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

1. 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

2. 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.

3. 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.

4. Palmen and Newton’s cartoon

5. #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. 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.

6. 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.

8. 0º 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.

12. 25 negative events 31 positive events

13. 45 days 72 days Meridional propagation Poleward propagation in the stratosphere Equatorward propagation in troposphere 2 Periods

14. 12 days 23 days Polar Cap (65-90N) Mid-lat. (40-55N) Sub-Tropics (10-25N) Downward propagation at different PV-lat bands

15. Explanation

16. 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

17. 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.

18. 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.

19. Why do stratospheric anomalies propagate poleward and downward simultaneously and tropospheric anomalies propagate equatorward? A global convection circulation paradigm

20. 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.

21. 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

22. Day_0 Day_50 Day_73 Day_50 Day_29 Day_0 Day-29 Day-73

23. 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

24. 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”.

25. 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

26. 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.

27. 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.