G109: Weather and Climate Wind Systems 4. Macroscale Winds 1. Global Circulation 1. Single-cell Model 2. Three-cell Model 3. Zonal Precipitation Patterns 4. Semi-Permanent Pressure Cells 2. Asian Monsoon 3. Jet Stream 4. Rossby Waves 5. El Niño – Southern Oscillation Readings A&B: Ch.8 (p. 213-247) CD Tutorial: El Niño – Southern Oscillation Topics 1. Concepts 1. Scale 2. Wind Direction 3. Differential Heating 2. Microscale Winds 3. Mesoscale Winds 1. Land & sea breezes 2. Mountain or valley winds 3. Chinook 4. Santa Ana winds 5. Katabatic winds G109: Weather and Climate 11: Wind Systems Concepts • Three major divisions Days 100 – 1000 km Synoptic Macro Days – Weeks >1000 km (global) Planetary Seconds – Hours Kilometers Meso Seconds – Minutes Meters Micro Time Space Scale Scale Wind Direction • Based on where the wind is Sea breeze: air coming from the sea Northwest wind: wind blowing from the northwest
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Low pressure system on the of a mountain barrier – pulls the air across
as it comes down mountainT can rise by 20oC Usually occur
G109: Weather and Climate 11: Wind Systems
Mesoscale Winds: Santa Ana Winds
• Santa Ana Winds – California High pressure system over the Rocky MountainsAir flows away from high, down western slopes
as it comes down mountainT can rise by 30oC Usually occur
Often contributes to spread of forest fires in CA
G109: Weather and Climate 11: Wind Systems
Mesoscale Winds: Katabatic Winds
• Katabatic Winds Cold downslope wind –Cold air sinks because more dense –
but still than lower elevation air it displacesIf channeled into narrow valleys → high velocitiesFrequently occur at edges of Greenland and Antarctic ice sheetsDifferent names in different places
• Bora: Balkans → Adriatic sea
• Mistral: Alps → France
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Global Circulation
• Synoptic and planetary (macroscale) winds influence the smaller scale (mesoscale and microscale) winds
• Global CirculationDifferential heating between equator and poles
→Global scale pressure differences
→Persistent large-scale motion
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Global Circulation
• Single Cell Model –Differential heatingAssumptions:
• Earth is uniformly covered with water
• Sun is directly over equator
→ Single-cell pattern of flow – Hadley Cell
• Warm air rises at
• Cold air sinks at
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Global Circulation
• Single Cell Model – Hadley CellEarth’s rotation →Coriolis force: winds deflected to right in Northern hemisphere, to left in Southern hemisphereWinds: winds from poles to equator
• Single-cell pattern is not what we observe
Breaks down due to:••
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Global Circulation
• Three-Cell Model – more realistic model
G109: Weather and Climate 11: Wind Systems
• Hadley Cell:
• Inter-Tropical Convergence Zone (ITCZ) (0o)
Very strong low pressure zone – rising air
Light winds: doldrums
• Sub-tropical High (30o N/S)
Sinking air
Light winds: horse latitudes
• Trade winds (0-30oN/S)
Macroscale Winds: Global CirculationThree-Cell Model – more realistic model
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Global Circulation
Three-Cell Model – more realistic model
• Ferrel cell –
Some of sinking air at subtropical high diverges poleward
• (mid-latitudes)
G109: Weather and Climate 11: Wind Systems
• Polar cell: high latitudesThermally driven circulation
• Polar High (90o)Very cold conditionsSinking, diverging air
• Sub-polar Low (60o N/S)Rising air
• Polar Flow from Very strong deflection by Coriolis force
Macroscale Winds: Global CirculationThree-Cell Model – more realistic model
• Three-cell model not quite true: doesn’t include land/water differences
• Three-cell model breaks down in upper-level winds – do not have the distinct structure of Ferrel cell and polar cell, although surface winds are correct there
• But it was a very useful starting point for considering global circulation
• In the real atmosphere, we instead find a number of semi-permanent High and Low pressure cells
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Global & Synoptic
• Semi-permanent Pressure CellsJanuary
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Global & Synoptic
• Semi-permanent Pressure CellsJuly
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Asian Monsoon
• Seasonal wind due to seasonal changes in mean pressure
• Winter: Sinking air from jet stream →
• Summer:Strong heating over continent → .
Draw moisture from warm Indian Ocean toward India and Asia
Himalayan Mountains cause strong orographicuplift
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Jet stream
• An area of increased wind speeds Narrow band: 100 - 500 km wideSpeeds: 200 - 500 km h-1
Height: 9 - 12 km ( )• Typically found above the largest horizontal T
gradient – e.g., at polar front• Move north and south with
the seasons • Stronger in the when
the T gradients are largest• Most powerful jet-stream:
• Weaker jet-stream:
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Rossby Waves
• Recall: Upper air (zones of low pressure extending equator-ward) and .
(zones of high pressure extending poleward)
→ Wavelike flow around earth at mid-latitudes
• Rossby• Rossby waves: “long waves”in flow
Usually 3-7 Rossby waves encircling earth
Migrate west to east
Change in wavelength and amplitude
G109: Weather and Climate 11: Wind Systems
Macroscale Winds: Rossby Waves
• Large amplitude Rossby waves ( .
flow) transport:Warm air from subtropics to high latitudesCold polar air to low latitudes
• Small amplitude Rossby waves ( flow)Flow is more westerly, less equator-pole exchange of heat
• Changes in the flow along the wave lead to:Divergence aloft
• Draws air • Leads to
Convergence aloft• Forces air • Inhibits
G109: Weather and Climate 11: Wind Systems
El Niño Southern Oscillation
• El Niño – weak warm current occurring along the west coast of South America (particularly Peru)
Appears every 3-7 years around Christmas time
Lasts about 1 year
Warm current is not good for fishing industry
1997-98 was warmest event ever recorded
• Occurs due to a reversal in “Walker Circulation” – the interaction between atmospheric circulation and ocean circulation in the equatorial Pacific
G109: Weather and Climate 11: Wind Systems
El Niño Southern Oscillation
• During a normal (non-El Niño) year:
Easterly trade winds drag warm surface water from East to West across Pacific
Upwelling of cold water along the west coast of South America
Low pressure area:
High pressure:
G109: Weather and Climate 11: Wind Systems
El Niño Southern Oscillation
• A normal (non-El Niño) year
G109: Weather and Climate 11: Wind Systems
El Niño Southern Oscillation
• During an El Niño year:Weakening or reversal of trade winds drag warm surface water from W to E across Pacific
No upwelling of cold ocean water
Sea Surface Temps (SST’s) in Eastern Pacific become warmer than normal
Low pressure area shifts to Eastern Pacific → .
along west coast of South America, Central America and even California
High pressure shifts from to western Pacific
The reversal in surface pressure is called the
G109: Weather and Climate 11: Wind Systems
El Niño Southern Oscillation
• During El Nino year:
G109: Weather and Climate 11: Wind Systems
El Niño Southern Oscillation
• When El Niño dissipates:Normal (non-El Niño) conditionsOR La Niña conditions
• During a La Niña year:Very strong easterly trade-winds in the PacificVery strong upwelling of cold water along the west coast of South AmericaSST’s become colder than normalIn Western Pacific: warm water promotes uplift, which intensifies surface low, and intensifies easterly trade windsAlong west coast of America’s: very High pressure →