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as illustrated by Fig. 13.6 . This mechanism accounts not only for the cooling on
the margins of the precipitating region but also the cooling over the Arabian Sea
during NH winter. During this season, dust concentrations are lower, but subsidence
occurs following the migration of precipitation across the equator into the Southern
Hemisphere (Miller and Tegen 1998 ): cooling results as a consequence of the
reduced overturning in response to dust atmospheric forcing. This mechanism
is also consistent with cooling over the subtropical Atlantic, where the dust
plume extends downwind from North Africa (Fig. 13.5 a-c). The dependence of the
surface temperature response upon the anomalous tropical overturning shows the
importance of the atmospheric forcing by dust, the sign of which varies among
current models according to Fig. 13.2 b, e.
Over the Sahara Desert, surface air temperature increases as the prescribed
shortwave absorption by dust is increased (Fig. 13.5 ), despite further dimming of
the surface (Fig. 13.1 ). During NH summer, a deep boundary layer that is well
mixed by dry convection is observed between the surface and the mid-troposphere
(Carlson and Prospero 1972 ). The low humidity allows the ground to radiate directly
to space, and the conditions leading to ( 13.1 )and( 13.2 ) are rarely satisfied. In this
region, the sign of the anomalous surface air temperature calculated by models is not
consistently related to the sign of the forcing (Table 13.2 ). For example, warming
is calculated by Yoshioka et al. ( 2007 )andYueetal.( 2011 ), despite forcing
whose sign differs between the two models at both TOA and the surface. This is
only possible if the perturbation to lateral energy transport varies among models,
demonstrating that this transport must be considered when attributing temperature
perturbations to dust forcing, especially in regions of weak vertical mixing.
13.3.2
Precipitation
13.3.2.1
The Global Anomaly
Aerosols alter precipitation by changing the regional distribution of diabatic
heating along with surface evaporation that supplies moisture to the atmosphere.
Evaporation is coupled to aerosol forcing through the surface energy balance, where
dimming beneath the aerosol layer is compensated by a reduction in the net flux of
heat from the surface into the atmosphere:
ıR LW .0/
F S D
C
ıLE
C
ıS E :
(13.4)
Here, ıR LW .0/ is the anomalous net upward flux of longwave radiation, while ıLE
and ıS E are the anomalous turbulent fluxes of latent and sensible heat, respectively.
Over most of the ocean, the net heat flux is dominated by the latent component,
and dimming of the surface is primarily compensated by a reduction in evaporation
(cf. Figure 12 from Perlwitz and Miller 2010 ). This is illustrated by the reduction
of precipitation within the tropical Atlantic ITCZ (Fig. 13.7 ) that is associated with
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