Atmospheric Circulation

Moving things around on present day Earth

Short-term cycles

Long term (organic)

Long term - rock(inorganic/tectonic)

Global cycles• Biogeochemical cycles are the major way that elements are

moved on Earth’s surface– Driven by solar input (primary production)– Elements cycle between reservoirs that operate on different

time scales– Cycles have positive and negative feedbacks and subject to

perturbations• Interaction with physical processes through tectonic/rock

cycles• Oceans and atmosphere are important conduits transporting

matter and energy

Oceans & Atmosphere

• Shorter timescales of exchange

• Exchange time in atmosphere – hours to decades– Mediates rapid cycling between oceans and

continents

• Exchange time in oceans – Surface and deep water – years– Deep circulation – 100’s to 1000’s of years

Atmosphere & Ocean

• Gases and water freely exchange at the ocean-atmosphere interface

• Movement of air (and water) by wind help minimize worldwide temperature extremes.

• Weather is influenced by the movement of water in air (state of the atmosphere at a specific time and place)

• Climate is the long-term average of the weather in an area

In general

• Atmosphere exchanges material with biota and oceans rapidly

• Cycles that include an atmospheric component tend to have more rapid recycling (N and C)

• Cycles without an atmospheric component can be slower (immobile) because tied to geological cycles (P)

Atmosphere

• Major conduit for transport between oceans and land– Major role in controlling climate (heat transport)

• Composition evolved as a result of evolution of life– Changing due to human activities

– Well-mixed so harbinger of global change

• Structure – layered

• Held on earth’s surface by gravity

Mt. Everest(8,850 m)

Atmosphere structure

Tropopause

Troposphere

20 km

40 km

10 mi

20 mi

30 mi

Weather zone Water Vapor

Dry

Ozo

ne

Stratosphere

Stratopause

Mesosphere

Tem

pera

ture

0oC

20oC

-55oC

80% of atmospheric mass is in the troposphere

Pressure decreases with altitude – 1 atmosphereof pressure at Earth’s surface at sea level.

Structure of the Atmosphere

• Troposphere is densest and is where our weather occurs

• Substances in the stratosphere persist for long periods because there are few removal processes

• In troposphere, temperature decreases with altitude• In stratosphere, temperature increases with altitude

due to interactions with particles and radiation from the sun

• The ozone layer is within the stratosphere– Ozone absorbs UV at top of stratosphere

Troposphere

• Well-mixed

• Limited exchange with overlying stratosphere

• Heated by long-wave radiation (heat) re-radiated from Earth’s surface– Temperature decreases with altitude in troposphere

• Heating from below results in convection, remember?– Rising warm air creates thermal instability

Composition of the atmosphere

• 78% nitrogen and 21% oxygen

• Other elements make up < 1%

• Air is never completely dry and water can be up to 4% of its volume.

• Residence time of water vapor in the atmosphere is ~10 days.

also H2S, H2, (CH3)S

Atmosphere

• N2 – fairly inert; long residence time (20 my)

• O2 – accumulated over time; complex controls; shorter residence time (~10,000 years)

• CO2 – trace constituent; complex controls; short residence time (~3 years)- Affected by processes with cycles at various timescales (from rock

to seasonal)

- Long-term variations

- Greenhouse

Atmosphere

• Trace constituents – reduced gases– Microbially produced at present and removed in

rain/oxidation– Greenhouse gases

• Ozone – stratosphere– Problematic in troposphere

• Water vapor– Varies tremendously– Important in distributing heat– Greenhouse gas

Properties of the atmosphere

• Air has mass (and density)• Molecular movement associated with heat causes

the same mass of warm air to occupy more space than cool air. So, warm air is less dense.

• Humid air is less dense than dry air at the same temperature because molecules of water vapor (H2O) weigh less than N2 and O2 molecules displaced.

Density structure of troposphere

• Influenced by temperature and water content• Water vapor is less dense than dry air so causes

density of air to decrease and air to rise• Warming air makes it less dense so it rises• Condensation of water vapor releases heat which

warms the air• Warm air can hold more water vapor than cold air

Air density affected by pressure

• Air lifted to altitude experiences less pressure so expands and cools

• Air compressed as it descends from altitude warms

Air movement

• Water vapor rises, expands and cools

• Condenses into clouds or precipitation (cooler air can’t hold as much water)

• Atmosphere can lose water by precipitation

• As air loses water vapor it becomes more dense and air will then fall, compress and heat

Atmospheric circulation

• Powered by sunlight – uneven solar heating

• About 51% of incoming energy is absorbed by Earth’s land and water

• Energy absorption varies depending on the angle of approach, the sea state and the presence of ice or other covering (e.g., foam)

Heat budget• Energy imbalance – more energy comes in at the equator

than at the poles• 51% of the short-wave radiation (light) striking land is

converted to longer-wave radiation (heat) and transferred into the atmosphere by conduction, radiation and evaporation.

• Eventually, atmosphere, land and ocean radiate heat back to space as long-wave radiation (heat)

• Input and outflow of heat comprise the earth’s heat budget• We assume thermal equilibrium (Earth is not getting

warmer or cooler) or the overall heat budget of the earth is balanced

Atmospheric circulation

• Uneven solar heating of earth– Atm and oceans move heat poleward– Air moves from high pressure to low pressure– Poleward movement of warm air (less dense)– Equatorward movement of cold air (more

dense)

Movement of heat

• Sensible heat– Transported by a body that has higher

temperature than its surroundings (conduction and/or convection)

• Latent heat– Phase changes of water– Evaporation takes up heat and condensation

releases heat

Uneven solar heating

• Heat budget for particular latitudes is NOT balanced

• Sunlight reaching polar latitudes is spread over a greater area (less radiation per unit area)

• At poles, light goes through more atmosphere so approaches surface at a low angle favoring reflection

• Tropical latitudes get greater radiation per unit area and light passes through less atmosphere so they get more solar energy than polar areas

Solar radiation•Radiation hits the earth in parallel rays•Incident angle varies with latitude•Energy is spread out over more area

–Less heat per area

•Passes through more atmosphere

–Which absorbs radiation

•Poles are cooler because they receive lower intensity solar radiation do to angle of incident radiation.

S

N

Solar radiation•Second reason the poles are cooler is the tilt of the earth on its axis

–Variation in daylength–Even when poles have long daylength, the incident angle is long.

•Third reason is that poles are farther from the sun S

N

23.5o

Fig. 4-1

Fig. 4-2

Seasons & solar heating

• Mid-latitudes – N Hemisphere receives 3x the amount of solar energy per day in June than in December

• Due to the 23.5o tilt of Earth’s rotational axis

• N Hemisphere tilts toward the sun in June and away in December

• Tilt causes seasons

Figs. 4-15 and 4-16

Circulation

• Atmospheric and oceanic circulation are governed by the redistribution of this energy

• Water moves heat between tropics to poles• Ocean currents and water vapor move heat. • Higher latent heat of vaporization means

vapor transfers more heat per unit mass than liquid water.

Atmospheric circulation

• Warm air rises and cool air sinks

• Warm air expands and rises

• Expansion causes cooling and contraction causing increasing density and sinking

• Air will rise where its warmer and sink where its cooler

Convection

Logically on the earth, one can imagine this

Fig. 13.11

Fig. 4-25

Air movement

• Air is warmed at equator so rises

• As it rises, it dumps its moisture because its expanded and cooled

• Air moves south to replace air that’s risen

• Creates zone of low pressure (sinking air creates high pressure and rising air creates low pressure.

Fig. 4-3

Atmospheric circulation

• But, this is NOT what happens• Atmospheric circulation is governed not

only by uneven solar heating but,• The Earth’s rotation• Eastward (CCW) rotation of the Earth on its

axis deflects moving air or water (or any object with mass).

• CORIOLIS effect (1835)

Coriolis Effect

• Rotation of the Earth CCW • Relative speeds of sphere at different latitudes• Caused by an observer’s moving frame of

reference on a spinning Earth• Curve is slightly to the right of initial path in the

northern hemisphere• Curve is slightly to the left of initial path in the

southern hemisphere

Relative speeds of objects at different radii moving at the same angular speed

AirplaneCoriolis

Coriolis effect and atmospheric circulation

• Coriolis effect influences wind direction• End up with 3 sets of cells – by 30 deg, flow has been

deflected 90 deg• Air is deflected before getting all the way from equator to

poles• Air only makes it about 1/3 of the way to the poles before it

becomes dense enough to sink• Descending air turns back toward equator when it reaches

the surface because it is again deflected to the right• Heats up when it gets back to equator and rises again.

Fig. 4-11

Fig. 4-7

Fig. 4-6

Hadley CellHadley Cell

Fig. 4-18

Fig. 4-19 Pressure at sea-level (mbars)

Atmospheric circulation & precipitation

• Air carries water vapor

• In general, uplift of air masses induces precipitation

• Conversely, descending air affects distribution of deserts

– Descending arms of Hadley cells

– Continental interiors

– Leeward (downwind) of mountains

– West coasts of major continents – due to upwelling and offshore currents

Deserts offset from convergence zones where there is high precipitation.Deserts at divergence zones.

Features of the model

• At boundaries, air is moving vertically– Surface winds are weak and erratic

• Equatorial region– Lots of rain as humid air rises and loses moisture (rain

forests)– Doldrums– Intertropical convergence zone (ITCZ) – winds converge

• 30oN and S region– Sinking air is arid and evaporation >> precipitation (deserts

and high salinity)– Horse latitudes

Features of the model

• Air moves horizontally within the cells from areas of high pressure to areas of low pressure

• Tropical areas – Hadley cells– Surface winds are strong and dependable– Trade winds or easterlies centered at ~15oN (northeast trade winds)

and ~ 15oS (southeast trade winds)– Surface wind moves from horse latitudes to doldrums so come out

of northeast in N hemisphere

• Mid-latitude areas – Ferrel cells– Westerlies centered at ~ 45oN and ~45oS– Surface wind moves from horse latitudes to polar cells so comes

out of southwest in the N hemisphere

The 6-celled model

• Not exactly correct either • North - South variation• Land versus water distribution

– Equator to pole flow of air different depending on amount of land at a particular longitude

– ITCZ narrower and more consistent over land than ocean– Seasonal differences greater in N hemisphere (remember,

more land)

• The ocean’s thermostatic effect reduces irregularities due to surface conditions at different longitudes

Distributions of land masses-Differential heating and cooling-Land heats up and cools more rapidly

North - South variation (cont)

• Offset at the equator – Geographical vs. meteorological equator – Meteorological equator is ~ 5oN of geographical

equator (thermal equilibrium between hemispheres)– Meteorological equator and ITCZ generally coincide

and change with seasons (moves N in northern summer)

– Atmospheric and oceanic circulation is symmetrical around the meteorological equator NOT the geographical equator!

• Seasonality

Seasonality importantShifts in polar front and the ITCZ – meteorological equator

West-East variations• Air over chilled continents becomes cold and dense in

the winter• Air sinks creating high pressure over continents • Air over relatively warmer waters rises (possibly with

water vapor) creating low pressure zones over water• Air flows from high pressure to low pressure

modifying air flow within cells• Reverse situation in summer• Effects pronounced in N hemisphere (mid-latitudes)

where there is about the same amount of land & water

Winds over the Pacificon two days in Sept1996

Stronger winds in red-orange

Notes:Deviates from 6-cellmodel

Strong westerlies hittingCanada

Strong tradewinds (easterlies) over Hawaii

Extratropical cycloneeast of New Zealand

Circulation of the Atmosphere

• Most of the variation from the 6-cell model is due to – Geographical distribution of landmasses– Different response of land and ocean to solar

heating– Chaotic flow

• Over long term – 6-cell model is pretty good for describing average flow

• Major surface wind and pressure systems of the world and their weather

• These wind patterns move 2/3 of heat from tropics to poles.

Monsoons

• Pattern of wind circulation that changes with the season

• Generally wet summers and dry winters

• Linked to different heat capacities of land and water and to N-S movement of the ITCZ

• In the spring, land heats (faster than water)

• Warm air over land rises creating low pressure

• Cool air flows from ocean to land

• This humid air heats and rises (rains form)

Wet season

• Land cools (faster than ocean)

• Air cools and sinks over land creating high pressure

• Dry surface wind moves seaward

• Warms and rises over water (with or without evaporation and rain over water)

Dry Season

• Most intense over Asia where you have a huge land mass in the N and a huge ocean to the S

• Monsoon over India causes wet season (summer) from April – October (up to 10 meters – 425 inches of rain per year)

• Smaller monsoon in N America (Gulf of Mexico and SE)

Monsoons

ITCZITCZ

Dry season Wet season

• Daily changes in wind direction due to unequal heating and cooling of land versus water

• Warm air during day on land rises and cool air from sea moves onshore (with or without water vapor)

• Warmer air over water rises and cool air on land during the night sinks and moves offshore

Sea and Land breezes

Daytime Onshore Breeze

Nighttime Offshore Breeze

Fig. 4-17