Atmospheric circulation

Moving air and heat around

Upper level flow• Lower level flow in “cells”; upper level

flow to poles• Warm air in tropics and cooler air at poles• Depth of troposphere changes with

latitude• Steepest pressure gradient at mid-

latitudes• At given altitude, pressure higher at

equator• High altitude flow is from equator to

poles down pressure gradient• Wind speeds greatest at mid-latitudes (jet

streams)

Circulation in Hadley cells• Buoyancy

– Vertical air movements related to moisture and heat (DENSITY)

• Pressure differences– Hadley cells– Horizontal air movements– Air move from high pressure to low pressure

• Coriolis effect– Apparent deflection to right or left– Vertical component – centrifugal– Pushes an object away from center of rotation

• ITCZ – convergence and uplift of hot, moisture laden air

• Subsidence at 30 degrees – cool, dry air• Divergence due to high pressure at Earth’s

surface

Meridonal circulation (N-S flow)Pressure

Coriolis effect and atmospheric circulation

• Coriolis effect influences wind direction• Air only makes it about 1/3 of the way to the poles

before it becomes dense enough to sink• End up with 3 sets of cells – by 30 deg, flow has been

deflected 90 deg• 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.

• On a CCW (eastward) rotating earth

• Change in rotation rate with latitude (angular momentum

• Winds due east or due west affected by centrifugal force – horizontal component is to the right in N hemisphere

• Coriolis effect increases as speed increases

• Coriolis effect increases with latitude - counterintuitive

• No Coriolis at the equator

Net result – sort of

• Very low temperature at poles• Increased air density near the surface• Movement of cold air outward (divergence)• Equatorward-moving cold air• Steep temperature gradient at polar front• Little mixing – wavelike structure around the hemisphere• Exact latitude varies

Fig. 4-7

Fig. 4-6

Hadley CellHadley Cell

Fig. 4-11

More realistic near poles

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

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

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

Circulation of the Atmosphere

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

• 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

The 6-celled model

• Not exactly correct either • North - South variation• East-West variation• Seasonality• 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)

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

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

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

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

Seasonality importantShifts in polar front and the ITCZ – meteorological equator

• 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

Weather and our model

• Cells are not really continuous features• Clusters of convective cells• Vary seasonally and locally and from day to

day• Exact location of subtropical highs and polar

front can create cyclonic flow

Weather

• A result of smaller atmospheric motions and eddies

• Usually caused by differences in atmospheric pressure, temperature and humidity (remember all of these affect density)

• Weather forecasts try to predict smaller scale air movements

Cyclonic flow

• Localized circular flow to the right (CCW – N hemisphere). Air moving into a low pressure center

• Anticyclonic (CW – N hemisphere) flow – air flowing out of a high pressure center

• Low pressure systems forming outside the tropics are extratropical cyclones– Extensive uplift of warm air– Features move along polar front

• Comparable to a water mass• Large body of air with uniform temperature

and humidity (so density) throughout• Air over land or water will take on

characteristics of surface below– Cold, dry land yields cold, dry air (high pressure)– Warm ocean surface yields warm, wet air (low

pressure)

Air masses

Air masses

• Air masses form over land and water acquiring characteristics of their sources– Dry, cold air forms over Canada and Siberia…– Wet, moist air forms over equatorial waters…

• When air masses move, they change characteristics– Temperature changes– Humidity or water content changes (lose water)

• Air masses can move within or between cells• Density differences prevent air masses from

mixing (like water) – dense air slides beneath• Turbulence at boundaries between air masses• Fronts are boundaries between air masses of

different densities– Fronts marked by changes in temperature and humidity

Air masses

Fronts

• A cold front is the leading edge of a cold-air mass advancing on a warm air mass– Displaces warm air– Cold air pushes under warm air (more toe shaped)– Get precipitation (rain or snow) just behind the front

• A warm front is the advancing edge of a warm air mass– Displaces cold air– Rises over cold air in a wedge shape– Drops water in front of its leading edge

Polar front• About 50o N and S• Persistent boundary between converging warm

and cold air masses• Get highly variable weather at these latitudes• Made up of a succession of waves that appear

on weather maps as warm or cold fronts– Succession of warm, moist, subtropical air

and cold dry polar air– Weather typical of N America and Europe

• Narrow bands of strong winds called jet streams at altitudes of about 10 km

Ocean influence on weather• At mid-latitudes, warm and cold water masses

steer weather patterns on land• Size and energy of water masses permits this• Large cold water masses in the N Pacific shift

prevailing westerlies blowing across E North America

• Cold, dry air from Canada displaces warm, moist air from the Gulf of Mexico and the tropical N Atlantic

• So get cooler winters in the SE USA• Shifts in positions of water masses can cause

changes in patterns• Warm equatorial surface waters in the Atlantic

cause prolonged drought in Africa?

Storms

• Regional atmospheric disturbances characterized by strong winds and, often, precipitation

• Cyclones are intense storms around low pressure centers

• Tropical disturbances (in Hadley cells/tropics) – cyclones (hurricanes)

• Extratropical disturbances (in Ferrel cells/mid-latitudes) – also cyclones, usually in winter

• Low pressure air• Rotates as winds converge and ascend (may

bring water with them so get precipitation)• Form between or within air masses

Cyclones

• Form at the boundary of polar and Ferrel cells (polar front) – mid-latitudes

• Occur mainly in winter when temperature and density differences across the front are most pronounced

• Cold air poleward of front is moving from the pole and east (more dense)

• Warm air equatorward of the front is moving from the equator and west (less dense)

• Cold air tries to slide below the warm air at the low pressure interface of a stationary front

Extratropical cyclones

Extratropical cyclone• May get alternating high and low pressure systems

that bend the front• May get a twist in front due to opposite wind

directions• Twisting air mass becomes cyclonic and circulates

CCW in the N hemisphere (opposite Coriolis)• CCW flow is Coriolis driven because of the dominant

flow of air masses at the edges• Part of the front is cut off• Wind speed increases as storm condenses• Air rushing toward center rises making a low

pressure zone (air rises and loses moisture)

Cold air tries to dive below or push under warm airHigher pressure N of cold front so bending is towards lower pressureCold air pushes warm air/frontLow pressure intrusion into cold frontWarm air rises (with or without water) at both fronts & yields precipitation at the frontsCold front pushes warm frontEventually part of front is cut off and moves east

• Cyclone gets embedded in the westerly winds so moves eastward

• Typically 1000-2500 km in diameter• Last 2-5 days

Extratropical cyclones

Extratropical cyclones

• Precipitation begins as circular flow develops• Precipitation caused by the lifting and cooling of the

mid-latitude air (warmer air from the Ferrel cell) involved in the twist

• Cold air advances behind it and does the lifting creating a cold front

• Warm front occurs as the warm air is lifted on top of the retreating cold edge

• Often these are called frontal storms and are the principle cause of weather in mid-latitudes

Nor’easters

• Most powerful wind approaches from the east (polar cells)

• Occur along the east coast of the US in winter

Tropical cyclones

• Masses of warm, humid, rotating air• Occur in all tropical oceans except the equatorial

South Atlantic• Large tropical cyclones (winds at least 119 km/hr)

are:– hurricanes in the North Atlantic & eastern Pacific (about

100/year)– Typhoons in the western Pacific– Tropical cyclones in the Indian Ocean– Willi-willis in the waters near Australia

• Smaller tropical cyclones are tropical storms or depressions

Tropical cyclones

• Masses of warm, humid, rotating air• Occur in all tropical oceans except the equatorial

South Atlantic• Large tropical cyclones (winds at least 119 km/hr) are:

– hurricanes in the North Atlantic & eastern Pacific (about 100/year)

– Typhoons in the western Pacific– Tropical cyclones in the Indian Ocean– Willi-willis in the waters near Australia

• Smaller tropical cyclones are tropical storms or depressions

• Appear as circular spirals• May be 1000 km in diameter and 15 km high• Calm center is the eye & can be 13-16 km• Occur June – November in N hemisphere

Internal structure of a mature hurricane.

Tropical cyclones

• Usually generated within one air mass• Usually generated between 10o and 25o latitude

(Coriolis effect closer to equator is too weak to initiate rotary motion)

• Typically last ~9 days• Origins not well understood

– Convergence of warm, wet winds that rise• Usually develop from a tropical depression• Power if from the condensing water vapor and rising

air currents at the eye

Tropical cyclones

• Tropical depressions form in easterly waves – areas of lower pressure within the easterly tradewinds– thought to originate over a large, warm land mass.

• Air containing the disturbance is heated over tropical water

• Circular winds begin to blow in the vicinity of the wave

• Some warm, humid air is forced upward• Condensation begins

Where hurricanes form (areas of high humidity and warm air over warm water)

Hurricanes

• Develop in 2-3 days from tropical cyclones under ideal conditions

• Centers move westward and poleward (within easterlies) in N hemisphere at 5 to 40 km/hr

• Poleward motion due to general atmospheric circulation

• Hurricanes lose strength over land (friction and loss of water vapor supply) or relatively cold surface water (decreases rising wind speed in eye)

Tropical cyclone tracks – breeding grounds shown in orange