Comparisons of Brewer-Dobson Circulations
Toshiki Iwasaki1, Hisashi Hamada1 and Kazuyuki Miyazaki2
12

Graduate School of Science, Tohoku University, Japan
Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science andTechnology, Japaniwasaki@wind.geophys.tohoku.ac.jp

1. INTRODUCTION
We are planning to make revised reanalysis of ozone by using a chemical transport model (CTM), where theCTM is driven by a general circulation model (GCM) assimilating the atmospheric reanalyses. In this system,
reproduced distributions of minor constituents are considerably subject to the quality of atmospheric reanalyses.
For stratospheric transport of minor constituents in a meridional plane, important is the Brewer- Dobson (BD)circulation as well as quasi-isentropic diffusions (Miyazaki and Iwasaki, 2005). In this work, we compare
Brewer-Dobson circulation among JRA25, NCEP/NCAR and ERA-40, where the mean-meridional circulation isanalyzed in terms of mass-weighted isentropic zonal means (MIM, Iwasaki, 1989).
Intercomparison study among the analyses is made of the climatological mean states of BD circulation and its
interannual variabilityies.
The secondary circulation of the southern hemispheric polar vortex (PV) isinvestigated based on the zonal mean variables.
2. CLIMATOLOGICAL MEAN STATES OF MASS STREAMFUNCTION
Figure 1 shows the mean-meridional circulation in boreal winter (DJF) and summer (JJA) averaging from 1979
to 2001. The MIM method diagnoses BD circulation-like poleward mass flux from all the reanalyses in the lower
stratosphere.
The stratospheric mean-meridional circulations diagnosed from reanalyses are, however,considerably different from each other particularly in JJA. ERA is much larger in the mass flux near the tropical
tropopause than other reanalyses. The NCEP/NCAR and NCEP/AMIP have relatively large northern hemisphericmass flux with the southern hemispheric mass flux. JRA has unrealistic structure in the middle stratosphere.

Fig. 1 Mass streamfunctions of JRA-25, ERA-40, NCEP/NCAR-1 and NCEP/AMIP2 averaging during December,January and February (DJF: on the left), and during June, July and August (JJA: on the right) 1979-2001 from 100hPa
to 10 hPa. Unit: 1010kg sec-1.

3. INTERANNUAL VALIABILITY AND TREND
Figure 2 shows the year-to-year variations of winter mean mass streamfunctions at 100hPa of some latitudes,
which indicate poleward mass flux above 100hPa. Note that negative values indicate poleward mass flux in thesouthern hemisphere. In low latitudes, the mass flux analyzed from ERA is the largest of those from the fourreanalyses, while in the middle latitudes that from NCEP2 is the largest. Probably, such differences come from thephysics parameterizations in the model. In the tropics, mean-meridional circulation is directly driven by latitudinal
gradient of diabatic heating. In the extratropics, it is indirectly driven by wave-mean flow interactions, both effects
of model resolvable waves and parameterized gravity waves.
In middle latitudes of both winter hemispheres, we have weak increasing trends of the mass streamfunctions at
100 hPa, corresponding to intensification of Brewer-Dobson circulations. The trend during 20 years is, however,smaller than interannual variability. It is also smaller than differences between the two of reanalyses. In thesouthern hemisphere, its rates analyzed from individual reanalyses have considerable discrepancy. All of thosesuggest that we are unable to make full discussions on the trend of Brewer-Dobson circulation.
Interannual variability seems to be fairly correlated with each other. Correlations of detrended interannualvariability are shown in Table 1. The correlation is better in the extratropics than in the tropics. It indicates thatmean-meridional circulation is primarily driven by wave-mean flow interactions of resolvable waves. This will bechecked directly by Eliassen-Palm flux and its divergence in the reanalyses. The correlation is worse in the southernhemisphere than in the northern hemisphere. The sparsity of the observation data degrades the quality of reanalyses
in the southern hemisphere.

Fig. 2. Year-to-Year variations of mass streamfunctions at 100 hPA in the winter hemisphere. Note that negativevalues indicate poleward flux in the southern hemisphere. Upper left, Upper right, lower left and lower right panels
are values at 15N for DJF, 45N for DJF, 15S for JJA and 45S for JJA, respectively. Colors of red, green, blue andlight blue indicate JRA, ERA, NCEP and NCEP2, respectively.
Unit: 109kg sec-1.

Table 1. Correlations of detrended interannual variablities of the mass streamfunctions
at 100 hPa in winter hemisphere(JRA v.s. ERA)
15N for DJF
0.50
30N for DJF
0.45
45N for DJF
0.73
60N for DJF
0.77
15S for JJA
0.08
30S for JJA
0.24
45S for JJA
0.19
60S for JJA
0.03(JRA v.s.NCEP)
0.49
0.73
0.59
0.78
-0.08
0.16
0.64
0.50

(NCEP v.s. ERA)
0.32
0.40
0.58
0.75
0.41
0.40
0.70
-0.03

4. SECONDARY CIRCULATION OF SOUTHERN HEMISPHERIC POLAR VORTEX
The polar vortex (PV) surrounded by very strong polar night jet stream is a unique structure in the stratosphere. Its
secondary circulation, which plays important roles in the transport of minor constituents, is of great concern to thestratospheric dynamics and chemistry. The vertical velocity is so weak that we hardly estimate it accurately.
Furthermore, the northern hemispheric PV can not be analyzed by means of zonal mean variables, because it is to acertain degree axially asymmetric. Thus, our discussions are focused on the southern hemispheric PV.
Figure 3 shows a GCM diagnosis based on MIM (Iwasaki, 1989), where the model is the MRI/JMA 1998 GSM(Shibata et al., 1999). The left and right hand side panels are diagnosed in the dynamical way from meridional windwith the continuity equation and in the diabatic way from radiative heatings with the thermodynamic equation. The
vertical velocity obtained with the two different data is very consistent with each other and indicates its reliability(Miyazaki and Iwasaki, 2008). Note that the transformed Eulerian Mean (TEM) provides too strong downward
velocity outside of PV and too weak one (sometimes upward) inside. It is interesting that the major downward
branch of the Brewer-Dobson circulation is located just outside of the polar night jet stream (Iwasaki, 1992), as far as
the model atmosphere is concerned.
Figure 4 shows the seasonality of the mean vertical velocity at 50hPa. In the GCM, the downward branch ofBrewer-Dobson circulation is located in middle latitudes and it slightly moves with season. On the other hand, thereanalyses are too noisy to see seasonality of the vertical velocity. JRA has a similar seasonality to the GCM but it isstill subject to large noises. Actual secondary circulation of the polar vortex is an important matter to be studied in
the future reanalyses.
5. CONCLUDING REMARKSThe reanalyses are still too noisy for us to use them for the assessment of Brewer-Dobson circulation. They also
have some discrepancies of trends. Considering the smoothness of GCM output, assimilation processes ofobservation data cause noises and discrepancies in the reanalyses. Model biases are most responsible fordiscrepancies among reanaleses. For stratospheric analyses, we should improve parameterization schemes ofradiation and gravity wave drag. Data assimilation causes dynamical imbalances which cause gravity wave noises.
Such imbalance may be removed by advanced data assimilation technique.
REFERENCES
Iwasaki, T., 1989: A diagnostic formulation for wave-mean flow interactions and Lagrangian -mean circulation
with a hybrid vertical coordinate of pressure and isentrope. J. Meteor. Soc. Japan, 67, 293-312.
Iwasaki, T., 1992: General circulation diagnosis in the pressure-isentrope hybrid vertical coordinate. J. Meteor.
Soc. Japan, 70, 673-687.
Miyazaki K., and T. Iwasaki, 2005; Diagnosis of meridional ozone transport based on mass weighted isentropic zonalmeans, J. Atmos. Sci., 62, 1192-1208.
Miyazaki K., and T. Iwasaki, 2008: On the secondary circulation of the Antarctic polar vortex, submitted to J. Atmos.
Sci.

Shibata, K., H. Yoshimura, M. Ohizumi, M. Hosaka, and M. Sugi, 1999: A simulation of troposphere,
stratosphere and mesosphere with an MRI/JMA 98 GCM. Pap. Meteorol.and Geophys., 50, 15–53.

Fig. 3 Mean vertical velocity reproduced in the GCM for JJA（mm sec-1）．
Left panel is dynamically derived from meridional velocity and right panel is thermodynamically derived fromdiabatic heatings.

GCM

JRA

NCEP2ERA

Fig. 4 Seasonality of mean vertical velocity at 50 hPa（mm sec-1）．
Upper left, upper right, lower left and lower right panels are analyzed from GCM, JRA, NCEP2 and ERA,respectively.