The Quasi Biennial Oscillation
Examining the link between equatorial winds
and the flow regime of the wintertime polar
stratosphere
Charlotte Pascoe
Layout of Talk
Introduction
 QBO history
 How does the QBO work?
 Why is the QBO important?
 Polar vortex and planetary waves
The Unified Model
• Experiments
• Results
Summary
QBO History
1883 Krakatau debris circles the globe
from east to west in two weeks:
Krakatau Easterlies
1908 Berson launches balloons from Lake
Victoria in Africa and finds lower
stratospheric winds blowing from west to
east:
Berson’s Westerlies
QBO History
1960 Reed (US) and Elbon (UK)
“The circulation of the stratosphere”
Balloon measurements reveal alternate
bands of easterly and westerly winds
originating above 30km and moving
downwards through the stratosphere at
~1km per month.
Bands appear at 13 month intervals
26 months required for a complete cycle
QBO History
1960s Lots of meteorologists get sun
tans whilst releasing balloons to
measure this strange new
phenomenon. All find slightly
different cycle periods.
1964 Angell and Korshover give the
cycle the name:
Quasi Biennial Oscillation
The Quasi Biennial Oscillation
60 km
40 m/s
height
-40 m/s
20 km
30 m/s
-30
m/s
1965
QBO phase denotes wind
direction in the lower
stratosphere
time
1987
top panel: equatorial zonal winds
from rocketsonde
middle panel: de-seasonalised
bottom panel: broad-band filtered
(18-36 month)
How does the QBO work?
Holton and Lindzen (1972) proposed a model of the QBO based on vertically
propagating waves. The mechanism was further explained by Plumb (1977).
Equatorially trapped Kelvin waves provide westerly momentum and Rossby-gravity
waves provide easterly momentum to produce the QBO oscillation.
Wavy blue and red lines indicate the penetration of easterly and westerly waves
Why is the QBO important?
Hurricane Forecasts
West: Increased activity in the Atlantic
and NW Pacific
East: Increased activity in the SW
Indian basin
Stratospheric Winter Warmers
Holton and Tan (1980)
West: Cold undisturbed polar vortex
More stratospheric Ozone loss
East: Warm disturbed polar vortex
More tropospheric `cold snaps’
Example of a Stratospheric Sudden Warming
PV on the 1250K isentropic surface (~42 km)
Planetary wave of wave number one
Vertical propagation of planetary waves
Planetary waves (aka Rossby waves) drift to the west
relative to the background flow at typical speeds
of a few metres per second.
The vertical propagation of planetary waves is only
possible under the condition that the zonal wind is
within the range:
0<u<B/(k2 + l2)
Under conditions of easterly background flow no
vertical propagation of planetary waves can occur.
(Westerly flow is never strong enough for the upper limit to be reached)
Charney and Drazin (1961) found no stratospheric
planetary waves in summer when the background
flow is easterly.
QBO as wave guide
The QBO phase determines the position of the zero-line in the
subtropics which acts as a critical line for planetary waves
propagating into the stratosphere.
QBO EAST
Critical line is
in northern
subtropics
QBO WEST
Critical line is
in southern
subtropics
Planetary waves
are free to
move into the
Southern
Hemisphere
Planetary wave
activity is
confined to high
northern
latitudes
Increased heat
and momentum
transport into
the polar
vortex region
Less wave
activity close to
the pole
WEAK POLAR VORTEX
STRONG
POLAR
VORTEX
However…
The Holton-Tan relationship is not exact, there are many
exceptions to this rule of thumb.
Gray, Drysdale, Dunkerton and Lawrence (2001) have suggested
that equatorial winds in the stratopause region are also
important and may help understand polar vortex variability.
J-F Polar temperature North of 62.5oN at 24km
correlated with equatorial winds
Holton-Tan
Negative
correlation
between polar
temperature
and equatorial
winds
Significant
correlation in
stratopause
region where
QBO and
SAO
interfere
Model Description
•
•
•
•
•
•
•
•
UKMO Unified Model (version 4.5)
Hydrostatic primitive-equation model
Run in atmosphere only mode
64 vertical levels: 1000-0.01 hPa (0-80 km)
X-direction (E-W): 96 columns (3.75o)
Y-direction (N-S): 73 rows
(2.5o)
Rayleigh friction imposed above 50 km
Ocean climatology repeated each year
Experiments
3 QBO profiles (period 27 months)
1 SAO profile (period 6 months)
QBO Thick: Large overlap with SAO
QBO Thin:
No overlap with SAO
QBO Normal: Moderate overlap with SAO
Algorithm
U=U–timestep/rlxtime(U-(UQBO+USAO))
Experiments
QBO and SAO forcing amplitudes
wrt height and latitude
QBO + SAO amplitude functions
70
60
Latitude dependence
1.2
1
40
magnitude
Height (km)
50
30
20
10
0.8
0.6
0.4
0.2
0
0
0
5
10
15
20
25
30
Magnitude (m/s)
35
40
-60
-50
-40
-30
-20
-10
0
10
latitude
20
30
40
50
60
Experiments
Relaxation time scale wrt
height and latitude
Latitude Dependence
Relaxation Time Scale
10000
70
y
1000
60
100
10
50
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
40
50
60
latitude
40
Latitude Dependence
30
300
250
20
200
y
height (km)
1
-60
10
150
100
50
0
0
2
4
tim e (days)
6
8
0
-60
-50
-40
-30
-20
-10
0
latitude
10
20
30
Results
SAO
QBO
Results
40hPa Equatorial wind & 10hPa Polar temperature
WEST
EAST
Results
January and February zonal wind composites
Results
J-F Polar temperature North of 60oN at 10hPa (~30km@70oN)
correlated with equatorial winds
Positive
correlation in
upper
stratosphere
Negative
correlation in
lower
stratosphere
Summary
• Need to run the simulation for longer
• We are finding the expected negative
correlation between lower stratospheric
equatorial winds and polar temperatures
• There is also a positive correlation in the
upper stratosphere
• No asymmetry about the mid summer
months (June and July) but this should
be fixed by including an annual cycle over
the equator