Meridional structure of the downwelling branch of the BDC Susann Tegtmeier

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1 Meridional structure of the downwelling branch of the BDC Susann Tegtmeier Helmholtz Centre for Ocean Research Kiel (GEOMAR), Germany SPARC Brewer-Dobson Circulation Workshop, Grindelwald, June 2012

2 no data [DU] Interannual variability of the late winter Arctic ozone column vortex average late March total ozone Year Tegtmeier et al., 2008

3 no data [DU] Interannual variability of the late winter Arctic ozone column vortex average late March total ozone range of October observations Year Tegtmeier et al., 2008

4 no data [DU] Chemical contribution to the interannual variability of the late winter Arctic ozone column vortex average late March total ozone range of October observations chemical loss Year Tegtmeier et al., 2008

5 no data [DU] Chemical and dynamical contribution to the interannual variability of the late winter Arctic ozone column vortex average late March total ozone range of October observations dynamical supply chemical loss Year Tegtmeier et al., 2008

6 no data [DU] Chemical and dynamical contribution to the interannual variability of the late winter Arctic ozone column vortex average late March total ozone range of October observations dynamical supply chemical loss Year Tegtmeier et al., 2008

7 Change of total ozone during winter [DU] Total ozone [DU] Chemical and dynamical contribution to the variability of the late winter Arctic ozone column 100 dynamical supply 500 late March ozone Interannual variability of chemical loss and dynamical supply is largely driven by variability of tropospheric wave forcing chemical loss range of October observations For the current halogen loading both contribute about half to the total ozone variability in Arctic spring EP-Flux (F z ) [10 5 kg s 2 ] EP-Flux (F z ) [10 5 kg s 2 ] Tegtmeier et al., 2008

8 How to derive the downwelling branch of the BDC from atmospheric data Observations of trace gases in the polar stratosphere based on the observed descent of tracer isopleths measured during field campaigns or by satellite instruments Uses tracers with long photochemical lifetime and strong concentration gradients If possible effects of meridional mixing have to be taken into account Diabatic heating rates derived from radiative transfer models and temperature fields from meteorological assimilation models Radiative transfer model and a one-dimensional vortex interior descent model Backward or forward trajectory calculations driven by wind fields from GCMs or meteorological assimilation models and by diabatic heating rates Different measures of downwelling: Mean subsidence of air parcels (trajectory calculations) Mean change of potential temperature (diabatic cooling rates) Mean change of tracer mixing ratio profiles with time (observations) Residual vertical velocities

9 NH diabatic descent based on backward trajectories PDF of starting and end potential temperatures for polar winter backward trajectories (red) great deal of structure and interannual variability Rosenfield et al., 2001

10 SH middle stratospheric descent based on HALOE CH 4 Descent rate versus E-P flux divergence of the zonal wavenumber 1 component at 1 hpa and 60S. Year-to-year variations Kawamoto and Shiotani, 2000

11 Climatology of NH vortex averaged diabatic descent Vortex-averaged diabatic descent shows very pronounced interannual variability which expresses the year-to-year variability of the polar branch of the residual circulation. Tegtmeier et al., 2008

12 Climatology of NH vortex averaged diabatic descent Tegtmeier et al., 2008

13 Meridional structure of downwelling Plumb, 2002 Rossby wave pumping in the surf zone produces most descent at the vortex edge Indications for vortex descent from the mesosphere all the way down to the lower stratosphere ( gravity wave pump )

14 Meridional structure of downwelling in the SH

15 Vertical residual velocity derived from tracer measurements strongest descent is along the jet axis meridional extent of the vertical velocity field narrows below 24 km above 24 km: greater eddy activity which extends toward the pole, creating a broader region of subsidence Schoeberl et al., 1992

16 Monthly mean heating rates for 1987 SH winter During polar night strongest descent along the vortex edge Rosenfield et al., 1994

17 Antarctic polar descent from zonal mean evolution of ISAMS CO Allen et al., 2000

18 Descent rates derived from ISAMS CO contours and diabatic trajectories mean descent rate: ~15 K/day. mean descent rate: ~20 K/day. Allen et al., 2000

19 Diabatic descent based on forward trajectories Above 900 K: Strongest descent in the center Below 900 K: Strongest descent near the vortex edge Manney et al., 1994

20 What about the NH?

21 Vertical residual velocity derived from tracer measurements Arctic: comparatively broader latitudinal width and generally stronger in the lower stratosphere Schoeberl et al., 1992

22 Monthly mean heating rates for 1991/92 NH winter (AASE II) Largest cooling exterior to the vortex Rosenfield et al., 1994

23 Diabatic descent based on forward trajectories Early winter: Strongest descent near the center of the vortex except during periods of particularly strong wave activity Late winter: Strongest descent near the vortex edge Manney et al., 1994

24 Averaged diabatic descent inside the polar vortex 1996/1997 vs. 1998/1999

25 Diabatic descent [K/day] as a function of potential temperature and equivalent latitude Winter 1996/97 Meridional structure varies over the course of the winter

26 Diabatic descent [K/day] as a function of potential temperature and equivalent latitude Winter 1998/99 Meridional structure varies over the course of the winter

27 Meridional structure of the diabatic descent for 1996/1997 and 1998/1999 Tegtmeier et al., 2008 Cold and undisturbed polar vortex: Strongest diabatic descent at the vortex edge (similar to SH) Stronger wave activity and disturbed polar vortex: Strongest diabatic descent in the vortex core

28 Summary Pronounced year-to-year variability of the polar branch of the residual circulation apparent from trace gas observations and diabatic descent estimates based on temperature fields from meteorological assimilation models Year-to-year variations of residual circulation determine dynamical ozone supply to the polar regions which explain large fraction of late spring total ozone in the Arctic Meridional structure of diabatic descent in the SH Above 900 K strongest descent in the center Below 900 K strongest descent near the vortex edge Meridional structure of diabatic descent in the NH stratosphere Cold and undisturbed polar vortex: strongest diabatic descent at the vortex edge (similar to SH) Stronger wave activity and disturbed polar vortex: strongest diabatic descent in the vortex core

29

30 / /1997

31 Monthly mean heating rates for 1991/92 NH winter (AASE II) Rosenfield et al., 1994

32 Climatology of NH vortex averaged diabatic descent r = 0.77 r = 0.82 r = K 450 K 460 K

33 How to derive the downwelling branch of the BDC from atmospheric data 1) Observations of trace gases in the polar stratosphere based on the observed descent of tracer isopleths measured during field campaigns or by satellite instruments Uses tracers with long photochemical lifetime and strong concentration gradients (e.g. CO) HALOE CH 4 in SH spring stratospheric vortex were as low as those found in the mesosphere (Russell et al., 1993) N 2 O and H 2 O from MLS: descent of mesospheric and upper stratospheric air to the middle and lower stratosphere (Lahoz et al., 1993) 2) Quantification based on calculated diabatic heating rates from radiative transfer models and temperature fields from meteorological assimilation models Radiative transfer model and a one-dimensional vortex interior descent model (Rosenfield et al., 1994) Backward or forward trajectory calculations driven by wind fields from GCMs or meteorological assimilation models and by diabiatic heating rates Different measures of diabatic descent have been used Mean descent of parcels as computed in forward trajectories Mean change of potential temperature calculated using diabatic cooling rates Mean change of tracer mixing ratio profiles with time.

34 Monthly and zonal mean diabatic ascent rate for December 1999 Plumb et al., 2003

35 Diabatic descent based on backward trajectories Most air masses originate from low and middle latitudes in the fall: Either from the upper stratosphere and mesosphere Or from lower in the stratosphere with mixing in during vortex formation Rosenfield and Schoeberl, 2001

36 Diabatic descent based on backward trajectories Significant fraction of the air in the springtime polar vortex has been mixed in from the low and middlelatitudes Rosenfield and Schoeberl, 2001 Forward calculations can give misleading information on the amount of descent, as well as on the degree of isolation of polar from midlatitude air

37 Diabatic circulation vertical velocities [cm/s] Rosenfield et al., 1987

38 Diabatic circulation vertical velocities [cm/s] Rosenfield et al., 1987

39 Zonal mean diabatic heating rates [degree/day] for 15 July 1991 Fisher et al., 1993