Chapter 9: Circulation of the Ocean

Transcription

1 Chapter 9: Circulation of the Ocean Tropical gardens on Britain s Scilly Isles. Only 48 kilometers (30 miles) off the coast of Cornwall at 50 N, these scenic islands lie in the path of the warm waters of the Gulf Stream. 1

2 Learning Goals: Circulation of the Ocean Ocean circulation is driven by winds and by differences in water density. Winds and ocean currents distribute heat worldwide. Surface currents: driven by wind, they move in circular patterns (gyres) around the edges of major ocean basins. Surface = top 100 meters Upwelling supports the most productive fisheries. Thermohaline currents: slow, deep currents that affect the bulk of seawater below the pycnocline. El Niño and La Niña affect ocean and atmosphere. In El Niño years, the normal wind and current flow changes near the coast of Peru (and influences the whole world). 2

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4 Winds, driven by uneven solar heating and Earth s spin, drive the movement of the ocean s surface currents. Two types of winds are most important to ocean circulation: 1) powerful westerlies and 2) persistent trade winds (easterlies). 4

5 Compare the two figures. 5

6 Hurricanes move with trade winds then westerlies at higher latitudes. Cannot form on equator (air is too still here). 6

7 Another The example low pressure of isobars cell in the northeast is Hurricane Sandy. 7

8 Hurricanes Low pressure over ocean picks up evaporation from the ocean. 8

9 In the Northern Hemisphere, hurricanes (tropical cyclones) turn counterclockwise, as winds become deflected to the right. In Southern hemisphere, winds spin opposite. 9

10 Two major things are needed for hurricane development: 1) High humidity 2) High temperatures (>79 F) Red zones = where hurricanes develop 10

11 Compare the two figures. 11

12 Winds, driven by uneven solar heating and Earth s spin, drive the movement of the ocean s surface currents. Two types of winds are most important to ocean circulation: 1) powerful westerlies and 2) persistent trade winds (easterlies). 12

13 Northern Hemisphere: Coriolis Effect moves surface waters to right of winds. 13

14 Gyres surface currents circulating around ocean basins Four forces cause gyres: 1.Surface winds 2.Sun s heat 3.Coriolis effect 4.Gravity 14

15 For example, the North Atlantic Gyre includes four currents. 15

16 Surface currents can be illustrated by sea surface temperature. Earth has 2 northern gyres, 4 southern. North Pacific Gyre North Atlantic Gyre South Pacific Gyre South Atlantic Gyre Indian Ocean Gyre Circum - Antarctic Gyre Red: Celsius (77 82 F). Yellow: C (68 77 F); Green: C (59 68 F); Blue: 0 15 C (32 59 F). Purple: below the freezing point of fresh water. Note the distortion of the temperature patterns we might expect from the effects of solar heating alone the patterns twist clockwise in the Northern Hemisphere, counterclockwise in the 16 Southern.

17 Surface currents, summarized with names and usual directions. Waters brought from equator warm-water current poles cold-water current 17

18 Ekman spiral two examples of what happens 1. How water is transported in the North Atlantic (as one example of an ocean gyre) 2. Upwelling vs. downwelling 18

19 Ekman spiral and Ekman transport: how the process works Coriolis Effects felt increasingly smaller below sea level. Net Effect 90⁰ motion of winds. 19

20 Ekman Spiral circular motion caused by geostrophic flow. Hill pushes mixed layer deeper. 20

21 Center of hill not at basin center as expected. 21

22 If the Coriolis effect didn t exist, ocean gyres would look like this: Because of the Coriolis effect, ocean gyres are curved : 22

23 If the Coriolis effect didn t exist, ocean gyres would look like this: Because of the Coriolis effect, ocean gyres are curved Key points: The Coriolis effect is strongest near the poles. Red arrows: Water flowing eastward at high latitudes turns sooner to the right (doesn t make a square) Green arrows: water flowing westward near the equator doesn t turn right until it hits a continent 23

24 Western boundary currents are stronger than eastern boundary currents. 24

25 Sverdrup 1 million m 3 of flow per second. Western part of gyre has stronger, narrower, deeper flow. Eastern section has weaker, wider, shallower flow. 25

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27 Upwelling Cold, nutrient-rich waters brought from below. Downwelling warm, nutrient-poor waters forced down. 27

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31 Slightly off equator with minimal Coriolis, upwelling caused by trade winds. 31

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33 Upwelling: cold water brings nutrients to the surface. California Current Canary Current Humboldt Current. Benguela Current There are four major upwelling zones, and they are eastern boundary currents: Where are they? 33

34 El Niño and upwelling Normally trade winds bring upwelled nutrientrich waters to Peru/Ecuador. 34

35 El Niño - When trade winds slow, upwelling stops. 35

36 A non-el Niño year 36

37 An El Niño year 37

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39 Deep bottom waters - Arctic caused by cooling waters 39

40 Thermohaline circulation also known as the global conveyor belt Fundamental process: Gulf Stream & conveyor belt: Conveyor belt animation (takes 1,000 years) 40

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43 Thermohaline circulation (temperature and salinity) influence sub-surface circulations. Distributes excess Earth s heat. 43

44 Deep bottom waters Antarctic caused by freezing ice increasing salinity 44