Chapter 9 Circulation of the OceanSurface Currents Are Driven by the WindsThe westerlies and the trade winds are two of the winds that drive the ocean’s surface currents.About 10% of the water in the world ocean is involved in surface currents, water flowing horizontally in the uppermost 400 meters (1,300 feet) of the ocean’s surface, driven mainly by wind friction.(left) Winds, driven by uneven solar heating and Earth’s spin, drive the movement of the ocean’s surface currents. The prime movers are the powerful westerlies and the persistent trade winds (easterlies).Surface CurrentsWhat are some effects of ocean currents?Transfer heat from tropical to polar regionsInfluence weather and climateDistribute nutrients and scatter organismsSurface currents are driven by wind:Most of Earth’s surface wind energy is concentrated in the easterlies and westerlies.Due to the forces of gravity, the Coriolis effect, solar energy,and winds, water often moves in a circular pattern called a gyre.Surface Currents Are Driven by the WindsA combination of four forces – surface winds, the sun’s heat, the Coriolis effect, and gravity – circulates the ocean surface clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, forming gyres.The North Atlantic gyre, a series of four interconnecting currents with different flow characteristics and temperatures.Surface Currents Flow around the Periphery of Ocean BasinsSurface water blown by the winds at point A will veer to the right of its initial path and continue to the east. Water at point B veers right and continues to the west.WindSurface waterNet Direction of Ekman transport45°Direction of motionFrictionThe Ekman spiral and the mechanism by which it operates. Surface Currents Flow around the Periphery of Ocean Basins90° to the right of winddirection is up hereAt 15°N30°–45°Trade windStepped ArtFig. 9-6, p. 237The effect of Ekman spiraling and the Coriolis effect cause the water within a gyre to move in a circular pattern.The movement of water away from point B is influenced by the rightward tendency of the Coriolis effect and the gravity-powered movement of water down the pressure gradient.Surface Currents Flow around the Periphery of Ocean BasinsSurface Currents Flow around the Periphery of Ocean BasinsThe surface is raised through wind motion and Ekman transport to form a low hill. The westward-moving water at B ‘feels’ a balanced pull from two forces: the one due to Coriolis effect (which would turn the water to the right) and the one due to the pressure gradient, driven by gravity (which would turn it to the left). The hill is formed by Ekman transport. Water turns clockwise (inward) to form the dome, then descends, depressing the thermocline.Consider the North Atlantic.Seawater Flows in Six Great Surface CircuitsGeostrophic gyres are gyres in balance between the pressure gradient and the Coriolis effect. Of the six great currents in the world’s ocean, five are geostrophic gyres. Note the western boundary currents in this map.Ocean CurrentsWestern boundary currents – These are narrow, deep, fast currents found at the western boundaries of ocean basins.zThe Gulf StreamzThe Japan CurrentzThe Brazil CurrentzThe Agulhas CurrentzThe Eastern Australian CurrentEastern boundary currents– These currents are cold, shallow and broad, and their boundaries are not well defined.zThe Canary CurrentzThe Benguela CurrentzThe California CurrentzThe West Australian CurrentzThe Peru CurrentBoundary Currents Have Different CharacteristicsBoundary Currents Have Different CharacteristicsThe general surface circulation of the North Atlantic. Unit for measuring flow rates (or volume transported by ocean currents):sverdrups1 sv = 1 million cubic meters of water per secondBoundary Currents Have Different CharacteristicsEddy formationThe western boundary of the Gulf Stream is usually distinct, marked by abrupt changes in water temperature, speed, and direction.(a) Meanders (eddies) form at this boundary as the Gulf Stream leaves the U.S. coast at Cape Hatteras. The meanders can pinch off (b) and eventually become isolated cells of warm water between the Gulf Stream and the coast (c). Likewise, cold cells can pinch off and become entrained in the Gulf Stream itself (d). (C = cold water, W = warm water; blue = cold, red = warm.)Eddies•Form•Migrate• Dissipate (friction)Boundary Currents Have Different CharacteristicsWater flow in the Gulf Stream and the Canary Current, parts of the North Atlantic gyre.Surface Currents Affect Weather and ClimateGeneral summer air circulation patterns of the east and west coasts of the United States. Warm ocean currents are shown in red; cold currents, in blue. Air is chilled as it approaches the west coast and warmed as it approaches the east coast.Surface Currents Affect Weather and ClimateWind induced vertical circulation is vertical movement induced by wind-driven horizontal movement of water.Upwelling is the upward motion of water. This motion brings cold, nutrient rich water towards the surface.Downwelling is downward motion of water. It supplies the deeper ocean with dissolved gases.Nutrient-Rich Water Rises near the EquatorEquatorial upwelling.The South Equatorial Current, especially in the Pacific, straddles the geographical equator. Water north of the equator veers to the right (northward), and water to the south veers to the left (southward). Surface water therefore diverges, causing upwelling. Most of the upwelled water comes from the area above the equatorial undercurrent, at depths of 100 meters or less.Wind Can Induce Upwelling near CoastsCoastal upwelling.In the Northern Hemisphere, coastal upwelling can be caused by winds from the north blowing along the west coast of a ccontinent. Water moved offshore by Ekman transport is replaced by cold, deep, nutriend-laden water. In this diagram, temperature of the ocean surface is shown in degrees Celsius.Wind Can Also Induce Upwelling Coastal DownwellingCoastal downwelling.Wind blowing from the south along a Northern Hemisphere west coast for a prolonged period can result in downwelling. Areas of downwelling are often low in nutrients and therefore relatively low in biological productivity.El Niño and La Niña Are Exceptions to Normal Wind and Current FlowIn an El Niño year, when the Southern Oscillation develops, the trade winds diminish and then reverse, leading to an eastward movement of warm water along the equator.