The coupling of stratospheric H2O and TTL thin cirrus clouds to

atmospheric circulations: Implication to climate sensitivity

Q. Fu*, Q. Ding, and J.M. Wallace

Department of Atmospheric Sciences

University of Washington

Seattle, WA, USA

Coupling of stratospheric H2O and TTL thin cirrus to atmospheric circulations

Long-term changes in the Brewer-Dobson circulations and equatorial planetary waves

A closer look at a recent study on stratospheric H2O feedback

Final remarks

Tropical tropopause layer(TTL)

Fueglistaler et al. (2009)

Coupling of stratospheric H2O and TTL …

Temperatures in the TTL largely govern stratospheric H2O (e.g., Brewer 1949; Fueglistaler et al. 2009) and TTL thin cirrus clouds associated with the H2O dehydration (e.g., Wang et al. 1996; Jensen et al. 1996).

Zonal asymmetries in the TTL temperature field are governed by the planetary equatorial waves (i.e., equatorial Rossby and Kelvin waves) that respond to latent heating from tropical deep convection (e.g., Highwood and Hoskins 1998).

The variations in zonal mean TTL temperatures are governed by the upwelling associated with the Brewer-Dobson circulations driven by the extra-tropical stratospheric waves (e.g., Yulaeva et al. 1994; Ueyama and

Wallace 2010).

Temperatures in the TTL are determined by a combination of tropospheric (bottom up) and stratospheric (top down) processes (e.g., Fueglistaler et al. 2009; Grise and Thompson 2013):

Fu (2013, NCC)

Li and Thompson (2013)

Virts et al. (2010)

Also see Virts and Wallace (2010)

The TTL thin cirrus clouds are largely regulated by both equatorial planetary waves and the strength of the Brewer-Dobson circulation;

The long-term changes in these circulations would lead to changes in TTL thin cirrus clouds as well as the H2O entering the stratosphere, which would feedback to the climate system through their radiative affects.

Brewer-Dobson circulation (BDC) (Brewer 1949; Dobson 1956)

Long-term changes in the Brewer-Dobson …

GCMs predict an increase in the strength of the BDC in response to an increase of greenhouse gas concentrations (e.g., Rind et al. 1990; Butchart and Scife 2001; Eichelberger and Hartmann 2005; Butchart et al. 2006; Butchart et al. 2010; Shepherd and McLandress 2011; Lin and Fu 2013).

The strengthening of the BDC for 1979-2009 based on observations is 5-8% (Fu et al. 2010; Fu et al. 2014), which supports the chemistry climate model simulations: The mean relative increase of tropical w* at 70 hPa is 6% (ranging from -0.3 to 18%) in last three decades from the 11 CCMVal-2 models.

The strengthening of the BDC will lead to a colder tropopause temperature, smaller entry value of stratospheric H2O, and more TTL thin cirrus clouds.

Garfinkel et al. (2013)

Warming SSTs in the Indian Ocean and warm pool have led to enhanced moist heating in the upper troposphere and thus a Gill-like response, subsequently less water vapor entering the stratosphere.

Projected future SSTs appear to drive a temperature and H2O response whose zonal structure is similar to the historical response.

Garfinkel et al. (2013)

A closer look at a recent study on stratos …

Dessler et al. (2013)

MCA1 between MLS H2O with SST (2005-2013)

MCA1 between MLS H2O with u50 (2005-2013)

MCA1 between MLS H2O with w70 (2005-2013)

Regression of tropical mean (30S-30N) MLS H2O using the three index, i.e., time series of SST, U50 and w70 of MCA1.

MLS=a*U50+b*w70+c*SST+r where a=0.07 b=0.15 c=0.007

The feedback due to the change in TTL thin cirrus clouds to the climate has not been evaluated yet.

All CMIP5 GCMs show a positive feedback associated with stratospheric H2O (e.g., Dessler et al. 2013) though this H2O response to atmospheric circulation changes suggest a negative feedback. There is still much to be learned about this aspect of climate feedback.

Both observational and modeling studies indicate a strengthening of the BDC as well as equatorial wave activities, which would lead to a colder tropical tropopause, smaller entry value of stratospheric H2O, and more TTL thin cirrus clouds.

Final remarks