1. Intraseasonal and Interannual Variability
Tropical
M. D. Eastin

2. 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

3. 30-60 Day Filtered OLR Anomalies (Blue = Convection Red = dry)
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 days
Discovered by R. Madden and P. Julian in 1971
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
30-60 Day Filtered OLR Anomalies
(Blue = Convection Red = dry)
Tropical
M. D. Eastin

4. 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
Causes (or Forcing Mechanisms)
Unknown
Currently believed to result from both external
forcing and internal instabilities
30-60 Day Filtered
OLR, Streamfunction, and Wind Anomalies
shading → convection
contours → streamfunction
vectors → winds
200 mb
Zonal Cross-Section
850 mb
Wave motion
Cool
Cool
Warm
Moist
Dry
Dry
Warm
Cool
Moist
Cool SST
Warm SST
Cool SST
Adapted from Kiladis et al. (2005) and Zhang (2005)
Tropical
M. D. Eastin

5. Madden-Julian Oscillation (MJO)
Current Status:
Let’s go to:
Tropical
M. D. Eastin

6. Time-Height (Pressure) diagram of Equatorial Zonal Wind
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 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
East
East
East
West
West
West
West
Tropical
M. D. Eastin

7. 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
“Critical Layer”
Wave Flow the waves Flow the waves
Propagation CAN vertically can NOT vertically Momentum
Wave type Direction propagate through propagate through Acceleration
Inertio-Gravity eastward westward strong eastward eastward
Kelvin
Inertio-Gravity
Rossby-Gravity westward eastward strong westward westward
Rossby
Tropical
M. D. Eastin

8. Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the East phase to the West phase
in the lower stratosphere
Time 0
Time 1
dissipate at
upper levels
start new
west phase
aloft
westward
flow max
descends
Eastward
Flow
eastward
flow max
descends
pass
unimpeded
damped and
accelerate
mean flow
Westward
Propagating
Waves
Eastward
Propagating
Waves
Tropical
M. D. Eastin

9. 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
Eastward
Propagating
Waves
Tropical
M. D. Eastin

10. 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
damped and
accelerate
mean flow
pass
unimpeded
Westward
Propagating
Waves
Eastward
Propagating
Waves
thin eastward max
destroyed by
viscous diffusion
Tropical
M. D. Eastin

11. Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the West phase to the East phase
in the lower stratosphere
Time 3
Time 4
dissipate at
upper levels
start new
east phase
aloft
Westward
Flow
eastward
flow max
descends
westward
flow max
descends
pass
unimpeded
damped and
accelerate
mean flow
Westward
Propagating
Waves
Eastward
Propagating
Waves
Tropical
M. D. Eastin

12. 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
Eastward
Propagating
Waves
Tropical
M. D. Eastin

13. 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
thin westward max
destroyed by
viscous diffusion
Tropical
M. D. Eastin

14. Quasi-Biennial Oscillation (QBO)
QBO Physics: Switching from the East phase to the West phase
in the lower stratosphere
Time 0 or Time 6
Time 1 or Time 7
dissipate at
upper levels
start new
west phase
aloft
westward
flow max
descends
Eastward
Flow
eastward
flow max
descends
pass
unimpeded
damped and
accelerate
mean flow
Westward
Propagating
Waves
Eastward
Propagating
Waves
Tropical
M. D. Eastin

15. Quasi-Biennial Oscillation (QBO)
Why is the QBO Important?
The phase of the QBO affects Atlantic TC activity
West phase → Increased TC activity
East phase → 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

16. Winter normalized NAO index: 1900-2012
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:
Source:
Tropical
M. D. Eastin

17. 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

18. 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

19. 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
The positive phase (warm North America and Europe) has dominated since ( )
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

20. Monthly values of the normalized PDO index: 1900-2012
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:
Source:
Tropical
M. D. Eastin

21. Anomalous SST, Pressure, and Winds
Pacific-Decadal Oscillation (PDO)
Positive PDO index (Warm phase)
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
Anomalous SST, Pressure, and Winds
Warm Phase of PDO
Correlation of Winter Surface Temperature and Precipitation with the PDO Index
Temperature
Precipitation
Tropical
M. D. Eastin

22. Anomalous SST, Pressure, and Winds
Pacific-Decadal Oscillation (PDO)
Negative PDO index (Cold phase)
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
Anomalous SST, Pressure, and Winds
Cold Phase of PDO
Correlation of Winter Surface Temperature and Precipitation with the PDO Index
Temperature
Precipitation
Tropical
M. D. Eastin

23. Global Warming? Reliable global observations:
It is well accepted that reliable
global weather observations
began in the late 1940s
Hence → global warming “evidence”
Monthly values of the normalized PDO index:
Winter normalized NAO index:
Tropical
M. D. Eastin

24. Global Warming? ? Reliable global observations:
Notice how the NAO and PDO exhibit
a negative trend from
Could the recent trends in ENSO
NAO and PDO be an oscillation?
Monthly values of the normalized PDO index:
Winter normalized NAO index: ?
Tropical
M. D. Eastin

25. 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
North Atlantic Oscillation (NAO)
Definition
Impacts during each Phase
Pacific Decadal Oscillation
Tropical
M. D. Eastin

26. References Tropical M. D. Eastin
Baldwin, M. P., and Coauthors, 2001: The Quasi-Biennial Oscillation. Reviews of Geophysics, 39,
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),
Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage
( )
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,
Zhang, C., 2005: Madden-Julian Oscillation, Reviews of Geophysics, 43, 1-36.
Tropical
M. D. Eastin