Oil spilled on the ocean travels

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How does oil spilled on the ocean travels

The main factor acting on the traveling of oil on the sea is the ocean current.

Ocean current is a continuous movement of ocean water generated by the forces acting upon this mean flow, caused by the gravitational pull of the Moon and the Sun.

They can be of two types:

Surface and deep, the work here will focus on the surface ones once oil stays at the surface.

The surface currents are mainly wind driven. On the Northern hemisphere they flow clockwise while in the Southern hemisphere, they flow counter-clockwise because of wind stresses. (100)

When wind blows across the seas, the surface layer of water moves, this layer of water generally has a thickness from 10-200m. The amount of water flowing on that layer depends upon the wind and the Coriolis force that act.

The currents along the Western boundaries of the oceans are narrower, stronger and deeper and are called streams while in the middle of oceans and at Eastern boundaries, they are broad, slow moving and shallow. (101)

How does ocean currents move and how wind affects it

The movement of ocean currents is around 2-4% of the wind velocity that influences it.

To have the same quantity of energy that the wind generates on the ocean currents is necessary an ocean current with minor velocity than the wind because of the differences in density from wind and sea water. An example of that is, a wind that travels at 176km/h has a corresponding ocean current of 19,2km/h. (102)

Considering that the oil spilled on the sea travel at the same velocity of the water where it lays on, if we have very strong winds action on the current, the currents will travel faster so is the oil.

Under natural conditions, the current velocity and wind are related, their relationship is determined by the variation of there parameters. The relation between the drift current velocity and the wind speed are dependent on the latitude.

Ug = KW = (0.127/ sin ϕ) W

Ug: drift current velocity, measured in m/s

W: wind speed

ϕ: latitude

Generally, the current velocities are around 10 to 20 cm/ but very strong winds can cause currents with velocities in the order of hundreds cm/s. (103)

Now, relating curl of wind stress with mass transport on the surface layer of water (equation of wind driven current) (104)

β ≡ ∂f / ∂y = 2 Ω cos ϕ / R

∂2f / ∂y2 = - f / R2

Where f is a function of latitude

β ≡ ∂f / ∂y: rate of change of Coriolis parameter with latitude

R: Earth's radius

ϕ: latitude

The unit of ocean currents is Sverdrup (Sv), where 1Sv is equivalent to a volume flow rate of 1,000,000m3 (35,000,000cuft) per second.

Surface currents within gyres vary considerably in strength, width, and depth. The northeastward flowing Gulf Stream of the northwestern Atlantic is one of the swiftest surface currents with velocities ranging from 3 to 4 km per hr (1.8 to 2.5 mph). Those currents are also relatively deep and narrow, usually measuring no more than 50 to 75 km (30 to 45 mi) across. On the eastern arms of these gyres, the southward flowing Canary and California Currents, respectively, are hundreds of kilometers wide and rarely flow at more than 1 km per hr (0.6 mph). (105)

One of the strongest ocean currents is the Golf current which has velocities ranging 2,5m/s and transport water masses in the order of 150SV and is as large as 200km). (106)

Velocities and ocean currents in the Norwegian Sea

The current system in the North Sea is dominated by three main branches of inflowing Atlantic Water and the outflow of the Norwegian Current (Mork 1981). The Norwegian Coastal Current flows northward along the west coast of Norway in the upper 50-100 m of the water column (Helland-Hansen and Nansen 1909, Ikeda et al. 1989), in this case, if a misfortune happens and oil is spilled on this area, the oil will follow the direction of the Coastal current.

Estimates for the velocities of the current in this area vary widely, ranging from 5 cm s-1 to 60 cm s-1 which suggests a characteristic speed of 30 cm s-1 and those velocities are not constant, they vary greatly with time. (107)

Examples of Large oil Spills

The largest accidental oil spill on record (Persian Gulf, 1991) put 240 million gallons of oil into the ocean near Kuwait and Saudi Arabia when several tankers, port facilities, and storage tanks were destroyed during war operations. The blowout of the Ixtoc I exploratory well offshore Mexico in 1979, the second largest accidental oil spill, gushed 140 million gallons of oil into the Gulf of Mexico. By comparison, the wreck of the Exxon Valdez tanker in 1989 spilled 11 million gallons of oil into Prince William Sound offshore Alaska, and ranks fifty-third on the list of oil spills involving more than 10 million gallons. The number of large spills (over 206,500 gallons) averaged 24.1 per year from 1970 to 1979, but decreased to 6.9 per year from 1990 through 2000. (108)

References

100 http://en.wikipedia.org/wiki/Ocean_current

101 http://www.ospar.org/eng/doc/pdfs/R2C2.pdf

102 http://www.dee.ufrj.br/lafae/txt_correntes.html

103 http://www.springerlink.com/content/t3734271580m0112/fulltext.pdf

104http://oceanworld.tamu.edu/resources/ocng_textbook/chapter11/chapter11_01.htm#top

105 (Adapted from DataStreme Ocean and used with permission of the American Meteorological Society)

106 http://www.igeo.pt/atlas/Cap1/Cap1b_2.html

107 http://oceancurrents.rsmas.miami.edu/atlantic/norwegian.html

108 http://www.waterencyclopedia.com/Oc-Po/Oil-Spills-Impact-on-the-Ocean.html

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