Transcript 17_Ocean126_2006

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 (midlatitudes) where there is about the same amount
of land & water
Winds over the Pacific
on two days in Sept
1996
Stronger winds in redorange
Notes:
Deviates from 6-cell
model
Strong westerlies hitting
Canada
Strong tradewinds
(easterlies) over Hawaii
Extratropical cyclone
east 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
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)
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)
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)
Dry season
Wet season
ITCZ
ITCZ
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
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