Precipitation changes in a GCM resulting from the indirect effects of anthropogenic aerosols

An atmospheric GCM coupled to a mixed‐layer ocean model is used to study changes in rainfall due to the indirect effects of anthropogenic aerosols. The model includes treatments of both the first and second indirect effects. The most striking feature of the equilibrium rainfall response to the indirect effects of anthropogenic aerosols is a southward shift of the equatorial rainfall. We hypothesize that this is caused by a hemispheric asymmetry in the cooling of the sea surface. This is supported by another pair of experiments, in which the model is run with prescribed present‐day SSTs. In one experiment the standard model is used, and in the other the SSTs in the Northern Hemisphere are increased by 1 K in the calculation of the surface fluxes, to mimic the effect of a return to pre‐industrial conditions. Compared to the run with increased NH SST, the standard run shows a southward shift of equatorial rainfall, similar to that obtained as a response to anthropogenic aerosols.

[1]  L. Ruby Leung,et al.  A physically based estimate of radiative forcing by anthropogenic sulfate aerosol , 2001 .

[2]  Leon D. Rotstayn,et al.  On the “tuning” of autoconversion parameterizations in climate models , 2000 .

[3]  Joyce E. Penner,et al.  Indirect effect of sulfate and carbonaceous aerosols: A mechanistic treatment , 2000 .

[4]  Rosenfeld,et al.  Suppression of rain and snow by urban and industrial air pollution , 2000, Science.

[5]  Lennart Bengtsson,et al.  Transient Climate Change Simulations with a Coupled Atmosphere–Ocean GCM Including the Tropospheric Sulfur Cycle , 1999 .

[6]  Leon D. Rotstayn,et al.  Indirect forcing by anthropogenic aerosols: A global climate model calculation of the effective‐radius and cloud‐lifetime effects , 1999 .

[7]  H. Treut,et al.  Sulfate Aerosol Indirect Effect and CO2 Greenhouse Forcing: EquilibriumResponse of the LMD GCM and Associated Cloud Feedbacks , 1998 .

[8]  L. Rotstayn.  A physically based scheme for the treatment of stratiform clouds and precipitation in large‐scale models. I: Description and evaluation of the microphysical processes , 1997 .

[9]  U. Lohmann,et al.  Impact of sulfate aerosols on albedo and lifetime of clouds: A sensitivity study with the ECHAM4 GCM , 1997 .

[10]  G. Meehl,et al.  Climate change from increased CO2 and direct and indirect effects of sulfate aerosols , 1996 .

[11]  Olivier Boucher,et al.  The sulfate‐CCN‐cloud albedo effect , 1995 .

[12]  P. Rowntree,et al.  A Mass Flux Convection Scheme with Representation of Cloud Ensemble Characteristics and Stability-Dependent Closure , 1990 .

[13]  B. Albrecht.  Aerosols, Cloud Microphysics, and Fractional Cloudiness , 1989, Science.

[14]  S. Twomey.  The Influence of Pollution on the Shortwave Albedo of Clouds , 1977 .

[15]  J. Warner,et al.  A Reduction in Rainfall Associated with Smoke from Sugar-Cane Fires--An Inadvertent Weather Modification?. , 1968 .

[16]  J. Janowiak,et al.  The Global Precipitation Climatology Project (GPCP) combined precipitation dataset , 1997 .

[17]  U. Lohmann,et al.  The sulfate-CCN-cloud albedo effect: a sensitivity study with two general circulation models , 1996 .

[18]  J. Penner,et al.  Global Emissions and Models of Photochemically Active Compounds , 1994 .