40-70 Day Meridional Propagation of Global Circulation Anomalies (A Global Convection Circulation Paradigm for the Annular Mode)

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40-70 Day Meridional Propagation of Global
Circulation Anomalies (A Global Convection
Circulation Paradigm for the Annular Mode)

• Ming Cai1 and R-C Ren1,2
• 1Department of Meteorology
• Florida State University, USA
• 2 LASG, Institute of Atmospheric Physics,
• CAS, Beijing, P. R. China

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The objective
To provide a physical explanation on the
dynamical nature of the annular mode by linking
the climate variability of the annular mode to
the collective effects of individual weather
circulation systems.
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Global zonal mean (mass) circulation viewed from
isentropic coordinate
Speed 1-3 m/s
NH winter NH
summer
Townsend and Johnson (1985)
Warm air is transported poleward at the upper
layer and cold air advances toward the equator
near the surface.
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Palmen and Newtons cartoon
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PV Latitudes (or Equivalent Latitudes, Norton
1994)
2 The mapping from PV contours to PV latitudes
is done progressively from large PV to small PV
till reaching the zero PV contour.
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Data

• NCEP/NCAR isentropic reanalysis II (1979-2003)
• Daily 2.5ºx2.5º gridded data on 11 isentropic
  surfaces.
• PV, U, V, W, Temp./pressure, RH, N2.
• NH and SH.

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Transformation from a 3-D field to 2-D field in
the ?-PVLAT coordinate
Zonally averaging a field along PV latitudes (or
PV contours), instead of real latitudes, gt
Mean meridional circulation in the ?-PVLAT
coordinate.
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Meridional propagation
Poleward propagation in the stratosphere
Equatorward propagation in troposphere
2 Periods
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Polar Cap (65-90N) Mid-lat. (40-55N) Sub-Tr
opics (10-25N)
Downward propagation at different PV-lat bands
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Explanation
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Relationship between temperature and PV anomalies
Generalized PV (Bretherton, 1966)
T gt 0 near the top boundary gt negative PV T lt
0 near the top boundary gt positive PV
T gt 0 at the lower boundary gt Positive PV T lt
0 at the lower boundary gt Negative PV
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Why do the wind anomalies follow the temperature
anomalies of the opposite sign in the
stratosphere?

• T gt 0 aloft gt negative PV anomaly gt positive
  Montgomery potential gt u lt 0 follows poleward
  propagating T gt 0
• T lt 0 aloft gt positive PV anomaly gt negative
  Montgomery potential gt u gt 0 follows poleward
  propagating T lt 0 by a quarter of period.
• OR (by thermal wind relation)
• T lt 0 gt more elevated isentropic surface.
• T gt 0 gt downwelling of isentropic surface.
• Poleward propagating T gt 0 gt more sloped
  isentropic surface at north and less sloped at
  south gt U lt 0 follows T gt 0 by a quarter
  period.

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Why do the wind anomalies lead to the temperature
anomalies of the same sign in the troposphere?

• T lt 0 at low levels gt negative PV source gt
  positive Montgomery potential anomaly gt u lt 0
• T gt 0 at low levels gt positive PV source gt
  negative Montgomery potential anomaly gt u gt 0
• Equatorward propagation gt U gt 0 leads to T gt 0
  by a quarter period and U lt 0 leads to T lt 0 by
  a quarter period.

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Why do stratospheric anomalies propagate poleward
and downward simultaneously and tropospheric
anomalies propagate equatorward?
A global convection circulation paradigm
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Semi-geostrophic frontogenesis theory (Hoskins
1972)
(Fig. 2.68 of the book by Bluestein)
Warm
Cold
Due to cross-frontal circulation, the baroclinic
zone becomes less vertically sloped gt or a more
leveled baroclinic zone gt upper level
frontolysis in the warm air sector and
frontogenesis in the cold air sector.
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Application of the semi-geostrophic frontogenesis
theory
After
Easterly anomalies
Westerly anomalies
Before
YS
YN
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Day_0 Day_50
Day_73
Day_50
Day_29
Day_0
Day-29 Day-73
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Slope of the extratropical baroclinic zone and
the annular mode variability
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Troposphere and Stratosphere coupling

• Tropospheric cold air in high latitudes starts to
  propagate equatorward as the arrival of the
  stratosphere warm air gt disruption of the
  downward propagation of temperature anomalies
  into troposphere.
• T gt 0 in the stratosphere gt negative PV
  anomalies gt U lt 0
• T lt 0 in the troposphere gt negative PV source
  gt Ult0
• wind anomalies APPEARS to propagate downward
  continuously.

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Time Scale
gt 1.6 m.s
gt 2.5 m.s
Poleward propagation in the stratosphere
Enhanced hemispheric mass circulation is faster
due to a stronger meridional temperature gradient
gt a stronger eddy forcing Weaker hemispheric
mass circulation is slower due to a weaker
meridional temperature gradient gt a weak eddy
forcing.
Equatorward propagation in troposphere
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Summary

• The annular mode variability is a manifestation
  of continuous and endless adjustments of mass,
  geostrophy, and static stability accompanying
  with the processes of transporting heat poleward.
• Global mass adjustment/circulation is carried out
  by a succession of cross-frontal circulations
  from the tropics to the pole and from the
  stratosphere to the troposphere gt Stratospheric
  circulation anomalies propagate poleward and
  downward whereas tropospheric anomalies propagate
  equatorward.
• The leveling of the vertically slopped baroclinic
  zone results in a reduction (an increase) of the
  meridional temperature gradient in the warm
  (cold) air sector. gt a weakening
  (strengthening) of the westerly jet in the warm
  air sector (cold air sector).
• A more sloped baroclinic zone in the polar area
  corresponds to the positive phase of the annular
  mode and a more leveled baroclinic zone
  corresponds to the negative phase of the annular
  mode.
• The propagation time scale is dictated by
  diabatic heating/cooling of both external
  thermal forcing and eddy-driven forcing.

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Climate prediction application

• The long time scale (40-70 days).
• The systematic poleward propagation from the
  tropics to the pole.
• The coupling of stratospheric and tropospheric
  anomalies.