Atmospheric circulation

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

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

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

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