A study of the formation and trend of the Brewer-Dobson circulation

Transcription

1 A study of the formation and trend of the Brewer-Dobson circulation Kota Okamoto 1, Kaoru Sato 1, and Hideharu Akiyoshi 2 1: The University of Tokyo 2: National Institute for Environmental Studies Okamoto et al., (JGR, in press) Okamoto et al., (GRL, in preparation)

2 Introduction The Brewer Dobson circulation is a general term to show transport in the stratosphere which consists of a slower part of residual circulation (RC) driven by wave forcings and a fast part of lateral mixing by breaking waves (e.g., Birner and Boenish, 2010) The RC is composed of 2 cells: tropical upwelling and extratropical downwelling, which is well examined by the TEM equation system (Andrews et al., 1987) The planetary waves are considered to be a main driver of the RC (Plumb, 2002). However, it is known that synoptic waves and/or gravity waves also influence the formation of the RC, which needs to be quantified. Stratosphere RC Summer Winter Troposphere SP EQ Hadley Circulation WP 3

3 Purpose Using CCSR/NIES CCM data, Downward Control Analysis was made to clarify the role of gravity waves in the residual circulation (RC) cf. McLandress and Shepherd (JC, 2009) Backward Trajectory Analysis in the field of residual circulation was made to show how the RC acceleration affects the transit time (TT) as a proxy of Age of Air cf. Rosenlof (JGR, 1995), Birner and Boenish (ACP, 2011) 4

4 Data description 1978~2100 Ver.5.4g of CCSR/NIES AGCM T42L31, top 0.01 hpa daily CO2 from IPCC (A1B,2000) Halogen gases from WMO (Ab,2003) SST from MIROC CCSR/NIES CCM REF2 data no QBO, no solar activity, and no volcanic eruption gravity wave parameterization orographic McFarlane (1987) nonorographic Hines (1997) ERA Interim data is also used for confirmation of the model results 5

5 Downward Control Analysis - to clarify the role of gravity waves in the BDC -

6 The downward control principle The downward control (DC: Haynes et al. 1991) Various wave forcing contributions to the residual stream function can be estimated because wave forcing F can be linearly divided into resolved wave forcing and unresolved gravity wave forcing. ψ direct = ψ dc if in the steady state. 7

7 Wave contribution to the residual circulation (a) (b) (a) = (c) + (f) (c) = (d) + (e) (f) = (g) + (h) ψ direct = ψ dc steady state! the planetary wave is the main driver in the stratosphere gravity wave contribution is significant in the mid latitudes of the lower stratosphere and in the summer hemispheric low latitude part of the winter circulation. 8

8 Gravity wave contribution in ERA-Interim data gravity wave contribution in ERA Interim data (gwd ) is estimated as a difference GWD is important for the formation of the BDC in low latitudes of the summer stratosphere and mid latitudes of the lower stratosphere 9

9 Wave contribution to the net upward mass flux TL TL 70hPa 10

10 Backward trajectory analysis - to show how the RC acceleration modifies the transit time as a proxy of Age of Air

11 Motivation of the backward trajectory analysis Models show that the age of air decreases probably because of the RC acceleration. However, such trend is not clear in observations. Structural change in the RC may be a key to explain the difference. Age of Air [Year] CMAM (McLandress and Shepherd [2009]) CC (Waugh [2009]) (McLandress and Shepherd [2009]) 3/25/

12 Estimation method (v*,w*) Pressure 100 hpa START Latitude We calculated backward trajectory at each point in the stratosphere in time dependent residual velocity field. The transit time (TT), as a kind of proxy of AOA, at a particular point is obtained as the time elapsed after an air entering into the stratosphere from the troposphere. 13

13 Change in the transit time TT ΔTT ΔTT TT increases with the height and latitude, reflecting a two celled structure of the residual circulation in the stratosphere. TTs in high latitudes of the middle stratosphere are significantly increased in the past 20 years in the CCM, although TTs decrease in most stratosphere in the 21 st century. 14

14 Factors Controlling ΔTT y z ΔTT is mainly defined by ΔL This result suggests that the change in the structure of RC is important for determination of the transit time (and also possibly for the age of air). 15

15 Structural change October April DJF The difference occurs when the tracers are in the subtropics. After April, the tracers are in the summer hemispheric part of the winter circulation. Thus the tracers go back more equatorward for stronger RC. After October, the tracers are in the winter hemispheric part of the winter circulation. Thus the tracers go forth more poleward for stronger RC. This is a mechanism why when the RC accelerates, trajectories get longer and consequently the TTs increase. Note that the summer hemispheric part of the winter circulation is mainly contributed by gravity waves. 17

16 Summary and Concluding Remarks Wave contributions to RC and a possible cause of change in TT are investigated using CCSR/NIES CCM data. The planetary waves are a main driver of the RC, but gravity wave contribution is dominant in the summer subtropical region in the stratosphere as well as in the winter middle latitude region in the lowermost stratosphere. The change in TT depends on the height The structural change of the RC is important, in particular in the part of the summer subtropical region which is largely affected by GWs 7 This mechanism may explain the unclear decrease in AOA 5 that the observations show Age of Air [Year] 3 3/25/2011 CC (Waugh [2009])