Strengthening of Brewer- Dobson circulation since 1979 seen from observed lower- stratospheric temperatures Qiang Fu Department of Atmospheric Sciences University of Washington, Seattle, WA

Brewer (1949); Dobson (195 6) Brewer-Dobson Circulation (BDC)

GCM studies predict an increase in the strength of the BDC in response to an increase of greenhouse gas concentrations and to the ozone depletion (e.g., Rind et al. 1990; Butchart & Scife 2001; Eichelberger & Hartmann 2005; Butchart et al. 2006; Austin et al. 2007; Garcia & Randel 2008; Oman et al. 2009; Butchart et al. 2010; Shepherd et al. 2011; Lin and Fu 2013). GCM studies predict an increase in the strength of the BDC in response to an increase of greenhouse gas concentrations and to the ozone depletion (e.g., Rind et al. 1990; Butchart & Scife 2001; Eichelberger & Hartmann 2005; Butchart et al. 2006; Austin et al. 2007; Garcia & Randel 2008; Oman et al. 2009; Butchart et al. 2010; Shepherd et al. 2011; Lin and Fu 2013). Long-term cooling trends in tropical lower stratospheric temperature may be partly associated with an increased tropical upwelling, consistent with the accelerated BDC (e.g., Thompson and Solomon 2005; Randel et al. 2006; Rosenlof and Reid 2008). Long-term cooling trends in tropical lower stratospheric temperature may be partly associated with an increased tropical upwelling, consistent with the accelerated BDC (e.g., Thompson and Solomon 2005; Randel et al. 2006; Rosenlof and Reid 2008).

Johanson and Fu (2007 ) Also see Hu and Fu (2009); Lin, Fu, Solomon, Wallace (2009) A broader look: SH high latitude lower stratospheric temperature (MSU TLS) trend patterns for  Stronger BDC -> faster tropical (high latitudes) upwelling (downwelling) -> colder (warmer) TLS in the tropics (high latitudes).

Monthly Trend Patterns of TLS and TOMS total Ozone for Fu, Solomon, and Lin (2010)

We separated the dynamic warming from the total observed trends using a regression of MSU TLS on a BDC index (i.e., a three-month mean eddy heat flux averaged between hPa over 40 o poleward), times the BDC index trend. We separated the dynamic warming from the total observed trends using a regression of MSU TLS on a BDC index (i.e., a three-month mean eddy heat flux averaged between hPa over 40 o poleward), times the BDC index trend. Fu, Solomon, and Lin (2010)

TLS trends due to the changes in the BDC in the SH high latitudes (40 o S-82.5 o S), NH high latitude (40 o N-82.5 o N), and the high latitudes (40 o N-82.5 o N and 40 o S-82.5 o S) for Fu, Solomon, and Lin (2010)

TLS Trends in Tropics (20 o S-20 o N) versus Dynamically Induced TLS Trends in HL (40 o N&S poleward)

Tropical annual mean MSU TLS trends Total: K/decade Dynamic: -0.11K/decade Radiative: -0.29K/decade  Our observational analyses indicate that dynamic cooling in the tropical lower stratosphere is closely coupled with the dynamic warming in the high latitude, which thus indicates a strengthening of the hemispheric-wide BDC.

Statistical Significance of Dynamical TLS Trend in Tropics

An Independent Check of the Empirically Derived Radiative Component of Tropical TLS Trend We used a radiation scheme (Fu and Liou 1992) with the seasonally evolving fixed dynamic heating approach (Forster and Shine 1997; 1999). Observed changes in O 3, H 2 O, CO 2 /CH 4 /N 2 O, stratospheric aerosols were considered.

TLS Trends in Tropics (20 o S-20 o N) versus Dynamically Induced TLS Trends in HL (40 o N&S poleward)

Tropical annual mean MSU TLS trends Total: K/decade Dynamic: -0.11K/decade Radiative: -0.29K/decade

Relationship between Tropical TLS and Vertical Velocity at 70 hPa

 w* = –c  TLS where c = (-0.044) mm/s/K from interannual (seasonal) variations Considering a  TLS of 3*(-0.11 K/decade) in last three decades, we have a  w* of (0.015) mm/s The observationally-derived relative change in BDC in last three decade is (0.015)/0.29 = 8 (5)%.

 For the comparison of simulated BDC changes with our observational analysis, we used the residual vertical velocity w* at 70 hPa, averaged over the tropics, for from the 11 CCMVal-2 models. The mean relative increase of tropical w* at 70 hPa is 6% (-0.3 to 18%) in last three decades for the 11 CCMs, consistent with observations.

Our observational analyses indicate that dynamic cooling in the tropical lower stratosphere is closely coupled with the dynamic warming in the high latitude; Our observational analyses indicate that dynamic cooling in the tropical lower stratosphere is closely coupled with the dynamic warming in the high latitude; Conclusions The dynamic cooling in tropical lower stratosphere indicates a strengthening of BDC in last three decades at a confidence level of more than 90%; The dynamic cooling in tropical lower stratosphere indicates a strengthening of BDC in last three decades at a confidence level of more than 90%;

Our observational analyses indicate that the BDC is strengthening by 5 to 8% in last three decades, supporting the GCM simulations from CCMVal-2. Our observational analyses indicate that the BDC is strengthening by 5 to 8% in last three decades, supporting the GCM simulations from CCMVal-2. Conclusions The partitioning of tropical lower- stratospheric temperature trend into dynamic and radiative parts is validated using a radiation model with observed changes in atmospheric compositions. The partitioning of tropical lower- stratospheric temperature trend into dynamic and radiative parts is validated using a radiation model with observed changes in atmospheric compositions.

Fu, Q., P. Lin, S. Solomon, and D.L. Hartmann, 2015: Acceleration of the Brewer-Dobson circulation derived from observations since J. Geophys. Res., (to be submitted). Lin, P., and Q. Fu, 2013: Changes in various branches of the Brewer- Dobson circulation from an ensemble of chemistry climate models. J. Geophys. Res., 118, 73-84, doi: /2012JD Lin, P., Q. Fu, and D.L. Hartmann, 2012: Impact of tropical SST on stratospheric planetary waves in the Southern hemisphere. J. Clim., 25, Fu, Q., S. Solomon, and P. Lin, 2010: On the seasonal dependence of tropical lower-stratospheric temperature trends. Atmos. Chem. Phys., 10, Lin, P., Q. Fu, S. Solomon, and J.M. Wallace, 2009: Temperature trend patterns in Southern Hemisphere high latitudes: Novel indicators of stratospheric change. J. Climate, 22, Hu, Y.Y., and Q. Fu, 2009: Stratospheric warming in Southern Hemisphere high latitudes since Atmos. Chem. Phys., 9, Johanson, C.M., and Q. Fu, 2007: Antarctic atmospheric temperature trend patterns from satellite observations. Geophys. Res. Lett., 34, L12703, doi: /2006GL

Fu (2013, Nature Clim. Change )

Engel et al. (2009)

Partitioning of BDC Strengthening between NH and SH Also see Garcia et al. (2011) on the issues in the determination of age of air trends from observations.