M. Garstang et al., HORIZONTAL AND VERTICAL TRANSPORT OF AIR OVER SOUTHERN AFRICA, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 101(D19), 1996, pp. 23721-23736
Tropospheric air trajectories that occurred during the Southern Africa
n Fire-Atmosphere Research Initiative (SAFARI) in August-October 1992
are described in terms of a circulation classification scheme and the
vertical stability of the atmosphere. Three major and frequently occur
ring stable discontinuities are found to control vertical transport of
aerosols in the subtropical atmosphere at the end of the dry season.
Of these, the main subsidence-induced feature is a spatially ubiquitou
s and temporally persistent absolutely stable layer at an altitude of
about 5 km (3.5 km above the interior plateau elevation). This effecti
ve obstacle to vertical mixing is observed to persist without break fo
r up to 40 days. Below this feature an absolutely stable layer at 3 km
(1.5 km above the surface) prevails on and off at the top of the surf
ace mixing layer for up to 7 days at a time, being broken by the passa
ge of regularly occurring westerly wave disturbances. Above the middle
-level discontinuity a further absolutely stable layer is frequently d
iscerned at an altitude of about 8 km. It is shown that five basic mod
es can be used to describe horizontal aerosol transportation fields ov
er southern Africa. Dominating these is the anticyclone mode which res
ults in frequent recirculation at spatial scales varying from hundreds
to thousands of kilometers. In exiting the anticyclonic circulation,
transport on the northern periphery of the system is to the west over
the Atlantic Ocean via a semistationary easterly wave over the western
part of the subcontinent. On the southern periphery, wave perturbatio
ns in the westerly enhance transports which exit the subcontinent to t
he east into the Indian Ocean. Independently derived data suggest that
during SAFARI only 4% of the total transport of air from three locati
ons south of 18 degrees 8 was into the Atlantic Ocean. Over 90% of the
transport was into the Indian Ocean across 35 degrees E. This result
reflects circulation fields typical of the extremely dry conditions pr
evailing in 1992. The integrated effect of the control exerted by atmo
spheric stability on vertical mixing, on the one hand, and the nature
of the horizontal circulation fields, on the other, is to produce a di
stinctive suite of transport patterns that go a long way to explain th
e observed high concentrations of tropospheric aerosols and trace gase
s observed over the subcontinent in winter and spring, as well as over
the tropical South Atlantic and southwestern Indian Oceans.