How Does Air Move Around the Globe?

Review of last lectureReview of last lecture

• Know 3 Forces that affect wind speed /direction

• Especially work on Coriolis force, as this is the hardest to understand. Which direction is air deflected to by Coriolis force?

• What is the geostrophic balance? At which level is it valid? Difference between upper level and surface winds

• Troughs, ridges, cyclones and anticyclones. Do they correspond to high or low surface pressure? Is the air moving clockwise or counter-clockwise around them?

The most common atmospheric The most common atmospheric circulation structurecirculation structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

CE

CE

Friction

IntroductionIntroduction• Well-defined heating, temperature and pressure patterns exist

across the globe• These define the general circulation of the planet• In describing wind motions:

– Zonal winds (east-west): flow parallel to lines of latitude• Flowing eastward: Westerly wind• Flowing westward: Easterly wind

– Meridional winds (north-south): flow parallel to lines of longitude

• Flowing northward: Southerly wind• Flowing southward: Northerly wind

Annual mean precipitation (heating)Annual mean precipitation (heating)

Extratropical stormtrack

Tropical rainfall

Extratropical stormtrack

Primary Highs and LowsPrimary Highs and Lows

Equatorial low

Subtropical high

Subpolar low

Polar high

Zonal mean circulation

Each hemisphere is divided into 3 distinct cells:

Hadley Cell

Ferrel Cell

Polar Cell

Three-cell modelThree-cell model

Vertical structure and mechanismsVertical structure and mechanisms

Hadley Cell (thermal): Heating in tropics forms surface low and upper level high air converges equatorward at surface, rises, and diverges poleward aloft descends in the subtropics

Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells

Polar Cell (thermal): Driven by heating at 50 degree latitude and cooling at the poles

HadleyPolar

Zonal mean structure of temperatureZonal mean structure of temperature

Two characteristics:

• Horizontally uniform in the tropics

• Steep gradient in the extratropics

Zonal mean structure of zonal windZonal mean structure of zonal wind

Two characteristics:

• Westerly winds in the extratropical troposphere

• Jet streams: local maximum of winds

• The existence of the upper level pressure gradient air is being pushed toward poles Coriolis effect deflects upper air (to the right) Westerlies dominate upper troposphere

• Strongest during winter thermal gradient is large

• Explains why storms move eastward, flight times

Westerly winds in the extratropical troposphereWesterly winds in the extratropical troposphere

• Caused by steep temperature gradients between cold and warm air masses

• Polar front - marks area of contact, steep pressure gradient polar jet stream

• Low latitudes subtropical jet stream

• Stronger in winter, affect daily weather patterns

The Jet StreamsThe Jet Streams

The subtropical jet is seen as a band of clouds extending from Mexico on an infrared satellite image

A Jet Stream seen from satelliteA Jet Stream seen from satellite

Video: The jet streamsVideo: The jet streams

Semipermanent Pressure CellsSemipermanent Pressure Cells

• Instead of cohesive pressure belts circling the Earth, semipermanent cells of high and low pressure exist; fluctuating in strength and position on a seasonal basis.

• These cells are either dynamically or thermally created.• Sinking motions associated with the subtropical highs promote

desert conditions across specific latitudes.• Seasonal fluxes in the pressure belts relate to the migrating Sun

(solar declination).

For NH winter:

1. Aleutian and Icelandic lows

2. Siberian and Bermuda-Azores highs

3. South Pacific, Atlantic, Indian highs

South Pacific high

South Atlantic high

South Indian high

For NH summer:

1. Tibetan low

2. Hawaiian and Bermuda-Azores highs

3. South Pacific, Atlantic, Indian highs

South Pacific high

South Atlantic high

South Indian high

Low pressure: clouds and precipitationLow pressure: clouds and precipitation

Extratropical stormtrack

Tropical rainfall

Extratropical stormtrack

High pressure: warm surface temperature, High pressure: warm surface temperature, drought and desertdrought and desert

Global distribution of deserts (all near high pressure cells)

• Ocean surface currents – horizontal water motions• Transfer energy and influence overlying atmosphere• Surface currents result from frictional drag caused by

wind - Ekman Spiral

General circulation of the oceansGeneral circulation of the oceans

• Water moves at a 45o angle (right) in N.H. to prevailing wind direction

• Due to influence of Coriolis effect

• Greater angle at depth

• Surface currents mainly driven by surface winds• North/ South Equatorial Currents pile water westward, create the Equatorial Countercurrent• western ocean basins –warm poleward moving currents (example: Gulf Stream)• eastern basins –cold currents, directed equatorward

Global surface currentsGlobal surface currents

SummarySummary• Three precipitation (heating) belts. Primary high and

lows• Three-cell model. Mechanism for each cell• Two characteristics of zonal mean temperature structure• Two characteristics of zonal mean wind structure. Why

does westerly winds prevail in the extratropical troposphere? What cause the jet streams?

• Semipermanent pressure cells. Low pressure is associated with clouds and precipitation. High pressure is associated with warm surface temperature, drought, and desert.

• What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind?

Works citedWorks citedImages•http://pulleysandgears.weebly.com/gears.html •http://visual.merriam-webster.com/earth/meteorology/meteorological-measuring-instruments/measure-wind-direction.php