The Somali Current flows along the coast of East Africa in the western Indian Ocean and is a major component of the region’s ocean circulation. It is considered unusual because it is one of the few western boundary currents in the world that completely reverses its direction twice a year. Other major currents, such as the Gulf Stream in the Atlantic, maintain a consistent flow direction. This complete seasonal reversal makes it a unique feature in global oceanography, driving environmental changes along the East African coast.
The Defining Feature of Seasonal Reversal
The Somali Current exhibits two distinct phases, defined by changes in direction and speed. During the northern summer (June to September), the current flows rapidly northeastward along the coast of Somalia. This powerful western boundary current achieves surface speeds exceeding 3.5 meters per second, making it one of the fastest currents known.
This summer phase is characterized by the formation of large, circulating eddies, most notably the massive anticyclonic structure known as the Great Whirl. During the northern winter (December to February), the current system reverses its flow. It shifts to a weaker, generally southward or southwestward drift, sometimes replaced by near stagnation.
The current’s reversal transitions through intermediate phases in the spring and autumn. The dramatic change from a high-velocity, poleward current to a slow flow highlights the immense seasonal variability. This mechanical oscillation is a direct response to the atmosphere.
The Direct Influence of Monsoon Winds
The complete reversal of the Somali Current is directly caused by the seasonal shift in the Asian Monsoon system, which dictates wind patterns over the Indian Ocean. The interaction between the wind field and the ocean surface is the primary forcing mechanism for the current’s bi-directional nature.
During the summer, the Southwest Monsoon brings strong, persistent winds that blow parallel to the East African coast, pushing surface water northeastward. This intense wind stress, often concentrated in the Findlater Jet, drives the current to exceptional speed and volume, generating the fast, northward flow and the associated Great Whirl.
In the northern winter, the monsoon reverses, becoming the weaker Northeast Monsoon. This shift removes the alongshore stress, causing the current to slow down, reverse, or dissipate entirely.
Ecological and Oceanographic Impact
The seasonal reversal and high velocity of the Somali Current profoundly impact the ocean environment, creating one of the world’s most intense seasonal coastal upwelling systems. During the peak of the Southwest Monsoon, the fast, northward-flowing current pulls surface water away from the coast via Ekman transport. This action draws deep, cold, and nutrient-rich water up to the surface to replace the warmer water.
This seasonal upwelling dramatically cools the sea surface temperature along the Somali coast, with drops of 5 degrees Celsius or more observed. The influx of cold water injects nutrients, such as nitrate, into the sunlit surface layer. This enrichment supports high biological productivity, making the offshore Somali area one of the most productive marine ecosystems globally.
When the current reverses and the monsoon shifts in winter, the upwelling process shuts down. Coastal waters become warmer, and biological productivity declines significantly due to the lack of nutrient supply. This annual cycle defines the ecological rhythm of the western Indian Ocean region.