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