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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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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.
is assigned to have a PV latitude
equal to the latitude.
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
30ºTransformation 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.
31 positive events
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
OR (by thermal wind relation)
A global convection circulation paradigm
ColdSemi-geostrophic frontogenesis theory (Hoskins 1972)
(Fig. 2.68 of the book by Bluestein)
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.
P2Application of the semi-geostrophic frontogenesis theory
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.
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.
P2Slope of the extratropical baroclinic zone and the annular mode variability
Minimum U anomalies
Montgomery potential anomalyTroposphere and Stratosphere coupling
72 daysTime 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