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Explainers: Air-Sea Interaction
Ocean-atmosphere interaction is a critical
component of oceanography. In many ways, the
ocean and atmosphere are just two parts of the
same coupled system. Both are subject to similar
physical fluid dynamics, and the state of each is
highly dependent upon the other. The ocean has
a strong influence on atmospheric conditions,
and the physical and chemical properties of the
atmosphere affect the currents, temperature, and
chemistry of the ocean.
I. Heat Transfer
An important function of atmosphere-ocean
interaction is the way that heat becomes
distributed around the planet through the
processes of evaporation, precipitation, wind and ocean currents. Your textbook (section 6.2)
has good diagrams and explanations of the variations in solar heating over the earth, and the
site Earth’s Climate Machine has a good explanation of the way heat moves around via winds
and currents. You may want to visit this diagram about latent heat also.
For you to think about:
A) Which regions of the earth receive more solar energy than they emit back -- in other words,
which regions experience a net GAIN of heat energy? Which regions experience a net LOSS
of heat?
B) How can the latent heat of evaporation move heat away from regions of net heat gain and
add heat to regions that have a net loss of heat? The answer includes the concepts of
evaporation, precipitation, and wind circulation!
II. The Coriolis Effect
The Coriolis effect is a very important concept for understanding the circulation of large scale
winds and ocean currents. It is also tricky to understand! Watch this video of earth scientists
being asked to explain it on live camera, and the lab demonstration of how it works.
BBC: Scientists Explain the Coriolis Effect (note: “roundabout” is British for merry-go-round)
You may want to also review the PDF handout Understanding the Coriolis Effect, or this
merry-go-round animation.
The earth’s surface is not stationary. It is rotating eastward, one rotation every 24 hours. This
is why we have night and day. When an object is launched into the air, it will continue to move
with the same velocity and direction as when it was on the ground: in other words, it keeps
moving at the same velocity as the earth below it.
If it didn’t, look what would happen if you tossed up a juggling ball:
• The earth, you, and the ball are all moving (with earth’s rotation) at, say, 720 km/hr. (It
actually depends on what latitude you’re at.)
• You toss the ball up and it is in
the air 3 seconds before you
catch it.
• In 3 seconds, you and the earth
have rotated for 3 seconds at
720 km/hr. How far is that?
Convert the speed to km/
second: 720 km/hr ÷ 60
minutes/hr ÷ 60 seconds/min
= 0.2 km/sec. Multiply by 3
seconds: 0.2 km/sec x 3
seconds = 0.6 km.
• You would be 600 meters from
where your ball landed!
Brain teasers to think about:
A) A launcher shoots a ball vertically, straight up in the air, from two different locations: one at
the equator and one at 60° North. It takes the ball an hour round-trip to reach its highest
altitude and come back down again. How far and in what direction has each ball moved
horizontally? (Hint: how far does a point on the earth rotate in an hour?)
For reference, you’ll need to know:
• circumference of the earth at the equator Ce = 40,000 kilometers
• circumference of the earth away from equator = Ce x cosine(LATITUDE)
(the circumference at the equator times the cosine of the latitude)
• earth rotates eastward 360° in 24 hours
• cosine(60°)=0.5
Now, just like when an object is
launched straight up vertically, if it
is launched horizontally it will not
only move in the direction it is
launched toward, it will also
move with the same velocity
and in the same direction as
the earth below it. Its course
will be the result of the SUM of the
two velocities, like at the right:
Brain teaser B)
The launcher now shoots the ball from the equator toward a target directly north at 60° N
latitude. By the time it lands at 60° N, it has been in the air for 5 hours. How far east or west
from its target does it land?
The key here is that in addition to going north in the direction it is launched, the ball also goes
east with the velocity it has from the earth’s rotation. However, the ground underneath it is
moving eastward more slowly the further north it goes. So it will end up overshooting the
target. Hints: To figure out how far it over- or under-shoots, you will need to: 1) calculate how
far east the velocity at the equator would take the ball, 2) calculate how far east the velocity at
60°N would move the target, and 3) find the difference between the two.
III. HURRICANES
The power of hurricanes is fed by warm ocean water. The video that follows, from NASA,
illustrates with satellite imagery and computer modeling what occurs to cause hurricanes to
develop and grow.
The latent heat of evaporation is one of the key processes. Remember this is the large quantity
of heat energy that is required for water to make the phase change from liquid to vapor. That
energy is stored as latent heat and when the vapor condenses back into liquid water (like cloud
droplets or rain) the heat is released back into the environment. With the huge quantity of
evaporation and rainfall associated with hurricanes, that is a LOT of latent heat.
Remember from the discussion of earth’s mantle what happens when materials are heated?
The molecules move faster, collide more, take up more space, and the material becomes less
dense. Less dense layers are more buoyant, so they rise (or the more dense layers sink). The
same applies to air masses. When lots of latent heat is released by condensation of water
vapor, the air mass becomes less dense and tends to rise.
You see this even in local
thunderstorms: the clouds
form from warm, humid air
(like when we have air masses
move in from Baja California).
As the vapor condenses it
releases latent heat which
causes the air in the cloud to
rise. As it rises, the high
altitude surroundings cool,
and this causes more water to
condense, releasing more
latent heat. This results in
more buoyancy, and the
whole process repeats until no
more moisture can be
condensed out of the air. The
process of condensing, heat release, and rising results in tall thunderhead clouds, often with
flat tops where the process stops.
Watch the NASA video. (If link does not work, enter this in your browser:
http://svs.gsfc.nasa.gov/vis/a000000/a003200/a003228/hurricanes_640x480.mpg. It may
play better if you download it to your hard drive first.)
Pay particular attention to these sections of the video:
•
•
•
•
What is meant by the “heat engine” of hurricanes?
How does the narrator describe rain? What does he mean?
What happens to the ocean in the wake of a hurricane?
How can this slow down or prevent the development of another hurricane?
Hurricane/Superstorm Sandy, U.S. East Coast October 2012
Hurricane Sandy was an extremely unusual event. So unusual that meteorologists don't even
have a name for what it eventually became. It started as a large hurricane, in the usual
manner, traveling up the U.S. east coast. But at the point where it would normally have drifted
out to sea in the Atlantic, it turned west and merged with a very large winter-type storm, called
in this part of the world a Nor'easter ( because the dominant wind direction over land is from
the northeast).
Merging storms are not all that unusual.
What was unusual in this case was that
the two storms were of opposite types,
both were very large to start with, and
they fed each other in a unique way.
Hurricanes, as we've seen, get their
energy from warm ocean water. Winter
storms get their energy from the pressure
gradient that forms between cold, dry air
from the polar regions and warmer, more
humid air from the south. At the point
where Hurricane Sandy would typically
have lost energy as it approached land, it
instead bumped up against this huge
winter storm front. That made the
pressure gradient even bigger, as the cold
polar air collided with the vast energy of warm humid air in the hurricane. In addition, several
other unusual conditions aided this process, and also made the effects on the coastline more
severe. One of the big lessons from Superstorm Sandy is that these unusual conditions are
ones that are expected as a result of our warming climate due to the burning of fossil fuels.
For more pictures and info about Sandy, and why it was so darn big, see
http://iod.ucsd.edu/~raz/OceanWeb/hurricane_sandy.html
Explainer Answer Guide
Ia.
Net heat Gain: occurs at approximately 40°S – 40°N, Net heat Loss at 40°-90° N/S
Ib. How can the latent heat of evaporation move heat away from regions of net heat gain and
add heat to regions that have a net loss of heat?
• Energy from the sun is absorbed by ocean water, warming it
• More energy is stored in the water molecules as latent heat as it makes the phase change
from liquid to gas (evaporates) from the ocean
• Wind blows the air and water vapor to cooler latitudes (or higher altitude)
• Water vapor condenses into clouds/rain and releases latent heat into cooler area.
IIa.
At the Equator, in 1 hour the ball travels 1/24th of a compete rotation, so the distance is
40,000 km ÷ 24 = 1667 km. Earth is rotating eastward, so it's 1667 km in one hour, to the
EAST.
At 60°N, the circumference around the earth is equal to the circumference around the equator
(40,000 km) times the cosine of the latitude. We have cosine(60°)=0.5, so the circumference
at 60°N is: 40,000 km x 0.5 = 20,000 km. So here the ball would travel 20,000 km ÷ 24 hrs
= 833.5 km in one hour, also to the EAST.
IIb.
To figure out how far: 1) calculate how far east the velocity at the equator would take the ball,
2) calculate how far east the velocity at 60°N would move the target, and 3) find the difference
between the two.
The ball launched from the equator has an eastward velocity of 1667 km/hr (from 2a) so it will
have gone east 1667 km/hr x 5 hrs = 8335 km. (Or you can say it has travelled 5/24 of a
rotation: 5 ÷ 24 x 40,000 km = 8335 km.)
The target has also moved with the rotating earth, but it’s moving at 833.5 km/hr, so it will
have gone east 833.5 km/hr x 5 hrs = 4167.5 km. (Or 5/24 of a rotation, where one full
rotation is only 20,000 km: 5 ÷ 24 x 20,000 = 4167.5 km)
By the time the ball reaches the latitude of the target, it has gone 8335 km east, and the target
has gone only 4167.5 km east, so the difference is 8335 – 4167.5 = 4167.5. It’s overshot the
target by 4167.5 km to the east.
In summary:
•
•
•
•
•
Target is moving only half as fast as something at equator (20,000 km/day vs 40,000 km/
day, or 1667 km/hr and 833 km/hr)
Ball sets off with eastward velocity of 40,000 km/day or 1667 km/hr
Ball travels east for 5 hours at 1667 km/hr = 8335 km east.
Target at 60° only goes half as fast (20,000 km/day), covering only half that distance:
4167.5 km east.
The amount it misses by is the difference: 4167.5 km to the east.
IIIa. The heat engine is what the narrator calls the process in which warm ocean water,
evaporation, and latent heat release powers hurricanes.
• warm ocean water evaporates
• latent heat is stored in vapor
• the warm moist air rises and the vapor condenses
• condensation releases latent heat
• the released heat causes further rising, and the cycle continues
IIIb. The narrator calls rain the “by-product” of the hurricane heat engine
• The “heat engine” is the process of warm ocean water evaporating, water vapor
condensing in the hurricane clouds, and latent heat being released that fuels the rising air
more
• Rain is just what falls out of the cloud as this process is going on -- the real action is in the
evaporation, condensation, and release of heat. Rain is just the “leftovers”.
NOTE: rain is the result, not the cause of condensation. Two things occur simultaneously
when vapor condenses: liquid water droplets are formed, and latent heat is released. When the
water droplets accumulate and get big enough, they fall as rain.
IIIc.
• The wake of a hurricane is where the warm surface ocean water has been disturbed by the
strong wind and waves, which can stir up colder water from below.
• This leaves colder water behind it, so there is less heat energy to fuel hurricanes that
follow