User:Sjyang80/sandbox

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. (*below information was cited from Technical Report AD-A276 238 )

(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.

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

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)

1971 Düing & Szekelda : the baroclinic mode is the dominant mode of the Somali current

1972,1973 Leetma : local winds are crucial to the onset of the Somali Current.

1975 Colborn : climatological analysis of the entire Indian Ocean from bathythermograph and hydrocast observations

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

1976 Bruce : time-series analyses, XBT cross-section measurement by the EXXON tankers )

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. )

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). 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 )

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.

Northeastward current (Jun-Sep): The Somali current begins developing its current from mid-May, 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 2.0m/s and in June, 3.5 m/s and more. Typically the Somali current moves through about 1500km northeastward, and near Somalia coast of 6~10°N, the current changes its direction to the east (at near the Cape Guardafui )), and merges to the Indian Monsoon current. )

Southwestward current (Dec-Feb): 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.

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. By the influence of the cold coastal upwelling, the coastal temperatures are lowered by 5°C and more during May to September.

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. 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 (between June to September). Typical size of eddy is 400~600 km in horizontal diameter, and typical surface current velocity is 1.5~2.0 m/s

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).

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.