as sardines and sardinellas, tunas, cephalopods, and zooplankton like copepods, which form the base of a rich food web supporting both endemic species and migratory populations.^[4] It plays a critical role in regional fisheries, contributing significantly to economic activities in bordering nations, though it faces pressures from climate variability, including sea surface temperature increases of about 0.48–0.59°C since the mid-20th century, which may alter upwelling dynamics and species distributions.^[1][4] The system's mesoscale features, such as eddies and filaments, facilitate nutrient exchange between coastal and open ocean waters, enhancing overall productivity.^[1]

Geography

Path and extent

The Canary Current originates as a southward-branching extension of the North Atlantic Current, typically splitting near 40°N off the western coast of the Iberian Peninsula, where it separates from the broader eastward flow of the Azores Current.^[5] This branch forms the eastern limb of the North Atlantic Subtropical Gyre, directing cooler waters equatorward along the continental margin, beginning along the coasts of Portugal and Spain before entering Moroccan waters.^[6] From its origin, the current flows southwestward, paralleling the northwest African coastline through the territorial waters of Morocco, Western Sahara, and Mauritania, while passing the Canary Islands archipelago around 28°N, which partially deflects the flow and generates mesoscale eddies and filaments.^[7] The islands' influence creates distinct branches, including a nearshore coastal branch that hugs the continental slope southward.^[6] The main flow continues equatorward, maintaining proximity to the coast (typically within 100–200 km offshore) until near Cape Blanc (21°N), where it detaches, turning westward toward the Cape Verde Islands and merging with the North Equatorial Current around 15°N.^[8] The current's extent spans approximately from 43°N to 10°N latitude, with its primary influence confined to the upper ocean layers of 500–1000 m depth, though the core transport occurs above 500 m.^[9]

Relation to broader circulation

The Canary Current serves as the eastern boundary current of the clockwise North Atlantic Subtropical Gyre, a major circulation feature driven by trade winds and the Coriolis effect that spans from the equator to about 40°N.^[10] It forms the southward return flow of this gyre, balancing the northward and eastward transports of warmer waters in the western and northern sectors.^[11] This integration positions the Canary Current as a critical component in maintaining the gyre's overall mass balance and heat distribution across the subtropical North Atlantic.^[12] Upstream, the Canary Current receives water from the North Atlantic Current, a warm inflow that branches eastward near the Azores before contributing to the Azores Current, which feeds directly into the Canary system around 35°N.^[8] Downstream, as it approaches the equator near 10°N, the Canary Current merges with the North Equatorial Current, facilitating the equatorial return flow that completes the gyre loop and transports subtropical water masses toward tropical latitudes.^[10] This connectivity ensures the southward advection of cooler, nutrient-rich waters, influencing inter-basin exchanges and the broader thermohaline circulation.^[13] The Canary Current interacts minimally with adjacent flows, including a subtle northern influence from the Portugal Current, which extends southward along the Iberian Peninsula and blends into the Canary regime near Cape St. Vincent.^[6] To the south, it encounters the Guinea Current, an eastward-flowing equatorial countercurrent that introduces warmer waters and generates convergence zones off northwest Africa.^[14] Additionally, the current's path is modulated by mesoscale eddies shed from the Canary Islands, which enhance offshore transport and variability in the surrounding waters.^[14] These interactions underscore the Canary Current's embedded role within the dynamic eastern North Atlantic boundary system. The name "Canary Current" originates from its passage along the Canary Islands archipelago, a strategic position in the subtropical gyre.^[8] This naming reflects the current's geographic alignment with the islands, which lie directly in its core path and influence its local dynamics through bathymetric effects.

Physical characteristics

Flow speed and volume

The Canary Current exhibits a typical surface velocity of 0.1–0.2 m/s (10–20 cm/s), reflecting its role as a relatively sluggish eastern boundary current compared to western counterparts like the Gulf Stream.^[15] Peak speeds reach up to 0.35 m/s in coastal branches, particularly near the African margin where boundary effects enhance flow intensification.^[8] These velocities are derived from a combination of satellite altimetry, moored current meters, and high-resolution numerical models, which capture the current's mesoscale variability.^[16] Volume transport estimates indicate an annual average southward flux of approximately 2–3 Sverdrups (Sv; 1 Sv = 10^6 m³/s), with the open-ocean branch contributing about 3 ± 1 Sv and an upwelling-related coastal component adding 1 ± 0.3 Sv.^[13] Seasonal peaks show enhanced southward transport of around 2.9 Sv in summer and 4.5 Sv in fall for the open-ocean segment, based on geostrophic calculations from hydrographic sections and inverse box models.^[13] One geostrophic estimate yields an average of 1.8 ± 1.4 × 10^9 kg/s for a specific coastal branch, underscoring the uncertainties in partitioning fluxes across the basin.^[14] The current displays vertical shear characteristic of geostrophic balance, with stronger near-surface flows decreasing with depth due to the thermal wind relation driven by horizontal density gradients.^[16] This structure is surface-intensified in summer, extending to about 700 m, while becoming more barotropic in winter, as revealed by moored observations and modeling.^[16] Spatially, the flow is slower in the open ocean (around 0.15 m/s) than near the coast (up to 0.25 m/s), influenced by topographic steering and eddy interactions along the eastern boundary.^[17] This variability aligns with the current's path, where offshore migration in certain seasons reduces coastal speeds.^[16]

Temperature, salinity, and upwelling

The surface waters of the Canary Current exhibit temperatures ranging from 15 to 20°C on average, which is cooler than adjacent subtropical waters primarily due to coastal upwelling processes that bring deeper, colder water to the surface.^[18] For instance, near the Canary Islands, surface temperatures typically average around 18°C in upwelling-influenced zones.^[19] Salinity levels in the current range from 35.5 to 36.5 practical salinity units (psu), reflecting its subtropical origin with relatively high values, though these are locally modified by coastal freshwater inputs such as river discharges along the Iberian and northwest African margins.^[19][20] Upwelling in the Canary Current is primarily driven by offshore Ekman transport, where persistent northeasterly trade winds force surface waters away from the coast, resulting in the ascent of nutrient-rich deep waters, particularly North Atlantic Central Water (NACW), to the euphotic zone.^[1] This process is most intense along the northwest African coast during spring and summer, enhancing vertical mixing and nutrient availability.^[18] Prominent upwelling filaments extend offshore from key capes, such as Cape Blanc (Mauritania) and Cape Beddouza (Morocco), transporting upwelled waters into the oligotrophic North Atlantic and supporting elevated biological productivity, as evidenced by chlorophyll-a concentrations reaching 2-5 mg/m³ in these zones.^[21][18] Over the period from 1982 to 2013, long-term trends in the northern limits of the Canary Current show a warming of approximately 0.25°C per decade in oceanic regions shallower than the permanent thermocline, accompanied by slight salinity increases of about 0.02 psu per decade, which are density-compensating with the temperature changes.^[22] These trends are less pronounced or insignificant directly within coastal upwelling areas, where upwelling intensity modulates surface properties.^[22] More recent analyses, including a 2023 study of data from 1987 to 2020 in the northern limit, found no statistically significant temperature trend and a negative salinity trend below 50 m depth, while observations from 2024 indicate sustained warming near the Canary Islands with surface temperatures reaching 20°C in seasonally cold months.^[20][23]

Formation and dynamics

Driving mechanisms

The Canary Current is primarily driven by the northeasterly trade winds, which generate Ekman drift in the surface layer, directing transport to the right of the wind in the Northern Hemisphere and contributing to southward flow along the eastern boundary of the North Atlantic subtropical gyre.^[24] These winds produce a negative wind stress curl over the subtropical region, inducing Ekman convergence and downwelling in the gyre interior, which sustains the equatorward circulation.^[25] At the gyre scale, the flow is governed by Sverdrup balance, which relates the meridional transport $V$V to the wind stress curl through the planetary vorticity gradient $\beta$β: $$\beta V = \curl\left(\frac{\tau}{\rho}\right)$$βV=\curl(ρτ​) where $\tau$τ is the wind stress vector and $\rho$ρ is the reference density.^[26] This balance describes the interior transport as southward in the subtropical gyre due to the negative curl from the trade winds transitioning to westerlies. Eastern intensification, as modeled by Stommel, results in a broad, relatively weak return flow along the eastern boundary, contrasting with the narrow, intense western boundary current, due to the effects of lateral friction and planetary vorticity advection.^[25] Boundary effects further shape the current, with coastal friction along the African margin creating a thin western boundary layer (relative to the gyre scale) that decelerates the flow, while the continent's coastline channels the southward momentum from the interior Sverdrup transport.^[25] The Canary Current forms the eastern limb of the North Atlantic subtropical gyre, closing the circulation driven by these wind forces. Thermohaline contributions play a minor role in the overall transport, arising from density gradients due to subtropical salinity variations, but wind forcing dominates the upper-ocean dynamics.^[11]

Seasonal and interannual variability

The Canary Current exhibits a pronounced seasonal cycle in its transport and path, primarily driven by variations in the intensity of the trade winds. Southward transport is stronger during summer and autumn, reaching magnitudes of approximately -4.1 ± 0.5 Sverdrups (Sv) in the oceanic region off northwest Africa based on 2013–2014 observations, due to enhanced northeasterly trade winds that intensify the alongshore flow.^[11] In contrast, winter conditions feature weaker southward flow, with transports reduced to around -2.5 to -3.3 Sv, and occasional reversals or northward components near the equator, particularly in intermediate waters where summer northward transport can reach up to 2.0 Sv before reversing.^[11][27] This seasonality also affects the current's path, with the flow positioned more eastward in winter, closer to the African coast at about 3.4 ± 0.3 Sv.^[28] On interannual timescales, the Canary Current's strength and variability are modulated by large-scale atmospheric and oceanic oscillations, notably the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO). Positive NAO phases are associated with stronger trade winds and enhanced southward transport, while negative phases lead to reductions; over the past 70 years, trade wind speeds have decreased by approximately 1 m/s, linked to NAO influences.^[29] The AMO contributes to longer-term modulations, with its positive phase correlating to weaker winds and reduced upwelling intensity in the Canary system.^[29] These oscillations introduce lags exceeding 10 years in their impact on wind patterns and current transport.^[29] Mesoscale eddy activity significantly contributes to the current's path variability, with eddies frequently generated in the lee of the Canary Islands, causing meanders and offshore deflections. These anticyclonic and cyclonic eddies, often with vertical extents of 500–600 m and lifetimes exceeding three months, propagate westward along a zonal corridor influenced by planetary Rossby waves.^[30][31] Rossby wave propagation further modulates the current's trajectory, interacting with the mesoscale field to alter the mean flow without net cross terms in momentum balances.^[32] Recent observations indicate a slight weakening of the Canary Current since the 1950s, consistent with the decline in trade wind intensity, though upwelling-favorable winds show no significant overall increase.^[29] Observations from 2013–2014 indicate transport anomalies with oceanic southward flow averaging -4.1 Sv contrasted by +3.7 Sv at the eastern boundary, reflecting boundary current reversals and eddy influences.^[11] More recent high-frequency measurements from November 2022 to December 2023 show a mean Canary Current transport of -1.4 ± 1.6 Sv near the Canary Islands, with seasonal values of -0.3 Sv (spring), -2.5 Sv (summer), -2.1 Sv (autumn), and -0.8 Sv (winter), and northward flows in the Lanzarote Passage during summer and autumn.^[33] Numerical modeling using the Regional Oceanic Modeling System (ROMS) confirms that nearshore wind stress curl variability is the primary modulator of these seasonal and interannual fluctuations, reproducing observed mesoscale features and transport cycles in high-resolution simulations of the Canary Basin.^[16]

Environmental and human impacts

Climatic influences

The Canary Current significantly moderates coastal climates along Morocco and the Canary Islands by upwelling cold subsurface waters, which lower sea surface temperatures and adjacent air temperatures, often stabilizing the lower atmosphere and inhibiting convective activity that could generate precipitation. This cooling influence is particularly pronounced in summer, contributing to milder coastal conditions compared to inland areas and reducing the potential for rainfall formation through enhanced atmospheric stability.^[34][35] The upwelling-driven cooling plays a key role in promoting arid conditions along the Sahara's western fringe, where the influx of cold water suppresses moist convection and limits precipitation to less than 100 mm annually in many affected zones. By cooling surface waters and the overlying air, the current reinforces subsidence associated with the subtropical high-pressure system, further drying the regional atmosphere and facilitating desertification processes.^[36][37] A positive feedback loop exists between the Canary Current and the northeasterly trade winds, wherein the current's cooling of sea surface temperatures enhances the land-sea thermal contrast, strengthening the winds that drive further upwelling and perpetuate the dry subtropical climate. This interaction maintains low evaporation rates over cooler waters, limiting moisture availability in the lower troposphere and sustaining aridity across northwest Africa.^[38][39] On broader scales, variations in the Canary Current influence Sahel drought variability by modulating regional sea surface temperatures and associated moisture transport pathways, with cooler upwelled waters potentially altering monsoon dynamics and reducing rainfall during dry phases. Historical proxy records from the Canary Islands indicate correlations between current-driven oceanic cooling and climate patterns during the Medieval Warm Period (circa 950–1250 CE), including shifts in surface temperatures that affected regional atmospheric circulation.^[40][41] Under projected climate change, the Canary Current may experience intensified upwelling due to stronger trade winds from amplified land-sea temperature gradients, potentially exacerbating coastal and inland aridity by further suppressing convection despite global warming trends. Model simulations suggest this could lead to enhanced atmospheric stability and reduced precipitation in northwest Africa, compounding drought risks in the Sahara-Sahel transition zone.^[42][43]

Ecological and economic roles

The Canary Current's upwelling processes enrich surface waters with nutrients from deeper layers, such as North Atlantic Central Water and South Atlantic Central Water, fueling elevated primary production rates that average around 2.4 g C m⁻² day⁻¹ in the euphotic zone.^[44] This nutrient supply sustains robust pelagic food webs, supporting high biomass of small pelagic fish like sardines (Sardina pilchardus), anchovies (Engraulis encrasicolus), and mackerel (Scomber colias), which form the base of the marine trophic structure in the Canary Current Large Marine Ecosystem (CCLME).^[45] The CCLME serves as a biodiversity hotspot for migratory species, attracting over 70% of tracked individuals from eight seabird species across the Atlantic during non-breeding periods, drawn by the enhanced ocean productivity.^[46] This region also supports diverse cetacean populations and migratory fish, contributing to its role as a critical habitat within eastern boundary upwelling systems.^[47] Economically, the Canary Current underpins major small pelagic fisheries, with annual catches in the CCLME exceeding 3.5 million tonnes as of 2017, dominated by sardines and related species that account for over 80% of Morocco's fishery production volume.^[45] However, sardine stocks off northwest Africa have declined by 34% as of 2024 due to overfishing and climate strain, with Morocco's sardine landings falling from 1.2 million tonnes in 2020 to 700,000 tonnes in 2024.^[48] Morocco's sardine catch alone typically ranges from 500,000 to 800,000 tonnes yearly, forming a cornerstone of the national economy through exports and processing.^[49] Historically, the current facilitated ancient trade by providing predictable coastal navigation, exemplified by Phoenician exploitation of murex snails (Bolinus brandaris) on the Iles Purpuraires near Essaouira for Tyrian purple dye production around the 1st millennium BCE.^[50] Human activities pose significant risks, including overfishing that exceeds sustainable levels for key stocks like sardines by up to 40%, and pollution from land-based sources such as pesticides, which degrade upwelling zones and threaten ecosystem productivity.^[49][51] Conservation efforts include the designation of Banco de la Concepción as a candidate Marine Protected Area within the Natura 2000 network, aimed at preserving the seamount's high-biodiversity habitats 90 km northeast of Lanzarote.^[52] In 2023, an International Scientific Symposium on the Sustainable Management of the CCLME was held in Bissau, Guinea-Bissau, to advance regional cooperation on reversing ecosystem degradation through improved governance and fisheries management.^[53] Climate-induced warming is driving poleward shifts in small pelagic fish distributions across the CCLME, potentially altering stock availability and challenging management.^[54]