Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 328

Fig. 13.3 Midday ( a ) forcing at the surface by a dust outbreak on April 12, 2002 (W m 2 ), and ( b )
contemporaneous change in surface air temperature attributed to dust by contrasting experiments
with radiatively active dust and no dust (Reproduced from Pérez et al. ( 2006 ). Copyright (2006)
with permission from the American Geophysical Union)

surface air temperature by up to 6 K coincident with the surface forcing, compared
to an experiment without dust radiative forcing (Fig. 13.3 b). Behind the dust plume,
the surface air temperature is quickly restored to its unperturbed value following the
passage of the dust layer.

13.3.1.1 Temperature Adjustment in Convectively Mixed Regions

On time scales longer than a few days, the atmospheric temperature adjusts to the
aerosol forcing at TOA and the perturbed energy exchange at the lateral margins of
the dust layer. In regions of frequent vertical mixing, forcing at TOA is an especially
strong constraint upon temperature at the surface.
Figure 13.4 shows the adjustment to a succession of weekly dust outbreaks in
a simple single-column model of a coupled atmosphere and ocean (Miller 2012 ).
During each outbreak, the prescribed radiative forcing peaks within a day, with
gradual decay during the subsequent week. The atmospheric temperature anomaly
is assumed to be identical at all levels as a result of vigorous vertical mixing
by convection. Figure 13.4 a shows that the atmosphere initially warms after each
outbreak, while cooling of the ocean is smaller in magnitude due to its greater
heat capacity. (Warming of the surface air in the simple model is in contrast to
the continental cooling beneath the dust plume in Fig. 13.3 b and is an artifact of
the instantaneous coupling between the dust layer and surface that is assumed by
the simple model.) In response to each dust outbreak, temperature anomalies in
both the ocean and atmosphere diminish within a few days (Fig. 13.4 a), but total
restoration of the unperturbed state occurs over a time scale of several months,
representing the time required for the ocean temperature to return to its original
value through reduced longwave radiation to space from the upper troposphere
(cf. Schopf 1983 ). Because the dust outbreaks are frequent compared to this
slow restoration, the residual cooling accumulates with each additional outbreak,