40-70 Day Meridional Propagation of Global Circulation Anomalies
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40-70 Day Meridional Propagation of Global Circulation Anomalies ( A Global Convection Circulation Paradigm for the Annular Mode) . Ming Cai 1 and R-C Ren 1,2 1 Department of Meteorology Florida State University, USA 2 LASG, Institute of Atmospheric Physics, CAS, Beijing, P. R. China.

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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 annul

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


The objective

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.


Global zonal mean mass circulation viewed from isentropic coordinate

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.


Palmen and newton s cartoon

Palmen and Newton’s cartoon


Pv latitudes or equivalent latitudes norton 1994

#1: The location of the PV contour

is assigned to have a PV latitude

equal to the latitude.

30º

30º

NP

NP

30º

For circularcontours of PV centered at the pole,

PV Latitudes = Latitudes

Area encircled by a PV contour

= Area encircled by a Latitude.

PV Latitudes (orEquivalent 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.


40 70 day meridional propagation of global circulation anomalies a global convection circulation paradigm for the annul

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.


Transformation from a 3 d field to 2 d field in the pvlat coordinate

30º

NP

30º

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, =>

Mean “meridional” circulation in the -PVLAT coordinate.


40 70 day meridional propagation of global circulation anomalies a global convection circulation paradigm for the annul

25 negative events

31 positive events


Meridional propagation

45 days

72 days

Meridional propagation

Poleward propagation in the stratosphere

Equatorward propagation in troposphere

2 Periods


40 70 day meridional propagation of global circulation anomalies a global convection circulation paradigm for the annul

12 days

23 days

Polar Cap

(65-90N)

Mid-lat.

(40-55N)

Sub-Tropics

(10-25N)

Downward propagation at different PV-lat bands


Explanation

Explanation


Relationship between temperature and pv anomalies

Relationship between temperature and PV anomalies

Generalized PV (Bretherton, 1966)

T’ > 0 near the top boundary => negative PV

T’ < 0 near the top boundary => positive PV

T’ > 0 at the lower boundary => Positive PV

T’ < 0 at the lower boundary => Negative PV


Why do the wind anomalies follow the temperature anomalies of the opposite sign in the stratosphere

Why do the wind anomalies follow the temperature anomalies of the opposite sign in the stratosphere?

  • T’ > 0 aloft => negative PV anomaly => positive Montgomery potential => u’ < 0 follows poleward propagating T’ > 0

  • T’ < 0 aloft => positive PV anomaly => negative Montgomery potential => u’ > 0 follows poleward propagating T’ < 0 by a quarter of period.

    OR (by thermal wind relation)

  • T’ < 0 => more elevated isentropic surface.

  • T’ > 0 => downwelling of isentropic surface.

  • Poleward propagating T’ > 0 => more sloped isentropic surface at north and less sloped at south => U’ < 0 follows T’ > 0 by a quarter period.


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

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

  • T’ < 0 at low levels => negative PV source => positive Montgomery potential anomaly => u’ < 0;

  • T’ > 0 at low levels => positive PV source => negative Montgomery potential anomaly => u’ > 0;

  • Equatorward propagation => U’ > 0 leads to T’ > 0 by a quarter period and U’ < 0 leads to T’ < 0 by a quarter period.


40 70 day meridional propagation of global circulation anomalies a global convection circulation paradigm for the annul

Why do stratospheric anomalies propagate poleward and downward simultaneously and tropospheric anomalies propagate equatorward?

A global convection circulation paradigm


Semi geostrophic frontogenesis theory hoskins 1972

Warm

Warm

Cold

Cold

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 => or a more leveled baroclinic zone => upper level frontolysis in the warm air sector and frontogenesis in the cold air sector.


Application of the semi geostrophic frontogenesis theory

3

2

1

P1

P2

3

2

1

P1

P2

Application of the semi-geostrophic frontogenesis theory

After

Easterly

anomalies

Westerly

anomalies

Before

YS

YN


40 70 day meridional propagation of global circulation anomalies a global convection circulation paradigm for the annul

Day_0 Day_50

Day_73

Day_50

Day_29

Day_0

Day-29 Day-73


Slope of the extratropical baroclinic zone and the annular mode variability

More leveled isentropic surfaces (a gently sloped baroclinic zone) => the negative phase of the annular mode. => surface cold air advances southward => cold episodes in mid latitudes.

NAM

3

2

P1

1

P2

Steeply sloped isentropic surfaces (a steeply sloped baroclinic zone) => the positive phase of the annular mode=> surface cold air mass remains in the polar region => a much warm SURFACE temperature in mid-latitudes.

+NAM

3

2

1

YS

YN

P1

P2

Slope of the extratropical baroclinic zone and the annular mode variability


Troposphere and stratosphere coupling

Polar cap

(65-90N)

mid-lat

(40-55)

Sub-tropics

10-25N

Minimum U anomalies

Montgomery potential anomaly

Troposphere and Stratosphere coupling

  • Tropospheric cold air in high latitudes starts to propagate equatorward as the arrival of the stratosphere warm air => “disruption” of the downward propagation of temperature anomalies into troposphere.

  • T’ > 0 in the stratosphere => negative PV anomalies => U’ < 0

  • T’ < 0 in the troposphere => negative PV source => U’<0

  • wind anomalies APPEARS to propagate downward “continuously”.


Time scale

47 days

45 days

72 days

Time Scale

=> 2.5 m.s

=> 1.6 m.s

Poleward propagation in the stratosphere

Enhanced hemispheric mass circulation is faster due to a stronger meridional temperature gradient => a stronger eddy forcing

Weaker hemispheric mass circulation is slower due to a weaker meridional temperature gradient => a weak eddy forcing.

Equatorward propagation in troposphere


Summary

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 => 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. ==> 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.


Climate prediction application

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.


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