Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 338

incident radiative flux by dust that reduces the total flux of energy back into the
atmosphere.
The increase of evaporation with dimming in Table 13.1 remains a provocative
result that has yet to be explained quantitatively. Nonetheless, ( 13.6 ) opens the
possibility that absorbing aerosols with sufficiently large F T can increase surface
evaporation and precipitation. (In this case, dimming of the surface must be
compensated by reduced longwave and sensible fluxes.) The more general point
is that the aerosol perturbation to the global hydrologic cycle and precipitation does
not depend solely upon surface forcing but depends additionally upon forcing at
TOA. Indeed, Ming et al. ( 2010 ) refer to the right side of ( 13.6 ) as “hydrologic
forcing”.
The influence of aerosol forcing upon the hydrologic cycle is mediated through
the surface energy balance ( 13.4 ). Models that prescribe SST as a lower boundary
condition lack this balance and fail to simulate the full influence of aerosol
radiative forcing upon the hydrologic cycle. Models with prescribed SST generally
underestimate the reduction in precipitation by dust forcing compared to companion
experiments with calculated SST (e.g., Miller and Tegen 1998 ; Yoshioka et al. 2007 ;
Yu e e t a l . 2011 ). Mixed-layer ocean models, where heat transport by ocean currents
is fixed (Miller et al. 1983 ), necessarily compensate the surface forcing with the
various fluxes comprising the surface energy balance. Inclusion of an ocean general
circulation model (OGCM) within an ESM allows the additional possibility of
compensating surface dimming with anomalous ocean heat import. OGCMs allow
for the most general response to aerosol forcing, but they have been used in only a
few cases (e.g., Ramanathan et al. 2005 ; Bollasina et al. 2011 ).

13.3.2.2 Regional Anomalies

Regional anomalies of precipitation forced by aerosols are also sensitive to the
implementation of a surface energy balance. Precipitation associated with the Asian
monsoon is increased by dust radiative forcing when SST is prescribed, but reduced
when a surface energy constraint is added as part of a mixed-layer ocean model
(Miller et al. 2004a ).
Only on shorter time scales of a few weeks after the onset of aerosol radiative
forcing, before SST has time to adjust, can the neglect of a surface energy balance
be justified. Stephens et al. ( 2004 ) consider the initial effect of radiative heating
by a dust outbreak using a cloud-resolving model within a tropical ocean domain.
Heating within the aerosol layer acts as an elevated “heat pump” (e.g., Schneider
1983 ; Lau and Kim 2006 ), driving ascent along with low-level convergence of
moisture and precipitation.
At equilibrium (that is achieved by the upper ocean after several months),
the transport of energy by the atmospheric circulation must compensate the loss
(or gain) of energy within the region of aerosol forcing, equal to the difference of
the total energy flux at TOA and the surface. Initially, this gain corresponds to the
atmospheric forcing F T F S , but as the circulation adjusts, the net flux imbalance