Atmospheric and Oceanic Movements
What drives their motion? Heat transport from hot to cold
There is an imbalance in heating of the Earth's surface (with > heating in the tropics), because:
1) Albedo is higher at high latitudes (reflective snow and sea ice, low angle of incidence).
2) Albedo is lower at low latitude (absorbant sea water, high angle of incidence).
3) Per unit area, high latitudes receive a smaller flux of solar radiation than low latitudes, because
the same amount of energy is spread over a greater area.
4) Emission rate of long wave radiation is a function of planetary temperature. With respect to the
Stefan-Boltzman Law (Energy flux = T4), polar and equatorial regions are almost the same
temperature. Therefore, they radiate energy back to space at about the same rate.
There is some heat flow between equator and pole (i.e., the outgoing heat flux at each latitude is not the
same as the incoming flux at that latitude). This heat is carrier by the oceans and atmosphere (more on
this below). Yet there still is an equator-to-pole temperature gradient, so heat flow is not infinitely
efficient. Indeed, the magnitude of the heat flux is proportional to the temperature gradient (i.e., heat
flow is more vigorous when the gradient is large; less vigorous when the gradient is small).
Thus the pole-to-equator temperature imbalance results from differential energy input that is not
completely erased heat flow.
Atmospheric Circulation: moves 50% of the excess heat from equator to pole
Sensible heat transport: describes the movement of hot air
Latent heat transport: describes heat moved by transport and phase changes of water
Global Atmospheric Circulation Cells
Air rises at equator and at polar front (~60° latitude)
Rising air cools, moisture condenses - rain or snow
Air sinks at 30° and 90° latitude
Sinking air warms, holds more moisture - no precipitation
Find deserts/dust/salty water where air falls, forests/swamps/brackish water where air rises
Coriolis Effect
A bit more complicate than just rising and falling, because the world turns on its axis.
There is an apparent deflection of flowing air due to planetary rotation: Coriolis Effect
Deflection is to the right in the Northern Hemisphere, to the left in Southern Hemisphere
Easterlie winds at equator and 60° latitude, Westerlie winds at 30° latitude
In Northern Hemisphere
Clockwise-rotating, falling air, high pressure cells: no rain
Counterclockwise-rotating, rising air, low pressure cells: lots of precipitation
Land-sea air flow
Water has a higher heat capacity than rock. Water can store more heat. Land cools/heats faster.
This difference creates the large thermal contrasts between land and sea that shifts seasonally.
Monsoons - seasonal shift in onshore moisture flow
Hot season: Low pressure develops over the rapidly heating land. This pulls warm, wet air
on to land; rain falls under the rising air. Dry high-pressure cell over the colder oceans.
Get summer, continental monsoonal rains. Examples: Asia and central North America
Cold season: Land cools fast, while the oceans remain relatively warm. A dry, high pressure
cell develops on land, while a wet/low pressure cell forms at the coast.
Get winter, coastal monsoonal rains. Examples: California, Peru
Ocean Circulation: moves 50% of the excess heat from equator to pole
Over most of its surface area, the ocean can be divided into warmer, brackish, less dense surface
ocean water and cooler, saltier, denser deep ocean water.
Surface oceans: involves about 20% of the Earth's ocean volume
Surface flow is driven by the winds.
Surface ocean currents follow trends in the wind field.
In Northern Hemisphere:
Strong, warm, northward flowing current on east coasts of continents
Broad cool, southward flowing currents on west coast of continents.
If the winds fail, circulation does too.
Ekman Spirals: Coriolis deflection of deeper layers of water drives offshore flow.
At depth, net motion of water is perpendicular to the winds
This surface water "volume deficit" is made up by Upwelling of deep ocean water.
Because deep water if so full of nutrients (remember the biological C pump), upwelling sites are
zones of spectacular biotic production.
Important in western coastal regions and at Equator: Examples: California, Peru, SW Africa
Deep ocean circulation: involves about 80% of the Earth's ocean volume
Driven by density contrasts in water (due to temperature and salinity).
Salty water at equator (lots of evaporation), fresher water at poles
Today, most deep water formation takes place in the North Atlantic (some in Antarctic)
Why is North Atlantic so salty?
Equatorial Easteries blow freshwater (not salt) over the Isthmus of Panama into the Pacific.
As it cools (loses heat to the atmosphere), Atlantic gets denser and denser until finally it sinks.
Salt flows in deep water currents to make up this the Atlantic/Pacific imbalance.
This sucks more warm, salty, surface water to the N. Atlantic.
Release of heat from this water accounts for 30% of the heat delivered to Europe at present.