Oceanography
My Blogs (olelog) are mainly based on my daily reading of earth science news. Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

Currents
 

Ocean Circulation

Ocean circulation is driven by energy from the sun and the rotation of the Earth.

(Solar) Energy

causes circulation of the atmosphere = winds

and variations in temperature and salinity of seawater -> temperature and salinity controls the water's density.

Earth rotation and the Coriolus Force

Due to the rotation of the earth, currents are deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This effect is known as the "Coriolis force."

In the Northern Hemisphere currents curve to the right (clockwise). In the Southern Hemisphere currents curve to the left (counter-clockwise)

Image from NASA

Generalized model of the thermohaline circulation: 'Global Conveyor Belt' This illustration shows cold deep high salinity currents circulating from the north Atlantic Ocean to the southern Atlantic Ocean and east to the Indian Ocean. Deep water returns to the surface in the Indian and Pacific Oceans through the process of upwelling. The warm shallow current then returns west past the Indian Ocean, round South Africa and up to the North Atlantic where the water becomes saltier and colder and sinks starting the process all over again.

Water is an enormously efficient heat-sink. Solar heat absorbed by bodies of water during the day, or in the summer, is released at night, or in winter. But the heat in the ocean is also circulating. Temperature & Salinity control the sinking of surface water to the deep ocean, which affects long-term climate change. Such sinking is also a principal mechanism by which the oceans store and transport heat and carbon dioxide. Together, temperature and salinity differences drive a global circulation within the ocean sometimes called the Global Conveyor Belt.

"The Global Conveyer Belt for Heat" represents in a simple way how ocean currents carry warm surface waters from the equator toward the poles and moderate global climate. This global circuit takes up to 1,000 years to complete.
The heat in the water is carried to higher latitudes by ocean currents where it is released into the atmosphere. Water chilled by colder temperatures at high latitudes contracts (thus gets more dense). In some regions where the water is also very salty, such as the far North Atlantic, the water becomes dense enough to sink to the bottom. Mixing in the deep ocean due to winds and tides brings the cold water back to the surface everywhere around the ocean. Some reaches the surface via the global ocean water circulation conveyor belt to complete the cycle.
During this circulation of cold and warm water, carbon dioxide is also transported. Cold water absorbs carbon dioxide from the atmosphere, and some sinks deep into the ocean. When deep water comes to the surface in the tropics, it is warmed, and the carbon dioxide is released back to the atmosphere.
Salinity can be as important as temperature in determining density of seawater in some regions such as the western tropical Pacific and the far North Atlantic. Rain reduces the salinity, especially in regions of very heavy rain. Some tropical areas get 3,000 to 5,000 millimters of rain each year. Evaporation increases salinity because as evaporation occurs, salt is left behind thus making surface water denser. Evaporation in the tropics averages 2,000 millimeters per year. This denser saltier water sinks into the ocean contributing to the global circulation patterns and mixing. Ocean salinity measurements have been few and infrequent, and in many places salinity has remained unmeasured. Remotely sensed salinity measurements hold the promise of greatly improving our ocean models. This is the challenge of project Aquarius, a NASA mission scheduled to launch in 2008, which will enable us to further refine our understanding of the ocean-climate connection.

There are two type of Ocean Currents:

1. Surface Currents

2. Deep Water Currents

Deep waters make up the other 90% of the ocean. They move around the ocean basins by density driven forces and gravity. The density difference is a function of different temperatures and salinity. These deep waters sink into the deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase.

Surface Currents

The Gulf Stream

is part of the North Atlantic Ocean Circulation System.

The Kuroshio Current

is the world's second-largest (after the Gulf Stream) ocean current. It is found in the western Pacific Ocean off the east coast of Taiwan and flowing northeastward past Japan, where it merges with the easterly drift of the North Pacific Current. It is analogous to the Gulf Stream in the Atlantic Ocean, transporting warm, tropical water northward towards the polar region. Kuroshio means Black Stream in Japanese - an allusion to the deep blue colour of its water. It's also sometimes known as the Japan Current.

It is a swift, intense current with an average sea surface temperature of 24°C. The Kuroshio is the western portion of a giant clockwise, horizontal circulation known as the North Pacific subtropical gyre. This circulation extends from 15° to 45°N across the entire width of the Pacific Ocean. It is driven by the large-scale winds the trades in the south and the westerlies in the north. As with all other western boundary currents, such as the Gulf Stream, the effect of the Earth's rotation and its spherical shape is to concentrate the Kuroshio flow into a current that is only about 100 km wide with speeds up to 2 m/s.

Kuroshio Current does not only have a large effect on the hydrological conditions around Japan, and is important for fishery and marine transportation, but it also affects large-scale climate around Japan, carrying much heat from the tropics. Although the previously accepted theory assumes that the large-scale wind over the Pacific Ocean drives the current, it is also possible to assume that the current drives the wind. However, the effect of Kuroshio Current on the wind had not been clearly understood.

In the recent twenty years, the Kuroshio path moved periodically, possibly (partly?) due to el Niño and la Niña events. The North Equatorial Current is weak during the period when la Niña is observed in western equatorial Pacific.

The Oyashio Current

(also named 'Oya Siwo') is a cold subarctic ocean current that flows south and circulates counterclockwise in the western North Pacific Ocean. It collides with the Kuroshio Current off the eastern shore of Japan to form the North Pacific Current.

The Antarctic Circumpolar Current (ACC)

is the most important current in the Southern Ocean, and the only current that flows completely around the globe. The ACC transports more water than any other ocean current. It extends from the sea surface to depths of 2000-4000 m and can be as wide as 2000 km. This cold current (shown in blue) isolates Antartica from any warm (red) current transporting heat southwards.

The Somali Current

As the major ocean (surface) currents are wind-driven currents, the currents obviously must behave differently where the monsoons, or seasonal winds, rule the waves - like in the northern part of the Indian Ocean, where the surface circulation change seasonally, in response to the monsoons.
The most spectacular seasonal change is the reversal of the Somali Current, off east Africa.

During the northern summer months and the South-West monsoon the coastal waters move northeastward with surface velocities reaching up to 14 km per hour. At longitude 6°–10° N (off Somalia), the northeastward Somali flow turns eastward as the Monsoon Current. During the South-West Monsoon the Somali Current is a major western boundary current, comparable with the Gulf Stream and the Kuroshio Current. Typical for western boundary currents is their high flow velocity. The Somali current is no exception - it can manifest velocities in excess of 3.5 m s-1.

With the monsoon's reversal to the northeast in September, the current begins to weaken until, in the winter, it disappears entirely, to be replaced by a slow southwestward drift. During the South-West monsoon a two gyre system develops in the region - the Great Whirl between 5-10°N with clockwise rotation and a secondary eddy towards its south. This two gyre system is stable until August or September, when the southern gyre propagates northward and merges with the Great Whirl.

The Somali Current is also known as the East Africa Coast Current.

Image from NASA

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