HIGHWAYS IN THE SEA

OCEAN CURRENTSand Tides

TIDES

The greatest tides in the world occur in the Bay of Fundy.... Burntcoat Head in the Minas Basin, Nova Scotia, has the greatest mean spring range with 14.5 metres (47.5 feet) and an extreme range of 16.3 metres (53.5 feet).”

Bay of Fundy High Tide

Bay of Fundy Low Tide

Major Surface Currents – generally occur from the surface to approximately 1300 feet. The three major factors that set deep and surface currents in motion: wind, changes in sea level, and variations in water density.

The Wind – The primary energy force for both surface currents and waves.The Trade Winds and the Westerlies account for most of the Earth’s wind energy, producing the surface currents.

Gyre (pronounced – Jier, Greek gyros meaning circle)-There are 5 major ocean gyres, which are huge circular flows of water found in each ocean basin.

The combination of the westerlies pushing water eastwardalong the upper Atlantic, the trade winds pushing it westwardnear the equator, and the Coriolis effect results in a

circular flow (gyre) in each ocean basin.

The Coriolis Effect – The ocean currents circle clockwise in the Northern Hemisphere and counter clockwise in the Southern hemisphere brought about by the rotation of the Earth

Of the western boundary currents, the Gulf Stream is the most studied.

Benjamin Franklin made one of the first detailed mapsof the Gulf Stream in the 18th century.

The Gulf Stream transports more than 55 sverdrup (sv).- The sverdrup is a unit of measure invented to express

volume transported in ocean currents. - Named for oceanographer Harald Sverdrup, one sverdrupequals 1 million cubic meters of water per second.

The gulf stream has 10 times the volume of the Niagara Falls

A tidal bore is a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travels up a river or narrow bay against the direction of the river or bay's current

Surfing a Tidal Bore

Amazon Tidal Bore

http://www.youtube.com/watch?v=56DMuu-eO2s

Amazon Tidal Bore

Cook Inlet Alaska

Eddies are swirling currents, that can pinch off of the main current. They can form large circular loops that break awaya. These are usually small and temporary eddies and vary in temperature depending on rotation.

Eddies are important because they can profoundly affect local temperatures and water conditions

The Horse Latitudes is an area in the Atlantic Ocean where the

trade winds die, this would stall early Spanish ships sailing to the

New World.

To lighten the ships, the sailors would throw heavy cargo

overboard, which would allow the weak sails to pull the ship out

of the doldrums.

This included furniture, crates of dishes, and even canons.

When particular areas were still too calm, they were forced to toss their frightened horses into the sea. The legend is that the horses would swim after them for miles before they drowned, and the superstitious sailors would hear the horse screams in their haunted dreams for the rest of the voyage.

Wind-driven currents sometimes flow vertically, causing upwellings and downwellings. These currents take oxygen rich surface waters to deep water, and bring nutrient rich water to the surface.

a. An upwelling is an upward vertical current that brings deep water to the surface.

b. A downwelling is a downward vertical current that pushes surface water deep into the ocean.

Coastal upwellings occur when the wind blows offshore or parallel to shore.

Upwellings and downwellings have strong biological effects.a. An upwelling tends to bring deepwater nutrients up into shallow

water. This can significantly increase biological productivity

For example, the productive fishing grounds off the South

American west coast exist due to frequent upwelling.

b. Phytoplankton production is also increased due to the nutrient

rich water brought upward.

Thermohaline circulation is water motion caused by differing water densities.

a. Thermohaline circulation drives most of the vertical motion of seawater and the ocean’s overall circulation.

b. It works because water density increases due to cooling, increased salinity, or both.

c. When water becomes denser than the water below it, the denser water sinks, causing a downward flow.

DownwellingWhat causes downwelling? Where does most downwelling when the

water on the surface of the sea becomes denser than the water

beneath it and so it sinks. Seawater gets denser when it gets colder or

saltier.

Where does most downwelling occur?

Two deep masses, Antarctic Bottom and North Atlantic Deep Water, make up most of the world’s cold deep water. Pacific Deep and Mediterranean Deep Water are also important.a. The densest ocean water forms in the Antarctic during the

winter. This Antarctic Bottom Water has a salinity of about 34.65‰ and temperature of (31°F).

b. As seawater freezes, it leaves salt behind. The water that remains therefore becomes saltier, explaining why Antarctic Bottom Water has such high salinity.

c. Antarctic Bottom Water is thought to reach about 40°north latitude, taking somewhat less than 1,000 years to get there.

What are the two distinct approaches to studying currents?The first is the Lagrangian method, also called the float method.a. With this approach, you study the current by tracking a

drifting object.b. The Lagrangian method involves floating something in the

current that records information as it drifts.

A more useful float is called a drogue. The advantage ofa drogue over a simple surface float is that the “holeysock” attached to it ensures that the current and not the wind determinewhere it drifts.

Float deployment

from vessel

c. Although this is normally an instrument, it can also be a vessel. The Ben Franklin, a specialized submersible, studied the Gulf Stream by floating in it for weeks with a team of scientists aboard.

Scientists are now using floats that transmit data to satellites.a. For example, the Argo float drifts at depths typically ranging from 5,000 to 6,500 feet, before periodically rising to the surface to transmit to a satellite a temperature and salinity profile of the water it rose through. b. Then, it sinks back to its drifting depth.

During a data collection and reporting cycle, a PALACE

(Profiling Autonomous LAgrangian Circulation Explorer) float

drifts with the current at a programmed depth, rises every

week or two by inflating the external bladder (recording

temperature and salinity profiles on the way up), spends a day

at the surface transmitting data, then returns to drift at depth

by deflating the bladder

FLOTSAM METHOD – ACCIDENTAL OPPORTUNITIES TO STUDY CURRENTS Scientists also take advantage of accidental opportunities to study currents. a. Two examples occurred in May 1990 and in January 1992. In both cases, ships in severe weather lost readily identifiable cargo overboard.b. In the first incident, a cargo carrier en route to Seattle, Washington, from Korea lost 30,000 pairs of athletic shoes. Six months later, they started washing up on thebeaches of British Columbia and Oregon.

c. Oceanographers asked the public to help by reporting the date, place, and time they found the shoes.d. Based on these data, they were able to improve computer models of currents in the North Pacific.

In the second case, in 1992 a ship lost 29,000 rubber ducks, frogs, and turtles while sailing from China to Seattle.The floating navy of 29,000 rubber duckies had quite a journey after that. Some arrived early in Canada, others were to have a farther journey and still others are still out there sailing their little hearts out.Enterprising little Duckies, known as Friendly Floatees, they actually went from the Pacific to the Atlantic.Curtis Ebbesmeyer is the famous oceanographer who tracked them to the coast of Britain in 2007 after a 15 year journey.

How Earth’s rotation beneath the tidal bulges produces high and low tides.