Transcript El Niño

Global Climate Systems
Chapter 10
Lecture 25
March 9, 2005
Final exam
• When: Wednesday, March 16th from 47pm
• Where: this room (Girvetz 1004)
• What to bring: a pencil, pink Scantron
• What it will look like: 70 or so multiple
choice questions as before
TA Review Session
• When:
• Where:
Global Climate Systems
Chapter 10
Climate classification systems
Introduction to El Nino/Southern Oscilation
What is climate?
weather - a short-term “snapshot” of our planet’s
atmospheric state and the atmosphere-earth energy
balance
climate - not simply a long-term average of weather,
climate is the spatial and temporal pattern of
atmospheric conditions that prevail in a certain
location
the components of climate include temperature,
precipitation, cloudiness, pressure, insolation, winds,
and air masses
Koppen Climate Classification
A-E designate latitudinal distribution
(H is altitudinal) based on
temperature
A
B
C
D
E
H
B is based on both temperature
and moisture
Modified Koppen climate classification
this classification is based on average monthly temperature,
precipitation, and total annual precipitation
the letters after A, B, C, D, and E are used to further specify
temperature and moisture conditions, such as the temperature
threshold of the warmest month or the minimum rainfall threshold
(e.g., always above 10 C and at least 3 cm rain)
Koppen-Geiger Climate Classification
Mediterranean Climates (Csa, Csb)
note the different annual
temperatures and
ranges
at least 70%
of precip.
in winter
summer
summer
brown,
brown,
winter green
winter green
recharge
P<E
recharge
P<E
note the amazing climate
diversity in North America
now look at CA (different
classification system)
ENSO (El Niño Southern Oscillation)
El Niño is the abnormal warming of surface waters in
the eastern Pacific – named by Peruvians who
noticed that it often occurred around Christmas (”the
boy”) - also it came when fish catches were low…
southern oscillation - shift in the pressure and trade
wind patterns over the Pacific - implications for
location of the warm water pool and convection
ENSO drives much of interannual weather variations
El Nino & weather
NH winter
southeast US and
western S. America
are both wetter in
winter
much of Alaska and
Pacific NW are
warmer in winter
NH summer
Australia, Indonesia,
and S. Africa are
drier
El Niño/Southern Oscillation
• Major climatic perturbation on the planet
• Coupled atmosphere-ocean process
• Key is the western tropical Pacific
• Changes strength of trade winds
• Makes ocean “slosh”
• www.pmel.noaa.gov/tao/elnino/nino-home.html
The Southern Oscillation
• Discovered in 1928 by
Sir Gilbert Walker
• Links D’s in global
climate indices
• Proposes a zonal
circulation cell over the
equatorial Pacific
The Southern Oscillation
The Southern Oscillation
• Pressure D’s regulate strength of the trades
• El Niño periods = weak trades
Southern Oscillation Index
• SOI = Tahiti - Darwin air pressure D’s
• Low SOI = El Niño conditions
Walker Circulation
• East-to-west pressure gradients drive trades
Figure credit: ‘Earth’s
Climate’ by W. Ruddiman
Ekman transport
away from Equator
the result is
cold water
along the
coast and a
tongue of
cold water
along the
eastern
Pacific
equator
Figure credit: ‘Earth’s
Climate’ by W. Ruddiman
S. America
this upwelling of cold, nutrient-rich water drives
microscopic plant (phytoplankton) growth
Western Pacific Warm Pool
Coupled Circulation
El Niño
Events
• Relaxation of
trades causes
warm pool to
slosh across
Pacific basin
El Niño Conditions
El Niño Conditions
La Nina
• Opposite phase of the
ENSO cycle
• Intensified trades &
Walker circulation
• Thermocline undergoes
maximum upwelling
Altimetry &
ENSO States
• Most “action” is in the
tropical Pacific
• Effects are seen in other
places
• East subtropics
• Western NECC region
Effects of an El Niño
• Region of ascending air
has moved to center of
equatorial Pacific
• Climate system shifts over
10,000 km to east
• Affects the entire planet
Effects of an El Niño
• Places that were wet are now dry and vice versa
Effects of an El Niño
Teleconnections
• Discovered in 1928 by
Sir Gilbert Walker
• Links D’s in global
climate indices
• Proposes a zonal
circulation cell over the
equatorial Pacific
El Niño & Subtropical Jet Stream
• Hadley cells intensify as source of heat is now in
central equatorial Pacific
• Jet stream intensifies & takes south path
El Niño in the United States
Winter
temp
anomalies
El Niño in the United States
Winter
precip
anomalies
El Niño in Australia
El Niño in
Australia
El Niño & Diseases
intense convection
low pressure
Normal State
convective loop
high pressure
upwelling of cold,
nutrient-rich water
(good fisheries)
El Niño State
convective loop reverses
increased
convection,
rainfall
thermocline is depressed in the
eastern Pacific and raised in the
western Pacific
fisheries along
the coast fail
rain falls on
dry coastal
deserts
Present Conditions
97/98 El Niño
• The 97/98 El Niño was biggest of all time
Present Conditions
• NOAA Climate Prediction Center - March 3, 2005
•
A transition from weak warm-episode (El Niño)
conditions to ENSO-neutral conditions is expected
to continue during the next three months.
Regions
west
to
east