Atmospheric & Ocean Circulation-mdmccar/migrated/ocea80b/public/lectures/... · "Image/Text/Data from the University of Illinois WW2010 Project." Flow around LOW Cyclonic flow Counter-clockwise

Atmospheric & Ocean Circulation-

Overview: Atmosphere & Climate

• Atmospheric layers• Heating at different latitudes• Atmospheric convection cells (Hadley, Ferrel,

Polar)• Coriolis Force• Generation of winds• Low pressure, wet, convergence• High pressure, dry, divergence• Climate zones

Recall: Atmospheric temp. vs. height

Heated atthe bottom:where the land iswarm

Heated from top:ozoneabsorbsenergy

Atmospheric layers have different stability

TROPOSPHERE:Unstable becauseatmosphere is heated from below:CONVECTINGCONDITIONS~80% of mass of atm

STRATOSPHERE:Stable because cool dense air is beneath warm dense air:STRATIFIEDCONDITIONS~19.9% of mass ofatm

overall: TROPOSPHERE

=the Weather Zone

Also in TROPOSPHERE:CONVECTION due to heating from below

STRATOSPHERE:STABLE because cool dense air is beneath warm dense air:STRATIFIEDCONDITIONS

STEPPING BACK:

Fundamentally, Why does the Atmosphere circulate at all?

What is energy source that sets it in motion?

Oceanic motion ultimately derives from the Sun’s rays

© Calvin J. Hamilton

NASA

http://www.solarviews.com/raw/misc/ss.gif

http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question31.html

Lights Please!

Uneven solar heating with latitude

• Solar energy in high latitudes:– Has a larger “footprint” – Is reflected to a greater

extent– Passes through more

atmosphere• Therefore, less solar

energy per square meter is absorbed at high latitudes than at low latitudes

Uneven solar heating with latitudeanother way to visualize: how “high” is sun in sky?

Recall: basic radiation budget

Earth

Incoming visible

• Energy arrives as visible (shortwave) sunlight

Reflected solar

• About 30% is reflected “albedo”

• Higher reflection “albedo” at higher latitudes

Outgoing infrared• The remainder leaves

as outgoing infrared (longwave) radiation

*+ GREENHOUSE GASSES : In atm. trap that outgoing long-wave radiation

This basic model does a good job of predicting AVG global temp..

BUT: Do you think it does a good job of predicting the temp at Equator vs. Santa Cruz?

Incoming visible

Outgoing infrared

Earth

Reflected solar

In a word: “NO”

Incoming visible

Outgoing infrared

Earth

Reflected solar

1. Curvature of the Earth’s surface (flashlight effect..)

2. Albedo increases with latitude (more snow in N..)

Recall Solar heating imbalance with latitude:

Predicts WAY too much heat at Equator (ie, predicts mid-latitudes warmer than they are..)

WAY too little heat at poles (ie, predicts them to be colder than they are)

• Means: a net heat gain is experienced in low latitudes

• A net heat loss is experienced in high latitudes

• ?

Actual- much more EVEN

How do we explain the global Heat transfer that must be happening?

Recall..

Convection:the soup analogy.!

As with earth’s crust: its still all about Density

If air mass WARMS•molecules move more quickly•air mass expands•DENSITY DECREASES•AIR MASS RISES

If air mass COOLS•molecules move less quickly•air mass contracts•DENSITY INCREASES•AIR MASS SINKS

Up in atm

osphere

Implications of differential warming: Convection in Troposphere*!

Figure 6-5

• Warm, low density air rises

• Cool, high density air sinks

• Creates circular-moving loop of air (convection cell)

* remember, this is lower layer that is heated from BELOW

One more thing: High vs. Low air Pressures

Figure 6-6

• A column of cool, dense aircauses high pressure at the surface, which will lead to sinking air

• A column of warm, less denseair causes low pressure at the surface, which will lead to rising air

A big result of Different “Pressure” zones: moisture.

Equator 30˚ N

LOW Pressure HIGH Pressure

As air rises, it cools, water condenses, lots of rain

As air sinks, it warms,lots of evaporation

Warm rising airWIND

WIND

Cold sinking air

What does water this transport have to do with heat?

Water phase changes require enormous energy (in part due to H-bonding)

Figure shows “latent heat” of each water phase change

Evaporation (liq.=> gas) removes heat fromatm.

Rain (gas => liquid) releases heat to atm.

So, thinking about a circulation cell,

WHAT WOULD YOU EXPECT THEM TO BE LIKE?

So: Basic Global wind patterns RESULT differential heating/cooling.

They then redistribute heat due

To review:Basic Convection Cell

“Ideal” Circulation for a non-rotating Earth

Fig. 6-7

• Warm air would rise at the equator

• Cold air would sink at the poles

• Single circulation cell with equator-ward flow

But, of course , it doesn’t work in the “ideal” way.

Fig. 6-7

• Why NOT?

• Density and pressure differences create smaller “cells” of circulation!

AtmosphericCells

90˚N

30˚N

60˚N

Equator L

H

HL

Rising air•Low pressure zone•Convergence•Wet - Tropical

Sinking air•High pressure zone•Divergence•Dry - Sub-tropical

Rising air•Low pressure zone•Convergence•Wet - Temperate / Sub-Polar

Sinking air•High pressure zone•Divergence•Dry - Arctic / Polar

Hadley Cell

Ferrel Cell

Polar Cell

Overview: Atmospheric Circulation

• 1) Think of density differences driving vertical movements

• Warm air rises• Cool air sinks

• 2) Think of pressure differences driving horizontal movements

• Air moves fromHIGH TO LOW pressure

• 3) Think of evaporation/ precipitation of water carried by winds as transporting heat.

Major circulation cells global moisture

bands

Equator 30˚ NLOW Pressure HIGH PresWarm rising air

WIND

WIND

Cold sinking a

Fig. 13.2

Rainy equator?ITCZ – Inter-tropical Convergence Zone

ITCZ - Intertropical Convergence Zone

More evidence of “Hadley” cell” Cooling as the air rises causes the water Cooling as the air rises causes the water vapor to condense as clouds andvapor to condense as clouds and rain rain -- releasing its latent heat. The heat can then releasing its latent heat. The heat can then transported to highertransported to higher latitudes by the Hadley cells (directly as warmer air, as well latitudes by the Hadley cells (directly as warmer air, as well

as indirectly as water vapor)as indirectly as water vapor)

BUT :

THERE IS YET ANOTHER COMPLICATION

Big Complication #2: The Earth rotates The “Coriolis” Effect

• Accounts for how things move relative to the earths surface (which is rotating underneath them!)

• Causes objects in motion to curve (relative to the earth!)– To right in the North– To left in the south

N. HemisphereDeviate to Right(relative to direction of motion)

S. HemisphereDeviate to Left (relative to direction of motion)

Coriolis effect

Coriolis EffectConsequence of something moving over a turning object..

Figure 6-9

The Earth rotates The Coriolis Effect

"Image/Text/Data from the University of Illinois WW2010 Project."

Flow around LOWCyclonic flowCounter-clockwise in NH(Clockwise in SH)

Flow around HIGHAnti - Cyclonic flowClockwise in NH(Counter-clockwise in SH)`

Also explains direction that storm winds circulate

Resulting Atmospheric

Cells & Winds90˚N

30˚N

60˚N

Equator L

H

HL

Hadley Cell

Ferrel Cell

Polar Cell

H HHHHH

L LL L LL

L LL L L

Northeasterly Trade Winds

Prevailing Westerlies

Polar Easterlies

Southeasterly Trade Winds

H HHHHH30˚S H

Resultant cells

Hadley Cell

Ferrel Cell

Polar Cell

Major ”Circulation cells”- start with idealCells, then add the “twisting” of coriolis!

IntertropicalConvergence Zone

Horse latitudes

Polar Front

Note: BOUNDARIES BETWEEN WINDBELTS

Finally, the real world naturally deviates even more from this ideal

“three cell” model

Real world Complications:

Regional or local pressure gradients can be influenced by:

• Seasons: Tilt of earth’s axis - latitude of max. heating changes through the year

• Land:– Variations in land topography and albedo– Land - Sea contrasts

Heat flux in July (W/m2)

Heat flux in January (W/m2)

Why does solar heating change seasonally?

Fig 6-1

Seasonal Heating Differences Due to TILT

Tilt = 23.5 ˚ , CAUSES SEASONS

PerihelionAphelion

• Heat capacity of rock is much less than that of water, so land heats up more quickly during the day than the water.

• Air above land warms and rises.

Land-driven Sea Breezes ( Very near to shore.)

• At nighttime, no solar influx, but outgoing radiation remains. So both land and sea cool.

• However, land cools more rapidly than water because of a lower heat capacity. Circulation reverses.

Can experience this on our Coast: leads to afternoon onshore sea breezes

(even better example is S. Cal coast)

Real world: complicated and varies by region

• Seasonal heating changes• Variations in land topography and albedo

These factors produce:- some strong High and Low Pressure Zones- lots of change from one Season to another

Next: Main wind bands lead to Ocean Circulation!