Intraseasonal and Interannual
Variability
Tropical
M. D. Eastin
Outline
Tropical phenomena
• Madden Julian Oscillation (MJO)
• Quasi-Biennial Oscillation (QBO)
Mid-Latitude phenomena with links to the Tropics
• North Atlantic Oscillation (NAO)
• Pacific Decadal Oscillation (PDO)
Comments on Global Warming and Data Records
Tropical
M. D. Eastin
Madden-Julian Oscillation (MJO)
Definition and Background
• The MJO is an intraseasonal oscillation
between periods of enhanced and suppressed
tropical rainfall with a period of 30-60 days
• Discovered by R. Madden and P. Julian in 1971
30-60 Day Filtered OLR Anomalies
(Blue = Convection Red = dry)
Evolution
• Each regime or phase (wet and dry) tends to
develop in the Indian Ocean and propagate
eastward at ~5 m/s into the central Pacific
• More pronounced in the eastern Hemisphere
• Upon crossing in the dateline, the convective
intensity decreases and the regimes begin
to propagate eastward at >10 m/s
• Often hard to “track” in the western Hemisphere
• Intensity is influenced by ENSO
Impacts
• Affects the intensity and breaks periods of the
Asian (Indian) and Australian monsoon
• Wet phase tends to enhance TC activity
Tropical
M. D. Eastin
Madden-Julian Oscillation (MJO)
Structure
• Wet phase is characterized by an “envelope”
of deep convection composed of numerous
higher frequency “events” (equatorial waves,
TCs, and tropical MCS) that propagate both
east and west through the envelope
30-60 Day Filtered
OLR, Streamfunction, and Wind Anomalies
shading → convection
contours → streamfunction
vectors → winds
200 mb
Causes (or Forcing Mechanisms)
 Unknown
• Currently believed to result from both external
forcing and internal instabilities
Zonal Cross-Section
850 mb
Wave motion
Cool
Cool
Warm
Moist
Dry
Cool
Cool SST
Tropical
Dry
Warm
Moist
Warm SST
Cool SST
Adapted from Kiladis et al. (2005) and Zhang (2005)
M. D. Eastin
Madden-Julian Oscillation (MJO)
Current Status:
Let’s go to: http://www.cpc.noaa.gov/products/precip/CWlink/MJO/mjo.shtml
Tropical
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
Definition and Background
• The QBO is a quasi-periodic interannual oscillation of the tropical stratospheric zonal winds
between easterlies and westerlies with a mean period of 28-29 months
• First recognized by R. J. Reed and R. A. Ebdon in 1960
• First referred to as the QBO by Jim Angell in 1964
• Each wind regime or phase (east and west) develops at the top of the stratosphere and
propagates downward at ~1 km per month until they dissipate at the tropical tropopause
• The west phase (westerly winds) descends faster than the east phase
• The east phase amplitude (20-30 m/s) is larger than the west phase amplitude (10-20 m/s)
• The phases are coherent through the entire equatorial belt at any given time
• Peak amplitudes are over the Equator at an altitude of 24 km (~30 mb)
• Amplitudes decrease away from the equator (i.e. “equatorially trapped” → Why?)
Time-Height (Pressure) diagram of Equatorial Zonal Wind
East
East
West
Tropical
East
West
East
West
East
West
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
Causes (or Forcing Mechanisms)
 QBO is equatorially trapped
→ Forcing is from equatorial waves (also trapped)
that vertically propagate into the stratosphere
→ QBO physics described by Holton and Lindzen (1972),
Plumb (1977), Plumb (1984), Baldwin et al. (2001)
Vertical Propagation of Equatorial Waves (the Basics)
• Waves can propagate vertically until they reach a “critical layer” (which acts like a sponge)
• Critical layers are determined by the mean flow characteristics and the wave type
• When a wave encounters a critical layer, the wave is first damped and then dissipates
• Damped waves deposit their propagation momentum into the mean flow, accelerating
the mean flow in the direction of wave propagation
Wave type
Inertio-Gravity
Kelvin
Inertio-Gravity
Rossby-Gravity
Rossby
Tropical
Wave
Propagation
Direction
Flow the waves
CAN vertically
propagate through
“Critical Layer”
Flow the waves
can NOT vertically
propagate through
Momentum
Acceleration
eastward
westward
strong eastward
eastward
westward
eastward
strong westward
westward
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the East phase to the West phase
in the lower stratosphere
Time 0
dissipate at
upper levels
Time 1
westward
flow max
descends
Eastward
Flow
start new
west phase
aloft
eastward
flow max
descends
pass
unimpeded
damped and
accelerate
mean flow
Westward
Propagating
Waves
Tropical
Eastward
Propagating
Waves
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the East phase to the West phase
in the lower stratosphere
Time 1
Time 2
Westward
Flow
damped and
accelerate
mean flow
westward
flow max
descends
Eastward
Flow
pass
unimpeded
eastward
flow max
descends
damped and
accelerate
mean flow
Westward
Propagating
Waves
Tropical
Eastward
Propagating
Waves
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the East phase to the West phase
in the lower stratosphere
Time 2
Time 3
Westward
Flow
westward
flow max
descends
damped and
accelerate
mean flow
pass
unimpeded
damped and
accelerate
mean flow
Westward
Propagating
Waves
Eastward
Propagating
Waves
Tropical
thin eastward max
destroyed by
viscous diffusion
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the West phase to the East phase
in the lower stratosphere
Time 3
Westward
Flow
Time 4
dissipate at
upper levels
eastward
flow max
descends
start new
east phase
aloft
westward
flow max
descends
pass
unimpeded
damped and
accelerate
mean flow
Westward
Propagating
Waves
Tropical
Eastward
Propagating
Waves
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the West phase to the East phase
in the lower stratosphere
Time 4
Time 5
Eastward
Flow
damped and
accelerate
mean flow
eastward
flow max
descends
westward
flow max
descends
Westward
Flow
pass
unimpeded
damped and
accelerate
mean flow
Westward
Propagating
Waves
Tropical
Eastward
Propagating
Waves
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the West phase to the East phase
in the lower stratosphere
Time 5
Time 6
Eastward
Flow
eastward
flow max
descends
damped and
accelerate
mean flow
damped and
accelerate
mean flow
pass
unimpeded
Westward
Propagating
Waves
Eastward
Propagating
Waves
Tropical
thin westward max
destroyed by
viscous diffusion
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the East phase to the West phase
in the lower stratosphere
Time 0 or Time 6
dissipate at
upper levels
Time 1 or Time 7
westward
flow max
descends
Eastward
Flow
start new
west phase
aloft
eastward
flow max
descends
pass
unimpeded
damped and
accelerate
mean flow
Westward
Propagating
Waves
Tropical
Eastward
Propagating
Waves
M. D. Eastin
Quasi-Biennial Oscillation (QBO)
Why is the QBO Important?
• The phase of the QBO affects Atlantic TC activity
West phase
East phase
→
→
Increased TC activity
Decreased TC activity
• West Pacific TC frequency increases during the West phase
• Southwest Indian Ocean TC frequency increased during the East phase
• Believed to impact the onset and intensity of monsoon circulations
• Used by some statistical predictions of ENSO to forecast strength and timing of events
• Believed to impact west African rainfall
• Decay of aerosol loading from volcanic eruptions depends on QBO phase
Tropical
M. D. Eastin
North Atlantic Oscillation (NAO)
Definition and Background
• The NAO is an interannual meridional oscillation of mass within the North Atlantic
defined by the normalized surface pressure difference between the Iberian Peninsula
and (minus) Iceland
• First recognized by the Danish in the 1770s
• First researched and defined by Sir Gilbert Walker in the 1920s
• Primarily influences the strength of mid-latitude westerly winds and storm tracks
• Influence is most prominent in the winter months (Nov-Apr)
• Impacts the seasonal climate from eastern North America to central Asia and
from tropical to polar latitudes
Winter normalized NAO index: 1900-2012
Source: http://www.cgd.ucar.edu/cas/jhurrell/indices.html
Tropical
M. D. Eastin
North Atlantic Oscillation (NAO)
Positive NAO Index
• Low pressure anomaly over Iceland (a
stronger Icelandic Low) and a high
pressure anomaly over the subtropical
Atlantic (a stronger Azore High)
• Enhanced mid-latitude westerlies
• Enhanced tropical trade winds
• More intense SAL outbreaks
• Warm over Europe and the eastern U.S.
• Cold over the polar North Atlantic and
the tropical North Atlantic
• Wet from Iceland to Scandinavia
(contributes to advancing glaciers)
• Dry over southern Europe
• More intense winter storms in
northern Europe
• Milder winters in eastern U.S.
 Dominant phase from the early 1980s
through the mid-2000s (will it continue?)
Tropical
M. D. Eastin
North Atlantic Oscillation (NAO)
Negative NAO Index
• High pressure anomaly over Iceland (a
weaker Icelandic Low) and a low
pressure anomaly over the subtropical
Atlantic (a weaker Azore High)
• Suppressed mid-latitude westerlies
• Suppressed tropical trade winds
• Less intense SAL outbreaks
• Cold over Europe and the eastern U.S.
• Warm over the polar North Atlantic
(enhanced ice sheet melting) and
the tropical North Atlantic
• Dry over Iceland to Scandinavia
• Wet over southern Europe and
eastern North America
• More intense winter storms in
the eastern U.S.
 Dominant phase from the mid 1950s
to the late 1970s
Tropical
M. D. Eastin
North Atlantic Oscillation (NAO)
Causes (or Forcing Mechanisms)
• Thought to be driven by “natural internal atmospheric processes” (i.e. unknown)
• High correlation with Atlantic SST and ice variability, but it’s unclear which causes which
Interesting “Relationship” to Global Warming
• The negative phase (cold North America and Europe) was dominant 1955-1979
• The positive phase (warm North America and Europe) has dominated since (1980-2005)
• The majority of “reliable” global surface observations are made over land
(in particular over North America, Europe, and Asia)
 Could the recent “global warming” be an artifact of the NAO and our observation networks?
Global Climate
Observing Network
981 total stations
Tropical
M. D. Eastin
Pacific-Decadal Oscillation (PDO)
Definition and Background
• The PDO is an multi-decadal (20-30 year) oscillation of SST within the North Pacific
defined by normalized (and de-trended) SST anomalies north of 20ºN
• First recognized by Steven Hare (a fisheries scientist) in 1996
• First related to climate by Yuan Zhang in 1997
• Influence is most prominent in the winter months (Oct-Mar)
• Impacts the seasonal climate from eastern Asia and North America
• Climate impacts that occur during each phase are similar to the ENSO impacts
but are less intense and occur over a longer timescale
• The cause of the PDO is unknown
Monthly values of the normalized PDO index: 1900-2012
Source: http://jisao.washington.edu/pdo/
Tropical
M. D. Eastin
Pacific-Decadal Oscillation (PDO)
Positive PDO index (Warm phase)
Anomalous SST, Pressure, and Winds
Warm Phase of PDO
• Colder SSTs in the central North Pacific
• Warmer SSTs along the west coast of
North America and in the equatorial
central Pacific (similar to El Nino)
• Low pressure anomaly over North Pacific
• Warm winters in the Pacific Northwest
• Cold winters in the southeast U.S.
• Wet winters in western U.S.
• Dry winters in U.S. Midwest
Correlation of Winter Surface Temperature and Precipitation with the PDO Index
Temperature
Tropical
Precipitation
M. D. Eastin
Pacific-Decadal Oscillation (PDO)
Negative PDO index (Cold phase)
Anomalous SST, Pressure, and Winds
Cold Phase of PDO
• Warmer SSTs in the central North Pacific
• Colder SSTs along the west coast of
North America and in the equatorial
central Pacific (similar to La Nina)
• High pressure anomaly over North Pacific
• Cold winters in the Pacific Northwest
• Warm winters in the southeast U.S.
• Dry winters in western U.S.
• Wet winters in U.S. Midwest
Correlation of Winter Surface Temperature and Precipitation with the PDO Index
Temperature
Tropical
Precipitation
M. D. Eastin
Global Warming?
Reliable global observations:
• It is well accepted that reliable
global weather observations
began in the late 1940s
• Hence → global warming “evidence”
Winter normalized NAO index: 1900-2012
Monthly values of the normalized PDO index: 1900-2012
Tropical
M. D. Eastin
Global Warming?
Reliable global observations:
• Notice how the NAO and PDO exhibit
a negative trend from 1920-1960
• Could the recent trends in ENSO
NAO and PDO be an oscillation?
?
Winter normalized NAO index: 1900-2012
?
Monthly values of the normalized PDO index: 1900-2012
?
Tropical
M. D. Eastin
Intraseasonal and Interannual Variability
Summary:
• Madden-Julian Oscillation (MJO)
• Definition and Evolution
• Causes / Forcing
• Impacts
• Quasi-Biennial Oscillation (QBO)
• Definition and Basic Characteristics
• Causes / Physics
• Impacts
• North Atlantic Oscillation (NAO)
• Definition
• Impacts during each Phase
• Causes / Forcing
• Pacific Decadal Oscillation
• Definition
• Impacts during each Phase
• Causes / Forcing
Tropical
M. D. Eastin
References
Baldwin, M. P., and Coauthors, 2001: The Quasi-Biennial Oscillation. Reviews of Geophysics, 39, 179-229.
Bond, N. A., and D. E. Harrison (2000): The Pacific Decadal Oscillation, air-sea interaction and central north Pacific winter
atmospheric regimes. Geophys. Res. Lett., 27(5), 731-734.
Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage
( http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl )
Hurrell, J. W., Y. Kushnir, M. Visbeck, and G. Ottersen, 2003: An Overview of the North Atlantic Oscillation. The North
Atlantic Oscillation: Climate Significance and Environmental Impact, J.W. Hurrell, Y. Kushnir, G. Ottersen,
and M. Visbeck, Eds. Geophysical Monograph Series, 134, 1-35.
Trenberth, K. E. and J. W. Hurrell, 1994: Decadal atmosphere-ocean variations in the Pacific. Clim. Dyn. 9, 303-319.
Zhang, C., 2005: Madden-Julian Oscillation, Reviews of Geophysics, 43, 1-36.
Tropical
M. D. Eastin