Ocean-atmosphere
interactions
ENSO (El Nino-Southern
Oscillartion) and PDO (Pacific
Decadal Oscillation)
NASA: 70% of global climate change due
to Pacific Ocean Oscillations
• “It can be shown with a simple climate model
that small cloud fluctuations assumed to
occur with two modes of natural climate
variability- ENSO and Pacific Decadal
Oscillations- can explain 70% of the warming
trend since 1990, as well as nature of that
trend, warming until the 1940’s, no warming
until the 1970’s, and resumed warming since
then.”
Dr. (Meteorology) Roy W. Spencer,
NASA lead investigator of U.S.
Science Team Leader for NASA Earth
Observervatory, July, 22, 2008 in
Congressional testimony
PDO
•
The "Pacific Decadal Oscillation" (PDO) is a long-lived El Niño-like pattern of Pacific
climate variability. While the two climate oscillations have similar spatial climate
fingerprints, they have very different behavior in time. Fisheries scientist Steven Hare
coined the term "Pacific Decadal Oscillation" (PDO) in 1996 while researching
connections between Alaska salmon production cycles and Pacific climate (his
dissertation topic with advisor Robert Francis). Two main characteristics distinguish
PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO "events"
persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months;
second, the climatic fingerprints of the PDO are most visible in the North
Pacific/North American sector, while secondary signatures exist in the tropics - the
opposite is true for ENSO. Several independent studies find evidence for just two full
PDO cycles in the past century: "cool" PDO regimes prevailed from 1890-1924 and
again from 1947-1976, while "warm" PDO regimes dominated from 1925-1946 and
from 1977 through (at least) the mid-1990's. Shoshiro Minobe has shown that 20th
century PDO fluctuations were most energetic in two general periodicities, one from
15-to-25 years, and the other from 50-to-70 years.
PDO
typical winter sea surface temps
Cool phase
warm phase
Ocean surface currents
Sea Surface Temperature Anomalies
Gulf Stream over North
Atlantic from NASA satellite
North Atlantic salinity sink/
North Pacific upwelling
Thermohaline circulation
Surface air temperature deviations
from zonal mean
El Nino-Southern Oscillation
(ENSO)
Normal conditions
Cartoon of normal
Normal conditions
• High pressure and dry conditions over
eastern Pacific
• Low pressure and moist conditions over
western Pacific, Australia, Indonesia, etc.
• Trade winds flow east to westward
• Thermocline is nearly 200 m deep in western
Pacific and near surface in eastern Pacific,
causing upwelling of Humbolt Current
• Fishing industry is good
Fishing industry is fine;
mackerel, anchovies, shrimp,
sardines, etc.
El Nino conditions
El Nino animation
Onset of El Nino
El Nino episode
El Nino characteristics
• E. Pacific ocean warmed by 4 to 10 deg Ccauses low pressure
• Thermocline drops in east Pacific
• East Pacific sea level 60 cm to 1 m higher
• Air pressure drops in E. Pacific, rises in W.
Pacific, Indonesia, Australia, Tahiti
• Pressure gradient weakens, trade winds
weaken or reverse direction
• Polar front jet stream shifts equatorward
• Heavy rains, floods to W. coasts of Americas
• Drought to western Pacific
• Increased number of icebergs south of 48
degrees N. and S. latitude
• Global reduction of clouds = drier and warmer
El Nino Impacts
• Rains, floods, and terrible fishing in E.
Pacific
• Agriculture, fisheries, food supply of
developing countries impacted
• Wetter conditions in southwestern and
northwestern US, southern India,
• Drier conditions W. Pacific, N. South
America, SE Africa, N. India,
More El Nino imacts
• 93% of Monsoon droughts occur during El
Nino events
• Number and duration of hurricanes
decreases
• Increases CO2 production by 1.4 ppm/ yr
• Clearer skies worldwide- global warming?
• Since 1972, El Nino has dominated over La
Nina
• 1998 El Nino caused 16% of world’s coral
reefs to die. Since all regions have suffered
“severe bleaching.”
El Nino regional impacts
La Nina
La Nina impacts
• Exaggerates normal conditions,
• SST’s are lower than usual in E. Pac
• Trade winds exceptionally strong
La Nina- regional impacts
ENSO Index- 1980-2005
Major El Nino Events
• 1790-93, 1828, 1876-78, 1891, 192526, 1982-83, 1997-98
• Other Recent El Nino Events:
• 1986-87, 1991-92, 1993, 1994, 199798, 2002-03, 2004-05, 2006-07
• El Ninos identified in early Holocene
records
Southern Oscillation Index
Calculated from air pressure difference between
Darwin, Australia and Tahiti.
SOI = 10 x Pdiff - Pdiffav
SD(Pdiff)
Pdiff = average monthly Tahiti pressure - average monthly
Darwin pressure
Pdiffav = long term average of pressure difference
SD (Pdiff) = long term average standard deviation of Pdiff for the
month in question
Sustained negative SOI values often indicate El Nino
ENSO Index using 6 variables: sea-level
pressure, zonal, meridional, components
of sfc winds, sst, and cloudiness fraction
What are precursors to failure
of Walker Circulation?
• More extensive sea-ice in the Antarctic and
snow cover in central Asia
• Two months before onset of El Nino high
altitude winds (easterly subtropical jet stream)
weaken
• Changes in behavior of southern Hemisphere
mobile polar highs
• Weakening of southern Hemisphere
westerlies between 35 and 55 degrees
• Particulates from volcanic eruptions (El
Chichon in March 1982, and Mount Pinatubo
in June, 1991)
NASA: 70% of global climate change due
to Pacific Ocean Oscillations
• “It can be shown with a simple climate model
that small cloud fluctuations assumed to
occur with two modes of natural climate
variability- ENSO and Pacific Decadal
Oscillations- can explain 70% of the warming
trend since 1990, as well as nature of that
trend, warming until the 1940’s, no warming
until the 1970’s, and resumed warming since
then.”
Dr. (Meteorology) Roy W. Spencer,
NASA lead investigator of U.S.
Science Team Leader for NASA Earth
Observervatory, July, 22, 2008 in
Congressional testimony