Download Air-Sea Interactions

OS 101—Marine Environment
Winter 2007
Air-Sea Interactions
I.
Solar Energy Received by Earth
A. Introduction
- The atmospheres and oceans are very similar in that they are both well
described by fluid mechanics, or the movement of liquid bodies
- However, the oceans are about 8000x as dense!
- Therefore, we have “weather” in the oceans just like in the atmosphere, but
it’s:
- slower
- contains more energy
- lasts longer
B. Sunlight
- The primary source of all energy on earth
- other sources we have discussed: radioactive decay, chemosynthesis
- Sunlight, by itself, is not enough…we also have the greenhouse gasses
- b/c of this, the average temperature of the planet is raised from ~ -18°C to
15°C, due to the “trapping” of heat
- There are many greenhouse gases, but water vapor accounts for 75% of
the trapped heat
- Most of the light hitting the earth is in the UV/visible spectrum, but most of
the electro-magnetic energy that leaves the earth is shifted towards the IR
- This is because of something called:
Blackbody Radiation: all objects emit electro-magnetic radiation, and the
wavelength emitted is dependent on the temperature of the object (hot
objects glow, really hot objects turn blue-white).
-
The earth is essentially of constant temperature (15°C), so it emits energy
uniformly!
The atmosphere and clouds don’t absorb visible/ultraviolet light very well, so
only about 23% of the sunlight hitting the earth is absorbed by the atmosphere
Although water vapor accounts for the greatest greenhouse warming, there are
other important gases. carbon dioxide is the most important gas which
humans have an affect on (anthropogenic changes)
- First noticed by Ralph Keeling, at the Mauna Loa (Hawaii) observatory:
CO2 in the atmosphere has increased steadily since the 1950s
- This has been extended backward in time by ice-core records
-
This is also confirmed by looking at other anthropogenic gases, such as
methane.
We all assume that this is causing global warming, and that it’s caused by
human intervention
Global Warming
- NOT the same as the greenhouse effect!
- We think it’s getting warmer, but it’s actually very hard to measure
- Two factors make atmospheric modeling difficult:
- water vapor, and aerosols
Trends
- Temperature Global temperatures are rising. Observations collected over
the last century suggest that the average land surface temperature has risen
0.45-0.6°C (0.8-1.0°F) in the last century.
- Precipitation Precipitation has increased by about 1 percent over the
world's continents in the last century. High latitude areas are tending to see
more significant increases in rainfall, while precipitation has actually
declined in many tropical areas.
- Sea Level Sea level has risen worldwide approximately 15-20 cm (6-8
inches) in the last century. Approximately 2-5 cm (1-2 inches) of the rise
has resulted from the melting of mountain glaciers. Another 2-7 cm has
resulted from the expansion of ocean water that resulted from warmer
ocean temperatures.
Fact or Fiction? http://www.marshall.org
- we know it appears to be getting warmer, but:
- Satellite and land temperatures don’t match
- bad satellite data (decaying orbits)
- Urban Heat Island Effects
- Uncertainties in the models
- aerosols are critical, but very hard to model
- errors could account for the changes in temperature
- It might not be so bad
II.
Distribution of Solar Energy
-
More solar energy is received at the equator, due to the angle of the sun, and
the intensity of sunlight is much higher because the same amount of energy
isn’t as spread out
Radiation at high latitudes must also pass through more atmosphere (that’s
why sunsets appear red)
At the poles, more energy is reflected, because of the reflectivity (albedo) of
snow/ice, and also because of the angle
Earth is tilted 23.5°….this has major effects on our climate!
The Tropic of Cancer, Tropic of Capricorn are at 23.5° N-S of the equator
-
III.
Heat Flow
-
IV.
- At the summer solstice, the sun is directly overhead at the T. of Cancer
- At the winter solstice, the sun is directly overhead at the T. of Capricorn
Arctic/Antarctic Circles are at 66.5 ° (N-S), which is 90 minus 23.5
- Boreal Winter, sun never crosses the Arctic Circle
- Austral Winter, sun never crosses the Antarctic Circle
Combining all of this information, we know that the band between the Tropics
gets more direct sunlight, lower angle of incidence, so heats up more
According to the Blackbody Radiation Law, outgoing radiation will be
essentially uniform
Therefore, the planet gains heat at the equator, and loses heat at the poles
- from measurements, at about 35° N or S, go from a heat sink to source
- Heat is transferred from low latitudes to high latitudes!
Heat Budgets
-
A Heat Budget is simply a description of the sources/sinks and fluxes of heat.
“Q” refers to the heat flux
Rate of Heat Gain – Rate of Heat Loss = Net rate of loss/gain
(Qsun + Qcurrent) – (Qradiation + Qevaporation + Qconduction) = Qnet
-
Can be calculated for any area, but we make the simplifying assumption that
for the earth, over thousands of years, it’s essentially constant, so:
Qsun = Qradiation + Qevaporation + Qconduction = 100%
Qsun = 46%
-
V.
+
53%
+ 6%
Remember that the atmosphere is 8000x less dense than water, so if we want
to move heat around, we use the atmosphere
As water evaporates, ocean H20 molecules displace O2 and N2 molecules in
the atmosphere…this makes the air less dense, and it rises. As it rises, the
water vapor condenses, releasing the latent heat of condensation, which
warms the air even more, so it keeps rising (think of a hurricane)
Idealized Atmospheric Circulation and the Coriolis Force
-
At the equator, heat rises, forming a low pressure zone
As it cools, it sinks forming high pressure zones—at about 30° (The Horse
Latitudes)
The poles have high pressure due to the cold (condensed) air
Between the poles and 30°, there is another “cell”
-
Air moves from High to Low pressure, so we get winds
If the earth were uniform (all water or all land), and not spinning, that’s all
there would be….two factors change this idealized circulation
1) Coriolis Force
- As the air rises and sinks, it tends to move to the right (in the N hemisphere)
- This is caused by differential rotation speeds. At the poles, the rotation falls to
zero. At the equator, rotation is maximum
- Can be described as:
Coriolis = 2Ωsinθ, where theta refers to the latititude
-
The East-West deflection is caused by Centripetal force….basically, if you
move due east, you are moving faster than the earth is spinning. To
compensate, you must move further away from the center of rotation (the
pole), so you move towards the equator. Since the earth is a sphere, that also
moves you away from the axis, and reduces the centripetal force
-
In a high pressure zone, the air is moving downward and turning to the right,
so you get clockwise (anticyclonic) rotation
In a low pressure zone, the air is moving up and turning to the right, so you
get counterclockwise (cyclonic) rotation
-
2) Differential heat capacity
- most of the continental land mass is in the northern hemisphere
- land heats up and cools off more than water
- the earth is tilted, so we get seasonality (winter/summer)
- so, you end up with strong high pressure (cold, compressed air) zones over the
continents in the winter, and strong low pressure (warm air) over the
continents in the summer
Winds
-Combining all of that, we get major wind patterns:
Trade Winds
Westerlies
Polar Easterlies
VI.
Ocean Climate Zones
-
Just like land, we can describe the oceans as divided into “climate” zones
It falls into latitudinal bands
o Equator: lots of heating, vertical air movement, high humidity. This results
in low winds, heavy precipitation, and low surface salinity
o Tropical: strong trade winds, high pressure, dry air, high evaporation. This
results in high surface salinity.
VII.
o Subtropical: high evaporation and salinity, weak winds and currents
(except for boundary currents)
o Temperate: strong westerly winds, high precipitation, violent storms
o Subpolar: winter sea ice, icebergs, water near freezing
o Polar: permanent ice
These patterns are relatively stable, and are controlled by the heat budget and
the wind patterns
Other Forms of Air-Sea Interactions
1) Fog
a. One kind of fog forms over land on clear nights, particularly in winter
over areas with high humidity during the day.
b. Coastal Fog: warm, moist air moves from a warm to cold water region.
c. Sea Smoke: cold air moves over warmer (about 10°C warmer) water
2) Sea Ice
a. Sea ice is frozen seawater. Icebergs are freshwater glacier chunks
b. As sea ice forms, it tends to extrude salts. This lowers the freezing
point of the water left behind, and it sinks. The warmer, fresher water
rises to replace it, producing more ice
c. Sea iceslushpancake iceice floes
d. Arctic ocean has pack ice, which forms around the margin or near the
shore of that ocean (reaches 2 m thick)
e. Polar ice covers most of Arctic (never melts completely), can reach 50
m thick
f. Fast ice connects the pack ice to the shore, and melts during summer
g. Antarctic is mostly pack ice, because there’s a continent there