Strengthening of Brewer-Strengthening of Brewer-Dobson circulation since Dobson circulation since 1979 seen from observed 1979 seen from observed

lower-stratospheric lower-stratospheric temperaturestemperatures

Qiang Fu Department of Atmospheric SciencesUniversity of Washington, Seattle, WA

Brewer (1949); Dobson (1956)

Brewer-Dobson Circulation Brewer-Dobson Circulation (BDC)(BDC)

GCM studies predict an increase in GCM studies predict an increase in the strength of the BDC in response the strength of the BDC in response to an increase of greenhouse gas to an increase of greenhouse gas concentrations and to the ozone concentrations and to the ozone depletion depletion (e.g., Rind et al. 1990; Butchart & (e.g., Rind et al. 1990; Butchart & Scife 2001; Eichelberger & Hartmann 2005; Scife 2001; Eichelberger & Hartmann 2005; Butchart et al. 2006; Austin et al. 2007; Garcia Butchart et al. 2006; Austin et al. 2007; Garcia & Randel 2008; Oman et al. 2009; Butchart et & Randel 2008; Oman et al. 2009; Butchart et al. 2010; Shepherd et al. 2011; Lin and Fu al. 2010; Shepherd et al. 2011; Lin and Fu 2013)2013)..

Long-term cooling trends in tropical Long-term cooling trends in tropical lower stratospheric temperature lower stratospheric temperature may be partly associated with an may be partly associated with an increased tropical upwelling, increased tropical upwelling, consistent with the accelerated BDC consistent with the accelerated BDC (e.g., Thompson and Solomon 2005; Randel et (e.g., Thompson and Solomon 2005; Randel et al. 2006; Rosenlof and Reid 2008)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 A broader look: SH high latitude lower stratospheric temperature lower stratospheric temperature

(MSU TLS) trend patterns for 1979-(MSU TLS) trend patterns for 1979-20062006

Stronger BDC -> faster tropical (high Stronger BDC -> faster tropical (high latitudes) upwelling (downwelling) -> latitudes) upwelling (downwelling) -> colder (warmer) TLS in the tropics colder (warmer) TLS in the tropics (high latitudes). (high latitudes).

Monthly Trend Patterns of TLS Monthly Trend Patterns of TLS and TOMS total Ozone for 1979-and TOMS total Ozone for 1979-

20072007

Fu, Solomon, and Lin (2010)

We separated the dynamic warming from the We separated the dynamic warming from the total observed trends using a regression of total observed trends using a regression of MSU TLS on a BDC index (i.e., a three-month MSU TLS on a BDC index (i.e., a three-month mean eddy heat flux averaged between 50-10 mean eddy heat flux averaged between 50-10 hPa over 40hPa over 40oo poleward), times the BDC index poleward), times the BDC index trend.trend.

Fu, Solomon, and Lin (2010)

TLS trends due to the changes in the BDC in the SH high latitudes (40oS-82.5oS), NH high latitude (40oN-82.5oN), and the high latitudes (40oN-82.5oN and 40oS-82.5oS) for 1980-2008

Fu, Solomon, and Lin (2010)

TLS Trends in Tropics (20TLS Trends in Tropics (20ooS-20S-20ooN) versus N) versus Dynamically Induced TLS Trends in HL Dynamically Induced TLS Trends in HL

(40(40ooN&S poleward)N&S poleward)

Tropical annual mean MSU TLS trendsTropical annual mean MSU TLS trends Total: Total: -0.40 K/decade -0.40 K/decade Dynamic: Dynamic: -0.11K/decade -0.11K/decade Radiative: Radiative: -0.29K/decade-0.29K/decade

Our observational analyses indicate Our observational analyses indicate that dynamic cooling in the tropical that dynamic cooling in the tropical lower stratosphere is closely coupled lower stratosphere is closely coupled with the dynamic warming in the high with the dynamic warming in the high latitude, which thus indicates a latitude, which thus indicates a strengthening of the hemispheric-strengthening of the hemispheric-wide BDC.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 O3, H2O, CO2/CH4/N2O, stratospheric aerosols were considered.

TLS Trends in Tropics (20TLS Trends in Tropics (20ooS-20S-20ooN) versus N) versus Dynamically Induced TLS Trends in HL Dynamically Induced TLS Trends in HL

(40(40ooN&S poleward)N&S poleward)

Tropical annual mean MSU TLS trendsTropical annual mean MSU TLS trends Total: Total: -0.40 K/decade -0.40 K/decade Dynamic: Dynamic: -0.11K/decade -0.11K/decade Radiative: Radiative: -0.29K/decade-0.29K/decade

Relationship between Tropical TLS and Vertical Velocity at 70

hPa

w* = –cTLS where c = -0.069 (-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 0.023 (0.015) mm/s

The observationally-derived relative change in BDC in last three decade is 0.023 (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 1980-2009 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 Our observational analyses indicate that dynamic cooling in the tropical that dynamic cooling in the tropical lower stratosphere is closely lower stratosphere is closely coupled with the dynamic warming coupled with the dynamic warming in the high latitude; in the high latitude;

Conclusions

The dynamic cooling in tropical The dynamic cooling in tropical lower stratosphere lower stratosphere indicates a indicates a strengthening of BDC in last three strengthening of BDC in last three decades decades at a confidence level of at a confidence level of more than 90%more than 90%;;

Our observational analyses indicate Our observational analyses indicate that the BDC is strengthening by 5 to that the BDC is strengthening by 5 to 8% in last three decades, supporting 8% in last three decades, supporting the GCM simulations from CCMVal-2.the GCM simulations from CCMVal-2.

Conclusions

The partitioning of tropical lower-The partitioning of tropical lower-stratospheric temperature trend into stratospheric temperature trend into dynamic and radiative parts is dynamic and radiative parts is validated using a radiation model validated using a radiation model with observed changes in with observed changes in atmospheric compositions.atmospheric compositions.

Fu, Q., P. Lin, S. Solomon, and D.L. Hartmann, 2015: Acceleration of the Brewer-Dobson circulation derived from observations since 1980. 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: 10.1029/2012JD018813.

Lin, P., Q. Fu, and D.L. Hartmann, 2012: Impact of tropical SST on stratospheric planetary waves in the Southern hemisphere. J. Clim., 25, 5030 - 5046.

Fu, Q., S. Solomon, and P. Lin, 2010: On the seasonal dependence of tropical lower-stratospheric temperature trends. Atmos. Chem. Phys., 10, 2643-2653.

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, 6325-6341.

Hu, Y.Y., and Q. Fu, 2009: Stratospheric warming in Southern Hemisphere high latitudes since 1979. Atmos. Chem. Phys., 9, 4329-4340.

Johanson, C.M., and Q. Fu, 2007: Antarctic atmospheric temperature trend patterns from satellite observations. Geophys. Res. Lett., 34, L12703, doi:10.1029/2006GL029108.

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.