Somali Current

What it is

The Somali Current is a boundary current of the western Indian Ocean that runs along the coasts of Somalia and Oman. It is compared to the Gulf Stream in the Atlantic, and it is the only major upwelling system on the western edge of an ocean (Wikipedia: Somali Current).

Unlike most ocean currents, it changes direction with the seasons because it is driven by the monsoon winds (Wikipedia: Somali Current; Schott and McCreary, Progress in Oceanography 2001).

The seasons

• March to May, before the monsoon: a shallow current 50 to 100 km wide flows north near the coast over a southward undercurrent. In the south, the East African Coast Current crosses the equator and continues north as the southern Somali Current to about 3 to 4°N (Wikipedia: Somali Current).
• June to September, south-west monsoon: the current strengthens from mid-May and flows north-east. Speeds are about 2 m/s in mid-May and 3.5 m/s or more in June, up to about 7 knots. It travels about 1,500 km north-east and turns east near Cape Guardafui to join the Indian monsoon current (Wikipedia: Somali Current).
• Northern winter, north-east monsoon: the winds reverse and the current weakens and turns south (Wikipedia: Somali Current).

The Somali year has four seasons. The Jilaal, from December to March, is the harshest dry season, and the Xagaa, from July to September, is the second dry season; they fall between the Gu and Deyr rains (Wikipedia: Geography of Somalia).

Why the sea off Somalia is rich

During the south-west monsoon, wind pushes coastal water offshore and cold water rises from below to replace it. A narrow low-level wind called the Somali Jet, or Findlater Jet, blows across the ocean parallel to the coasts of Somalia and Oman and causes more upwelling out at sea. The two together bring up very large amounts of cold, nutrient-rich water, creating one of the most productive ocean ecosystems in the world (Wikipedia: Somali Current). This is the base of Somalia's fish stocks; see fishing companies in Somalia.

Size and speed

The current is shallow, often less than 150 m deep south of 5°N. In mid-September it typically carries about 37 million cubic metres of water a second (37 sverdrups), weaker than the Gulf Stream, but flows of 60 to 70 sverdrups have been measured south of Socotra (Wikipedia: Somali Current; Beal and Chereskin, Deep-Sea Research 2003).

Two large eddies form in summer: the Great Whirl off the Somali coast and the Socotra Eddy. The US ship Wilkes recorded both in August and September 1979 (Wikipedia: Somali Current; Bruce 1979, Journal of Geophysical Research).

How it was studied

Scientists agreed on the current's existence by 1966 (Warren et al. 1966). Lighthill in 1969 explained it by waves at the ocean's western edge (Lighthill 1969), and studies through the 1970s showed that local winds set off the current each year (Leetmaa 1972).

Sources

• Wikipedia: Somali Current
• Wikidata Q1267550
• Schott and McCreary, "The monsoon circulation of the Indian Ocean", Progress in Oceanography, 2001
• Beal and Chereskin, Deep-Sea Research II, 2003
• Lighthill, Philosophical Transactions of the Royal Society A, 1969