Properties of the Three Cells

Lecture 5: Atmospheric General Circulation Single-Cell Model: Explains Why There are Tropical Easterlies

JS JP

Without Earth Rotation With Earth Rotation Hadley Cell Ferrel Cell Polar Cell (driven by eddies) LHL H

Basic Structures and Dynamics Coriolis Force General Circulation in the Troposphere General Circulation in the Stratosphere Wind-Driven Ocean Circulation

ESS55 (Figures from Understanding Weather & Climate and The Earth System) ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

Breakdown of the Single Cell Î Three-Cell Model Properties of the Three Cells

Absolute angular momentum at Equator = Absolute angular momentum at 60°N thermally indirect circulation thermally direct circulation

The observed zonal velocity at the equatoru is ueq = -5 m/sec. Therefore, the total velocity at the equator is U=rotational velocity (U + u ) 0 Eq JS JP

The zonal wind velocity at 60°N (u ) can be determined by the following: 60N Hadley Cell Ferrel Cell Polar Cell (U0 + uEq) * a * Cos(0°) = (U60N + u60N) * a * Cos(60°) (driven by eddies)

(Ω*a*Cos0° - 5) * a * Cos0° = (Ω*a*Cos60° + u60N) * a * Cos(60°)

u60N = 687 m/sec !!!! LHL H Equator 30° 60° Pole This high wind speed is not (warmer) (warm) (cold) (colder) observed!

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1 Atmospheric Circulation: Zonal-mean Views The Three Cells

Single-Cell Model Three-Cell Model ITCZ

(Figures from Understanding Weather & Climate and The Earth System) Subtropical midlatitude High Weather system

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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.

(from The Earth System) ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

2 Semi-Permanent Pressure Cells Is the Three-Cell Model Realistic?

Yes and No! The Aleutian, Icelandic, and Tibetan lows (Due to sea-land contrast and topography) – The oceanic (continental) lows achieve maximum strength during winter (summer) months – The summertime Tibetan low is important to the east-Asia Yes: the three-cell model explains reasonably well the monsoon surface wind distribution in the atmosphere. Siberian, Hawaiian, and Bermuda-Azores highs – The oceanic (continental) highs achieve maximum strength during No: the three-cell model can not explain the circulation summer (winter) months pattern in the upper troposphere. (planetary wave motions are important here.)

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January July

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3 Sinking Branches and Deserts Global Distribution of Deserts

(from Weather & Climate)

(from Global Physical Climatology) ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

Upper Tropospheric Circulation Subtropical and Polar Jet Streams

Subtropical Jet Only the Hadley Cell can be Located at the higher-latitude end of the identified in the lower latitude Hadley Cell. The jet obtain its part of the circulation. maximum wind speed (westerly) due the conservation of angular momentum. Circulation in most other latitudes are dominated by Polar Jet westerlies with wave patterns. Located at the thermal boundary Dominated by large-scale between the tropical warm air and the waver patterns (wave number 3 polar cold air. The jet obtain its maximum wind speed (westerly) due the in the Northern hemisphere). latitudinal thermal gradient (thermal wind relation).

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4 Thermal Wind Relation Thermal Wind Equation

∂U/∂z ∝ - ∂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).

(from Weather & Climate)

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Jet Streams Near the Western US Parameters Determining Pineapple Express Mid-latitude Weather

Temperature differences between the equator and poles Both the polar and subtropical jet streams can affect weather and climate in the western US (such as California). The rate of rotation of the Earth. El Nino can affect western US climate by changing the locations and strengths of these two jet streams.

(from Riehl (1962), Palmen and Newton (1969))

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5 Rotating Annulus Experiment Cooling New Understanding of Cyclone after WWII Outside Carl Rossby mathematically expressed relationships Heating between mid-latitude Inside cyclones and the upper air during WWII. Mid-latitude cyclones are a large-scale waves (now called Rossby waves) that grow from the “baroclinic” instabiloity associated with Carl Gustav Rossby (1898-1957) the north-south temperature differences in middle latitudes.

(from “Is The Temperature Rising?”) ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

Polar Front Theory El Nino and Southern Oscillation

Jacob Bjerknes was the first one to Bjerknes, the founder of the recognizes that El Nino is not just an oceanic phenomenon (in his 1969 Bergen school of meteorology, paper). developed polar front theory during WWI to describe the In stead, he hypothesized that the formation, growth, and warm waters of El Nino and the pressure seasaw of Walker’s Southern dissipation of mid-latitude Oscillation are part and parcel of the cyclones. same phenomenon: the ENSO.

Bjerknes’s hypothesis of coupled Jacob Bjerknes atmosphere-ocean instability laid the foundation for ENSO research. Vilhelm Bjerknes (1862-1951) ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

6 Walker Circulation and Ocean Temperature East-West Circulation

(from Flohn (1971))

The east-west circulation in the atmosphere is related to the sea/land distribution on the Earth.

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Walker Circulation and Ocean

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7 Scales of Motions in the Atmosphere 1997-98 El Nino

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Monsoon: Sea/Land-Related Circulation How Many Monsoons Worldwide?

North America Monsoon Asian Monsoon Monsoon (Arabic “season”) Monsoon is a climate feature that is characterized by the seasonal reversal in surface winds. The very different heat capacity of land and ocean surface is the key mechanism that produces monsoons. During summer seasons, land surface heats up faster than Courtesy of Kevin G. Cannariato the ocean. Low pressure center is established over land while high pressure center is established over oceans. Winds blow from ocean to land and bring large amounts of water vapor to produce heavy precipitation over land: A rainy season. Australian During winters, land surface cools down fast and sets up Monsoon a high pressure center. Winds blow from land to ocean: a dry season. South America Monsoon Africa Monsoon ESS55 ESS55 Prof. Jin-Yi Yu (figure from Weather & Climate) Prof. Jin-Yi Yu

8 Seasonal Cycle of Rainfall Temperatures in Stratosphere (from IRI) Northern Winter Northern Summer

Indian Monsoon mesosphere

Australian Monsoon stratosphere

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Ozone Distribution Stratosphere: Circulation and Temperature

Zonal Wind Temperature

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9 Circulation in Stratosphere Zonal-Mean Circulation in the Stratosphere

Northern Winter Northern Summer mesosphere stratosphere

winter summer (from Dynamic Meteorology) ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

Ozone Production and Destruction Sudden Warming

(from The Earth System) Every other year or so the normal winter pattern of a cold polar stratosphere with Photodissociation a westerly vortex is (or photolysis) interrupted in the middle winter. The polar vortex can completely disappear for a period of a few weeks. During the sudden warming period, the stratospheric temperatures can rise as much as 40°K in a few days!

visible light destroy O3 permanently ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

10 Antarctic Ozone Hole The 1997 Ozone Hole

Mean Total Ozone Over Antarctic in October The decrease in ozone near the South Pole is most striking near the spring time (October). During the rest of the year, ozone levels have remained close to normal in the region.

(from The Earth System)

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Why No Ozone Hole in Artic? Polar Stratospheric Clouds (PSCs)

In winter the polar stratosphere is so cold (-80°C or below) that certain trace atmospheric constituents can condense. These clouds are called “polar stratospheric clouds” (PSCs). The particles that form typically consist of a mixture of water and nitric acid (HNO3). The PSCs alter the chemistry of the lower stratosphere in two ways: (1) by coupling between the odd nitrogen and chlorine cycles (Sweden, January 2000; from NASA website) (2) by providing surfaces on which (from WMO Report 2003) heterogeneous reactions can occur.

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11 Ozone Hole Depletion Winds and Surface Currents

Long Antarctic winter (May through September) Î The stratosphere is cold enough to form PSCs Î PSCs deplete odd nitrogen (NO) Polar Cell Î Help convert unreactive forms of chlorine (ClONO2 and HCl) into more reactive forms (such as Cl2). Ferrel Cell Î The reactive chlorine remains bound to the surface of clouds particles. Î Sunlight returns in springtime (September) Hadley Cell Î The sunlight releases reactive chlorine from the particle surface. Î The chlorine destroy ozone in October. Î Ozone hole appears. Î At the end of winter, the polar vortex breaks down. Î Allow fresh ozone and odd nitrogen to be brought in from low latitudes. Î The ozone hole recovers (disappears) until next October. (Figure from The Earth System) ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

Basic Ocean Structures Global Surface Currents Warm up by sunlight!

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.

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12 Six Great Current Circuits in the World Ocean Characteristics of the Gyres

5 of them are geostrophic gyres: (Figure from Oceanography by Tom Garrison) Currents are in geostropic balance North Pacific Gyre Each gyre includes 4 current components: two boundary currents: western and eastern South Pacific Gyre two transverse currents: easteward and westward North Atlantic Gyre Western boundary current (jet stream of ocean) South Atlantic Gyre the fast, deep, and narrow current moves warm water polarward (transport ~50 Sv or greater) Indian Ocean Gyre Eastern boundary current the slow, shallow, and broad current moves cold The 6th and the largest current: water equatorward (transport ~ 10-15 Sv) Trade wind-driven current Antarctic Circumpolr Current the moderately shallow and broad westward (also called West Wind Drift) current (transport ~ 30 Sv) Westerly-driven current the wider and slower (than the trade wind-driven (Figure from Oceanography by Tom Garrison) Volume transport unit: current) eastward current 1 sv = 1 Sverdrup = 1 million m3/sec ESS55 ESS55 Prof. Jin-Yi Yu (the Amazon river has a transport of ~0.17 Sv) Prof. Jin-Yi Yu

Major Current Names Surface Current – Geostrophic Gyre

Mixed Layer Western Boundary Current Trade Wind-Driven Current Currents controlled by frictional force + Coriolis force Gulf Stream (in the North Atlantic) North Equatorial Current Æ wind-driven circulation Kuroshio Current (in the North Pacific) South Equatorial Current Brazil Current (in the South Atlantic) Æ Ekman transport (horizontal direction) Eastern Australian Current (in the South Pacific) Æ convergence/divergence Agulhas Current (in the Indian Ocean) Æ downwelling/upwelling at the bottom of mixed layer

Eastern Boundary Current Westerly-Driven Current Canary Current (in the North Atlantic) North Atlantic Current (in the North Atlantic) Thermocline California Current (in the North Pacific) North Pacific Current (in the North Pacific) downwelling/upwelling in the mixed layer Benguela Current (in the South Atlantic) Peru Current (in the South Pacific) Æ pressure gradient force + Coriolis force Western Australian Current (in the Indian Ocean) Æ geostrophic current Æ Sverdrup transport (horizontal)

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13 Step 1: Surface Winds Winds and Surface Currents

Polar Cell Ferrel Cell

Hadley Cell

(Figure from The Earth System) (Figure from Oceanography by Tom Garrison) ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

Step 2: Ekman Layer Step 2: Ekman Layer Ekman Spiral – A Result of Coriolis Force (frictional force + Coriolis Force)

(Figure from Oceanography by Tom Garrison) ESS55 (Figure from The Earth System) ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

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

NASA-TOPEX Observations of Sea-Level Hight

(from Oceanography by Tom Garrison)

(Figure from The Earth System) ESS55 ESS55 Prof. Jin-Yi Yu Prof. Jin-Yi Yu

Thermohaline Circulation Thermohaline Conveyor Belt

Typical speed for deep ocean current: Thermo Î temperature 0.03-0.06 km/hour. Antarctic Bottom Water takes some 250- Haline Î salinity 1000 years to travel to North Atlantic and Pacific.

Density-Driven Circulation

Cold and salty waters go down (Figure from Oceanography by Tom Garrison) Warm and fresh waters go up

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15 It Takes ~1000 Years for Deep Ocean Waters to Travel The Most Unpolluted Waters are.. Around… the waters in the deep northern Pacific.

The man-released CFC and the chemical tritium If we date a water parcel from the time that it and C14, which were released through atmospheric leaves the surface and sink into the deep ocean atomic bomb test in the 1950s and 1960s, entered the deep ocean in the northern Atlantic and are still moving southward slowly. Î Then the youngest water is in the deep north Atlantic, and the oldest water is in the deep Those pollutions just cross the equator in the northern Pacific, where its age is estimated to be Atlantic Î They have not reached the deep 1000 year. northern Pacific yet!!

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Global Warming and Thermohaline Circulation

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

If the warming is fast Surface ocean becomes so warm (low water density) ÎNo more thermohalione circulation ÎThe rate of global warming in the atmosphere will increase.

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