1. Atmospheric circulation

4. 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

5. 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

7. 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

9. 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

11. 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

12. 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

13. 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

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. 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

16. 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)

17. 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)

19. 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)

21. Dry season
Wet season
ITCZ
ITCZ

22. 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

24. Daytime Onshore Breeze
Nighttime Offshore Breeze

25. Take home points
Major latitudnal cells and their approximate boundaries
N-S and E-W variation in cell circulation
Air movements at cell boundaries versus within cells
Heating and cooling of air masses
How distribution of land and water affect air movement
Monsoons and onshore-offshore wind patterns