Friday, March 19, 2010
Gulf Stream and the planet.http://images.google.com/imgres?imgurl=http://kingfish.coastal.edu/gulfstream/path.jpg&imgrefurl=http://kingfish.coastal.edu/gulfstream/p2.htm&usg=__jhg6E2LlO_Cfc-ETCRFsWsh7OFI=&h=546&w=729&sz=500&hl=en&start=7&sig2=CI4Z-qnXBU8tFy28Grjluw&um=1&itbs=1&tbnid=fSJQ8s-Z-moZIM:&tbnh=106&tbnw=141&prev=/images%3Fq%3Dgulf%2Bstream%2Bcuba%26um%3D1%26hl%3Den%26sa%3DN%26rls%3Dcom.microsoft:en-us:IE-SearchBox%26rlz%3D1I7GGLL_en%26tbs%3Disch:1&ei=_Z2jS-OcOI6Vtgfd6oGHCg
The Gulf Stream, located along the eastern coast of the United States, is a fast, intense current known as a western-boundary current. These currents are located on the western side of every ocean basin. The Gulf Stream is a result of the wind pattern acting on most of the North Atlantic Ocean. The combination of the Trade Winds (10°- 25°N) blowing to the west and the Westerlies (35°- 55°N) blowing to the east cause the North Atlantic to rotate clockwise. This basin wide, clockwise flow is referred to as the Subtropical Gyre. Because of the Earth's rotation, the poleward flow in the western Atlantic is constrained to a narrow current on the western boundary of the ocean basin. This is the Gulf Stream. While western boundary currents exist in all the major ocean basins, the two in the Northern Hemisphere (including the Kuroshio Current in the Pacific) are better developed and more intense than their Southern Hemisphere counterparts. The current that makes up the southern portion of the Subtropical gyre is the North Equatorial Current
Under the trade winds the broad, slow North Equatorial Current flows to the west. As the current approaches the Caribbean Sea, water is funneled through the many channels of the Antilles and Caribbean Islands, and through the Yucatan Channel - where it narrows and gains strength - and then loops into the Gulf of Mexico. This is where we can first observe an organized flow on satellite images. This flow enters the Gulf of Mexico at the Yucatan Channel and exits the Gulf at the Straits of Florida. The current is commonly referred to as the Loop Current while it's in the Gulf of Mexico and the Florida Current when it leaves the Gulf between Florida and Cuba.
Along the east coast of Florida the current is fed by the Antilles Current, and the flow, now called the Gulf Stream, runs parallel to the coast until reaching Cape Hatteras where it leaves the coast and enters deeper water. While flowing in deep water the Stream often forms large meanders or flucuations in its path. At approximately 50°W, the Gulf Stream splits into several currents the largest being the North Atlantic Current. The North Atlantic Current then feeds both the Norwegian Current which transports water northward along the west coast of Europe and the Canary Current which flows equatorward on the eastern side of the Atlantic. These currents are labeled in red in the figure below: Gulf Stream (GS), North Atlantic Current (NAC), Norwegian Current (NOR), Canary Current (CC).
The Gulf Stream is the fastest ocean current in the world with peak velocities near 2m/s. Shown to the left are velocity profiles across the Gulf Stream at the Straits of Florida and Cape Hatteras. In both sections the Gulf Stream is flowing into the page.
As the velocity profiles indicate, the maximum velocities are confined to the top 200 meters of the water column. Velocity decreases with depth, and the flow below 1000 meters is usually less than 10 cm/s.
Transport and Velocity figure from: Regional Oceanography: an Introduction by M. Tomczak and S. J. Godfrey
The figure below depicts the surface velocity of the North Atlantic during September 2000. Gulf Stream velocity fields are derived from near-realtime radar altimeter data measured from a satellite. The velocities of the currents are represented by colors as well as by arrows. Brighter colors indicate higher velocities. As the color bar at the bottom shows, velocities are in the range of 0 to 1 m/s (meters per second), which corresponds roughly to 0 to 2 knots (nautical miles per hour). The size of the arrows also refer to the magnitude of the velocity, an arrow of 1 m/s (approx. 2 knots) is drawn in the lower right corner, next to the color bar. The directions in which the arrows point is the direction of the current. Thus, an arrow pointing to the right indicates an eastward flow.
The highest velocities are observed both in the Gulf Stream current and south of the current in eddies that formed from the Gulf Stream. The formation of these eddies will be discussed later.
Image courtesy of Delft Institute for Earth Oriented Space Research
The width of the Gulf Stream can also be seen by looking at a temperature or density section. In this temperature section, west is to the left and east is to the right, so the Gulf Steam is flowing into the page.
Sloping isotherms (or isopycnals on a density section) indicate where the current is flowing. The isopycnals are horizontal on both sides of the Gulf Stream, indicating no flow in these regions.
A corresponding density section is shown below.
In the ocean, light reflects off particulates suspended in the water, and absorption of light is mainly due to chlorophyll present in phytoplankton. Chlorophyll is very good at absorbing blue light, thus a high level of chlorophyll will depelete the availability of blue light. Radiometers measure radiance at different wavelengths, and these measured radiances may then be quantitatively related to various constituents in the water column. The satellite can detect very small changes in the ocean color as a result of the chlorophyll in phytoplankton.The basic principle behind the remote sensing of ocean color from space is this; The more phytoplankton in the water, the greener it is; the less phytoplankton, the bluer it is.
To the north of the Subtropical Gyre is the Subpolar Gyre. The Subpolar Gyre which flows counterclockwise consists of much colder water than that found in the Subtropical Gyre.
As the North Atlantic Current splits into many smaller currents, some of these smaller currents flow toward Greenland and meet the southward flowing East Greenland Current (EG). This flow then meets the West Greenland Current (WG) to form the Labador Current (LAB) which flows back into the North Atlantic Current. Together all these flows comprise the Subpolar Gyre.
Transport of the Gulf Stream
The transport of the Florida Current is approximately 30 Sverdrups (Sv). One Sverdrup is one million cubic meters of water per second! The transport of the Gulf Stream increases to 85 Sv near Cape Hatteras, peaks at 150 Sv near 65°W, then decreases eastward of 65°W.
One of the primary reasons that the Gulf Stream increases in transport between Florida and 65°W and decreases in transport downstream of 65°W is the Recirculation Gyre. As the figure below shows, water leaves the Gulf Stream downstream of 65°W, and enters the Sargasso Sea. This water is recirculated clockwise throughout the Sargasso and eventually re-enters the Gulf Stream. This flow is known as the Recirculation gyre.