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Somali Current

The Somali Current is a seasonal western boundary current in the western Indian Ocean, flowing along the northeastern coast of Africa from the equator to approximately 12°N off Somalia and Oman, and characterized by its reversal in direction driven by the Indian Ocean's monsoon winds.^[1] During the boreal summer monsoon (June–September), the current intensifies and flows northward across the equator, achieving peak surface speeds of up to 1 m/s and volume transports ranging from 20 Sv to as high as 70 Sv, primarily in the upper 1,500 m of the water column.^[1] This northward phase is fed by cross-equatorial inflow from the East African Coastal Current and remote forcing via Rossby waves and equatorial dynamics, while local alongshore winds like the Findlater Jet contribute to its acceleration.^[1]^[2] In the boreal winter monsoon (November–February), the surface flow reverses to southward, with transports of approximately 5–10 Sv feeding the South Equatorial Countercurrent near 2–3°S, though a persistent northward undercurrent continues beneath the surface layer.^[1] The timing and dynamics of this annual reversal vary by latitude: north of 5°N, it occurs in boreal spring due to propagating Rossby waves; between 2°N and 5°N, it aligns with local wind reversals; and south of 2°N, it is influenced by the persistent northward East African Coastal Current.^[2] A defining feature of the summer phase is intense coastal upwelling, where cold waters (as low as 17°C) rise from depths of 200–300 m, cooling sea surface temperatures to below 26°C (and down to 19–23°C in typical cases) and supporting high biological productivity in the region.^[1] The current also spawns large mesoscale eddies, including the anticyclonic Great Whirl (centered at 5–10°N) and the Southern Gyre (near the equator to 4°N), which trap waters, enhance mixing, and modulate the overall circulation.^[2]^[1] Overall, the Somali Current facilitates inter-hemispheric exchange of water masses subducted in the Southern Hemisphere, influences heat storage and salinity stratification in the Arabian Sea, and exhibits interannual variability in eddy positions and strengths, impacting regional climate, monsoon precipitation, and marine ecosystems.^[1]^[2]

Physical Characteristics

Location and Extent

The Somali Current serves as a prominent boundary current along the eastern coast of Africa, specifically tracing the Somali shoreline in the northwestern Indian Ocean. It originates near the equator and flows northward, demarcating the western edge of the Somali Basin. This positioning situates it within the broader Arabian Sea circulation, where it acts as a key conduit for water mass exchange between the southern Indian Ocean and northern regions.^[1] The current's latitudinal extent spans from the equator (0°N) northward to approximately 12°N, forming a coastal pathway of roughly 1,300 km in length. Offshore, it extends up to several hundred kilometers, typically 100–500 km depending on seasonal and mesoscale influences, before interacting with broader oceanic features. To the north, it connects with the Gulf of Aden inflow and the Arabian Sea, facilitating transport into these semi-enclosed basins. In its southern reaches, the Somali Current is fed by the northward-flowing East African Coastal Current (EACC), particularly during the summer monsoon when the EACC crosses the equator to supply the northward flow, while an equatorward countercurrent contributes to the complex near-equatorial dynamics.^[1]^[3]^[4] Vertically, the Somali Current is surface-intensified, with its core flow confined to the upper 200 m, though it exerts influence down to about 500 m in intensified regions due to baroclinic adjustments. This shallow structure aligns with its role as a wind-driven boundary current, primarily responding to monsoon forcing. Geographically, it flows northeastward past Ras Hafun at approximately 10.5°N, 51.5°E, before approaching Socotra Island near 12°N, 53°E, where it contributes to local eddy formation and recirculation.^[1]^[3]^[4]

Properties and Measurements

The Somali Current exhibits significant volume transport, with an average of 37 ± 5 Sverdrups (Sv) during the summer monsoon, driven by northward flow across the equator.^[5] This transport peaks at 60-70 Sv near Socotra, reflecting the intense cross-equatorial exchange comparable to major western boundary currents.^[1] In contrast, winter monsoon transports are substantially lower, typically 5-10 Sv in the upper layers, highlighting the current's seasonal variability.^[6] Surface velocities reach up to 1-2 m/s during the summer monsoon, particularly in the core of the northward jet between 5°N and 10°N.^[7] The flow reverses southward in winter, with velocities of 0.5-1 m/s, confined to shallower depths and influenced by Ekman dynamics.^[2] This reversal aligns with the monsoon wind shifts, underscoring the current's wind-driven nature. Temperature and salinity profiles vary markedly by season due to upwelling and water mass mixing. During summer, upwelled waters along the Somali coast range from 17-22°C with salinities of 35-36 psu, originating from subsurface layers and contributing to cooler surface conditions.^[1] In winter, surface waters warm to 25-28°C, with higher salinities exceeding 35.5 psu from Arabian Sea High Salinity Water influence.^[6] The current spans a width of 100-300 km, narrowing to about 200 km in its intense core during summer.^[8] It displays a baroclinic structure, with the strongest flows in the upper 150 m south of 5°N, where the current remains shallow and surface-intensified year-round.^[2] Recent satellite altimetry and ARGO data (as of 2024) confirm these transport estimates, with interannual variations linked to the Indian Ocean Dipole affecting eddy influences on the current's extent and strength.^[9] These properties have been quantified through historical ship-based Acoustic Doppler Current Profiler (ADCP) measurements, such as those from WOCE cruises in the 1990s, which provided direct velocity and transport profiles.^[1] Recent satellite altimetry data, including from AVISO and ARGO floats, have confirmed these transport estimates by deriving geostrophic currents from sea surface height anomalies.^[2]

Dynamics and Formation

Monsoon-Driven Circulation

The Somali Current is primarily driven by the seasonal monsoon winds of the Indian Ocean, with the southwest monsoon (June–September) exerting the dominant influence on its poleward flow. During this period, southeasterly winds along the Somali coast induce Ekman transport, where surface waters are deflected to the right of the wind in the Northern Hemisphere, resulting in offshore Ekman transport and divergence near the coast, driving upwelling and contributing to the northward-directed current. This wind forcing generates a strong western boundary current with transports reaching approximately 20 Sv in the upper 500 m, integrating local wind stress and remote signals to form the core of the summer circulation.^[1] In contrast, the northeast monsoon (December–February) reverses the Somali Current's direction through changes in wind stress curl, shifting the flow southward and integrating it into the broader equatorial current system. The positive wind stress curl during this season reduces northward momentum and promotes equatorward transport, with southward flows exceeding 20 Sv in Sverdrup-balanced interior regions, thereby linking the western Indian Ocean to the South Equatorial Current. This reversal highlights the Somali Current's role as a dynamic response to seasonal wind variability rather than a persistent gyre feature.^[1] The Somali Current interacts closely with the broader Indian Ocean gyre, particularly through connections to the West India Coastal Current (WICC) and equatorial dynamics. During the southwest monsoon, northward Somali transport feeds into the WICC via Rossby wave propagation and seasonal monsoon currents, while equatorial undercurrents and zonal flows modulate the supply of water masses. This integration ensures that variations in the Somali Current propagate across the basin, influencing the annual mean cross-equatorial exchange of approximately 10 Sv.^[1] The large-scale forcing of the Somali Current adheres to Sverdrup balance, where planetary vorticity input from wind stress curl governs the interior transport. Integrating this balance meridionally yields the streamfunction \psi = \frac{1}{\beta} \int \text{[curl](/page/Curl)}\left(\frac{\tau}{\rho f}\right) \, dx, which quantifies the vorticity-driven transport and explains the seasonal poleward and equatorward flows as direct responses to monsoon wind curls.^[1] Remote influences further shape the Somali Current, notably through cross-equatorial flows from the Southern Hemisphere during the summer monsoon. Subducted waters from southern subtropical convergence zones supply the northward current via interior pathways, contributing up to 20 Sv of transport and ensuring mass balance across the equator despite local wind forcing. This southern influx underscores the Somali Current's embedding within the Indian Ocean's interhemispheric circulation.^[1]

Seasonal Reversal and Upwelling

The Somali Current exhibits a marked seasonal reversal in its flow direction, transitioning from a southwestward direction during the boreal winter to a northeastward direction during the boreal summer. This annual cycle is a direct response to the reversing monsoon wind patterns over the northern Indian Ocean. In the winter phase, spanning December to February, the current flows southward along the Somali coast under the influence of northeasterly winds, promoting downwelling conditions with limited vertical motion and warmer surface waters near the shore.^[2]^[1] In contrast, the summer phase from June to September features a robust northeastward flow, intensifying to its maximum in July and August as southwesterly winds strengthen. This period triggers vigorous coastal upwelling through offshore Ekman transport, where surface waters are diverged away from the coast due to the wind's alongshore component acting to the right of the flow in the Northern Hemisphere. The upwelling lifts subsurface waters, recognized as one of the most intense seasonal wind-driven systems globally.^[2]^[10]^[5] The upwelled waters originate from depths of roughly 100–300 m, where nutrient levels are elevated, resulting in surface nitrate concentrations of 5–20 μM upon shoaling. The Somali coastal topography, with its northeastward slant, amplifies this process by channeling the winds and constraining the flow, thereby intensifying the offshore divergence and vertical uplift compared to straighter coastlines.^[11]^[12]^[13]^[2] Transition periods between monsoons, especially the spring inter-monsoon from March to May, are characterized by weak winds and subdued currents, leading to variable flow regimes that differ by latitude—such as delayed reversals near the equator influenced by Rossby waves. The underlying momentum balance governing the summer coastal jet involves a steady-state approximation where the alongshore acceleration from wind forcing is countered by the Coriolis effect. This relation highlights the wind's role in driving the poleward jet while the Coriolis force deflects it, sustaining the cross-shore pressure gradient essential for upwelling. Nonlinear effects and geostrophy become prominent farther offshore, modulating the jet's structure.^[2]^[1]

Mesoscale Features

The Great Whirl

The Great Whirl is a prominent anticyclonic eddy within the Somali Current system, emerging off the coast of Somalia in June between approximately 5° and 10°N and 48° and 52°E.^[14] It typically attains a diameter of 400–600 km and persists through September, forming as part of the seasonal southwest monsoon circulation.^[9] This feature arises primarily from remote forcing by annual Rossby waves propagating westward from the eastern Indian Ocean, which precondition the region before the monsoon's intensification.^[14] Structurally, the Great Whirl exhibits clockwise rotation, with surface velocities of 1.5–2.0 m/s concentrated along its periphery, and it extends vertically to depths exceeding 1000 m during its mature phase.^[14] The eddy traps nutrient-rich upwelled waters from the Somali coastal region within its core, creating a distinct cool and fresh signature compared to the surrounding Arabian Sea.^[14] This recirculation inhibits the offshore transport of upwelled material, influencing local ocean dynamics.^[15] The eddy's evolution begins with growth driven by barotropic instabilities in the intense coastal jet of the northward-flowing Somali Current, leading to detachment and spin-up around early summer.^[16] It frequently interacts with the Socotra Eddy to the southeast, sometimes merging or exchanging water masses, which modulates its intensity and position.^[14] Decay occurs in late September to November as the southwest monsoon weakens, reducing wind stress and allowing the eddy to dissipate through frictional damping and advection.^[14] In terms of fluxes, the Great Whirl intensifies air-sea interactions by enhancing vertical mixing and momentum transfer, with its cool core featuring sea surface temperature (SST) anomalies of -2 to -4°C relative to ambient waters, sustained by the advection and entrapment of upwelled cold water.^[14] Satellite observations, including altimetry-derived sea surface height and chlorophyll data from 1993 onward, highlight interannual variability, with the eddy's position shifting by up to 100 km and its strength fluctuating in response to monsoon intensity and large-scale climate modes like the Indian Ocean Dipole.^[17]^[9]

The Southern Gyre

The Southern Gyre is a semi-permanent anticyclonic gyre in the Somali Current system, located near the equator extending to approximately 4°N off the coast of Somalia.^[18] It has an average spatial scale of about 400 km and forms in early boreal summer (May–June) as a result of barotropic instabilities in the northward-flowing Somali Current, influenced by the cross-equatorial East African Coastal Current (EACC) turning offshore.^[19] The gyre deepens from around 100 m in June to 300 m by July–August and contributes to enhanced northward transport, merging with the main Somali Current and the Great Whirl by July to form a continuous northward flow.^[18] It typically dissipates by late August as monsoon dynamics shift.^[19]

Eddies and Rings

The Somali Current features a variety of mesoscale eddies and rings, primarily anticyclonic rings that detach from the Great Whirl and cyclonic eddies generated through baroclinic instability. Anticyclonic rings form when portions of the intense northward-flowing Somali Current retroflect or detach from the Great Whirl during the summer monsoon, creating coherent vortices that contribute to the region's circulation variability. Cyclonic eddies, in contrast, arise from baroclinic instability in the stratified waters influenced by upwelling and shear in the current, often appearing along the flanks of the main jet.^[20] These features exhibit a frequency of 10-20 events per monsoon season, with typical diameters ranging from 100 to 300 km and lifetimes of 1-3 months. The anticyclonic rings generally have larger scales, around 250-300 km in diameter, while cyclonic eddies tend to be smaller but more numerous. Their persistence is influenced by interactions with the ambient flow, including mergers with neighboring eddies like the Southern Gyre.^[20] Propagation of these eddies and rings varies by type and season; anticyclonic rings often move northwestward into the Gulf of Aden at speeds of 5-8 cm/s, sometimes pairing with cyclonic eddies on their eastern flanks for joint westward translation. Others propagate southeastward across the equator, influenced by the β-effect and cross-equatorial flow. Cyclonic eddies typically remain closer to the Somali coast, advected by the jet before dissipating.^[21]^[20] Instability mechanisms driving eddy formation include barotropic Rayleigh instability in the Somali jet, arising from horizontal shear, and baroclinic instability from vertical shear in the current. Barotropic modes dominate during monsoon transitions, with growth facilitated by the jet's velocity profile. Recent observations using Argo floats and glider deployments have quantified elevated eddy kinetic energy in the upper 300 m near the Somali coast, highlighting their role in vertical mixing and transport during the southwest monsoon.^[20]^[22]

Ecological Impacts

Nutrient Dynamics and Productivity

The Somali Current's upwelling regime supplies nutrients from intermediate depths of approximately 100–300 m, primarily drawing from oxygen minimum zone waters rich in nitrates and phosphates. Surface nitrate concentrations in upwelled waters along the northern Somali coast and within the Great Whirl front reach 15–20 µmol L⁻¹ during the summer monsoon, approaching deep-ocean values of around 20 µM, while phosphate levels elevate to 0.8–1.2 µM.^[23]^[9] These nutrients, advected offshore via upwelling filaments, create spatial heterogeneity, with higher accumulations north of 9°N compared to nitrate-depleted conditions (<2 µmol L⁻¹) farther south.^[24] This nutrient influx drives elevated primary productivity, with chlorophyll-a concentrations peaking above 5 mg m⁻³ in the northern upwelling zones during July, though values remain below 1 mg m⁻³ in central and southern regions. Annual primary production exceeds 300 g C m⁻² year⁻¹, supported by daily rates of 1.2–1.8 g C m⁻² day⁻¹ during peak blooms. Phytoplankton communities are diatom-dominated in these nutrient-enriched areas, forming intense blooms tied to upwelling filaments and exhibiting spatial patterns that align with the reversal of the Somali Current.^[23]^[24]^[9] Enhanced carbon cycling results from the trapping of organic matter within mesoscale eddies, promoting export fluxes estimated at 50–100 g C m⁻² year⁻¹, equivalent to daily rates of 0.5–1.0 g C m⁻² day⁻¹ during active upwelling. Diatom valves contribute significantly to this flux, with late-summer exports reaching 12.6 × 10⁶ valves m⁻². Productivity is nevertheless constrained by iron availability (typically 1.2–1.8 nM in northern waters, though limiting in depleted zones) and light penetration in turbid, sediment-laden coastal waters south of 9°N, where silicate and nitrate co-limitations further cap bloom intensity.^[23]^[24] Recent observations as of 2024 indicate a decline in upwelling intensity and phytoplankton productivity over the past two decades, particularly linked to the absence or weakening of the Great Whirl, resulting in reduced chlorophyll-a levels in the upwelling zones.^[9]

Fisheries and Biodiversity

The fisheries of the Somali Current region are dominated by pelagic species, including yellowfin tuna (Thunnus albacares), skipjack tuna (Katsuwonus pelamis), longtail tuna (Thunnus tonggol), and mackerels, which thrive in the nutrient-enriched waters driven by seasonal upwelling.^[25] Artisanal fleets, operating primarily from coastal communities in Somalia, target these small pelagics and contribute the majority of domestic catches, with small pelagic species accounting for around 80% of the total artisanal harvest in the surrounding regions.^[26] Reconstructed catch data indicate that artisanal fisheries have yielded approximately 1.3 million tonnes cumulatively from 1950 to 2010 as of 2010, underscoring the current's role in supporting local food security and livelihoods amid limited industrial infrastructure.^[27] Extensions of these reconstructions to 2020 estimate annual catches in Somali waters at 200,000–300,000 tonnes, with significant contributions from illegal, unreported, and unregulated (IUU) fishing by foreign industrial fleets exploiting the region's political instability.^[28] Biodiversity hotspots in the Somali Current's upwelling zones exhibit elevated biological diversity due to the influx of nutrients that sustain complex marine communities, including high abundances of zooplankton such as copepods (Calanoides carinatus and Eucalanus elongatus), which form the base for higher trophic levels.^[29] These areas host notable concentrations of cetaceans, including the Endangered Indian Ocean humpback dolphin (Sousa plumbea), which frequents coastal habitats influenced by the current's dynamics, and seabirds that forage on the abundant prey aggregates during peak productivity seasons.^[30] While endemism is more pronounced in associated coastal ecosystems like mangroves, the upwelling fosters transient biodiversity hotspots for migratory species, enhancing regional ecological connectivity.^[31] The trophic interactions in the Somali Current are structured around upwelled nutrients that fuel a pelagic food web spanning 3-4 levels, from phytoplankton to zooplankton, small pelagic fish, and apex predators like tunas and cetaceans.^[32] This nutrient-driven productivity supports efficient energy transfer, with zooplankton biomass increasing significantly during the southwest monsoon upwelling, providing a critical link between primary producers and commercially important fish stocks. The system's dynamics promote rapid biomass accumulation in mid-trophic levels, sustaining the fisheries' productivity while maintaining ecological balance in this highly dynamic environment.^[29] Conservation challenges in the Somali Current fisheries include overfishing by both artisanal and foreign fleets, leading to declining stocks of small pelagics and large predators, as well as bycatch of vulnerable species such as the Indian Ocean humpback dolphin, which faces threats from entanglement in fishing gear. IUCN assessments highlight the dolphin's Endangered status, with populations impacted by habitat degradation and incidental capture in the region's unregulated waters. Efforts to address these issues emphasize sustainable management of artisanal fisheries and monitoring of bycatch to protect the biodiversity supported by the current's ecological productivity.^[27]

Research and Observations

Historical Studies

Early nautical observations of seasonal winds and currents along the Somali coast date back to the 19th century, primarily derived from British Admiralty surveys and ship drift records compiled by seafaring nations. These accounts, documented in Sailing Directions and pilot charts, noted the reversal of surface flows influenced by the monsoon regime, with southward currents during the northeast monsoon and indications of northward shifts in summer, based on logs from vessels navigating the western Indian Ocean.^[33] The first systematic scientific measurements of the Somali Current occurred during the 1964 International Indian Ocean Expedition (IIOE), a multinational effort involving over 40 research vessels that provided the initial hydrographic sections across the region. Bruce A. Warren's analysis of IIOE data revealed the current's intense northward flow during the southwest monsoon, with surface speeds exceeding 100 cm/s and volume transports up to 50 Sv, highlighting its role as a western boundary current driven by monsoon winds.^[34]^[35] Theoretical advances in the late 1960s explained the current's seasonal reversal through models of wind-driven circulation. Michael J. Lighthill's 1969 study demonstrated how the onset of southwest monsoon winds generates Rossby waves that propagate westward, depositing mass flux along the East African coast to form the northward Somali Current, with the reversal occurring rapidly over weeks.^[36] Complementing this, Walter Düing's 1970 work on monsoon upwelling described cold water rising along the Somali shelf due to Ekman transport and offshore advection, supported by temperature profiles from monsoon cruises.^[37] In the 1970s and 1980s, dedicated programs like the Monsoon Experiment (MONEX) in 1979 and the INDIGO cruises (1986–1987) supplied extensive ship-based data on current transport and mesoscale features. MONEX vessels, including Soviet and U.S. ships, measured velocities up to 150 cm/s and identified eddy formations during peak monsoon, while INDIGO hydrographic sections quantified volume transports of 30–60 Sv and documented anticyclonic rings detaching from the current, enhancing understanding of its variability.^[38]^[39] Prior to the satellite era in the 1980s, studies relied on sparse hydrographic sections from opportunistic ship transects, limiting resolution of temporal and spatial dynamics and often missing short-lived eddies or reversal transitions.^[1]

Modern Observations and Modeling

Modern observations of the Somali Current have been significantly advanced by satellite remote sensing, particularly through altimetry data from the Jason series, which has provided continuous measurements of sea surface height anomalies since Jason-1 launched in 2001, enabling precise tracking of mesoscale eddies and current variability in the western Indian Ocean.^[40] These datasets have revealed the seasonal intensification of the Somali Current during the southwest monsoon, with eddy amplitudes reaching up to 15 cm and influencing transport patterns along the Somali coast.^[41] Complementing altimetry, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite has delivered chlorophyll-a concentration maps since 2002, highlighting intense phytoplankton blooms along the Somali upwelling zone during summer monsoons, where chlorophyll levels exceed 10 mg/m³ due to nutrient upwelling.^[24]^[42] In-situ observations have been bolstered by the Argo float array, which, following its full deployment around 2005, has delivered over 100 temperature and salinity profiles annually in the northwestern Indian Ocean region encompassing the Somali Current, capturing subsurface structures such as the undercurrent and cold wedges during inter-monsoon periods.^[43]^[44] These autonomous platforms have documented persistent cold anomalies in the Somali system, with profiles showing near-surface cooling and shoaling of the thermocline during drought-influenced years like 2014. While glider deployments remain limited in the Somali upwelling zones, regional efforts have explored their potential for high-resolution sampling of vertical velocities and nutrient fluxes in monsoon-driven circulations. Numerical modeling has played a crucial role in resolving mesoscale features of the Somali Current, with the Hybrid Coordinate Ocean Model (HYCOM) simulations demonstrating eddy-resolving capabilities that accurately reproduce the annual reversal and subgyre formations, validating peak transports of around 60 Sverdrups during strong southwest monsoons.^[2] Similarly, the Regional Ocean Modeling System (ROMS) has been applied to simulate instabilities in the northward Somali Current, generating the Southern Gyre and associated eddies with realistic amplitudes and propagation speeds.^[19] Key studies include a 2018 analysis in the Journal of Geophysical Research: Oceans using HYCOM to detail subgyre dynamics, such as the Great Whirl and Socotra Gyre interactions, which drive much of the current's variability.^[2] A 2020 investigation of eddy variability in the Somali Coastal Current Large Marine Ecosystem further quantified eddy formation rates and lifetimes, showing higher eddy counts during monsoon peaks compared to adjacent systems.^[45] Recent satellite-based studies (2023–2025) have highlighted interannual variability, including the occasional absence of the Great Whirl, which has led to a approximately 10% decline in upwelling productivity over the past two decades, as evidenced by reduced chlorophyll concentrations and altered circulation patterns.^[9] Additionally, analyses of upwelling and mixing processes in the Somali coastal region during the summer monsoon, using satellite data and Lagrangian methods, have revealed enhanced vertical mixing and nutrient fluxes influencing biological productivity.^[46] Data integration through reanalysis products like the Global Ocean Data Assimilation System (GODAS) has facilitated the study of interannual variability in the Somali Current by assimilating satellite altimetry, Argo profiles, and in-situ measurements into a consistent framework, revealing modulations linked to Indian Ocean Dipole events with transport fluctuations of 10-20%.^[47] GODAS outputs have been particularly useful for hindcasting subsurface pathways and validating model transports against observed monsoon reversals.^[48]

Climate and Human Implications

Climate Change Effects

A 2019 study highlighted that the upwelling system is largely confined to the early phase of the summer monsoon, leading to its "annihilation" in later stages and reduced nutrient supply to surface waters. This has resulted in diminished summer nitrate fluxes, supporting lower phytoplankton productivity in the region.^[5] Sea surface temperatures (SST) in the Somali Current region exhibit a dipole trend during the southwest monsoon season (1982–2013), with warming of approximately 0.37°C per decade south of about 5°N and cooling of 0.43°C per decade north of 10°N, contributing to overall changes in thermal stratification that suppress vertical nutrient transport in some areas.^[49] This warming in southern areas is linked to potential delays in monsoon onset, altering the seasonal timing of current reversal and upwelling initiation. The primary mechanism driving these changes is enhanced upper-ocean stratification due to surface warming, which inhibits Ekman pumping and reduces the upward advection of nutrients. Ekman pumping velocity is described by the equation w_e = \frac{1}{\rho f} \curl(\tau) where \rho is seawater density, f is the Coriolis parameter, and \tau is wind stress; alterations in \tau from changing monsoon dynamics lead to diminished w_e. Recent 2024 analyses of mixing processes confirm that such modifications alter vertical nutrient fluxes, resulting in reduced plankton productivity and smaller chlorophyll blooms, particularly when the Great Whirl is absent or weakened. Upwelling intensity has decreased, leading to about a 10% decline in productivity over the past two decades (as of 2024).^[5]^[9]^[46]^[9]

Socioeconomic Relevance

The Somali Current poses significant navigation hazards, particularly during the summer monsoon when its northward flow intensifies to speeds of up to 7 knots along the Somali coast and into the Gulf of Aden, complicating shipping routes and increasing risks for vessels transiting to and from the Red Sea.^[50] These strong currents generate confused seas, especially when opposing winds, and have historically contributed to shipwrecks near key coastal points such as Muqdisho and Marka, where stranded vessels and reefs exacerbate dangers.^[50] Mariners are advised to maintain a good offshore distance and adjust courses to account for these variable flows, which can set vessels toward hazardous shoals and overfalls in the region.^[50] The current modulates regional climate patterns by influencing heat flux in the western Indian Ocean, where its seasonal reversal affects sea surface temperatures and evaporation rates that contribute to the Indian summer monsoon dynamics and associated rainfall over Somalia.^[2] During periods of persistent northward flow, such as in drought years like 2014, cooler wedges in the current system can suppress atmospheric moisture, exacerbating aridity and contributing to prolonged Somali droughts that impact agriculture and water resources.^[51] In winter, the southward-flowing Somali Current promotes downwelling along the coast, which alters local hydrodynamics and can lead to intensified erosion and episodic flooding in low-lying areas, compounding vulnerabilities in Somali coastal communities.^[52] The Somali Current supports vital fisheries by enhancing nutrient upwelling during the summer monsoon, sustaining marine productivity that underpins livelihoods for over 500,000 people involved in direct and related activities along Somalia's 3,300 km coastline.^[53] The sector generated $51.3 million in fish exports in 2022, with conservative estimates of annual value around $135 million, though it has potential to reach $300-500 million if fully managed and illegal fishing addressed.^[53]^[54]^[55] This sector represents a key economic pillar amid broader instability, though illegal fishing has historically undermined these gains. Management of the Somali Current falls under the Agulhas and Somali Current Large Marine Ecosystems (ASCLME) framework, a UN-backed initiative through the Global Environment Facility and UNDP that promotes transboundary ecosystem-based approaches to address overfishing, pollution, and climate threats across participating countries including Somalia.^[56] This program facilitates policy coordination, capacity building, and sustainable resource use, emphasizing the current's role in regional biodiversity and economic resilience while integrating international conventions like the UN Convention on the Law of the Sea.^[57]

References

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[PDF] The monsoon circulation of the Indian Ocean
In the Somali Current section, we describe the ocean's responses to the summer and winter monsoon winds, and outline ... Winter monsoon circulation of the ...<|control11|><|separator|>
[2]
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Schematic representation of coastal currents off Somali at the surface. Shaded regions represent wedges of upwelled water. O.E. Esenkov, D.B. Olson / Deep-Sea ...
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3). In July, according to our estimates, the maximal transport of the Somali Current can exceed 70 Sv. The authors of [9] ...<|separator|>
[8]
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The Somali Current, with a volume transport comparable to that of the Gulf Stream, changes direction with the monsoon winds. It flows northeastward during the ...
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[19]
Somali Current rings in the eastern Gulf of Aden - AGU Journals
Sep 30, 2006 · New satellite-based observations reveal that westward translating anticyclonic rings are generated as a portion of the Somali Current accelerates northward.
[20]
Eddy‐Induced Temperature and Salinity Variability in the Arabian Sea
Feb 19, 2019 · These strong eddies strengthen local currents like the Somali Current and strongly impact regional upper-level vertical and lateral advection.Missing: post- | Show results with:post-
[21]
Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean
Below is a merged summary of the nutrient dynamics and related processes in the Somali Current Upwelling, consolidating all information from the provided segments into a comprehensive response. To maximize detail and clarity, I will use a table in CSV format to present the data systematically, followed by a narrative summary that integrates additional context and qualitative information not easily captured in the table.
[22]
Biophysical Interactions in Driving the Summer Monsoon ...
Mar 3, 2020 · The intense chlorophyll bloom along the Somali coast during summer monsoon is attributed to the upwelled nutrients from the subsurface.
[23]
[PDF] THE SOMALI REPUBLIC
The large pelagic species are tuna and big mackerels, mainly yellowfin tuna (Thunnus albacares), longtail tuna (Thunnus tonggol), bonito (Sarda orientalis), ...
[24]
[PDF] World Bank Document
Sep 13, 2023 · Small pelagic fisheries are critical for food security in the regions around the Somali coast. They represent around 80 percent of the total ...
[25]
[PDF] Reconstruction of Domestic Fisheries Catches in Somalia 1950-2010
However, the region is known for the seasonally high abundance of large pelagic fish. (tuna and billfishes) that has attracted Distant Water Fleets (DWF), ...
[26]
[PDF] II-5 Somali Coastal Current LME - IW:LEARN
The Somali Current LME is a highly productive ecosystem (>300 gCm-2year-1 ... Within the upwelling zone, the dominant zooplankton species include the large ...Missing: numerical | Show results with:numerical
[27]
(PDF) Higher trophic levels: offshore seabirds and marine mammals
Mar 28, 2025 · As a result of the Somali Current upwelling , internationally important foraging areas and feeding aggregations of apex predators may be ...
[28]
Somalia Biodiversity | Nairobi Convention Secretariat
The offshore Somali coastal area is one of the most productive marine ecosystems in the world. These ecosystems include mangrove swamps, estuaries, rocky shores ...
[29]
[PDF] Physical and biogeochemical processes associated with upwelling ...
Suginohara, N.: Coastal upwelling: Onshore-offshore circulation, equatorward coastal jet and poleward undercurrent over a conti- nental shelf-slope, J. Phys ...
[30]
[PDF] WORLD SURFACE CURRENTS FROM SHIP'S DRIFT ...
The improvement of manufacturing techniques in the early 19th Century ... The Somali Current is atypical in that it is almost as wide as it is long. A ...
[31]
[DOC] \noindent{\bf Bruce Alfred Warren}\newline - Woods Hole ...
Warren, Bruce A., 1965. The Somali Current. Oceanus, 12(1), 27. Warren, Bruce, 1966. Articles on “Coriolis force,” “Gulf Stream,” “Oceanic circulation,” and ...
[32]
On the occurrence of oxygen maxima and minima in the upper 500 ...
The data collected by various ships during the International Indian Ocean Expedition ... Warren, B. .. “The Somali Current,”Oceanus,12 (1), 2–7.
[33]
Dynamic response of the Indian Ocean to onset of the Southwest ...
Of particular interest is the strong Somali Current, that flows northward along the Somali coast only during the northern hemisphere summer (after monsoon ...
[34]
Large-scale variations of the Somali current during the southwest ...
Hydrographic measurements of the Somali Current between the equator and 12°N during the Southwest monsoon in August and September, 1970, indicate that the ...
[35]
S-MONEX | Earth Observing Laboratory
The research ships in the S-MONEX region provided oceanographic measurements (e.g. sea surface temperature) as well as upper-air observations. Sixteen ships ...Missing: Current | Show results with:Current
[36]
Penetration of bomb radiocarbon in the tropical Indian Ocean ...
Seventy-four AMS14C determinations on samples collected in the tropical-equatorial Indian Ocean during the second leg of the INDIGO program (1986) are ...
[37]
https://www.sciencedirect.com/science/article/pii/0011747173900053
[38]
Variability of the Somali Current and eddies during the southwest ...
Somali Current and eddies during strong, weak, and normal SW ... The GW is an almost-closed anticyclonic eddy with transports of approximately 60 Sv (x106 ...
[39]
Comparison of Seasonal Cycles of Phytoplankton Chlorophyll ...
During summer monsoon season, the Somalia coastal region is characterized by strong upwelling with high primary productivity due to the swift Somali current ...
[40]
Argo data products
Argo's more than 3500 floats provide 100,000 plus temperature and salinity profiles each year which create a large data set available on the Argo GDACs.
[41]
Persistence of Cold Wedges in the Somali Current System
The cold wedge was also observed by an Argo float and its upper-ocean temperature profiles show the progression of the near-surface cooling as well as shoaling ...
[42]
A numerical investigation of the Southern Gyre using ROMS
The model results indicate that the Southern Gyre appears in early June in the upper ocean as a result of instability in the northward flowing Somali Current.
[43]
Comparative oceanographic eddy variability during climate change ...
The Agulhas Current LME and the Somali Coastal Current extend along the east coast of Africa as depicted in Fig. 1. The main mesoscale features are geostrophic ...
[44]
Impact of an upgraded model in the NCEP Global Ocean Data ...
Oct 13, 2016 · This study shows how upgrades to the forward model simulations from MOM4p0d to MOM4p1 impact ocean analyses over the tropical Indian Ocean in GODAS.
[45]
Full article: Improved ocean analysis for the Indian Ocean
In order to evaluate how well the analysis reproduces the Wyrtki jet, we compare the GODAS surface current with the OSCAR analysis during spring and fall.
[46]
https://www.sciencedirect.com/science/article/abs/pii/S0967064524000250
[47]
A dipole‐like SST trend in the Somalia region during the monsoon ...
Jan 7, 2015 · The GW, which can transport up to 70 Sv under fully developed ... Chereskin (2003), The volume transport of the Somali Current during ...
[48]
Understanding Biases in Indian Ocean Seasonal SST in CMIP6 ...
Feb 20, 2024 · Coastal surface currents like the Somali Current show particularly large biases in CMIP6; in JJA the northward Somali current is too strong ( ...<|control11|><|separator|>
[49]
Study of upwelling and mixing process in the Somali coastal region ...
As a result, the northern Somali coast, especially around 9°N and 10°N, showcases the most extensive offshore upwelling, reaching as far as 55°E.Missing: depth | Show results with:depth
[50]
[PDF] Pub. 171, Sailing Directions (Enroute) East Africa and the South ...
... Somali Current is used by oceanographers for that part of the coastal current that under- goes a seasonal reversal. When the Somali Current is S it turns. E ...
[51]
Persistence of Cold Wedges in the Somali Current System
Feb 22, 2023 · Argo data 1999–2019: Two million temperature-salinity profiles and subsurface velocity observations from a global array of profiling floats.<|separator|>
[52]
The biogeochemistry and oceanography of the East African Coastal ...
During the NE monsoon months (Dec-Mar) downwelling is established over much of the Somalian coastal region, the Somali Current flows southwards restricting the ...<|control11|><|separator|>
[53]
Somalia's Fisheries Sector: A Primer on the Sector
Mar 12, 2025 · Bargal: Artisanal fishers target pelagic species including tuna, with some processing capabilities; Bolimog: Focus on small pelagic fish and ...
[54]
Somalia's Maritime Resources: A Treasure Worth Protecting
Dec 24, 2024 · ... Somalia's fisheries sector could generate between $300 and $500 million annually. This revenue would not only drive economic growth but also ...
[55]
Somali piracy at 7-year low, new tactics by militants onshore -U.N.
Oct 23, 2013 · Somalia struggles to rebuild after decades of lawlessness · Pirates took up to $413 million in ransoms in 2005-2012 · African peacekeepers need ...
[56]
The Western Indian Ocean Large Marine Ecosystems SAPPHIRE
SAPPHIRE project builds on the previous work completed under the UNDP supported GEF financed Agulhas and Somali Current Large Marine Ecosystems (ASCLME) project ...
[57]
Agulhas and Somali Current Large Marine Ecosystems Project ...
The objective of the project is to work with two other thematic GEF international waters projects in the area as part of a strategic approach.Missing: UN | Show results with:UN