Transcript Air Mass

NATS 101-05
Lecture 16
Air 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.
El Nino 3.4
Upwelling Regions
weather.unisys.com
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
Darwin
Tahiti
Aguado & Burt, p230
Walker Circulation oscillates with
a quasi-period of every few years.
Oscillation is very evident in SLP
records for Darwin and Tahiti.
Southern Oscillation Index (SOI)
SST
SLP
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
Ocean currents, upwelling
SLP, winds, storms, etc.
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 Currents
Driven 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 Oscillation
Occurs every few years
Central equatorial Pacific warms
Low SLP, T-storms move with warm water
Upwelling weakens along Peru coast
Can be predicted up to one-year in advance
Modulates 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
Lutgens & Tarbuck, p 230
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.
Creation of mT and cT Air Masses
Williams p24
Contrasting Air Masses
17 Apr 1976
Ahrens Fig 8.9
Paths of cP Air Masses
Ahrens Fig 8.3
cP Air Mass
24 Dec 1983
Ahrens Fig 3 p203
Modification of cP Air Mass
cP
cP
warm ocean
warm ocean
mP
mP
Ahrens Fig 8.4
mP Air Masses
Air mass modified
further as it crosses
several mountain
ranges of West U.S.
Ahrens Fig 8.5
Ahrens Fig 8.7
cT Air Mass
29-30 Jun 1990
Ahrens Fig 8.10
Winter mT Pacific Air Masses
mP
mT
Ahrens Fig 8.8
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