Coriolis Effect Modifies Hadley Circulation
Modified Hadley Circulation
*
Horizontal motions
convergence: coming together
divergence: spreading apart
Vertical motions
upwelling: rising air
subsidence: sinking air
“Seeing” Hadley Circulation
Mean Surface Pressure Contours
Lines of constant
pressure (isobars).
More closely spaced
lines: steeper change
in pressure
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H
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Announcements
• Posters: March 12 – 13 during lecture; buy
one poster board (~ 4’x4’) per group
– Groups to be solidified by next Friday
– SEE WEBSITE FOR MORE DETAILS
• Office Hours today 4-5 pm (506 or 510
ATG) and 5-6 pm (406 ATG)
• Another problem set will be posted soon.
Beware of quizzes in discussion sections…
This Week
• Finish Chapter 4 of text
• Regional Climates Continued
– Midlatitude Circulation
– Land/Ocean Contrasts
• Global Water Cycle
Hadley Circulation and Regional Climates
• World’s largest deserts
• Wet and dry seasons in the Tropics
• The Trade Winds
World’s Deserts
Not shown: Polar Regions!
World’s Deserts
Desert dust blows from
W. Sahara and N.
Morocco over Canary
Islands.
Desert dust is a source
of nutrients to ocean
and land biota (often a
world away).
All desert areas (low annual precipitation)
occur in regions of general subsidence
1. True
2. False
78%
ls
e
Fa
Tr
ue
22%
Suppose you wanted to the take Presidents’ Day
vacation (Feb) in the sunny and warm tropics.
Because you want sunny dry weather you choose
60%
1. Costa Rica (NH)
2. Amazonia (SH)
on
m
az
A
C
os
ta
R
ic
a
ia
(S
H
(N
H
)
)
40%
Seasonal Shift in Hadley Circulation
ITCZ location shifts N-S depending on season.
Leads to wet and dry seasons in the tropics.
Surface Pressure and Winds Summary
Hadley Circulation Summary
Low surface pressure near equator  convergence
ITCZ
ITCZ: rainy! location moves N or S with seasons
causing WET and DRY seasons in the tropics
High altitude branches WESTERLY @ 15 – 30 N/S
due to Coriolis Force and PGF (geostrophic flow)
Subsiding branches located around 30o N and S
 DESERTS and high surface pressure
Surface flow towards equator is northeasterly in
NH and southeasterly in SH TRADE WINDS
Mid-latitude Circulation
• Westerly flow both NH and SH
• Strong temperature gradient gives rise
cold/warm fronts (moving air masses)
• Cyclones and Anticyclones
Mid-latitude Westerlies
Warm/Cold Fronts
Subsiding
Hadley Brach
Strong T gradients
Higher P
Lower P
Higher P
Flow Around High and Low Pressure Centers
Upper-level flow geostrophic: parallel to isobars.
In NH flow counterclockwise
around Lowcyclonic flow
L
Low Pressure Center
In NH flow clockwise around
Highanticyclonic flow
H
High Pressure Center
Surface Flow Impacted by Friction
Forces
PGF
Coriolis
Friction
Actual Flow direction
Low Pressure
High Pressure
Friction causes flow to move away from high pressure,
but towards low pressure.
Surface-level Flow affected by Friction
Centers of low or high pressure at surface induce flow that
spirals in or out, respectively.
Convergence/uplift—Stormy
L
Low Pressure Center
cyclones
Divergence/Subsidence - Nice
H
High Pressure Center
anticyclones
Tropical Cyclone—Hurricane Gordon
Midlatitude Cyclones
Midlatitude Average Circulation (Summary)
• Westerly flow 35 – 55 N/S
• Large latitudinal temperature gradients –
warm and cold fronts induce storminess
• Low pressure centers are wet/rainy (storms)
high pressure centers are dry/sunny
Where would you expect
“continentality” to be greatest?
89%
1. Northern
Hemisphere
2. Southern
Hemisphere
rn
ut
he
So
N
or
th
e
rn
H
H
em
is
p
em
is
ph
er
e
he
re
11%
Continentality-Find the Continents
Contours show annual temperature range: Tsummer - Twinter
44
4
56
Diurnal (Daily) Sea Breeze
Day
Night
Monsoonal Circulation
Similar to diurnal
sea breeze but on
larger spatial and
temporal (seasonal)
scales.
H
January
July
L
Atmospheric Circulation Summary
• Three major N – S circulation cells in each
hemisphere (Hadley, Midlatitudes, Polar)
• Tropics: surface level easterlies (trades),
ITCZ, and subsidence zones (30N/S)
• Midlatitudes: westerly flow, frontal storms
• Land/Ocean contrasts: monsoonal circulation,
diurnal sea breeze, continentality
Ocean Circulation and Climate
Reading:
Chapter 5
Atmosphere-Ocean Couplings
1.Heat Exchange
2.Momentum Exchange (surface wind
stress)
3.Moisture/Gas Exchange (water and
carbon cycles)
Heat Transport by Ocean and Atmosphere
Tropics
Midlatitudes
Polar regions
Key Ocean Properties
• Ocean water is salty ~ 30 g salt in 1 liter
• Ocean heated from above  warm
surface water, cold deep ocean
• Vertical mixing determined by buoyancy
– warm water less dense, saltier water more
dense
• Vertical mixing suppressed: surface vs.
deep circulation
Wind-driven Surface Ocean Circulation
Surface Ocean Circulation
Gulf Stream
western branch of mid-Atlantic gyre
AVHRR Satellite measurement of Sea Surface T
Convergence And Divergence
Net convergence of surface water in center of gyres
Net divergence at eastern ocean boundaries and equator
Surface ocean
Wind
Equator
Divergence Causes Upwelling
Coastal Upwelling/Downwelling
Marine Stratus Clouds
Surface winds
Ocean surface flow
Eastern-boundary
coastal upwelling
Cold water cools air
Cloud formation
Where do you expect the coldest sea
surface temperatures?
1. Eastern ocean
boundaries
2. Western ocean
boundaries
50%
oc
er
n
es
t
W
Ea
st
er
n
oc
e
an
ea
n
bo
bo
u
un
d
n.
..
...
50%
Sea Surface Temperatures
monthly mean SST animation
Observe the following
1. Latitudinal distribution of solar radiation
2. Heat exchange with atmosphere
3. Circulation patterns (e.g. upwelling)
Deep Ocean Circulation
• A SLOW process
– Timescale to overturn ~ 1000 years
– Lots of water (1.37x109 km3) and suppressed
vertical mixing
• Driven by formation of cold salty surface
water
Physical Properties versus Depth
Salinity
Salinity is measured
in parts per thousand
Thermo-haline Circulation
(temperature-salty)
Ocean-Atm heat transfer
Cold salty water
Mixed layer ~ 1 km deep
Sea ice
Middle and deep ocean
Net sinking: Deep
Water formation
Lower latitudes
High latitudes
Sea ice influence on the ocean
•Reduces the influence of the winds
•Insulates the ocean (prevents heat loss)
•Rejects salt when it grows / Adds
freshwater when it melts
Thermo-haline Circulation (THC)
Marine Chlorophyll From Space
Thermohaline Circulation Importance
•
Deep ocean is an enormous reservoir for
heat and dissolved gases like CO2
•
Overturning brings nutrients up to surface
biota  photosynthetic uptake of CO2
•
Maintains transport of heat to higher
latitudes, moderate latitudinal T gradients