oceanography quotations
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ch_6_modified_lecture_slides.pptxChapter 6
Air – Sea Interactions
Weather = present state of the atmosphere
Climate = long term average weather in a particular area
The FIRST picture sent back from the International Space Station http://www.technochitlins.com/archives/space /
Heat in = heat out when averaged for the whole world (ignoring global warming)
Earth has maintained an average temperature of 16° C for many thousands of years.
BUT…
There are significant departures from this balanced heat budget model!
http://connect.in.com/snowman/images.html
http://image-go.net/palm-tree-pictures.html
Departures from the balanced heat budget:
1) variation in solar heating with latitude. You know it is warmer in the tropics and colder near the poles. Here’s why – the angle the sun’s rays hit the Earth:
a) The sun’s rays hit the surface at 90 ° in the tropics,
1) concentrates the energy
2) less reflection of light back to space
3) less time in atmosphere = less absorption and back scattering
b) lower angle of incidence near the poles,
1) spreads the solar energy over a larger area
2) more light reflected back into space
3) more time in the atmosphere = more backscattering / absorption
4) snow and ice at the poles reflects the sun back to space
Fig. 6.3, page 167
Winds and ocean currents move heat from the tropics (too much solar heating) to the poles (too little solar heating.
Departures from the balanced heat budget:
1) variation in solar heating with latitude – the angle the sun’s rays hit the Earth.
2) the seasons.
Here’s why – the 23.5 ° tilt of the Earth on its axis of rotation.
Fig. 6.2, page 164
Fall (Autumnal) Equinoxes = the first day of fall
equal day and night everywhere
the sun is opposite the equator.
Winter solstice = the first day of winter.
North has the shortest days and the longest nights.
South has longest days and shortest nights (and summer).
The sun is opposite 23.5 ° S.
North of the Arctic Circle at 66.5 ° N, there is 24 hour darkness,
South of the Antarctic Circle at 66.5 ° S, there is 24 hour light.
Spring (Vernal) Equinox = the first day of spring
equal day and night everywhere
the sun is opposite the equator.
Summer solstice = the first day of summer
North has the longest days and the shortest nights.
South has the longest nights and the shortest days (and winter).
The sun is opposite 23.5 ° N.
North of the Arctic Circle at 66.5 ° N, there is 24 hour light,
South of the Antarctic Circle at 66.5 ° S, there is 24 hour darkness.
Departures from the balanced heat budget:
1) variation in solar heating with latitude.
2) the seasons.
3) the land responds to seasonal variations more drastically than the ocean does.
a) the ocean has thermal inertia – it resists changes in temperature.
b) the land responds to solar heating about 4 times faster than water does
Average yearly difference in temperature by latitude.
Note change in scale for land (right margin) and ocean (left margin)
Departures from the balanced heat budget:
1) variation in solar heating with latitude.
2) the seasons.
3) The thermal response of land vs. sea.
3) Day and night variations in solar heating.
http://www.andrewkelsall.com/spectacular/category/photos/page/2/
Composition of the lower atmosphere – a nearly homogeneous mixture of:
Nitrogen (N2) = 78%
Oxygen (O2) = 21 %
Argon (Ar) = 0.9%
Carbon Dioxide (CO2) = 0.04%
Air also contains water vapor (humidity) in amounts that vary from place to place and day to day (depends on temperature and pressure). It can make up as much as 4% of the total volume.
Fig. 6.5, p 168
Does air have mass?
Yes! Since air is matter, it has mass (it weighs something).
A 1 cm x 1 cm column extending from the surface to the top of the atmosphere weighs over 2 pounds.
A 1 ft x 1 ft column extending through the atmosphere weighs over a ton!
SO… If there is a ton of pressure pushing down on us all the time, why aren’t we all as flat as pancakes?
Because – we have a ton of pressure pushing back out again. Since pressure in = pressure out, we don’t feel a thing!
This is the same mechanism used by organisms living at the bottom of the ocean.
So, for all organisms, living under pressure is not the issue, it is change in pressure that can make us sick (or kill us)!
Just as with water, the density of air can change. What factors will affect the density of air?
1) Temperature – as T density will
-as T density will
2) Humidity – as Humidity density will
as Humidity density will (dry air is more dense)
a) AND warm air can hold more water vapor than cold air, so it becomes even less dense!
3) Pressure – as Pressure density will
(falling air = compressing air = warming air)
as Pressure density will
(rising air = expanding air = cooling air)
Low Pressure
High Pressure
Remember this important rule:
As air rises, it cools and expands => it can’t hold as much water vapor.
Where air is rising (low pressure), the climate tends to be rainy! http:// www.cksinfo.com/nature/weather/rain/index.html
As air falls, it warms and compresses => it can hold more water.
Where air is falling (high pressure), the climate tends to be warm and dry! http:// www.edupic.net/Images/Biomes/desert_saquaro167.JPG
Air moves / winds form because of
differences in density from place to place!
Fig. 6.7, p. 169
Low Pressure
High Pressure
High Pressure
Low Pressure
Fig. 6.8, p. 169
Convection in the atmosphere due to differences in air pressure!
http://www.ux1.eiu.edu/~cfjps/1400/circulation.html
Our first model – Convection on a non-rotating, water-covered Earth, assuming no loss of heat to space.
Air rises at the equator and sinks at the poles. Horizontal winds exist to connect these areas of vertical air motion.
What happens at the equator where warm, humid air rises?
RAIN!!!
Effect of the Earth’s rotation = The Coriolis Effect
Different latitudes at the Earth’s surface are at different distances from the Earth’s axis of rotation
Each latitude has to spin at a different speed to make one rotation per day.
This causes an apparent deflection of the direction of movement of an object moving free of friction with the Earth’s surface.
But the Earth does rotate, and it does have land, and seasons, and…
How does this change our model?
The Coriolis Effect causes objects moving without friction with the Earth’s surface to apparently deflect from their straight-line path.
Objects north of the equator will always deflect to the right
Objects south of the equator will always deflect to the left
The Coriolis Effect is 0 near the equator and increases the closer to the poles you get.
Another factor we ignored in our perfect world model of the Earth’s winds was the constant loss of heat from the upper atmosphere into space.
What happens to air as it gets colder?
Cold air gets more dense and it sinks!
Now let’s see what happens when we include the Coriolis Effect and heat loss in our wind model (we are still ignoring seasons and the effect of land for now).
Fig. 6.12, p. 174
Vertical air motion (up or down) occurs at:
The equator (0 °) where warm, humid air rises. The Doldrums.
As the air rises, it cools and expands, causing the water vapor to condense and fall as rain. Think Hawaii…
Vertical air motion (up or down) occurs at:
30 ° N and 30 ° S, where cold, dry air sinks. The Horse Latitudes.
The air warms and compresses as it sinks, allowing it to absorb water vapor, There is little rainfall here. Think the Sahara Desert…
Vertical air motion (up or down) occurs at:
60 ° N and 60 ° S, where warmish, humid air rises.
As the air rises, it cools and expands, causing the water vapor to condense and fall as rain. Think Seattle….
Vertical air motion (up or down) occurs at:
90 ° N and 90 ° S, where extremely cold, dry air sinks.
Again, there is little precipitation here. The polar regions are technically deserts based on their average precipitation.
Winds are named for the direction they blow from!
Looking at the Northern Hemisphere, horizontal air motion occurs between:
0 ° and 30 °N (the Hadley Cell). Surface winds here want to blow toward the equator but are deflected to the right by Coriolis. In the Northern Hemisphere they blow from NE to SW and are called the NE Trade Winds.
30° N and 60° N (the Ferrel Cell). Surface winds here want to blow toward the pole but are deflected to the right by the Coriolis Effect. In the Northern Hemisphere they blow from SW to NE and are called the Westerlies.
60° N and 90 ° N Surface winds here want to blow from the pole toward the equator but are deflected to the right by Coriolis. In the Northern Hemisphere they blow from NE to SW and are called the Polar Easterlies.
In the Southern Hemisphere, the wind pattern is a mirror image of the winds in the Northern Hemisphere. WHY?
Because Coriolis deflects things to the left here!
We’ve ignored a few things with our 3-convection cell per hemisphere model!
1) Local variation in wind patterns due to diurnal (day / night) variations.
In the day,
the land warms > than the sea. Warm air rises over land
cool air sinks over the ocean, onshore wind develops.
At night,
the land cools > than the sea. Cool air sinks over land
warm air rises over the ocean, offshore wind pattern develops.
We’ve still ignored a few things with our 3-convection cell per hemisphere model!
2) The seasons change the pattern of solar heating and length of day vs. night
| Latitude | Shortest day | Longest day |
| 0 | 12:07 | 12:07 |
| 10 | 11:32 | 12:42 |
| 20 | 10:56 | 13:20 |
| 30 | 10:14 | 14:04 |
| 40 | 9:20 | 14:00 |
| 50 | 8:05 | 16:21 |
| 60 | 5:54 | 18:49 |
| 70 | 0:00 | 24:00 |
| 80 | 0:00 | 24:00 |
| 90 | 0:00 | 24:00 |
July = summer in the north.
ITZC (inter-tropical convergence zone ≈ meteorlogical equator) moves slightly north of the equator.
January = summer in the south. ITZC moves slightly south of the equator.
Sea level pressure climatology, averaged over 1958 – 1997, from NCEP-NCAR reanalysis projects. Units are millibars. High pressure occurs where relatively cold air sinks, low pressure happens where relatively warm air rises. Take away: the ocean is colder than land in summer, warmer than land in the winter.
3) We also need to worry about the effect of land, which warms up and cools down faster than the ocean does.
In the Northern Hemisphere, winds swirl clockwise around high pressure (sinking) air, and counterclockwise around low pressure (rising) air.
3) We also need to worry about the effect of land, cont.
January sea-level atmospheric pressure and the resulting wind patterns, figure 6.13, p. 177.
Regional variation in wind patterns due to the influence of the seasons combined with the presence of land vs. sea:
Summer-time pattern
L
H
Non-rotating world wind
Real world wind
L
H
Regional variation in wind patterns due to the influence of the seasons:
Winter-time pattern
H
H
Definitions
Storms = regional atmospheric disturbances, characterized by strong winds and, usually, precipitation.
These can be very powerful:
11/70 Hurricane in Bangladesh – winds to 120 mph, storm surge 40’ high (in a country only 8’ tall on average), killed > 300,00 people
8/2005 Hurricane Katrina – winds to 125 mph, rain up to 10 inches per hour. Loss of life and damage were exacerbated by poor design and maintenance of the levee system.
Storms are cyclonic – huge, rotating masses of low pressure air. There are two main types of storms.
extratropical cyclones (form outside of the tropics)
tropical cyclones (form in the tropics)
Air mass = a large body of air with nearly uniform temperature and humidity so density.
For example, the polar air mass (cold and dry)
Tropic air mass (warm and humid)
Front = a boundary along which 2 air masses interact or collide.
example: the polar front between polar and temperature air masses
Along a front, the air masses don’t mix. Instead the denser air slides under and
lifts the less dense air upward.
There are two kinds of fronts – cold fronts and warm fronts.
Figure 6.16, p. 181
Cold fronts form when the cold air runs into warm air.
Warm fronts form when the warm air runs into cold air.
Occluded fronts form where a warm front, a cold front, and a very cold front collide.
Extratropical Cyclones – Our Storms
An extra-tropical cyclone primarily gets its energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones (also known as mid-latitude storms) are low pressure systems with associated cold fronts, warm fronts, and occluded fronts.
L
Stage 4
Extratropical cyclones form over small bends in the front caused by local low pressure zones.
1) They gain their energy from the temperature differences across the developing fronts.
2) ET cyclones begin to lose energy and die away once an occluded front forms and/or once the storm travels over land
3) They last between 2-5 days after they form.
4) Most of “ours” form in the Gulf of Alaska and are blown by the jet stream over the west coast and then across the continent.
http://www.weatherquestions.com/What_is_a_cyclone.htm
Tropical cyclones (in the tropics), in contrast, typically have little to no temperature differences across the storm at the surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics http://earthobservatory.nasa.gov/IOTD/view.php?id=43154
Tropical Cyclones
(Small) tropical storms<tropical depressions<hurricanes (Large)
huriccanes = typhoons = cyclones = williwilli (depending on where you are)
hurricanes are swirling masses of air up to 9 miles tall
central low pressure “eye” of rising air up to 10 miles wide
surrounded by bands of rain clouds hundreds of miles across
travel at east and north at speeds of 5 – 35 mph
a single hurricane contains more energy than the US uses in 20 years.
The formation of Tropical Cyclones is not well understood:
They form within a single air mass between 10 – 25 ° N and S
Originate with a bend in the winds over a local low pressure (usually over warm land)
Grow once over warm water (why?)
Rising humid air cools, condenses and releases latent heat to power the winds.
Once started, it takes 2-3 days for a tropical storm to grow to a hurricane
Worldwide, we average 100+ hurricanes (cyclones/typhoons) per year.
Conditions needed to grow a hurricane:
Warm water (the warmer the stronger the storm)
Warm, moist air to supply the latent heat
The Coriolis Effect to cause the winds to twist!
No hurricanes will start at the equator. Why?
No Coriolis Effect there
Few hurricanes in the South Atlantic due to high wind shear and few “starter” storms
Tropical Cyclone Tracks for the last 150 years.
How Hurricanes Kill:
1) strong winds up to 200 mph
2) rain fall of up to 1” per hour or more
3) storm surge = a bulge of water pulled up by a combination of the low atmospheric pressure vacuuming the water upward and the strong winds pushing the water onshore (usually the most deadly)
