Ocean Currents Highways of the Sea

Current – water moving 1 knot or faster 1 knot = 1 nautical mile per hour (nmph) 1 nautical mile = 1 minute of latitude = 1.852 km = 1.15 mile Drift – water moving less than 1 knot Two major types of ocean currents: Geostrophic currents – wind-driven currents found in surface waters Thermohaline currents – caused by differences in water density found in deep water flow

Wind-driven Surface Currents Caused by the wind and effected by the Coriolis effect and drag. Drag is friction between wind and water molecules and water molecules with other water molecules. Ekman Spiral: direction of water motion changes with depth in a spiral motion. The smaller the arrow the slower the speed. Surface currents move at 45° to the RIGHT of the wind direction in the Northern Hemisphere and 45° LEFT in the Southern Hemisphere.

The Ekman transport (or net transport) is 90° to the RIGHT or LEFT of the wind direction when you average all the motion of the water at different depths

Western vs. Eastern Boundary Currents Surface currents form huge circular patterns of water movement called gyres. Gyres move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. There are 5 major ocean gyres (2 in the N.Hemisphere, and 3 in the S.Hemisphere) The speed/depth/width of the currents that make up the gyres are not symmetrical. There is intensification of the western boundary currents (especially in the 2 northern hemisphere gyres)

Western Boundary Currents Fast Narrow Deep Examples: Gulf Stream and Kuroshio Current Eastern Boundary Currents Slow Wide Shallow Examples: Canary Current and California Current Western intensification is in some way the result of the fact that Coriolis varies with latitude and the rotation of the Earth cause water to “pile” up on the western shores of the ocean basin.

Warm Currents vs. Cold Currents Warm currents move transverse (parallel) with the equator or move away from the equator Ex. North Equatorial Current and Gulf Stream Cold currents move towards the equator (away from the poles) or move transverse at higher latitudes. Ex. North Atlantic Current and Canary Current

Upwelling vs. Downwelling Surface currents along coastlines of continents cause coastal upwelling or downwelling of water depending on the direction with wind is blowing and the hemisphere. Occur when the wind is blowing parallel to the shore. Coastal upwelling occurs when the net transport of surface water moves away from the coastline and deep, colder, more nutrient rich water flows up to replace it Coastal downwelling occurs when the net transport of surface water moves toward the coastline and as water builds up it is pushed downward.

Eddies As surface currents move past one another (i.e. in the opposite direction or a faster moving current going past a slower current) it can cause the current to meander back and forth. Sometimes this meandering gets so coiled they can actually “pinch off” from the main current. These are called eddies. Eddies are packets of water moving with a circular motion.

Eddies commonly form in the Gulf Stream As the warm fast moving water comes in contact with colder slower waters. Warm-core and cold-core eddies are easily visible using satellite imaging

Deep-Sea Currents Deep-sea currents, also called THERMOHALINE currents, are driven by differences in the density of the water. Density differences are primarily caused by changes in temperature and/or salinity. The differences in density are usually so small, that these currents move VERY slowly Cold, salty water will have the highest density and sinks from surface waters to the deep-sea and moves toward the equator.

In the Atlantic Ocean deep-sea currents form in two locations: In the North Atlantic between Iceland and Greenland In the South Atlantic in the Weddell Sea There are three major deep-sea currents in the Atlantic Ocean AABW – Antarctic Bottom Water NADW – North Atlantic Deep Water AAIW – Antarctic Intermediate Water

The Ocean Conveyor Belt Interconnected flow of the currents helps transport heat energy around the earth and helps moderate the climate. Some theorize that disruption of this conveyor belt has been linked to coldest intervals within the ice-ages of our past.

Studying Ocean Currents Lagrangian method – also called the “float” method. Study a current by tracking a drifting object. Message in a bottle – put object in water and track when and where it is located. Drogue – weighted “holey sock” is submerged enough which ensures the current is responsible for the movement and not the wind. Flotsam method – tracking accidental opportunities (ex. shoes from cargo container)

Eulerian method – also called the “flow” method Eulerian method – also called the “flow” method. Study a current by staying in once place and measuring the current speed and direction as is flows past. Flow meter – various types can be used to gather information that can be either transmitted electronically or stored for retrieval later. Doppler Acoustic Current Meter – uses sound and the change in frequency to determine speed and direction. They are simple and very accurate