Chapter 19

Air Pressure and Wind

19.3

Regional Wind Systems

Local Winds

 The local winds are caused either by topographic effects or by variations in surface composition —land and water—in the immediate area.

 Land and Sea Breezes • In coastal areas during the warm summer months, the land surface is heated more intensely during the daylight hours than an adjacent body of water is heated. As a result, the air above the land surface heats, expands, and rises, creating an area of lower pressure. At night the reverse takes place.

Sea and Land Breezes

Sea and Land Breezes, a local phenomenon

19.3

Regional Wind Systems

Local Winds

 Valley and Mountain Breezes • In mountainous regions during daylight hours, the air along the slopes of the mountains is heated more intensely than the air at the same elevation over the valley floor. Because this warmer air on the mountain slopes is less dense, it glides up along the slope and generates a valley breeze. After sunset the pattern may reverse.

Valley and Mountain Breezes

19.3

Regional Wind Systems

How Wind Is Measured

 Wind Direction • The

prevailing wind

is the wind that blows more often from one direction than from any other.

• In the United States, the westerlies consistently move weather from west to east across the continent.

19.3

Regional Wind Systems

How Wind Is Measured

 Wind Speed • An

anemometer

is an instrument that resembles a cup and is commonly used to measure wind speed.

Devices to measure Wind

19.3

Regional Wind Systems

El Ni ñ o and La Ni ñ a

 El Ni ño •

El Ni ñ o

is the name given to the periodic warming of the ocean that occurs in the central and eastern Pacific. • At irregular intervals of three to seven years, these warm countercurrents become unusually strong and replace normally cold offshore waters with warm equatorial waters.

• A major El Ni ñ o episode can cause extreme weather in many parts of the world.

Normal Conditions

El Niño Conditions

History of El Ni ño

• • • •

El Ni ño, as a oceanic phenomenon along the coasts of northern Peru and Ecuador, has been documented since the 1500s.

Originally, the term El Ni ño was used to describe the annual appearance of warm waters along the coast of northern Peru around Christmastime.

In some years the warm waters appeared earlier and lasted longer. Eventually, the term El Ni ño was applied to the periods of anomalous warming. The stronger events disrupted local fish and bird populations.

History of the Southern Oscillation

• • •

Beginning in the late 1800s scientists began to describe large-scale pressure fluctuations.

Sir Gilbert Walker and colleagues extended the early studies and determined that a global-scale pressure fluctuation (the Southern Oscillation) is related to rainfall anomalies in many areas of the Tropics (e.g., India and South America).

The SO was used as the basis for seasonal rainfall predictions (ca 1930s).

The ENSO Cycle

• • • •

Naturally occurring phenomenon Equatorial Pacific fluctuates between warmer-than average (El Niño ) and colder-than-average (La Niña) conditions The changes in SSTs affect the distribution of tropical rainfall and atmospheric circulation features (Southern Oscillation) Changes in intensity and position of jet streams and storm activity occur at higher latitudes

El Ni ño/ Low Southern Oscillation Phase VS.

La Ni ña/ High Southern Oscillation Phase

• • • • •

Signals in Tropical Pacific: Sea surface temperatures (SSTs) Precipitation Sea Level Pressure The Southern Oscillation (High vs. Low Phases) Low-level Winds and Thermocline Depth

Equatorial cold tongue is weaker than average or absent during El Niño, resulting in positive SST anomalies

Sea Surface Temperatures

Equatorial cold tongue is stronger than average during La Niña, resulting in negative SST anomalies

Precipitation

Enhanced rainfall occurs over warmer than-average waters during El Niño.

Reduced rainfall occurs over colder than-average waters during La Niña.

Sea Level Pressure

El Niño: Positive SLP anomalies over the western tropical Pacific, Indonesia and Australia. Negative SLP anomalies over eastern tropical Pacific, middle and high latitudes of the North Pacific, and over U.S. Opposite pattern for La Niña. The pressure see-saw between the eastern and western tropical Pacific is known as the Southern Oscillation.

Low-Level Winds & Thermocline Depth

La Niña

: stronger-than-average easterlies lead to a deeper (shallower)-than-average thermocline in the western (eastern) eq. Pacific.

El Niño

: weaker-than-average easterlies lead to a deeper (shallower)-than-average thermocline in the eastern (western) eq. Pacific.

ENSO: A Coupled Ocean Atmosphere Cycle

ENSO is a “coupled” phenomenon: atmosphere drives the ocean and the ocean drives the atmosphere. “Positive Feedback” between ocean and atmosphere. Example: Weaker equatorial trade winds east will decrease

 

cold water upwelling in the surface warming of the ocean



reduced east-west temperature gradient



Weaker equatorial trade winds

What is “Average?”

Warm Winds and Sea Surface Temperature are COUPLED. The SSTs influence the winds and vice versa.

(1) Easterly trade-winds help push warm water to the western Pacific and upwell cold water along the equator in the eastern Pacific Ocean.

Cold

(2) Warm water heats the atmosphere, the air rises, and low-level trade winds converge toward the warm water. Subsiding air occurs in the eastern Pacific basin.

December-February Average Conditions Warm Cold

“El Niño”

Warm

•

Easterly trade winds weaken

•

Thermocline deepens and the cold water upwelling decreases in the eastern Pacific.

Warm Cold NOTE: Location of the warmest SSTs (>~28°C) determines where tropical convection will be located.

•

Convection shifts eastward over the central and/or eastern Pacific Ocean. Convection becomes suppressed over the far western Pacific/ Indonesia.

Warm Cold

More Convection

“La Niña”

Enhanced Warm Stronger Stronger Upwelling Cold Cold becomes more shallow

•

Easterly trade winds strengthen

•

Thermocline becomes more shallow and the cold water upwelling increases in the eastern Pacific.

Warm

•

Convection becomes stronger over the far western Pacific Ocean/ Indonesia and more suppressed in the central Pacific.

Cold

Global El Niño Impacts

Impacts are generally more extensive during the northern winter.

Typical Global El Niño

Region

Impacts

Period Impact

Indonesia Northeast Brazil Life of event March-May Drier Drier Central America /Mexico West Coast South America Central South America Southeast Africa May-October March-May June-December December-February Drier Wetter Wetter Drier

Global La Niña Impacts

Mid-latitude impacts generally occur during the winter season (NH – DJF; SH JJA).

Typical Global La Niña Impacts

Region

Indonesia Northeast Brazil Central America /Mexico West Coast South America Central South America Southeast Africa

Period

Life of event March-May May-October March-May June-December December-February

Impact

Wetter Wetter Wetter Drier Drier Wetter

Summary

•

ENSO is a naturally occurring phenomenon.

•

Equatorial Pacific fluctuates between warmer-than average (El Niño ) and colder-than-average (La Niña) conditions.

•

The changes in SSTs affect the distribution of tropical rainfall and atmospheric circulation features (Southern Oscillation).

•

Many areas of the Tropics and Subtropics experience significant impacts during the extreme phases (El Ni ño and La Niña) of the ENSO cycle.

•

Changes in intensity and position of jet streams and storm activity occur at higher latitudes.

19.3

Regional Wind Systems

Global Distribution of Precipitation

 Global precipitation can be explained if knowledge of global winds and pressure systems are applied.