We will pick up where we left off on Tuesday, with the great circular currents in the surface ocean called Gyres

1.  Gyres in the Ocean-Large-scale winds set up large-scale circular ocean currents called gyres•  See Figures 9-3 and 9-12
a.  Flow is to the right of the wind in the northern hemisphere and to the left of the wind in the southern hemisphere
a. Flow is driven by the Coriolis effect.
b. Gyres consist of ocean currents with distinct properties
e.g.  North Atlantic Gyre, which consists of the Gulf Stream – narrow, fast & warm, and the Canary Current (broad, slower, cold) connected by the North Atlantic and the North Equatorial Currents.
 

2.  Wind driven circulation map:  Figure 9-12   Note circular motion of the gyres.  Note direction of warm water transport, and cold water transport.  Important currents to know (location, cold or warm)
a. the Gulf Stream and the Canary Current
b. the Peru-Humboldt Current
c. the Antarctic Circumpolar Current (aka the West Wind Drift)

Compare figures 9-5 and 9-12 Is there a problem? 

3.  The wind can only reach down and stir the surface oceans to a depth of about 600 meters (deeper in some places than others).  So, the surface circulation patterns will not be seen at depth (below about 600 m) – that is where we see thermohaline circulation patterns.
 
 

4.  Why are surface currents so important?
a. Transport heat from tropics to poles
b. Influence weather, climate, and commerce
d. Distribute nutrients and scatter organisms
 

5.  The Franklin-Folger Map of the Gulf Stream, 1769 --See Figure 9-1
a. Ben Franklin and his cousin Tim Folger mapped the Gulf Stream in 1769
b. Demonstrated transportation efficiency in traveling to England from eastern North America
 

6.  The Kon-Tiki Expedition in the Peru-Humboldt Current
 a.  Thor Heyerdahl’s expedition in a Peruvian sailing raft, with a crew of 6
c. Used the Peru-Humboldt cold water current to reach islands in the Pacific
 

7.  Ernest Shackleton, the James Caird, and the Antarctic Circumpolar Current
 a.  The polar research ship Endurance was smashed in the Antarctic pack ice.
 b.  Ernest Shackleton’s crew escaped to Elephant Island, a rocky island off    Antarctica.
d. Shackleton used his knowledge of the Antarctic Circumpolar Current to rescue the crew by rowing and sailing to South Georgia Island, 800 miles away to the east of Elephant Island in the open ocean, in a small leaky boat, the James Caird.  He made it and all of the crew were rescued.
 

8.  The structure of Gyres is important. 
There are several points to note about them.  One is the “mound” of water in the middle.
a. The gyres have higher elevation in the middle (a “mound” of water) than on the edges-so there are differences in sea level in the gyres.
b.  Sea level varies by up to 2 meters in the ocean, with the steepest slopes in regions of fast currents
c.  High sea level occurs toward the west side  of subtropical gyres
e. Low sea level occurs around the edges of subtropical gyres
 

9.  Why do the gyres have these mounds of water in the middle? 
a. wind driven currents flow perpendicular to the wind direction (that is, 90 degrees off from the wind direction) because of the effect of friction the water layers have on each other.  The direction of that 90 degree turn is always to the RIGHT in the northern hemisphere, and the LEFT in the southern hemisphere.  Look carefully at figure 9.4 and how the water flow changes with depth.
b. This “spiral” effect in the water is called the Ekman spiral.  The net effect of the motion can be seen when you add up all those directions-the net effect is 90 degrees off the wind direction.  This is called Ekman transport.
c. The gyres can maintain their “mound” because of the balance of the Coriolis effect and the “pressure gradient”.  This balance is called Geostrophic Flow.
 

10.  Surface water currents and deep ocean currents work together to drive ocean circulation.  The horizontal movement of water in the surface ocean, driven by wind, can cause vertical motion, too.  When water moves up to the surface, we call it UPWELLING, and when water sinks down, we call it DOWNWELLING.  See Figure 9-10
a. Ocean water masses form at the surface, take on their characteristics, and then some will sink far down to form deep bottom water layers, like North Atlantic Deep Water, and Antarctic Bottom Water. See Figure 9-21
b. The surface circulation and thermohaline circulation are connected.
c. Take a good look at Figure 9.24 to see the global pattern.

11.  Next Lecture:  Ocean Waves
Focus Question:  Why do waves break?