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Atmospheric circulation

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Presentation on theme: "Atmospheric circulation"— Presentation transcript:

1 Atmospheric circulation
Moving air and heat around

2 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)

3 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

4 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

5 Meridonal circulation (N-S flow) Pressure

6 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.

7 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

8 Net result – sort of

9 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

10 Fig. 4-7 Hadley Cell Hadley Cell Fig. 4-6

11 More realistic near poles
Fig. 4-11

12 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

13 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

14 Major surface wind and pressure systems of the world and their weather
These wind patterns move 2/3 of heat from tropics to poles.

15 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

16 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)



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

20 Winds over the Pacific on two days in Sept 1996 Stronger winds in red- orange Notes: Deviates from 6-cell model Strong westerlies hitting Canada Strong tradewinds (easterlies) over Hawaii Extratropical cyclone east of New Zealand

21 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

22 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

23 Seasonality important
Shifts in polar front and the ITCZ – meteorological equator

24 Wet season 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)

25 Dry 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)


27 Monsoons 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)


29 Dry season Wet season ITCZ ITCZ

30 Sea and Land breezes 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


32 Daytime Onshore Breeze
Nighttime Offshore Breeze

33 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

34 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

35 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

36 Air masses 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)

37 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)

38 Air masses 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

39 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

40 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

41 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?

42 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

43 Cyclones Low pressure air
Rotates as winds converge and ascend (may bring water with them so get precipitation) Form between or within air masses

44 Extratropical 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

45 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)

46 Cold air tries to dive below or push under warm air
Higher pressure N of cold front so bending is towards lower pressure Cold air pushes warm air/front Low pressure intrusion into cold front Warm air rises (with or without water) at both fronts & yields precipitation at the fronts Cold front pushes warm front Eventually part of front is cut off and moves east

47 Extratropical cyclones
Cyclone gets embedded in the westerly winds so moves eastward Typically km in diameter Last 2-5 days


49 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

50 Nor’easters Most powerful wind approaches from the east (polar cells)
Occur along the east coast of the US in winter

51 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

52 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

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

54 Internal structure of a mature hurricane.

55 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

56 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

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

58 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)


60 Tropical cyclone tracks – breeding grounds shown in orange

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