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Currents, Climate, and Life Unit (Topic 9A-2) – page 1
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Currents, Climate,
and Life Unit
Currents and Climate
Warm currents typically warm the air above them, and cold currents typically cool the air above
them. In addition, warm water evaporates more easily than cold water, making the air above
more humid. As we saw in the last section on the atmosphere (8A), if this air rises, it will cool
and its moisture will condense into rain. Thus, the southeast coast of the United States is more
humid and gets more rain than the southwest coast of the United States, because they live next to
the warm Gulf Stream while we live by the cool California Current.
Warm water and cold water do not make the local climate “warm” or “cold;” they make it
warmer or colder and more humid or less humid than it would otherwise be. So, the cold water of
the California Current does not make California a cold place, but it keeps California from being
even warmer and more humid. I don’t know about you, but I dislike humidity and think
California is warm enough already. Thank you, California Current!
As we’ve seen the gyres carry warm water towards the Poles in western boundary currents and
cold water towards the Equator in eastern boundary currents. Thus, the motion of the ocean (lots
of fun to say aloud) keeps the Poles from becoming too cold and the Equator from becoming too
hot, making them both nicer places to live.
1. How does a warm current affect the climate along a nearby coast?
Does it make it warmer or cooler? Does is make it more humid or less humid?
2. How does a cold current affect the climate along a nearby coast?
Does it make it warmer or cooler? Does is make it more humid or less humid?
3. Overall, do ocean currents warm or cool the Equator?
Do ocean currents warm or cool the Poles?
Currents, Climate, and Life Unit (Topic 9A-2) – page 2
Upwelling and Downwelling
Upwelling occurs when ocean currents bring water upwards from below, and downwelling
occurs when ocean currents push surface water downwards.
Downwelling occurs where winds cause ocean currents to come together (to “converge”) or run
into land. The water has nowhere to go, so it starts piling up, raising the sea surface. Eventually,
the weight of the “hill” of water starts pushing the water below it downwards (causes
“downwelling”).
Upwelling occurs where winds cause ocean currents to move apart (to “diverge”) or pull water
away from the land. This creates a “hole” or “gap” in the surface of the ocean. Eventually, it
becomes so deep that water from below starts rising up to replace the water that is pushed away
by the winds. (Water sinks on either side of the gap owing to the higher pressure: more water =
more weight pressing down. The water that is already below the surface has to go somewhere,
so water rises where the pressure is lower.)
The upwelling water is cold (it comes from below ), so it does not “want” to rise owing to its
higher density; ocean currents have to force the water to rise. If the ocean currents stop or change
direction, the cold upwelled water will sink back down again.
Notice that the sea surface goes up where there is downwelling, and goes down where there in
upwelling. Remember that the change in the sea surface CAUSES upwelling or downwelling. In
other words, deeper water is REACTING to changes at the surface.
Upwelling can occur for reasons other than those discussed so
far. For example, an ocean current can run into an underwater
hill (a “seamount”) and be forced up and over it. We do not
have time and space here to discuss all the causes of upwelling,
but please be aware that other causes exist.
seamount
Currents, Climate, and Life Unit (Topic 9A-2) – page 3
4. What is downwelling?
5. Does downwelling happen where sea level goes up or down?
6. What causes the sea level to change and produce downwelling?
7. Why does the change in sea level cause downwelling?
8. Does upwelling happen where sea level goes up or down?
9. What causes the sea level to change and produce upwelling?
10. Why does the change in sea level cause upwelling?
Currents, Climate, and Life Unit (Topic 9A-2) – page 4
Effects of Upwelling
Upwelling water is colder and saltier than surface water, making it possible for oceanographers
to detect where and when upwelling occurs. It is also rich in nutrients, so upwelling causes the
growth of lots of phytoplankton (who need the nutrients to carry out photosynthesis and for their
shells) who become food for zooplankton. Zooplankton, of course, are in turn food for small fish,
and small fish are eaten by larger fish.
Note: Deep water tends to have more nutrients, because there are no phytoplankton down deep
using them up, and new nutrients are added by bacteria decomposing (“breaking down”) the
dead, decaying matter sinking down from the surface.
Upwelling Zones
Upwelling is common in many places throughout the
world. I would like you to know about 3 of the most
important upwelling zones:
• along the west coasts of the continents =
on the east sides of the oceans (in the subtropics)
• at the Equator
• in the middle of the Southern (Antarctic) Ocean
11. Does upwelling make the water at the surface of the ocean warmer or colder?
12. Does upwelling make the water at the surface of the ocean fresher (less salty) or saltier?
13. Does upwelling increase or decrease the amount of nutrients at the surface of the ocean?
14. Why does the water below the surface typically have more nutrients than the water at the
surface?
15. True or false? “Fish are more abundant in upwelling zones.”
Currents, Climate, and Life Unit (Topic 9A-2) – page 5
16. True or false? “Upwelling is typically more common along the west coasts of
continents.”
17. True or false? “The warm water of the Equator prevents upwelling along the Equator.”
18. True or false? “Upwelling occurs in middle of both the Arctic Ocean and the Antarctic
Ocean.”
Winds and Currents along the Coast of California
In this section, I want to review the winds and currents along the coast of California, something
that I think all of my students should be familiar with by the end of the course.
The westerlies push water to the east across the Pacific Ocean. By the time it reaches the coast of
California, this water has cooled down, and it turns to the south, becoming the California
Current. This water helps cool the climate of California (it does not make the climate warm, but
keeps it from being even warmer than it is now) and helps the reduce the amount of evaporation,
making the coastal air unusually dry for a coastal environment. The current begins to leave the
coast near southern California (Santa Barbara) where the trade winds begin to push it westward
across the ocean. This allows warmer water to sneak northward up the coast to southern
California, where it meets and mixes with the cooler water from the north, producing turbulence
which stirs up nutrients and ultimately leads to more fish. Southern California has a very
complex and constantly shifting ocean environment.
Coastal winds tend to blow south along the coast of
California particularly during the spring and summer. (The
land is warmer than the ocean, so the warmer air over the
land rises, and the cooler air from the ocean comes in to
replace it. However, the Coriolis effect and coastal
mountains cause this air to bend towards the south along the
coast.) The coastal winds push water (including the
California Current) away from coast, and cold, nutrient-rich
water from down deep come up to replace it (“upwelling”).
The nutrients fertilize algae like phytoplankton, allowing
them to carry out photosynthesis and grow. The
phytoplankton become food for zooplankton who in
Winds = Green Arrows with Tails
turn are food for fish. Thus, upwelling creates
Ekman Transport = Purple Dashed Arrows
Ocean Currents = Solid Arrows, No Tails
abundant life along the coast of California.
Currents, Climate, and Life Unit (Topic 9A-2) – page 6
During the winter, coastal winds tend to blow northward along the coast. These winds can create
the “Davidson Current,” warm water flowing northward along the coast of California, and shut
down coastal upwelling. (The California Current does not stop or reverse; it is pushed away from
the coast.)
19. Does the California Current carry water to the north or south?
20. Is the California Current a warm current or a cold current?
21. How does the California Current affect the climate of California?
22. Does upwelling occur along the coast of California?
23. What is the Davidson Current?