Somali Current

The Somali Current is an ocean boundary current that runs along the coast of Somalia and Oman in the Western Indian Ocean and is analogous to the Gulf Stream in the Atlantic Ocean.^[1] This current is heavily influenced by the monsoons and is the only major upwelling system that occurs on a western boundary of an ocean. The water that is upwelled by the current merges with another upwelling system, creating one of the most productive ecosystems in the ocean.^[2]

The Somali current is characterized by seasonal changes influenced by the Southwest monsoon and the Northeast Monsoon. During the months of June to September, the warm Southwest monsoon moves the coastal waters northeastward, creating coastal upwelling. The upwelled water is carried offshore by Ekman transport and merges with water that was brought to the surface by open-ocean upwelling. The Findlater jet, a narrow low-level, atmospheric jet, also develops during the Southwest monsoon, and blows diagonally across the Indian Ocean, parallel to the coasts of Somalia and Oman. As a result, an Ekman transport is created to the right of the wind. At the center of the jet, the transport is maximum and decreases to the right and left with increasing distance. To the left of the jet center, there is less water movement toward the center than is leaving, creating a divergence in the upper layer and resulting in an upwelling event (Ekman suction). In contrast, to the right of the center of the jet, more water is coming from the center than is leaving, creating a downwelling event (Ekman pumping). This open-ocean upwelling in combination with the coastal upwelling cause a massive upwelling.^[1][2] The Northeast monsoon, which occurs from December to February, causes a reversal of the Somalia current, moving the coastal waters southwest. Cooler air causes the surface water to cool and creates deep mixing, bringing abundant nutrients to the surface.^[2]

History

The history of physical oceanographic approaches to the Somali current has begun from mid 1960s with serious interests. From the mid-1960s until the late 1970s several magnificent theoretical studies had been proposed and gave physical answers of the current behaviors and formations. After the late 1970s, the physics of the Somali current enhanced by ocean data analyses with outstanding field measurements of the current properties. The research footprints during the early 1960s to the late 1970s are presented as below.^[3]

(early research histories before 1981)

1966 Warren et al. : Oceanographers and Meteorologists agreed the existence of the Somali current and its behaviors, but its exact processes and involved nature sources hadn’t been clearly understood.^[4]

1969 Lighthill : the source of the Somali current is mass flux deposited by baroclinic and barotropic waves in the western boundary region.^[5]

1970 Düing : presented the presence of alternative cyclonic and anti-cyclonic gyre, the Indian Ocean Gyres. Found eddy size of the Indian Ocean Gyres are much larger than the gyres of the other mid-latitude western boundary currents (the Indian Ocean Gyres ~ 300-500 nmi > the Gulf Stream / Kuroshio ~ 50 – 100 nmi) ^[6]

1971 Düing & Szekelda : the baroclinic mode is the dominant mode of the Somali current ^[7]

1972,1973 Leetma : local winds are crucial to the onset of the Somali Current.^[8][9]

1975 Colborn : climatological analysis of the entire Indian Ocean from bathythermograph and hydrocast observations ^[10]

1976 Hurlburt & Thompson : Characterize the Somali current as a time-dependent, baroclinic, inertial boundary current.^[11]

1976 Bruce : time-series analyses, XBT cross-section measurement by the EXXON tankers ^[3]

1979 USNS WILKES : the Great Whirl (Prime Eddy) and the Socotra Eddy together with the strong shear zone along the eastern edge of the Great Whirl were observed during late August and early September of 1979.^[3]

Properties

The Somali Current is rapid response and shallow, shift the direction seasonally. Especially, regions from the 5°N to the south, the Somali current is extremely shallow (below 150m depth southward flow for whole year). But further north ocean, the jet is getting deeper, and reaches to the permanent thermocline. Structure of the current on equator is extremely complex and has similar layers with the equatorial flows, but the Somali current oriented northward-southward instead of eastward-westward.

Typical water volume transport of the Somali current is 37+/- 5 Sv (during mid-September).^[12] In generally understanding, the current circulation is weaker than other mid-latitude western boundary currents (such as the Gulf Stream, the Kuroshio current), but highest transport have been measured even between 60-70 Sv volume transports (comparable to the Gulf Stream) at around south of the Socotra Island ^[3]

Northward Somali Current during Southwest Indian Monsoon

Southward Somali Current during Northeast Indian Monsoon

Formation and behavior

The Somali current is driven principally by the monsoon winds. The characteristic behaviors of the Somali current also strongly depend on the local monsoon winds blows. In the west Indian Ocean area, the southwest monsoon blows along to the east African coast and Oman in northern hemisphere summer (between May – September). Passing through September (during fall) the monsoon changes direction to the opposite way, and the northeast monsoon blows in the northern hemisphere winter.

Before the onset of the monsoon (March-May): During this season, shallow northward coastal current flows with 50-100 km width near the Somali coast, overlying a southward undercurrent. By alongshore winds, upwelling current flows to the coast. Near the equator, the East African Coast Current (EACC) flows northward across the equator. The southern Somali current flows northward as an extension of the EACC from south to the 3-4°N.^[13]

Northeastward current (Jun-Sep, Summer Monsoon): The Somali current begins developing its current from mid-May with the summer monsoon onset, and the current velocities rapidly grow up to the maximum until June and September with the southwest monsoon blowing. During this season, the current direction is northeastward, and the velocity during in mid-May is about 2.0 m/s and in June, 3.5 m/s and more. Typically the Somali current moves through about 1500 km northeastward, and near Somalia coast of 6~10°N, the current changes its direction to the east (at near the Cape Guardafui^[3] ), and merges to the Indian Monsoon current.^[3][14]

According to the works of Friedrich A. Schott and Julian P. McCreary Jr., the Northeastward Somali current has following two sub-season periods. During June-July, the Great Whirl grows at 4-10°N, and a cold wedge shape water mass develops to the offshore at latitude 10-12°N. The size and strength of the Great Whirl during the summer monsoon, measured as about 1000 m depth with 10 cm/s velocity, and also some visible gyre structure observed at further deeper depth. The upper layer Somali current flows with northward direction along to the east African coast, and finally comes into the Gulf of Aden through between the Socotra and Horn of Africa. The mean flow velocity of this outgoing flow is about 5 Sv.

The period between August-September is the late phase of the summer monsoon. During this period, the Great Whirl forms almost closed circulation, and strong upwelling streams (colder than 17°C, typical upwelling water temperature ~ 19-23°C) develop near the Northern Somalia coast.^[13]

After the receding of the summer monsoon (Oct-Nov): During this period, the Southwest monsoon winds are weakened and diminished, so the Northward Somali current (extension flow of the EACC) is no longer crossing the equator, but turns to the east at around 3°N. However, the Great Whirl still remains.^[13]

Southwestward current (Dec-Feb, Winter Monsoon): During the Northeast Monsoon (Northern Hemisphere Winter, DEC –FEB) the Somali current changes its direction to the opposite way and becomes southward flow (limited region south of 10°N). After September, the northeast monsoon influences to the current, the Somali current get weaker and slower. Finally, in early December the flow direction changed to the south at south of 5°N, and expands rapidly to the 10°N until January (velocities of 0.7 – 1.0 m/s). In March, the southward flow contracts again to 4°N, until the surface flow reverses in April.^[14]

During winter monsoon, the southward Somali Current comes into a confluence with a Northward current EACC and flows to the East, when after this southward current across the equator. Near the equator, the southward Somali Current flows with shallow depth (above 150m) because of the undercurrent which flows to the northward.^[13]

Upwelling behavior: One of significant characteristics of the Somali current is the presence of strong coastal upwelling. The direction of the upwelling follows Ekman transport. Since the southwest monsoon blows along to the Somali coastline, the upwelling direction is to the offshore during the summer. During this season, the warm and salty northward Somali current flows from the south-hemisphere by the southwest monsoon, and turn to the east at near the Cape Guardafui. This flow deflection causes strong upwelling flow along the Somali coast. By the influence of the cold coastal upwelling, the coastal temperatures are lowered by 5°C and more during May to September.^[14] Entering to the winter season, the southwest monsoon winds changes its direction to the northeastward, and finally the northward Somali coastal current is no longer exist, but the southward current flow near the Somali coast. By the southward current, the coastal upwelling is shutdown.^[15]

Somali Undercurrent: April – early June ; southward under current develop underneath the northward surface current (depth 100-300m, speed Monthly Avg. 20 cm/s (Max 60 cm/s)). Stretch to the near 4°N, and turn to offshore, and terminated by establishment of the deep –reaching Great Whirl. Fall – Winter; southward under current develop underneath the northward surface current in latitude 8-12°N

Winter; Northward Cross-equatorial undercurrent (depth 150-400m) flows, underneath southward surface Somali Current. And this undercurrent flow balanced with upper surface southward Somali current.^[13]

Upwelling current at the Somali coast during Southwest Monsoon

Affect to the marine ecosystem

The offshore Somali coast is one of the highest productive regions in the world. Especially, during the southwest Indian monsoon, strong upwelling flow to the Somali coast line brings cold and highly nutrient rich water to the coastal area. During this season, the mean phytoplankton density and productivity is high because of the coastal upwelling and the activities of offshore eddy.^[16] After this season, from the beginning of the Northeast monsoon, primary productivity decreases, but the zooplankton density doesn’t decreases significantly. During the summer monsoon (upwelling flow season), total zooplankton biomass is consist of about 25% of Euphausids, and rest of Copepods. (dominant zooplankton species in the region ~ Calanoides carinatus and Eucalanus elongates)^[16][17]

Two gigantic eddies at the north west Indian Ocean during Southwest Monsoon

Great Whirl

The Great Whirl is a huge anti-cyclonic eddy generated by the Somali current flows in northern hemisphere summer, and which one of the two gigantic Indian Ocean Gyre (another one is the Socotra Gyre). The Great Whirl can be observed at the region between 5-10°N and 52-57°E off the Somali coast in the summer season which location is typically around southwestward 200 km away from the Socotra Gyre (between June to September).^[18][13] However, from previous researches, the Great Whirl and also the Socotra Gyre collapsed in some years, and the locations of each different year haven’t been regular. Typical size of eddy is 400~600 km in horizontal diameter,^[6] and typical surface current velocity is 1.5~2.0 m/s^[19]

The forming mechanism of the Great Whirl hasn’t been clearly solved yet, but the analytical approach by applying the Rossby wave theory can explain its basic formation mechanism. By observation and analysis of [Schott and Quadfasel (1982)], the summer monsoon developed suddenly during June-July, and drove westward water flow in the location. Schott and Quadfasel applied the first-mode Rossby waves to the water flows, and finally concluded “the formation of the Great Whirl is a response to the very strong anti-cyclonic wind-stress curl”.^[13]

Since the Somali current has seasonal changes, the Great Whirl also has seasonal behaviors along to the monsoon winds changes. The season when the eddy comes up is around between June and September. For 1995 monsoon, the Somali current wasn't evident in the June, so that the eddy occurred with very week strength and size (the onset phase). As the Somali current get developed through the summer, finally, in September, the Great Whirl maximized and began to disperse entering the winter season (the wane).^[20][21] According to previous observations, the Great Whirl remains until mid-October with still having large structure and its original location. And sometimes the curl structure can be remained and observed underneath the winter Somali Current during the winter season.^[13]

The seasonal behaviors of the Great Whirl affect to the local coastal ocean flows and the Arabian Sea ecosystem. During the summer season, strong coastal upwelling flows occur to the northwest of the Great Whirl, and these coastal upwelling flows are strongly depending on the shape and behaviors of the Great Whirl. In concludes, the controlled behaviors of coastal upwelling, by the Great Whirl, affect to the Arabian Sea ecosystem and heat flux budget in the North Indian Ocean.^[22]

See also

• Ocean current
• Oceanic gyres
• Physical oceanography

References

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