Chapter 9 Circulation of the Ocean

1. Oceanography An Invitation to Marine Science, 7th Tom Garrison Chapter 9 Circulation of the Ocean

2. 9.1: Mass Flow of Ocean Water is Driven by the Wind and Gravity 2 major types of currents: 1. Surface currents: wind-driven water at or near the ocean’s surface. • Flowing horizontally in the uppermost 1,300 feet • About 10% of the water in the world ocean is involved 2. Thermohaline currents: slow, deep currents that affect seawater below the pycnocline. • Depend on density differences based on temperature and salinity

3. 9.2: Surface Currents Are Driven by the Winds • What are some effects of ocean currents? • Transfer heat from tropical to polar regions • Influence weather and climate • Distribute nutrients and scatter organisms

4. 9.2: Surface Currents Are Driven by the Winds • Surface currents are driven by wind: • Most of Earth’s surface wind energy is concentrated in the easterlies (trade winds) and westerlies. • Due to the 4 forces of - gravity, the Coriolis effect, solar energy, and solar winds - water often moves in a circular pattern called a gyre. • Circulates the ocean surface clockwise in the Northern Hemisphere • Counterclockwise in the Southern Hemisphere

5. Each gyre is a series of four interconnecting currents with different flow characteristics and temperatures. 9.2: Surface Currents Are Driven by the Winds

6. 9.2: Surface Currents Flow around the Periphery of Ocean Basins • Surface water blown by the winds at point A will veer to the right of its initial path and continue eastward. • Water at point B veers right and continues westward.

7. 9.2: Surface Currents Flow around the Periphery of Ocean Basins • 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.

8. 90° to the right of wind direction is up here At 15°N 30°– 45° Water continues clockwise? Trade wind Direction of water movement Stepped Art Fig. 9-6, p. 236

9. 9.2: Seawater Flows in Six Great Surface Circuits • Geostrophic 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. 1. North Atlantic Gyre 2. South Atlantic Gyre 3. North Pacific Gyre 4. South Pacific Gyre 5. Indian Ocean Gyre

10. 9.2: Seawater Flows in Six Great Surface Circuits • 6. West Wind Drift (Antarctic Circumpolar Current): not around the periphery of a basin, flows endlessly eastward around Antarctica, driven by powerful westerly winds

11. 5 Geostrophic Gyres

12. 9.2: Boundary Currents Have Different Characteristics • There are 4 major currents found within each geostrophic gyres. • Eastern Boundary Current • Western Boundary Current • Northern Transverse Current • Southern Transverse Current

13. 9.2: Boundary Currents Have Different Characteristics • Eastern boundary currents –These currents are cold, shallow and broad, and their boundaries are not well defined. Found at eastern boundaries of ocean basins. • The Canary Current, Benguela Current, and the California Current

14. 9.2: Boundary Currents Have Different Characteristics • 2. Western boundary currents –These are narrow, deep, fast currents found at the western boundaries of ocean basins. • The Gulf Stream, Japan Current, and the Brazil Current • Westward intensification: Increase in speed of geostrophic currents as they pass along the western boundary of a basin.

15. 9.2: Boundary Currents Have Different Characteristics SUMMARY: Which currents are warmer? Colder?

16. 9.2: Boundary Currents Have Different Characteristics The numbers indicate flow rates in sverdrups. • 1 sv = 1 million cubic meters of water per second • 55sv = ?? SUMMARY: Which currents move faster? • Eastern? • Western?

17. 9.2: Boundary Currents Have Different Characteristics Eddy: meanders that form turbulent rings of water that trap cold or warm water and separate them from the main flow. The 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.)

18. Gulf Stream from space. • One of strongest WBC, 5mph and 43 miles wide. • #3 and #4 indicate cold and warm-core eddies

20. 9.2: Boundary Currents Have Different Characteristics • Transverse currents: Currents that flow east to west or west to east that connect the eastern and western boundary currents. • Northern Transverse Current or Southern Transverse Current

21. 9.2: Boundary Currents Have Different Characteristics • Countercurrents: currents that flow the opposite direction from the main current • Equatorial region caused by doldrums • Undercurrents: countercurrents that are beneath the surface currents.

22. 9.6: ThermohalineCirculation Affects All the Ocean’s Water • In the deep-ocean layers, water density variation, not wind, is the primary cause of current. • Thermohaline circulation: the movement of water due to different densities • When water becomes dense, it sinks, flowing downward. • This means water in some other place must rise to replace it, causing an upward flow.

23. 9.6: ThermohalineCirculation Affects All the Ocean’s Water • There are five common water masses based on differences in density: • Surface water – top 200m • Central water – to the bottom of the thermocline • Intermediate water – to about 1500m • Deep water – to about 4000m • Bottom water – in contact with seafloor

24. 9.6: The Formation and Downwelling of Deep Water Occurs in Polar Regions • The intermediate, deep, and bottom water masses form primarily, but not entirely, at high latitudes (around 70° North and South).

25. 9.6: The Formation and Downwelling of Deep Water Occurs in Polar Regions • Antarctic Bottom Water (AABW) : the densest water in the ocean (30oF) • Forms off the Antarctic coast during winter • Sinks to the bottom and begins to travel towards the equator. • North Atlantic Deep Water (NADW): dense bottom water formed at North Pole

26. 9.6: The Formation and Downwelling of Deep Water Occurs in Polar Regions • The enormous water quantities sinking at the poles and create the thermohaline circulation pattern. • Dense water descends and travels towards the equator. • As it warms, it rises and enters wind-driven currents and is carried to the poles. • There it cools, becomes more dense, and sinks again, repeating the process.

27. 9.6: ThermohalineFlow and Surface Flow: The Global Heat Connection • The global pattern of deep circulation resembles a vast “global conveyor belt” that carries surface water to the depths and back again. • The ocean conveyor belt is important because it moderates the world’s climate.