Coriolis Effect Modifies Hadley Circulation
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Transcript of Coriolis Effect Modifies Hadley Circulation
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Coriolis Effect Modifies Hadley Circulation
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Modified Hadley Circulation
Horizontal motionsconvergence: coming
togetherdivergence: spreading
apart
Vertical motionsupwelling: rising
airsubsidence:
sinking air
*
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“Seeing” Hadley Circulation
Lines of constant pressure (isobars).
More closely spaced lines: steeper change in pressure
H H
H H H
H
Mean Surface Pressure Contours
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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…
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This Week
• Finish Chapter 4 of text
• Regional Climates Continued– Midlatitude Circulation– Land/Ocean Contrasts
• Global Water Cycle
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Hadley Circulation and Regional Climates
• World’s largest deserts• Wet and dry seasons in the Tropics• The Trade Winds
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World’s Deserts
Not shown: Polar Regions!
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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).
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All desert areas (low annual precipitation) occur in regions of
general subsidence
True False
78%
22%
1. True2. False
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Suppose you wanted to the take Presidents’ Day vacation (Feb) in the sunny and warm
tropics. Because you want sunny dry weather you choose
Costa Rica (NH) Amazonia (SH)
60%
40%1. Costa Rica (NH)2. Amazonia (SH)
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Seasonal Shift in Hadley Circulation
ITCZ location shifts N-S depending on season. Leads to wet and dry seasons in the tropics.
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Surface Pressure and Winds Summary
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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 tropicsHigh altitude branches WESTERLY @ 15 – 30 N/S due to Coriolis Force and PGF (geostrophic flow)
Surface flow towards equator is northeasterly in NH and southeasterly in SH TRADE WINDS
Subsiding branches located around 30o N and S DESERTS and high surface pressure
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Mid-latitude Circulation
• Westerly flow both NH and SH
• Strong temperature gradient gives rise cold/warm fronts (moving air masses)
• Cyclones and Anticyclones
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Mid-latitude Westerlies
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Warm/Cold Fronts
Strong T gradients
Higher P Higher PLower P
Subsiding Hadley Brach
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Flow Around High and Low Pressure Centers
Upper-level flow geostrophic: parallel to isobars.
High Pressure CenterLow Pressure Center
L H
In NH flow counterclockwise around Lowcyclonic flow
In NH flow clockwise around Highanticyclonic flow
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Surface Flow Impacted by Friction
High Pressure
Low Pressure
PGF Coriolis
Actual Flow direction
Friction
Forces
Friction causes flow to move away from high pressure, but towards low pressure.
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Surface-level Flow affected by Friction
Centers of low or high pressure at surface induce flow that spirals in or out, respectively.
High Pressure CenterLow Pressure Center
L H
Convergence/uplift—StormyDivergence/Subsidence - Nice
cyclones anticyclones
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Tropical Cyclone—Hurricane Gordon
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Midlatitude Cyclones
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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
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Where would you expect “continentality” to be greatest?
Northern Hemisphere Southern Hemisphere
11%
89%1. Northern
Hemisphere2. Southern
Hemisphere
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Continentality-Find the Continents
Contours show annual temperature range: Tsummer - Twinter
4
44 56
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Diurnal (Daily) Sea Breeze
Day Night
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July
January
Similar to diurnal sea breeze but on larger spatial and temporal (seasonal) scales.
Monsoonal Circulation
H
L
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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
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Ocean Circulation and Climate
Reading: Chapter 5
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Atmosphere-Ocean Couplings
1.Heat Exchange
2.Momentum Exchange (surface wind stress)
3.Moisture/Gas Exchange (water and carbon cycles)
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Heat Transport by Ocean and Atmosphere
Tropics Midlatitudes Polar regions
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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
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Wind-driven Surface Ocean Circulation
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Surface Ocean Circulation
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Gulf Stream
western branch of mid-Atlantic gyre
AVHRR Satellite measurement of Sea Surface T
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Convergence And Divergence
Net convergence of surface water in center of gyres
Net divergence at eastern ocean boundaries and equator
Equator
Wind
Surface ocean
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Divergence Causes Upwelling
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Coastal Upwelling/Downwelling
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Marine Stratus Clouds
Eastern-boundary coastal upwelling
Cold water cools air
Cloud formation
Surface winds
Ocean surface flow
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Where do you expect the coldest sea surface temperatures?
Eastern ocean bound... Western ocean boun...
50%50%1. Eastern ocean boundaries
2. Western ocean boundaries
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monthly mean SST animation
1. Latitudinal distribution of solar radiation
2. Heat exchange with atmosphere
3. Circulation patterns (e.g. upwelling)
Sea Surface Temperatures
Observe the following
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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
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Physical Properties versus Depth
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Salinity is measured in parts per thousand
Salinity
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Thermo-haline Circulation(temperature-salty)
Mixed layer ~ 1 km deep
Middle and deep ocean
Lower latitudes High latitudes
Net sinking: Deep Water formation
Ocean-Atm heat transfer
Sea ice
Cold salty water
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•Reduces the influence of the winds
•Insulates the ocean (prevents heat loss)
•Rejects salt when it grows / Adds freshwater when it melts
Sea ice influence on the ocean
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Thermo-haline Circulation (THC)
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Marine Chlorophyll From Space
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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