The General Circulation of the Atmosphere and Oceans

ATS 351

Lecture 9

November 2, 2009

The Global Heat Budget

• The Atmosphere has a radiative/convective equilibrium in the vertical– Incoming and outgoing radiation– Covection and subsidence

• It also has a similar equilibrium in the horizontal

Collects in tropics Escapes near poles and aloft

Annual Radiation Budget

• Averaged over a year, there is a surplus of energy at the equator

• Since energy flows from high to low, it must go to the poles.

Single-Cell Model

Assumptions: Earth’s surface uniformly covered

with water Sun is always directly over

equator The earth does not rotate

A huge thermally direct cell develops in each hemisphere

Warm air rises and cold air sinks

Single-Cell Model

Assumptions: Earth’s surface uniformly covered

with water Sun is always directly over

equator The earth does not rotate

A huge thermally direct cell develops in each hemisphere

Warm air rises and cold air sinks

Three-Cell Model

Add in rotation…Three cells in each hemisphere

Hadley cell Ferrel cell Polar cell

Weak winds at the Equator (doldrums) and 30 degrees (horse latitudes)Boundary between cold polar air and mid-latitude warmer air is the polar front.

Three-Cell ModelHadley Cell

Thermally direct Driven by meridional gradient in heating Air rises near the equator and sinks near 30 degrees Explains deserts, trade winds, ITCZ

Ferrel Cell Thermally indirect Driven by heat transports of eddies (storms) Air rises near 60 degrees and sinks near 30 degrees

Polar cell Thermally direct

Inside the Ferrel Cell

• Westerly momentum is transferred from the earth to the atmosphere in the trade wind belt

• Westerly momentum is transferred from the atmosphere to the earth in the midlatitudes

• Why don’t the midlatitude westerlies slow down over time?• Eddies (storms) transfer momentum poleward in the upper

troposphere• This momentum transfer weakens the Hadley circulation, but

drives the Ferrel Cell

Three-Cell Model

3-Cell Model Pressure

The Real World• It’s much more messy!

– Continents, mountains, ice fields and forests all get in the way.

• The real wind patterns form around quasi-permanent high and low pressure systems rather than the banded ones discussed previously.– During winter, highs form over the land, lows over the

oceans. This flips in the summer. WHY?– The Bermuda high and Pacific High form near 30 degrees

north in response to convergence aloft (mainly in the NH summer)

• ITCZ moves south in January and north in July

Global Pressure Patterns

Winter Summer

Global Precipitation PatternsJanuary

Very Wet in the tropics (ITCZ) Monsoon Regions Seasonal Shift of the ITCZ Mid latitudes get more rain in the summer Storm tracks More variability in the NH

July

Take Home Concepts

1. Driven by differential solar heating between the equator and the poles. Acts to move heat poleward.

2. In Hadley cell, warmer air rises and moves poleward.3. Ferrel cell is driven by heat and momentum fluxes by eddies.4. In the NH, air is deflected to the right as it moves. Opposite in SH.5. Pole-to-pole Hadley Cell is unstable in the presence of rotation,

hence the single cell model breaks down.6. Rotation makes the trade winds, surface westerlies, and jet

streams

But What about the Oceans?

But What about the Oceans?

Heat still needs to be transported poleward

Sea Surface Temperatures

Temperature Structure of the Ocean

• Mixed Layer– Temperature in the top 50

m doesn’t change much with depth

• Thermocline– Rapid temperature drop

• Deep ocean

Temperature Structure of the Ocean

Ocean Currents

What causes ocean circulation?• The direction of most ocean

currents is determined largely by the wind

• Coriolis force acts to turn the moving water to the right (in the NH)

• Friction directly opposes motion

• Resulting current at the surface moves 45° to the right of the surface wind (in the NH)

Ekman Flow

• As you get deeper into the ocean, the flow is weaker and curved more toward the right

• Mean motion is 90° to the right of the wind

Ekman Pumping

• Ekman flow turns ocean to the right of the wind

• Cyclonic wind leads to divergence of water and upwelling in the center

• Anticyclonic wind leads to convergence of water and downwelling in the center

Ocean Gyres

• Idealized gyre is a rotating “dome” of water • Real world gyres are asymmetric

– Eastern boundary current is wide and moves slowly– Western boundary current is much faster

(e.g. the Gulf Stream)

Idealized vs. Real World

Coastal Upwelling

• Along west coast of U.S. prevailing wind is from the north in the summer

• Ekman pumping along western coasts leads to ocean moving away from the land

• Need upwelling to compensate – colder deepwater moves up near the coast

• Ocean temperatures off the coast of California are typically much colder than at similar latitudes on the east coast

El Nino & La Nina

El Nino La Nina

El Nino & La Nina•Non-El Nino:• Upwelling and cooler water in eastern Pacific • Warmer water in western Pacific

•El Nino:• Change in pressure causes trades to reverse• Reverses figure above - warmer in eastern Pacific

North America and ENSO

El Nino Wet weather and storms into California and the southern part

of US Weak polar jet stream over Canada creates warmer than

normal weather over a large part of NALa Nina

Moist air from ocean is directed into the Pacific NW: wet winter for that region

Winter months in southern part of US tend to be warmer and drier than normal

Thermohaline Circulation

• Circulation in the ocean is also caused by density differences• Density differences come from differences in temperature (thermo)

and salinity (haline)• In the north Atlantic, freezing leads to high salinity content• Cold, salty water sinks and forms North Atlantic Deep Water• Warmer water moves in at the surface to replace the mass lost

Thermohaline Circulation

Thermohaline Circulation