1	Lecture 5: Atmospheric General Circulation Basic Structures and Dynamics General Circulation in the Troposphere General Circulation in the Stratosphere Wind-Driven Ocean Circulation
2	Single-Cell Model: Explains Why There are Tropical Easterlies
3	Breakdown of the Single Cell è Three-Cell Model
4	Properties of the Three Cells
5	Atmospheric Circulation: Zonal-mean Views
6	The Three Cells
7	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.
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9	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.)
10	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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13	Sinking Branches and Deserts
14	Global Distribution of Deserts
15	Upper Tropospheric Circulation
16	Subtropical and Polar Jet Streams
17	Thermal Wind Relation
18	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).
19	Jet Streams Near the Western US
20	Parameters Determining Mid-latitude Weather Temperature differences between the equator and poles The rate of rotation of the Earth.
21	Rotating Annulus Experiment
22	Carl Gustav Rossby (1898-1957) Carl Rossby mathematically expressed relationships between mid-latitude 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 the north-south temperature differences in middle latitudes.
23	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.
24	El Nino and Southern Oscillation
25	Walker Circulation and Ocean Temperature
26	East-West Circulation
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28	Walker Circulation and Ocean
29	1997-98 El Nino
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31	Monsoon: Sea/Land-Related Circulation
32	How Many Monsoons Worldwide?
33	Seasonal Cycle of Rainfall
34	Temperatures in Stratosphere
35	Ozone Distribution
36	Stratosphere: Circulation and Temperature
37	Circulation in Stratosphere
38	Zonal-Mean Circulation in the Stratosphere
39	Ozone Production and Destruction
40	Sudden Warming Every other year or so the normal winter pattern of a cold polar stratosphere with a westerly vortex is 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!
41	Antarctic Ozone Hole 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.
42	The 1997 Ozone Hole
43	Why No Ozone Hole in Artic?
44	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 (2) by providing surfaces on which heterogeneous reactions can occur.
45	Ozone Hole Depletion Long Antarctic winter (May through September) The stratosphere is cold enough to form PSCs PSCs deplete odd nitrogen (NO) Help convert unreactive forms of chlorine (ClONO2 and HCl) into more reactive forms (such as Cl2). The reactive chlorine remains bound to the surface of clouds particles. Sunlight returns in springtime (September) The sunlight releases reactive chlorine from the particle surface. The chlorine destroy ozone in October. Ozone hole appears. At the end of winter, the polar vortex breaks down. Allow fresh ozone and odd nitrogen to be brought in from low latitudes. The ozone hole recovers (disappears) until next October.
46	Winds and Surface Currents
47	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.
48	Global Surface Currents
49	Six Great Current Circuits in the World Ocean
50	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
51	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)
52	Surface Current – Geostrophic Gyre Mixed Layer Currents controlled by frictional force + Coriolis force à wind-driven circulation à Ekman transport (horizontal direction) à convergence/divergence à downwelling/upwelling at the bottom of mixed layer Thermocline downwelling/upwelling in the mixed layer à pressure gradient force + Coriolis force à geostrophic current à Sverdrup transport (horizontal)
53	Step 1: Surface Winds
54	Winds and Surface Currents
55	Step 2: Ekman Layer (frictional force + Coriolis Force)
56	Ekman Spiral – A Result of Coriolis Force
57	Ekman Transport
58	Step 3: Geostrophic Current (Pressure Gradient Force + Corioils Foce)
59	Thermohaline Circulation Thermo è temperature Haline è salinity
60	Thermohaline Conveyor Belt
61	It Takes ~1000 Years for Deep Ocean Waters to Travel Around… If we date a water parcel from the time that it leaves the surface and sink into the deep ocean è Then the youngest water is in the deep north Atlantic, and the oldest water is in the deep northern Pacific, where its age is estimated to be 1000 year.
62	The Most Unpolluted Waters are.. the waters in the deep northern Pacific. The man-released CFC and the chemical tritium and C¹⁴, which were released through atmospheric atomic bomb test in the 1950s and 1960s, entered the deep ocean in the northern Atlantic and are still moving southward slowly. Those pollutions just cross the equator in the Atlantic è They have not reached the deep northern Pacific yet!!
63	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.