Coriolis Effect Modifies Hadley Circulation
Feb 09, 2016
Coriolis Effect Modifies Hadley Circulation
Modified Hadley Circulation
Horizontal motionsconvergence: coming
togetherdivergence: spreading
apart
Vertical motionsupwelling: rising
airsubsidence:
sinking air
*
“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
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
True False
78%
22%
1. True2. False
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)
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 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
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
Strong T gradients
Higher P Higher PLower P
Subsiding Hadley Brach
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
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.
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
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?
Northern Hemisphere Southern Hemisphere
11%
89%1. Northern
Hemisphere2. Southern
Hemisphere
Continentality-Find the Continents
Contours show annual temperature range: Tsummer - Twinter
4
44 56
Diurnal (Daily) Sea Breeze
Day Night
July
January
Similar to diurnal sea breeze but on larger spatial and temporal (seasonal) scales.
Monsoonal Circulation
H
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
Equator
Wind
Surface ocean
Divergence Causes Upwelling
Coastal Upwelling/Downwelling
Marine Stratus Clouds
Eastern-boundary coastal upwelling
Cold water cools air
Cloud formation
Surface winds
Ocean surface flow
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
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
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 is measured in parts per thousand
Salinity
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
•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
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