Chapter 8: Atmospheric Circulation and Pressure Distributions

General Circulation in the Atmosphere

General Circulation in Oceans

Air-Sea Interaction: El Nino

Thermally Direct/Indirect Cells

Thermally Direct Cells (Hadley and Polar Cells)

Both cells have their rising branches over warm temperature zones and sinking braches over the cold temperature zone. Both cells directly convert thermal energy to kinetic energy.

Thermally Indirect Cell (Ferrel Cell)

This cell rises over cold temperature zone and sinks over warm temperature zone. The cell is not driven by thermal forcing but driven by eddy (weather systems) forcing.

Is the Three-Cell Model Realistic?

Yes and No!

    (Due to sea-land contrast and topography)

   Yes: the three-cell model explains reasonably well the surface wind distribution in the atmosphere.

     No: the three-cell model can not explain the circulation pattern in the upper troposphere. (planetary wave motions are important here.)

Bottom Line

Pressure and winds associated with Hadley cells are close approximations of real world conditions

Ferrel and Polar cells do not approximate the real world as well

Surface winds poleward of about 30^o do not show the persistence of the trade winds, however, long-term averages do show a prevalence indicative of the westerlies and polar easterlies

For upper air motions, the three-cell model is unrepresentative

The Ferrel cell implies easterlies in the upper atmosphere where westerlies dominate

Overturning implied by the model is false

The model does give a good, simplistic approximation of an earth system devoid of continents and topographic irregularities

Semi-Permanent Pressure Cells

The Aleutian, Icelandic, and Tibetan lows

The oceanic (continental) lows achieve maximum strength during winter (summer) months

The summertime Tibetan low is important to the east-Asia monsoon

Siberian, Hawaiian, and Bermuda-Azores highs

The oceanic (continental) highs achieve maximum strength during summer (winter) months

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Global Distribution of Deserts

Sinking Branches and Deserts

Thermal Wind Relation

Thermal Wind Equation

¶U/¶z µ ¶T/¶y

The vertical shear of zonal wind is related to the latitudinal gradient of temperature.

Jet streams usually are formed above baroclinic zone (such as the polar front).

Subtropical and Polar Jet Streams

Jet Streams Near the Western US

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Cold and Warm Fronts

Tropical Hurricane

The hurricane is characterized by a strong thermally direct circulation with the rising of warm air near the center of the storm and the sinking of cooler air outside.

They Are the Same Things…

Hurricanes: extreme tropical storms over Atlantic and eastern Pacific Oceans.

Typhoons: extreme tropical storms over western Pacific Ocean.

Cyclones: extreme tropical storms over Indian Ocean and Australia.

Monsoon: Another Sea/Land-Related Circulation of the Atmosphere

How Many Monsoons Worldwide?

Sea/Land Breeze

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Basic Ocean Structures

Upper Ocean (~100 m)

Shallow, warm upper layer where light is abundant and where most marine life can be found.

Deep Ocean

Cold, dark, deep ocean where plenty supplies of nutrients and carbon exist.

Basic Ocean Current Systems

Six Great Current Circuits in the World Ocean

Characteristics of the Gyres

Currents are in geostropic balance

Each gyre includes 4 current components:

      two boundary currents: western and eastern

      two transverse currents: easteward and westward

Western boundary current (jet stream of ocean)

       the fast, deep, and narrow current moves warm water polarward (transport ~50 Sv or greater)

Eastern boundary current

      the slow, shallow, and broad current moves cold water equatorward (transport ~ 10-15 Sv)

Trade wind-driven current

       the moderately shallow and broad westward current (transport ~ 30 Sv)

Westerly-driven current

       the wider and slower (than the trade wind-driven current) eastward current

Major Current Names

Western Boundary Current

     Gulf Stream (in the North Atlantic)

       Kuroshio Current (in the North Pacific)

       Brazil Current (in the South Atlantic)

       Eastern Australian Current (in the South Pacific)

       Agulhas Current (in the Indian Ocean)

Eastern Boundary Current

      Canary Current (in the North Atlantic)

       California Current (in the North Pacific)

       Benguela Current (in the South Atlantic)

       Peru Current (in the South Pacific)

      Western Australian Current (in the Indian Ocean)

Step 1: Surface Winds

Winds and Surface Currents

Step 2: Ekman Layer
(frictional force + Coriolis Force)

Ekman Spiral – A Result of Coriolis Force

Ekman Transport

Step 3: Geostrophic Current
(Pressure Gradient Force + Corioils Foce)

Thermohaline Conveyor Belt

Thermohaline Circulation

Thermo è temperature

Haline è salinity

Global Warming and Thermohaline Circulation

If the warming is slow

The salinity is high enough to still produce a thermohaline circulation

The circulation will transfer the heat to deep ocean

The warming in the atmosphere will be deferred.

If the warming is fast

Surface ocean becomes so warm (low water density)

No more thermohalione circulation

The rate of global warming in the atmosphere will increase.

Mid-Deglacial Cooling: The Younger Dryas

The mid-deglacial pause in ice melting was accompanied by a brief climate osscilation in records near the subpolar North Atlantic Ocean.

Temperature in this region has warmed part of the way toward interglacial levels, but this reversal brought back almost full glacial cold.

Because an Arctic plant called “Dryas” arrived during this episode, this mid-deglacial cooling is called “the Younger Dryas” event.

Interactions Within Climate System

This hypothesis argues that millennial oscillations were produced by the internal interactions among various components of the climate system.

One most likely internal interaction is the one associated with the deep-water formation in the North Atlantic.

Millennial oscillations can be produced from changes in northward flow of warm, salty surface water along the conveyor belt.

Stronger conveyor flow releases heat that melts ice and lowers the salinity of the North Atlantic, eventually slowing or stopping the formation of deep water.

Weaker flow then causes salinity to rise, completing the cycle.

East-West Circulation

Precipitation Climatology

Walker Circulation and Ocean Temperature

Walker Circulation and Ocean

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El Nino and Southern Oscillation

Polar Front Theory

Bjerknes, the founder of the Bergen school of meteorology, developed polar front theory during WWI to describe the formation, growth, and dissipation of mid-latitude cyclones.

Coupled Atmosphere-Ocean System

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Delayed Oscillator: Wind Forcing

The delayed oscillator suggested that oceanic Rossby and Kevin waves forced by atmospheric wind stress in the central Pacific provide the phase-transition mechanism (I.e. memory) for the ENSO cycle.

The propagation and reflection of waves, together with local air-sea coupling, determine the period of the cycle.

Wave Propagation and Reflection

Why Only Pacific Has ENSO?

Based on the delayed oscillator theory of ENSO, the ocean basin has to be big enough to produce the “delayed” from ocean wave propagation and reflection.

It can be shown that only the Pacific Ocean is “big” (wide) enough to produce such delayed for the ENSO cycle.

It is generally believed that the Atlantic Ocean may produce ENSO-like oscillation if external forcing are applied to the Atlantic Ocean.

The Indian Ocean is considered too small to produce ENSO.

North Atlantic Oscillation

The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia.

The NAO is a large scale seesaw in atmospheric mass between the subtropical high and the polar low.

The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years.

Positive and Negative Phases of NAO

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North Atlantic Oscillation
= Arctic Oscillation
= Annular Mode


	General Circulation in the Atmosphere
	General Circulation in Oceans
	Air-Sea Interaction: El Nino


	Thermally Direct Cells (Hadley and Polar Cells)
	Both cells have their rising branches over warm temperature zones and sinking braches over the cold temperature zone. Both cells directly convert thermal energy to kinetic energy.
	Thermally Indirect Cell (Ferrel Cell)
	This cell rises over cold temperature zone and sinks over warm temperature zone. The cell is not driven by thermal forcing but driven by eddy (weather systems) forcing.


	Yes and No!
	(Due to sea-land contrast and topography)

	Yes: the three-cell model explains reasonably well the surface wind distribution in the atmosphere.

	No: the three-cell model can not explain the circulation pattern in the upper troposphere. (planetary wave motions are important here.)



			Pressure and winds associated with Hadley cells are close approximations of real world conditions
			Ferrel and Polar cells do not approximate the real world as well
			Surface winds poleward of about 30^o do not show the persistence of the trade winds, however, long-term averages do show a prevalence indicative of the westerlies and polar easterlies
			For upper air motions, the three-cell model is unrepresentative
			The Ferrel cell implies easterlies in the upper atmosphere where westerlies dominate
			Overturning implied by the model is false
			The model does give a good, simplistic approximation of an earth system devoid of continents and topographic irregularities


	The Aleutian, Icelandic, and Tibetan lows
		The oceanic (continental) lows achieve maximum strength during winter (summer) months
		The summertime Tibetan low is important to the east-Asia monsoon

	Siberian, Hawaiian, and Bermuda-Azores highs
		The oceanic (continental) highs achieve maximum strength during summer (winter) months


	¶U/¶z µ ¶T/¶y

	The vertical shear of zonal wind is related to the latitudinal gradient of temperature.
	Jet streams usually are formed above baroclinic zone (such as the polar front).


	The hurricane is characterized by a strong thermally direct circulation with the rising of warm air near the center of the storm and the sinking of cooler air outside.


	Hurricanes: extreme tropical storms over Atlantic and eastern Pacific Oceans.
	Typhoons: extreme tropical storms over western Pacific Ocean.
	Cyclones: extreme tropical storms over Indian Ocean and Australia.


	Upper Ocean (~100 m)
	Shallow, warm upper layer where light is abundant and where most marine life can be found.

	Deep Ocean
	Cold, dark, deep ocean where plenty supplies of nutrients and carbon exist.


	Currents are in geostropic balance
	Each gyre includes 4 current components:
	two boundary currents: western and eastern
	two transverse currents: easteward and westward
	Western boundary current (jet stream of ocean)
	the fast, deep, and narrow current moves warm water polarward (transport ~50 Sv or greater)
	Eastern boundary current
	the slow, shallow, and broad current moves cold water equatorward (transport ~ 10-15 Sv)
	Trade wind-driven current
	the moderately shallow and broad westward current (transport ~ 30 Sv)
	Westerly-driven current
	the wider and slower (than the trade wind-driven current) eastward current


	Western Boundary Current
	Gulf Stream (in the North Atlantic)
	Kuroshio Current (in the North Pacific)
	Brazil Current (in the South Atlantic)
	Eastern Australian Current (in the South Pacific)
	Agulhas Current (in the Indian Ocean)
	Eastern Boundary Current
	Canary Current (in the North Atlantic)
	California Current (in the North Pacific)
	Benguela Current (in the South Atlantic)
	Peru Current (in the South Pacific)
	Western Australian Current (in the Indian Ocean)


	If the warming is slow
	The salinity is high enough to still produce a thermohaline circulation
	The circulation will transfer the heat to deep ocean
	The warming in the atmosphere will be deferred.

	If the warming is fast
	Surface ocean becomes so warm (low water density)
	No more thermohalione circulation
	The rate of global warming in the atmosphere will increase.


	The mid-deglacial pause in ice melting was accompanied by a brief climate osscilation in records near the subpolar North Atlantic Ocean.
	Temperature in this region has warmed part of the way toward interglacial levels, but this reversal brought back almost full glacial cold.
	Because an Arctic plant called “Dryas” arrived during this episode, this mid-deglacial cooling is called “the Younger Dryas” event.


	This hypothesis argues that millennial oscillations were produced by the internal interactions among various components of the climate system.
	One most likely internal interaction is the one associated with the deep-water formation in the North Atlantic.
	Millennial oscillations can be produced from changes in northward flow of warm, salty surface water along the conveyor belt.
	Stronger conveyor flow releases heat that melts ice and lowers the salinity of the North Atlantic, eventually slowing or stopping the formation of deep water.
	Weaker flow then causes salinity to rise, completing the cycle.


	Bjerknes, the founder of the Bergen school of meteorology, developed polar front theory during WWI to describe the formation, growth, and dissipation of mid-latitude cyclones.


	The delayed oscillator suggested that oceanic Rossby and Kevin waves forced by atmospheric wind stress in the central Pacific provide the phase-transition mechanism (I.e. memory) for the ENSO cycle.
	The propagation and reflection of waves, together with local air-sea coupling, determine the period of the cycle.


	Based on the delayed oscillator theory of ENSO, the ocean basin has to be big enough to produce the “delayed” from ocean wave propagation and reflection.
	It can be shown that only the Pacific Ocean is “big” (wide) enough to produce such delayed for the ENSO cycle.
	It is generally believed that the Atlantic Ocean may produce ENSO-like oscillation if external forcing are applied to the Atlantic Ocean.
	The Indian Ocean is considered too small to produce ENSO.


	The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia.

	The NAO is a large scale seesaw in atmospheric mass between the subtropical high and the polar low.

	The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years.