How Does Air Move Around the Globe?

Review of last lectureReview of last lecture

• Thickness of the atmosphere: less than 2% of Earth’s thickness

• Definition of temperature. 3 units. • Four layers of the atmosphere, what separate them?• Definition of pressure and its unit. • Definition of pressure gradient. Pressure gradient sets

the air in motion.• Equation of state (Relationship between P, ρ, and T)• Vertical Pressure Distribution. How does pressure

change with height? What is the hydrostatic equilibrium?

Review of last lecture (cont.)Review of last lecture (cont.)

• 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

• Does cyclones correspond to high or low surface pressure? Is the air moving clockwise or counter-clockwise around them? How about anticyclones?

• What are the troughs and ridges?

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

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

Vertical structure of zonal windVertical structure of zonal wind

Two characteristics:

• Westerly winds in the extratropical troposphere (caused by the Coriolis force). Explains why storms move eastward, flight times

• Jet streams: local maximum of winds (caused by sharp pressure gradient across the boundary between warm tropical air and cold polar air)

• 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

Low pressure centers: weather and precipitation

High pressure centers: warm, drought and desert

South Pacific high

South Atlantic high

South Indian high

Semipermanent Pressure Cells: NH winter

South Pacific high

South Atlantic high

South Indian high

Semipermanent Pressure Cells: NH summer

• 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