The Quasi Biennial Oscillation Examining the link between equatorial winds and the flow regime of the wintertime polar stratosphere Charlotte Pascoe
Jan 08, 2016
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 • ResultsSummary
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 History1960 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 History1960s 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
top panel: equatorial zonal winds from rocketsondemiddle panel: de-seasonalisedbottom panel: broad-band filtered (18-36 month)
height
time
20 km
60 km
1965 1987
40 m/s
-40 m/s
30 m/s
-30 m/s
QBO phase denotes wind direction in the lower stratosphere
How does the QBO work?
Wavy blue and red lines indicate the penetration of easterly and westerly waves
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.
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 guideThe 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.
Planetary wave activity is confined to high northern latitudes
Increased heat and momentum transport into the polar vortex region
WEAK POLAR VORTEX
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
Less wave activity close to the pole
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.
Holton-Tan
Negative correlation between polar temperature and equatorial winds
Significant correlation in stratopause region where QBO and SAO interfere
J-F Polar temperature North of 62.5oN at 24km correlated with equatorial winds
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
AlgorithmU=U–timestep/rlxtime(U-(UQBO+USAO))
Experiments
Latitude dependence
0
0.2
0.4
0.6
0.8
1
1.2
-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60
latitude
mag
nit
ud
e
QBO + SAO amplitude functions
0
10
20
30
40
50
60
70
0 5 10 15 20 25 30 35 40
Magnitude (m/s)
Heig
ht
(km
)QBO and SAO forcing
amplitudes wrt height and latitude
Experiments Relaxation time scale wrt height and latitude
Relaxation Time Scale
0
10
20
30
40
50
60
70
0 2 4 6 8
time (days)
hei
gh
t (k
m)
Latitude Dependence
1
10
100
1000
10000
-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60
latitude
y
Latitude Dependence
0
50
100
150
200
250
300
-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60
latitude
y
Results
QBO
SAO
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
Negative correlation in lower stratosphere
Positive correlation in upper 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