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Transcript
Gyres of the world oceans http://staffwww.fullcoll.edu/tmorris/elements_of_ecology/images/ocean_currents.jpg Gyres of the World Oceans: Wind-driven Ocean Circulation http://www.physicalgeography.net/fundamentals/8q_1.html Ekman Transport - p. 138-139 Geostrophic Currents (gyres) - p. 140-141 Wind-Driven Upwelling - p. 142-143 p. 138 The prevailing winds create a drag (wind stress) on the ocean surface. The momentum gained at the surface is transferred deeper, but energy is lost with increasing depth. The Coriolis Effect deflects the moving water of the upper 75-150 m. A decrease in current speed coupled with continuous deflection with increasing depth creates a theoretical spiral of moving water called the Ekman Spiral. p. 138 The surface current is ~45o to the prevailing wind. Adding all the vectors (magnitude and direction) of the Ekman Spiral yields a net current direction that is ~90o to the prevailing wind. This composite current is called Ekman Transport and it controls the motion of the surface ocean. what hemisphere does this example come from? Northern Hemisphere - to the right of the prevailing wind Ekman Spiral change in direction and current velocity with increasing depth prevailing wind surface Surfacecurrent current (~45 to to thetheprevailing (~45° prevailingwind) wind) Ekman Transport p. 138 = composite of Ekman Spiral; ~90o to the prevailing wind Seeing the Ekman Spiral in 3-D: each layer of surface water moves in a different direction due to the Coriolis Effect Ekman Transport ~90o to the prevailing winds 90° N 90N (North Pole) (North Pole) H L Polar Easterlies Polar Easterlies 60N Westerlies Westerlies 30N H TradeWinds Winds Trade NE(NE TradeTrades) Winds L ITCZ 0(Equator) TradeWinds Winds Trade SE(NE TradeTrades) Winds H 30S Westerlies Westerlies L p. 133 Polar Polar Easterlies Easterlies 60S H 90S 90° (South Pole) S (South Pole) to the right of the prevailing winds in the N. Hemisphere, to the left of the prevailing winds in the S. Hemisphere Predict ocean circulation based on your knowledge of the prevailing winds and Ekman transport In the Northern Hemisphere: clockwise circulation in the low to mid-latitudes; counter-clockwise circulation in the high latitudes Circulation has the opposite sense in the S. Hemisphere 60N Gulf of Mexico m aam trree S lf Guu G Caribbean Sea nt Cu rre North Atlantic Subtropical Gyre CCaa nnaa r ryy C urrr u reennt t No lan tic rth A t Mediterranean Sea 30N N. Equatorial Current Subtropical Gyre South Atlantic West Wind Drift 60S East Wind Drift p. 132-133 t B Been ng ue la Cuurr gu rren entt 30S re n Eq. Countercurrent l Cu r ia r to ua S. Eq BBra razzil C il Cur u rrree n ntt 0(Equator) Circulation of the upper water masses is set in motion by the energy of the prevailing winds Ekman transport is responsible for this winddriven circulation Gyres are large horizontal current systems The subtropical gyres transport warm waters poleward along the western edges of the ocean basins, and cool waters equatorward along the eastern sides http://www.miracosta.cc.ca.us/home/kmeldahl/currents/world_circulation.jpg Ekman transport and the Coriolis effect cause surface waters to converge (“pile-up”) in the subtropics, and diverge (move apart) at the equator and in subpolar waters. Convergence and divergence of surface water masses create subtle relief (“domes” and “depressions”) on the ocean surface (<2 m, or <6 ft.). Gravity acts on the water to pull it downslope, while the Coriolis effect works in the opposite direction. Geostrophic flow represents the partial balance between gravity and Coriolis. p. 134 p. 133 divergence Ekman transport causes surface waters to converge (pile-up) in subtropical regions creating a hill or dome convergence divergence convergence divergence Ekman transport causes surface waters to diverge (move apart) in subpolar regions & at the equator creating a valley or depression Divergence creates valleys or depressions, Convergence produces hills or ridges. divergence of water masses results in upwelling and high productivity in subpolar waters convergence of water masses causes near-surface waters to pile-up in the subtropics divergence of water masses results in upwelling and high productivity at the equator convergence of water masses causes near-surface waters to pile-up in the subtropics divergence of water masses results in upwelling and high productivity in subpolar waters p. 134-135 Profile roRotation ta tio n ooff Earth E a rth Northeast Trad es Geostrophic currents represent the partial balance between gravity & Coriolis; most surface currents are geostrophic. Earth’s rotation from west to east, compounded by the typically strong Trade Winds, cause tropical & subtropical surface waters to “pile-up” on the western sides of the ocean basins The “hill” is displaced to the west of center causing water on the western side of the hill to flow faster than the eastern side In other words, the surface currents are forced through a narrower passage between the continents and the crest of the hill causing them to flow faster; this is called “western intensification”. Strong western boundary currents mark the western sides of the subtropical gyres. p. 134 p. 135 Map View Note: the dome is not in the center, but has been offset to the west "hill" warm core ring The Gulf Stream is a western boundary current of the subtropical gyre cold core ring “Sargasso Sea” http://veimages.gsfc.nasa.gov/1187/S1999148171230.png The Gulf Stream and North Atlantic Drift warm Europe http://www.ospar.org/grfx/Q.F.2.4.jpg Notice the warm currents flowing poleward on the western sides of the ocean basins and the cool currents flowing equatorward on the eastern sides. Also note the Antarctic Circumpolar Current that flows around the isolated continent of Antarctica; this current serves to connect ocean circulation between the three major ocean basins. The north Pacific Geostrophic Subtropical Gyre Nike shoes and toys found along North American beaches. Lots of garbage collects here! http://en.wikipedia.org/wiki/Image:North_Pacific_Gyre.png Map View Coastal Upwelling Profile p. 136-137 Ekman transport causes surface waters to be displaced seaward (i.e., pushed away from the coast) Surface waters are replaced by nutrient-rich deeper waters resulting in high biological productivity over the outer shelf and upper slope Dark greens on land and light greens & reds in the oceans are areas of highest chlorophyll and plant/algae abundances. In the oceans this means that either upwelling or continental runoff has enriched the ocean in nutrients. Map View Oceanic Divergence Profile (e.g., Equatorial Upwelling) p. 136-137 Change in the direction of the Coriolis effect at the equator causes divergence of the surface waters Surface waters are replaced by nutrient-rich deeper waters resulting in high biological productivity chlorophyll concentrations on land & sea Northern Hemisphere summer notice strong equatorial upwelling as well as coastal upwelling off Africa & Peru 2 1 2 2 3 3 1. equatorial upwelling (divergence) 2. coastal upwelling off Africa (several locations) 3. high latitude upwelling (divergence) around Antarctica p. 132-135 divergence convergence divergence convergence divergence divergence of water masses causes upwelling of deeper waters Summary of Wind-Driven Circulation 1. Prevailing winds (Trades, Westerlies, Polar Easterlies) impart energy to set the upper water masses in motion (nearsurface circulation) 2. Due to the Coriolis Effect, net transport of water is approximately at a right angle (~90o) to the direction of the prevailing winds (=Ekman Transport) 90o to the right in the N. Hemisphere 90o to the left in the S. Hemisphere 3. Ekman Transport causes near-surface waters to pile-up (converge) in subtropical regions (creating “hills”), and to move apart (diverge) in subpolar regions and at the equator (creating “valleys”) Nike shoes and toys found along North American beaches. Lots of garbage collects here! http://en.wikipedia.org/wiki/Image:North_Pacific_Gyre.png The “Great Pacific Garbage Patch” http://knowmanythings.files.wordpress.com/2009/07/great_pacific_garbage.jpg The remnants of a Laysan Albatross chick which was fed plastic by its parents resulting in death. http://en.wikipedia.org/wiki/Great_Pacific_Garbage_Patch Geostrophic currents flow around subtle “hills” and “valleys” on the ocean surface Summary of Wind-Driven Circulation 60N Caribbean Sea Cu rr North Atlantic Subtropical Gyre Mediterranean Sea 30N Ca na ry Gulf of Mexico m aam trree S lf Guu G en t nt Cu rre ic t n la At No rth N. Equatorial Current Subtropical Gyre South Atlantic rr en t t B en gu e la Cu 30S Eq. Countercurrent u rre n ria l C to a u S. Eq BBra razzil C il Cur u rrree n ntt 0(Equator) West Wind Drift 60S East Wind Drift 6. Western boundary currents transport warm waters poleward; eastern boundary currents transport cool waters equatorward. p. 132-135 4. The major near-surface currents are geostrophic, that is, the water masses flow around subtle “hills” and “valleys” on the ocean surface (balance between Coriolis effect & gravity) e.g., the subtropical gyres flow around “hills” of the Atlantic, Pacific & Indian oceans 5. Near-surface currents are stronger on the western sides of the subtropical gyres (=western boundary currents) the water is piled-up towards the west because of the Earth’s rotation and the strong Trade winds; currents flow faster where the “hills” are steepest & narrowest (i.e., the west side of the hill)
