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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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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

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.

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.

#1: The location of the PV contour

is assigned to have a PV latitude

equal to the latitude.

0º

0º

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.

#2: The mapping from PV contours to PV

latitudes is done progressively from large

PV to small PV till reaching the zero PV

contour.

- 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.

0º

30º

NP

30º

Zonally averaging a field along

PV latitudes (or PV contours),

instead of real latitudes, =>

Mean “meridional” circulation in the -PVLAT coordinate.

25 negative events

31 positive events

45 days

72 days

Poleward propagation in the stratosphere

Equatorward propagation in troposphere

2 Periods

12 days

23 days

Polar Cap

(65-90N)

Mid-lat.

(40-55N)

Sub-Tropics

(10-25N)

Downward propagation at different PV-lat bands

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

- 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.

- 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.

A global convection circulation paradigm

Warm

Warm

Cold

Cold

(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.

3

2

1

P1

P2

3

2

1

P1

P2

After

Easterly

anomalies

Westerly

anomalies

Before

YS

YN

Day_0 Day_50

Day_73

Day_50

Day_29

Day_0

Day-29 Day-73

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

Polar cap

(65-90N)

mid-lat

(40-55)

Sub-tropics

10-25N

Minimum U anomalies

Montgomery potential anomaly

- 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”.

47 days

45 days

72 days

=> 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

- 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.

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