page_title: Somali Current
section_title: Great Whirl
hierarchical_section_title: Somali Current / Great Whirl
caption_attribution_description: English: Two gigantic eddies at the north west Indian Ocean during Southwest Monsoon
context_page_description: The Somali Current is a cold 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. 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.
  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.
context_section_description: The Great Whirl is a huge anti-cyclonic eddy generated by the Somali current flowing in (northern) summer, and one of the two gigantic Indian Ocean Gyres (the other is the Socotra Gyre). The Great Whirl can be observed between 5-10°N and 52-57°E off the Somali coast in the summer season, a location typically around 200 km southwest of the Socotra Gyre (between June to September). However, in the past both the Great Whirl and the Socotra Gyre have been known to occasionally collapse, and their exact locations differ from year to year. The Great Whirl's typical size is 400–600 km in horizontal diameter, and typical surface current velocity is 1.5-2.0 m/s.
  How the Great Whirl forms exactly isn't yet fully understood, but the analytical approach of applying Rossby wave theory can explain its basic formation mechanism. By observation and analysis of [Schott and Quadfasel (1982)], the summer monsoon develops suddenly during June–July, and drives westward water flow in the location. Schott and Quadfasel applied first-mode Rossby waves to the water flows, and concluded that “the formation of the Great Whirl is a response to the very strong anti-cyclonic wind-stress curl”.
  Since the Somali current changes seasonally, the Great Whirl also displays seasonal behaviour in keeping with the monsoon cycle. The eddy is usually evident between June and September, however e.g. in 1995, the Somali Current did not develop in June at all, so that during that time the eddy was severely weakened and of very small size - a prolonged onset phase. As the Somali Current developed through the summer, it was only in September that the Great Whirl finally maximized and began to disperse, entering the winter season (the wane). Conversely, the Great Whirl may also continue to exist until mid-October, still exhibiting large size and its curl structure may remain evident even longer, underneath the winter Somali Current.
  This seasonal behaviour of the Great Whirl affects the local coastal ocean flows and thus the Arabian Sea ecosystem: During the summer season, coastal upwelling currents strongly dependent on the shape and behaviour of the eddy are observed to the northwest of the Great Whirl. Not only does the biological productivity of the region depend on these upwelling currents, they (and thus, the Great Whirl itself) also play a large part in regulating the heat flux budget of the North Indian Ocean.
caption: Two gigantic eddies at the north west Indian Ocean during Southwest Monsoon
exif: {}