Lecture 11 Wind Driven
Circulation, Part II
Focus Question: How do Surface circulation and thermohaline
circulation work together?
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 Graphics:
Garrison, Fig. 9.1, pg. 235 (pg. 201), and Figs. 9.2 and 9.3, pg. 235 (pg.
202).
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: Garrison, Figure 9.8, pg. 239 (pg.
205). 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)
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
a. Ben Franklin and his cousin Tim Folger mapped the
Gulf Stream in 1769
b. Demonstrated transportation efficiency in traveling to
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
d. Shackleton used his knowledge of the Antarctic
Circumpolar Current to rescue the crew by rowing and sailing to
8. The structure of Gyres is important. Graphic: Garrison, Fig.
9.7a, pg. 238 (pg 204) and Fig. 9.13, pg. 243 (pg.
210).
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?
Garrison 9.5 a,b,c on pg. 237
(pg. 203)
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 9.5 a,b,c, pg. 237, (pg. 203).
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 Figs. 9.16, 9.17, 9.19, and 9.20.
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.
b. The surface circulation and thermohaline
circulation are connected.
c. Take a look at figure 9.22 on page 219 (it is not in 6th edition)
to see the global pattern.
11. Next Lecture: Ocean Waves
Focus Question: Why do waves break?