Transcript g104_class16_ENSO

Geography 104 - “Physical Geography of the World’s Oceans”
El Nino – Southern Oscillation
- Southern Oscillation (atmosphere)
- El Nino (ocean)
- change in equatorial Pacific circulation
- produces global climate changes
- can significantly alter weather in California
Pacific Decadal Oscillation (PDO)
variations in Pacific ocean climate on decadal scales
El Nino – Southern Oscillation (ENSO)
variations in Pacific ocean climate on few-year scales
The Southern Oscillation
• Discovered in 1928 by
Sir Gilbert Walker
• Proposes a zonal
circulation cell over the
equatorial Pacific
• Links changes to global
climate
Walker Circulation
• East-to-west pressure gradients drive trades
Walker Circulation
• Vertical cell driven by warmest waters
Southern Oscillation correlation coefficients are equal
in magnitude, opposite in sign
atmospheric pressure: Darwin vs. Tahiti
Southern Oscillation Index
• SOI = Tahiti - Darwin air pressure D’s
• Low SOI = El Niño conditions
Southern Oscillation & warm water off S. America
Multivariate ENSO Index
• Combines observations of air pressure, zonal &
meridional components of wind, sea surface
temperature, air temperature and total cloudiness.
• Positive MEI = ENSO conditions
Pacific Ocean sea surface temperature
Western Pacific Warm Pool
normal equatorial conditions
El Nino conditions
El Niño
• Relaxation of
trade winds
causes warm
pool to slosh
esatward
across Pacific
Fig. 8.32.b
La Nina conditions
normal
La Nina
El Nino
normal
El Nino
An El Niño History
• late 1800s Fishermen name El Niño to the
periodic warm waters that appear off the coasts of
Peru and Ecuador around Christmas.
• 1928 Gilbert Walker describes the Southern
Oscillation.
• 1957 A large El Niño is observed which affects not
just the coasts of Peru and Ecuador but the entire
Pacific Ocean.
• 1969 Jacob Bjerknes links the Southern Oscillation
with El Niño events.
An El Niño History
• 1975 Wyrtki uses island sea level to show how
eastward flow causes SST’s to rise in east Pacific.
• 1976 An idealized computer model demonstrates
that winds over the far western Pacific can change
SST off Peru.
• 1982 A severe El Niño develops unexpectedly, but
is recorded in detail with newly developed ocean
buoys.
• 1985 Several nations launch the Tropical OceanGlobal Atmosphere (TOGA) program.
An El Niño History
• 1986 First coupled model of ocean & atmosphere
predicts El Niño event.
• 1988 Researchers explain how the "memory" of the
ocean--the lag between a change in the winds and
the response of the ocean--influences terminations
of El Niño and the onset of La Niña.
• 1996-1997 The array of instruments monitoring the
Pacific, plus coupled ocean-atmosphere models,
enable scientists to warn the public of an impending
El Niño event.
Dec 1997
Dec 1996
El Niño
SOI
La Niña
Sea surface temperature: el Niño vs normal conditions
la Niña
el Niño
Sea surface temperature: el Niño vs normal conditions
normal
el Niño
el Niño
anomaly
El Niño in Australia
Anchovy (Anchoa lucida - family: Engraulidae)
http://www.charterboats-uk.co.uk/Photographs/Fish/Anchovy.gif
el Niño effects on Peruvian fishery
El Niño Temperature and Precipitation Patterns
La Niña Temperature and Precipitation Patterns
El Niño
• 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
La Nina in the United States
El Niño in the United States
La Niña in the United States
El Niño effects on Southern California Bight
TAO-Triton mooring array
http://www.pmel.noaa.gov/tao/
TAO-Triton mooring
EL NIÑO/SOUTHERN OSCILLATION (ENSO)
DIAGNOSTIC DISCUSSION
issued by
CLIMATE PREDICTION CENTER/NCEP/NWS
9 October 2008
Synopsis: ENSO-neutral conditions are expected to
continue into early 2009.
ENSO-neutral conditions continued during September 2008,
as sea surface temperatures (SSTs) remained near-average in
the east-central equatorial Pacific Ocean. SSTs remained
slightly below-average in the central Pacific, and slightly
above-average in the eastern Pacific. From west to east, the
latest weekly SST index values range from −0.2°C in the
Niño-4 region to +0.3°C in the Niño 1+2 region. The
subsurface oceanic heat content (average temperatures in the
upper 300m of the ocean, continued to decrease in response
to the strengthening of negative temperature anomalies at
thermocline depth in the east-central Pacific.
ENSO Summary Slide
Readings (Ocean and Atmosphere):
Text Chapter 8 (pgs 138 – 163)
Reader pgs. 51 - 61
Reader Maps pgs 200 - 204
Midterm on Wednesday 5 Nov 2008