GEO-SCI 103 1nd EditionExam # 3 Study Guide Lectures: 12 - 15Lecture 12: Ekman Transport and Wind Driven Circulation - recall: how the prevailing winds move (p. 130-131) - what is Ekman transport? - Wind-driven circulation = motion of the surface ocean, caused by the prevailing winds - are near-surface water masses influenced by the Coriolis effect? - what are the subtropical gyres? - what are geostrophic currents? - western boundary currents (e.g., Gulf Stream in the N. Atlantic or the Kuroshio Current in the N. Pacific) vs. eastern boundary currents (e.g., Canary Current in the N. Atlantic or the California Current in the N. Pacific); how are the western and eastern sides of ocean basins different? What causes “western intensification”? - what special conditions cause costal upwelling?- Where does divergence of surface water masses occur and why- Why does upwelling result in high biological productivity- Wind Driven Circulation of the Surface OceanPrevailing winds provide the energy to drive the surface currents of the world ocean. Ekman transport and Coriolis Effect cause the surface waters toconverge (pile up) in the subtropics and diverge (move apart) at the equator and in subpolar waters. This creates subtle hills and valleys on the surface ocean of less than 2 meters (6 feet)- Prevailing Winds- Prevailing Winds create a drag (wind stress) on the ocean surface and some of this momentum is transferred to the water, causing it to move- Velocity of the current at the surface decreases with greater depth (because of the friction among the water molecules)- Coriolis Effect- The Coriolis effect causes the moving water to be deflected away from its direction of travel (to the right—Northern Hemisphere)- Surface current is deflected approx. 45 degrees from the direction to the prevailing wind- Causes the Ekman transport and drives circulating currents to form loops or gyres. - Ekman Spiral: change in DIRECTION and current VELOCITY with increasing depth- A decrease in current speed coupled with continuous Coriolis Deflection with increasing depth in an apparent spiral of moving water is called the Ekman Spiral- Ekman Transport: Composite of the entire Ekman Spiral, which controls the motion of the surface ocean and yields a 90-degree angle to the prevailing winds. - Adding vectors (magnitude and direction) of the Ekman spiral yields a current that is approximately 90 degrees to the prevailing winds which is known as the Ekman Transport- The prevailing wind moves water at the surface ocean, and this ocean current is deflected at an angle to the wind as determined by the Coriolis effect. Water flowing at greater depth is also deflected, so that the net transport (Ekman Transport) of water is at right angles to the prevailing wind. - Causes near surface waters to converge (pile up) in subtropical regions to create subtle hills, domes or ridges on the ocean surface- Causes waters to diverge (move apart) in sub polar regions and along the equator, creating depressions or valleys. Causes upwelling of deeper waters- The subtle highs and lows on the ocean surface are not visible because the relief is less than 2 meters higher or lower than the average level of sea over broad areas of the ocean- Caused by the Coriolis Effect, causes water to “pile up” in some areas and diverge in others- Wind Driven Circulation: the movement of upper water masses (motion moves to the right in Northern Hemisphere and to the left of the prevailing winds in the Southern Hemisphere. - Affects only the upper ocean to a depth of about 150 meters because the pycnocline (or permanent thermocline) provides a stable density barrier between less dense (warmer) near surface waters and the more dense (cold) deep waters. - The presence of continents restricts the wind driven circulation of the oceans to individual basins- Gyres: large horizontal wind driven current systems that circulate around the subtle domes and depressions on the ocean surface. The Coriolis Effect drives these circulating currents to form loops or gyres.- Subtropical Gyres: - Represent large circulation cells around the hills created by convergence in the subtropics - Transport warm waters to the pole along the western sides of the ocean basins, and cool waters toward the equator along the eastern sides - Prominent features of circulation in the North and South Atlantic, North and South Pacific and the Indian Ocean- Ocean Currents: Coriolis and Gravity in Balance- Gravity- Acts on the water to pull it back from hills or into valleys. - This continuous tug of war between opposing forces results in:- Partial balance or equilibrium that keeps water moving around these subtle domes or valleys - This geostrophic Flow is represented by most surface currents in the ocean (ex: subtropical gyres are geostrophic currents that represent the flow of upper water and in the Indian Ocean)- Earth’s Rotation- Earth’s rotation from West to East, assisted by the strong Trade Winds in the tropics, causes the ocean to “pile up” towards the western sides of the ocean basins (ex: pushing a pan or pail of water across roomwater rises on side you pushed from and sloshes out of bucket if you push hard enough)- As earth rotates, the water in the ocean starts to slosh towards the western sides of the basins, and the ocean surface stands slightly higher on the western sides of ocean bases- The dome of water created by convergence in te subtropics is not located in the center of the ocean basin but is displaced toward the western side - Westward Intensification: the offset dome causes water flowing on the western side of the dome to flow faster than the eastern side- Surface currents are forced through a narrower passage between the continents and the crest of the dome causing them to flow faster- Gulf Stream in the North Atlanticthe flow is narrower and stronger on the western side of the gyre where it transports warm topical water pole ward- Brazil Current in the South Atlantic- Kuroshio Current in the North Pacific- East Australian Current in the South Pacific- Agulhas Current in the Indian Ocean- Wind Driven Upwelling: Gateway to Biological Productivity- Upwelling and Downwelling- The wind driven surface currents of the world ocean including gyres, represent the horizontal flow of water masses.- Upwelling and downwelling are the