NATS 101-05Lecture 16Air Masses

Supplemental References for Today’s Lecture

Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Introduction to the Atmosphere, 8th Ed. 484 pp. Prentice Hall. (ISBN 0-13-087957-6)

Ocean Currents of World

Ahrens Fig. 7.24

Upwelling from Alongshore Winds

Ahrens Fig 7.25

Wind pushes surface water southward. Coriolis force deflects water to the right. Cold water from below rises to surface. Fog persists over the cold water.

weather.unisys.com

Upwelling RegionsUpwelling Regions

El Nino 3.4

El Nino-Southern Oscillation (ENSO)

An important atmospheric-ocean feedback

Normal conditions in tropical Pacific:

-Warm SST, low SLP, and T-storms in W Pacific

-Strong subtropical highs in E Pacific

-Easterly winds and cool upwelling water along equator in East Pacific

-Prevailing southerly winds off of Peru produce cold upwelling and excellent fishing

El Niño-Southern Oscillation (ENSO)

Every few years (4-5 years):

-Equatorial Central Pacific warms

-Low SLP, T-storms shift to Central and East Pacific (the Southern Oscillation)

-Trades and southerly winds off Peru weaken

-Upwelling ceases off Peru, warming leads to massive kill off of fish. Typically occurs around Christmas (an El Niño event)

Alters global patterns of wind, temp and rain

Walker Circulation

Pushes water westward

DarwinDarwin TahitiTahiti

Walker Circulation oscillates with a quasi-period of every few years. Oscillation is very evident in SLP records for Darwin and Tahiti.

Aguado & Burt, p230

Southern Oscillation Index (SOI)

SLPSLP

SSTSST

See current conditions

El Nino Precipitation Extremes

La Nina Precipitation Extremes

El Nino Precipitation Extremes

La Nina Precipitation Extremes

ENSO-Atmosphere Coupling

Ocean temperature pattern

SLP, winds, storms, etc.Ocean currents, upwelling

By observing SST’s and surface winds in equatorial Pacific, we are able to forecast ENSO events with considerable skill (much better once they have started to form). ENSO forecasts lead to skillful seasonal forecasts for the US several months in advance (e.g. 1997-1998 winter).

Ocean and ENSO Summary

• Major Ocean CurrentsDriven by prevailing wind

• Upwelling Regions

Occurs along west coasts of continents

Cold water rises from below to surface

Nutrient rich, excellent fishing regions

Summary

• El Nino-Southern OscillationOccurs every few yearsCentral equatorial Pacific warmsLow SLP, T-storms move with warm waterUpwelling weakens along Peru coastCan be predicted up to one-year in advanceModulates global patterns of wind, temp, rain

What is an Air Mass?

• Air Mass

Large area (>1600 km by 1600 km) of air that contains relatively uniform, horizontal distributions of temperature and moisture.

How Air Masses Form

• If surface air resides in a region for a few days, it acquires the thermal and moisture characteristics of the underlying surface.

• Source regions for Air Masses are:

Big in area [ >>(1600 km)2 ]

Dominated by persistent high pressure and light winds

Air Mass Source Regions

• Contrasting source regions are

Continents versus Oceans

Tropics versus Poles

• An Air Mass is designated in terms of its Source Region

Air Mass Characteristics

Ahrens Table 8.1

Air Mass Source Regions for NA

Ahrens Fig 8.2

Creation of cP Air Mass

Williams p22

Creation of cP Air Mass

Williams p23

Creation of mP Air Mass

Lutgens & Tarbuck, p 230

Lake Effect Snows

As cP air flows over the warmer, open Great Lakes, it is warmed and moistened. When the modified cP air flows onshore, prodigious snows of several feet can result.

Lutgens & Tarbuck, p 230

Creation of mT and cT Air Masses

Williams p24

Contrasting Air Masses

Ahrens Fig 8.917 Apr 1976

Paths of cP Air Masses

Ahrens Fig 8.3

cP Air Mass

Ahrens Fig 3 p20324 Dec 1983

Modification of cP Air Mass

Ahrens Fig 8.4Ahrens Fig 8.4

cPcP

cPcP

mPmP

mPmP

warm oceanwarm ocean

warm oceanwarm ocean

mP Air Masses

Ahrens Fig 8.7Ahrens Fig 8.5

Air mass modified Air mass modified further as it crosses further as it crosses several mountain several mountain ranges of West U.S.ranges of West U.S.

cT Air Mass

Ahrens Fig 8.1029-30 Jun 1990

Winter mT Pacific Air Masses

mTmT

mPmP

Ahrens Fig 8.8Ahrens Fig 8.8

“Pineapple Express”

Weather Map with Air Masses

Ahrens Fig 8.11

Summary

• Air Masses

Large (>1000 miles) regions with “uniform” temperature and moisture characteristics

• Classified by Source Region Continental (c) or Maritime (m)Polar (P) or Tropical (T)

• Source Regions

Big in area (>>1600 km by 1600 km)

Dominated by light winds (long resident times)

Assignment for Next Lecture

• Topic - Fronts • Reading - Ahrens pg 214-231• Problems - 8.12, 8.13