Circulation of the Ocean

Ocean Currents The mass movement of water, either vertically or horizontally, in the ocean Surface currents are wind-driven movements of water at or near the surface of the ocean Thermohaline currents are the slow, deep currents that affect the vast bulk of water beneath the pycnocline Named this because they depend on density differences caused by variations in water’s temperature and salinity

Surface Currents About 10% of the water in the ocean is involved in surface currents A surface current includes the water that is flowing horizontally in the uppermost 1,300 ft Most surface currents move water above the pycnocline These currents are driven mainly by wind friction Most of the wind energy is concentrated in the trade winds and westerlies Due to the forces of gravity, the Coriolis effect, solar energy, and solar winds, water often moves in a circular pattern called a gyre.

Winds Cause Surface Currents

Surface Currents and the Coriolis Effect Gyres in the N. Hemisphere move clockwise Gyres in the S. Hemisphere move counterclockwise

Gyres Surface currents flow around an ocean basin to create a gyre, which is typically made up of multiple surface currents Gyres flow continuously without obvious changes between currents The end of one current blends into the start of the next current Each current has distinct characteristics which is why each is considered a separate current Geostrophic gyres are gyres in balance between the pressure gradient and the Coriolis effect The geostrophic gyres are independent of one another due to wind patterns and the positions of the continents

Gyres There are 6 great surface circuits, but only 5 of these are considered geostrophic gyres. N. Atlantic S. Atlantic N. Pacific S. Pacific Indian Ocean The 6th circuit does not travel around an ocean basin (it travels around a continent) so it’s not considered a gyre Antarctic Circumpolar Current (aka West Wind Drift) Flows endlessly around Antarctica and is driven by the Westerlies

Western Boundary Currents Found on the western edge of each gyre (off the east coast of a continent) Fastest, deepest, and narrowest currents Move warm water from the equator to the poles 5 of them: Gulf Stream (N. Atlantic) Kuroshio Current (N. Pacific) Brazil Current (S. Atlantic) Agulhas Current (Indian) East Australian Current (S. Pacific)

Eastern Boundary Currents Found at the eastern edge of ocean basins (off west coast of continents) Shallow, slow moving, and broad Carry cold water from poles toward equator There are 5: Canary Current (N. Atlantic) Benguela Current (S. Atlantic) California Current (N. Pacific) West Australian Current (Indian) Peru Current (S. Pacific)

Transverse Boundary Currents Most of the power for currents is derived from the trade winds at the fringes of tropics and from mid-latitude Westerlies The stress of winds on the ocean in these bands gives rise to the transverse currents These currents flow from east to west and west to east, linking the eastern and western boundary currents on both the north and south ends of each gyre.

Transverse Currents

Ekman Spiral So in a gyre, why does water flow around the periphery of the ocean basin instead of spiraling to the center? Not only does the Coriolis play a role in this movement, so do wind patterns When driven by the wind, the topmost layer of ocean water in the N. Hemisphere flows at about 45˚ to the right of the wind direction.

Ekman Spiral What about the water below the top layer? The lower layers can’t “feel” the wind at the surface; However, it “feels” only the movement of the water immediately above The deeper layer of water moves at an angle to the right of the overlying water (due to Coriolis) The same thing happens in the layer below that and so on and so on This continues to happen until a depth of about 100m This process is known as Ekman Spiral

Ekman Spiral Even though its known as the Ekman Spiral, the water itself is not spiraling. The term spiral refers to what you would see on a diagram

Ekman Spiral

Ekman Transport Ekman Transport = the net motion of water down to about 100m after the allowance for the summed affects of Ekman spiral In theory, the direction of Ekman transport is 90˚ to the right of the wind direction in the N. Hemisphere and 90˚ to the left in the S. Hemisphere In actuality, the Ekman transport in gyres is 45˚ or less This difference is caused by the interaction of the Coriolis effect and the pressure gradient

Ekman Transport So, why doesn’t the water spiral to the center of each ocean basin? There are hills of water at the center of each ocean basin In order for the water to spiral toward the center, it would have to “climb” this hill and go against the gravity

Countercurrents and Undercurrents Usually found near equatorial currents Flow on the surface in the opposite direction of the main current Caused by the lack of persistent winds at the equator Undercurrents A countercurrent that occurs beneath the surface currents Found under most major currents

Surface Currents and Climate Surface currents (along with wind) distribute tropical heat worldwide Warm water flows to higher latitudes, transfers heat to the air and cools, moves back to low latitudes, and absorbs heat again The climate along a coastline greatly depends on the nearby surface current A warm current typically leads to a warm and humid climate (Florida and any of the Gulf states) A cold current typically leads to cooler temperatures with lower humidity (California and the other west coast states)

Surface Currents and Climate

Vertical Movement of Water The wind-driven horizontal movement of water can sometimes cause vertical movement of water The movement is called wind-induced vertical circulation 2 types Upwelling Downwelling

Upwelling Upwelling is the upward movement of water The water from within and below the pycnocline is often rich in the nutrients needed by marine organisms for growth Coastal upwelling - friction of wind blowing along the ocean surface parallel to the shore causes water to begin moving, the Coriolis effect deflects it to the right and the resulting Ekman transport moves it off shore Fuels primary productivity (photosynthesis) Equatorial upwelling - water moving in the currents on either side of the equator is deflected slightly poleward and replaced by deeper water

Coastal Upwelling

Coastal Upwelling (in red)

Equatorial Upwelling

Downwelling Water driven toward a coastline will be forced downward, returning seaward along the continental shelf Helps supply the deeper ocean with dissolved gases and nutrients Moves oxygen gas from the photic zone to the aphotic zone, which assists in distribution of living organisms Has no direct effect on climate or productivity of the coast

Downwelling

El Niño vs Normal Conditions during a normal year (a non-El Nino yr): Surface winds across most of the tropical Pacific normally move from east to west Trade winds blow from the normally high-pressure area over the eastern Pacific (S. America) to the normally stable low-pressure area over the western Pacific (N. Australia) Strong upwelling currents in the western Pacific Nutrient rich waters off the coast – leads to great biological diversity Sometimes these conditions are reversed for unknown reasons Occurs at irregular intervals of roughly 3-8 yrs

El Niño - The Conditions Conditions during an El Nino year: Pressure systems reverse High pressure builds in the western Pacific; low pressure dominates the eastern Pacific Winds across the tropical Pacific reverse direction and blow from west to east; the trade winds either weaken or reverse The change in atmospheric pressure is called the Southern Oscillation

El Niño – What are the Effects? The reversed winds move the warmest waters from the western Pacific to the eastern Pacific The eastern Pacific is greatly affected: Disrupts upwelling which causes a change in nutrients found in the waters, which affects the animals living in the area – they either die or migrate Causes high precipitation in normally dry areas Coastal storms are intensified Sea levels can rise up to 8 inches Water temp can increase up to 13˚F How is the western Pacific affected? Long periods of drought Crops don’t grow; people starve

Non El Niño Year El Niño Year Animations of Normal vs El Nino vs La Nina: http://esminfo.prenhall.com/science/geoanimations/animations/26_NinoNina.html https://www.youtube.com/watch?v=Ct1fXhTv3UA

La Niña Normal circulation returns with surprising strength Produces strong currents, powerful upwelling; chilly and stormy conditions along S. American coastline The powerful return that causes colder than normal events is called La Nina. As conditions in the eastern Pacific cool off, the ocean in the western Pacific warms rapidly Trade winds become stronger than normal resulting in above average upwelling in the eastern Pacific Increased upwelling pushes cold water further out into the Pacific, pushing the warm waters closer to the edge of the western Pacific

La Niña

Thermohaline Circulation Thermohaline circulation is the movement of water due to differences in density This type of circulation is mainly found below the pycnocline in the deep zone and moves water vertically and horizontally within this zone This process is responsible for the large-scale vertical movement of ocean water and the circulation of the global ocean as a whole

Thermohaline Circulation

Thermohaline Circulation

Water Masses Each water mass has specific temperature and salinity characteristics These differences prevent water masses from easily mixing with each other Water masses usually flow above or beneath each other These differences also lead to density stratification of the ocean

Water Masses Water masses are named according to their relative position In temperate and tropical waters, there are 5 layers: Surface water – goes to a depth of about 200 meters Central water – from approx. 200m down to the bottom of the thermocline Intermediate water – from bottom of thermocline down to about 1500m Deep water – water below the intermediate water, but not in contact with the bottom (to a depth of approx. 4000m) Bottom water – water in contact with the ocean floor These layers don’t exist near the poles. Why?

AAIW = Antarctic Intermediate Water. AABW = Antarctic Bottom Water AAIW = Antarctic Intermediate Water AABW = Antarctic Bottom Water NADW = N. Atlantic Deep Water

Studying Currents – Float Method Depends on the movement of a drift bottle or other free- floating object Drift bottles and drift cards – for surface currents Especially useful in determining coastal circulation Do not record the path taken Drift buoys – for surface currents Path can be tracked for more precise circulation patterns Tracked by radar or satellite Swallow float – for intermediate currents Detects the drift of intermediate water masses Descends to a specific density Emits sonar “pings” as it drifts so that it can be tracked

Drift Bottles

Drift Buoys

Swallow Float

Studying Currents – Flow Method Flow meters measure the speed and direction of a current from a fixed or stationary position Ekman flow meter Uses rotating vanes to measure speed and a recording compass to measure direction

Studying Currents – Slocum Glider A glider that “flies” smoothly up and down through the water column Collects data on temperature, salinity, density, direction and speed of deep water currents Powered by gravity and buoyancy Reach the surface periodically to transmit data to satellites before returning to the depths of the sea to collect more data Can spend years at sea before being collected

Slocum Glider