Part 3. Distribution and Movement of Air

Chapter 8

Atmospheric Circulation and Pressure Distributions

Planetary WindsWell-defined pressure patterns exist across the Earth that induce the global wind patterns on the planet

Idealized Single-Cell Convection Model for a Planet

The ITCZ is a band of clouds across the tropics

The Three-Cell Model

Intertropical convergence zone (ITCZ) -- surface low pressure with clouds and rain

Subtropical high -- Air subsides (dry climate)

Hadley cell -- tropical convection cell

Ferrel cell -- southwesterly winds at surface

Polar cell -- northeasterly winds at surface

The three-celled model vs. reality:

• Hadley cells are close approximations of real world equatorial winds

• Ferrel and polar cells do not approximate the real world winds very well at all

• Model is unrepresentative of westerly flow aloft

• Continents and topographic irregularities cause significant variations in real world wind patterns compared to the model

Semi-Permanent Pressure Cells are large areas of higher or lower atmospheric pressure than the surface average

They may be thermally induced (rising warm air or subsiding cold air) or they may be caused dynamically by converging or diverging wind patterns)

They fluctuate seasonally

Northern hemisphere semi-permanent cellsThe Aleutian, Icelandic, and Tibetan lows

Siberian, Hawaiian, and Bermuda-Azores highs

ITCZ (low)

Average atmospheric air pressure and wind patterns in January

Average atmospheric air pressure and wind patterns in July

The Sahelreflects seasonal migrationof the ITCZ

The Sahel is rainy during northern hemisphere summer and dry during northern hemisphere winter as the ITCZ shifts north and south through the year. It is the reason for annual flooding of the Nile River.

Shifts in the ITCZ affect the Sahel

Mid-latitude winds in the middle and upper troposphere are controlled b the pressure gradient force and the Coriolis force, giving the westerly winds

Stronger pressure gradients in winter give stronger westerlies

Westerly winds in the upper atmosphere• Air motions directed towards poles • Redirected by Coriolis deflection

– Westerly winds aloft result

The polar front and jet streams• Fast stream of air in upper troposphere

– Above polar front– Stronger in winter

Polar jet profile

The fastest middle and upper troposphere mid-latitude winds (the jet stream) are at polar front. There is a sharp temperature contrast and horizontal pgf to the south and north of the polar front.

300 mb

500 mb

800 mb

Strongest pgf here

The subtropical jet transports low latitude moisture and energy

Subtropical front and jet

Ridges and trough profileRidge Ridge

Trough

A trough over the mid-continent in (b)

Rossby waves are the wave-like pattern of ridges and troughs in the upper troposphere winds. Ridges and troughs (Rossby waves) will migrate either east or west with time

Sequence of Rossby wave migration -- the dashed line shows the migration and evolution of a trough with time across the country

Ocean currents are driven by wind stresses and are deflected by the Coriolis force. Thus, the water moves at about a 45° angle to the winds. The diection of movement of the ocean changes with depth following a pattern called the Ekman

spiral.

• Western basins usually experience warm ocean currents

• Eastern basins usually experience cold ocean currents

Upwelling in along-shore winds• Cold waters rise due to Ekman transport

Global Ocean Circulation

Infrared Satellite Image of the Gulf Stream

Major Wind Systems of the EarthMonsoons• Thermal induced seasonal wind patterns

associated with shifts of the ITCZ

• Monsoons are characterized by dry offshore winter flow and wet onshore summer flow

• The monsoon in East Asia experiences orographic enhancement

Winter monsoon

Summermonsoon

Topography enhances monsoonal effects

Foehn winds are strong, downslope winds that adiabiatically compress, raising the air temperature. Foehn winds are associated with hot, dry, clear weather

Chinook winds are foehn winds along the east slope of the Rockies (“snow eaters”)

Santa Ana winds are foehn winds that blow from the deserts and over the mountains into the valleys of southern and central California

Katabatic winds are cold, dense winds that flow down mountain slope. They warm as they descend, but they are still colder than the surrounding air.

• Boras and mistral winds are forms of katabatic winds in Europe

Southern California Santa Ana induced fires

Sea and land breezes form due to temperature differences over land and sea. Sea breezes form during the day, and land breezes form at night.

Valley and mountain breezes form due to heating and cooling on mountain sides. Valley breezes form during the day, and mountain breezes form at night (similar to katabatic winds)

Sea breezedevelopment

Sea breeze-initiated clouds over Hawaii

Valley and mountain breeze development

Air-Sea Interactions in the Equatorial PacificEl Niño, La Niña, and the Walker circulation• El Niño events

– Unusually warm water in the eastern equatorial Pacific Ocean

– Linked to global weather anomalies – 2 to 5 year recurrence

• La Niña events -- wind and temperature patterns reversed of El Niño patterns

• Walker circulation – Vertical and horizontal tropospheric flow in

the equatorial Pacific that controls areas of heavy rainfall

The “Normal” Walker Circulation (no El Niño conditions)

ENSO (El Niño/Southern Oscillation) events -- oscillations between El Niño and La Niña conditions• ENSO results in global teleconnection

patterns (weather effects far from the equatorial Pacific)

El Niño SST Anomalies

La Niña SST Anomalies

The Southern Oscillation Index

The PDO Index

Warmer waters in western tropical Pacific

Cooler waters in western tropical Pacific