3. Midlatitude Storm Tracks and the North Atlantic Oscillation

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3. Midlatitude Storm Tracks and the North Atlantic Oscillation Copyright 2006 Emily

2 Review of key results from last lecture 1. Basic midlaitude flow in lower atmosphere: jetstream with horizontal and vertical mean-flow shears. 2. Barotropic instability: associated with horizontal shear. Extracts KE from mean flow. 3. Baroclinic instability: associated with vertical shear. Converts PE associated with mean horizontal temp gradient (vertical shear). 4. From Eady problem: max growth rate baroclinic instability when no cross-flow structure). Max growth rate 0.31 f 0Λ N. Length scale of fastest growing mode just under L R. 5. For atmosphere: e-folding time for fastest growing waves 18 hours; Length scale of fastest growing waves 1000 km Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/2

Baroclinic life cycle Instability theories only deal with initial development of small disturbances. Role of eddies in general circulation depends on mean effect over life cycle.

3 Baroclinic life cycle Instability theories only deal with initial development of small disturbances. Role of eddies in general circulation depends on mean effect over life cycle. Simmons and Hoskins, 1980 Horizontal temperature mainly confined to 30-60, giving jet centred on 45 and 200 mb. Evolution of initial disturbance with structure of fastest growing mode of zonal wavenumber 6 with small amplitude (maximum disturbance pressure 1mb). How does it develop? Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/3

4 initial final Near-surface temperature after 5 days Meridional cross sections of zonal-mean potential temperature and zonal wind Effect of baroclinic instability is to flatten potential temperature surfaces and deepen and intensify the jet. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/4

5 Eliassen-Palm flux Linearised QGPV equation:{ } q + u q + v = 0 t x y 1 q + u q + q v = 0. t x 2 y multiply by q to give { } 2 Average wrt x over a wavelength & assuming wavelike q form of solution, follows that if 0 y then q v = 0, ie if wave amplitude not changing significantly with time, meridional flux of QGPV is zero. Can be re-written as: ( ) ρ0 q v = F y + ( F z ) where F y = ρ 0u v and y z v θ F z = ρ0 f0 are components to Eliassen-Palm flux. θ 0z If conditions steady & dissipation-free then, as above, flux is divergence-free: no effect of waves on mean flow. But in general divergence and EP flux useful in assessing effects on mean flow. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/5

6 initial Vertical arrows; poleward heat flux, no momentum flux Day 8 Planetary waves, upward and equatorward poleward transfer of westerly momentum Eliassen-Palm flux (arrows) and divergence (contours) QG EP flux: horizontal proportional to horizontal momentum flux; vertical proportional to horizontal heat flux & divergence proportional to QGPV flux. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/6

7 Effect on the mean flow Deduced from the effect of saturating instability on PV and surface temperature fields. Net effect is to accelerate the flow (in the vertical average) near the centre of the jet. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/7

North Thorncroft et al, 1993 Copyright 2006 Emily Shuckburgh, University

9 LC1 LC2 10ms -1 westerly at 20 N 10ms -1 easterly at 50 N Barotropic decay mechanism less active in LC2 Mean jet poleward ~10 (equatorward ~2 ) LC1 (LC2). Eady model: baroclinic normal modes tilt westward with height. Jet f-plane (symmetric): crescent shape in horizontal. Sphere: most eddy KE on cyclonic (north) side of jet hence LC1 & LC2 lie NW-SE. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/9

LC1 Copyright 2006 Emily Shuckburgh, University of Cambridge.

LC2 Copyright 2006 Emily Shuckburgh, University of Cambridge.

But need to account for strong westerly acceleration at entry (thermally direct) and

12 Jet Streams 200mb Planetary-scale flow highly longitude dependent influenced by orography and continentocean heating contrasts. But need to account for strong westerly acceleration at entry (thermally direct) and deceleration at exit (eddy fluxes). Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/12

14 Dec-Feb (ECMWF data ) Hoskins and Valdes, 1990 Diabatic heating (column mean) Horizontal temperature flux 700mb Vertical temperature flux 700 mb Height variance at 250 mb Heating regions off E coasts, to Sth of storm track sensible heating at surface & shallow convection. Diabatic heating maxima extend westwards & slightly poleward latent heat release. Hence diabatic heating owes existence partly to presence of storm track. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/14

Altantic storm track Pacific storm track Measure of baroclinicty: maximum Eady growth rate 0 0.

15 Altantic storm track Pacific storm track Measure of baroclinicty: maximum Eady growth rate f N Λ If midlatitude weather system grows in region of high baroclinicity, by transporting heat poleward it acts to reduce this baroclinicity. Are storm tracks self-maintaining? Simple model linearised about observed Dec-Feb timeaveraged flow. Include forcings from N, 80 W- 20 E region. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/15

Horizontal vorticity flux convergence Vorticity flux gives maximum in baroclinicity in Atlantic

16 Horizontal temperature flux convergence Vertical potential temperature flux convergence Total direct thermal effect Direct thermal effects give minimum in baroclinicity in Atlantic storm-track. Horizontal vorticity flux convergence Vorticity flux gives maximum in baroclinicity in Atlantic storm-track. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/16

Response in barocinicity to total forcings Copyright 2006 Emily

17 Mean diabatic heating Diabatic heating gives maximum in baroclinicity in Atlantic storm-track region. Response in barocinicity to total forcings Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/17

Blocking Large-scale movements timescales > baroclinic instability (1 week) significant impact on weather. Block like dipole in flow. Nth side clear sky & cold, Sth wet & warm. Can last ~20 days.

18 Blocking Large-scale movements timescales > baroclinic instability (1 week) significant impact on weather. Block like dipole in flow. Nth side clear sky & cold, Sth wet & warm. Can last ~20 days. Why so long? LC1 has features in common with events that occur during anticyclone blocking situations. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/18

Different timescales of circulation Copyright 2006 Emily Shuckburgh,

High index NAO winters: westerlies onto Europe stronger (warmer than usual temperatures) and anomalous

20 North Atlantic Oscillation (NAO) NAO index (pressure difference Iceland and Portugal) Average surface temperature anomalies (Dec-Mar) (High NAO) compared to mean. High index NAO winters: westerlies onto Europe stronger (warmer than usual temperatures) and anomalous flow over Nth America Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/20

21 High A, B: Moisture transport Normal/low High index: less precip Southern Europe and Greenland (ice sheet); more precip Scandinavia. C: Evap - precip Shift storm track & associated synoptic eddy activity. Changes in flow (mean & eddy) affect transport & convergence of moisture tied to regional precip. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/21

Structure defined on basis of point correlation.

22 +ve Temporal variability characterised by differences in sea level pressure between Iceland & Azores/Portugal. Structure defined on basis of point correlation. -ve Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/22

Northern Annular Mode (NAM) Thompson and Wallace, 2000 Leading empirical orthogonal function (EOF) of NH sea level pressure (SLP) anomalies. Strong zonally symmetric component.

23 Northern Annular Mode (NAM) Thompson and Wallace, 2000 Leading empirical orthogonal function (EOF) of NH sea level pressure (SLP) anomalies. Strong zonally symmetric component. Dipole fluctuations in mass between entire Arctic basin & surrounding zonal ring. Northern Annular Mode (NAM). Variances in NAM are reflected in frequency of occurrence of blocking & cold air outbreaks. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/23

NAM deep, nearly barotropic structure with zonal wind perturbations of opposing sign along ~55 and ~35. Zonally symmetric component, but largest variability in Nth Atlantic sector.

24 NAM deep, nearly barotropic structure with zonal wind perturbations of opposing sign along ~55 and ~35. Zonally symmetric component, but largest variability in Nth Atlantic sector. During winter NAM strongly coupled to circulation of NH stratosphere. In troposphere, NAM fluctuates on timescales ~10 days & associated with anomalous fluxes of zonal momentum by baroclinic waves across ~45 N. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/24

Positive temperature anomalies at 45 in troposphere & negative in stratosphere - lifting of tropopause.

25 Equatorward flow across maximum westerly wind anomaly upper troposphere - easterly acceleration, balanced by anomalous eddy flux convergence + opposite at lower latitudes. Hence anomalous poleward eddy flux of westerly momentum. Positive temperature anomalies at 45 in troposphere & negative in stratosphere - lifting of tropopause. Also modulation of Lagrangian mean meridional circulation. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/25

26 Zonally asymmetric thermal structure induced by land-sea distribution. Thermal advection. (contours climatological temperature, arrows wind anomalies) Regions of cold advection tend to coincide with negative thickness anomalies. Suggest temperature advection by zonal component of anomalous flow important in establishing & maintaining stationary wave pattern. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/26

27 Stratosphere Tropospheric variability in NAM driven by interactions between zonal flow and baroclinic waves. Stratospheric variability driven by slower interactions between zonal flow & planetary-scale waves. Tropospheric NAM: exists in models without stratosphere or ocean. Stratospheric NAM: annular mode like variations models without troposphere. But apparently dynamical coupling between stratosphere and troposphere. Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/27

Forecasting European winters Composite of: 1958, 1962, 1966,

distribution used as forcing anomaly in atmospheric global

May SST 2005 Copyright 2006 Emily Shuckburgh, University of

28 Forecasting European winters Composite of: 1958, 1962, 1966, 1969, 1980, 1995 (contour 95% significant) Idealised distribution used as forcing anomaly in atmospheric global circulation model experiment (HadAM3, 40 year integration) May SST 2005 Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/28

Copyright 2006 Emily Shuckburgh, University of Cambridge.

29 Circulation anomaly (anticyclone) Reinforced by cold SSTs Rossby wave propagation into Europe Precipitation anomaly Copyright 2006 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without permission. EFS 3/29

2. Baroclinic Instability and Midlatitude Dynamics

2. Baroclinic Instability and Midlatitude Dynamics Midlatitude Jet Stream Climatology (Atlantic and Pacific) Copyright 26 Emily Shuckburgh, University of Cambridge. Not to be quoted or reproduced without