Geoscience Reference
In-Depth Information
25
41
39
20
37
35
15
33
10
31
29
5
27
0
25
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Months
Figure 20.8
Monthly mean values for satellite-derived estimates of dust over the Bodele Depression, Chad: TOMS aerosol optical
thickness (hatched) (1979-1992), number of large dust plumes over the Bodele (2002-2005 January to September, 2002-2004
October to December) (black), TOMS aerosol index (1979-1992) (white) using right-hand vertical axis.
dust above the surface layers (Herman
et al.
, 1997; Ma-
howald and Dufresne, 2004).
Dust production from the Bodele may be explained by
the colocation of the strong surface wind and a vast supply
of erodible material in the Bodele Depression. The coloca-
tion results from alternating cycles of deflation, which led
to the formation of the depression during arid times and
subsequent filling of the depression with erodible material
during pluvials (Washington
et al.
, 2006b).
Washington and Todd (2005) point to a pronounced
easterly low-level jet (LLJ) feature evident in the reanal-
ysis wind data, peaking at close to the surface, overlying
the Bodele region near 19
◦
E. The Bodele LLJ is strongest
in January and is present in all months of the year except
August, when the northeasterlies are weak or absent. The
jet is part of the northeasterly Harmattan winds of North
Africa. Regional model experiments with model topog-
raphy included and then excluded have shown that the
northeasterly near-surface flow is likely to be accelerated
between the Tibesti and Ennedi massifs, which lie 2600 m
and 1000 m above the flat terrain in the Djourab Desert
of Chad, respectively (Washington
et al.
, 2006b). As a re-
sult, the Bodele LLJ is a feature that uniquely overlies the
greater Bodele region. It is absent to the west and weak-
ens to the south, disappearing south of 10
◦
N and north
of 22
◦
N. Sampling based on extreme dust years shows
that interannual variability is associated with a strength-
ening of the LLJ during dusty years in the Bodele and
dust loadings. Intraseasonal variability of dust over the
Bodele occurs contemporaneously with the ridging of the
Libyan High and pulsing of the pressure gradient, which
drives the northeasterlies in which the LLJ is embedded
(Washington and Todd, 2005).
From measurements of wind speed, AOT and visual
observations during BoDEx 2005, the specific threshold
wind speed (15 minute average at 2 m height) for dust
emission was found to be 10.0 m/s. This value is un-
usually high in comparison to other dust source regions.
Dust emission events in the course of BoDEX 2005 were
found to be associated with the pulsing of the Bodele
LLJ. Between January and March, dust from the Bodele
is transported south-southwest, crossing the coast of west
Africa within 5 days. The dust is transported in a layer
between 800 and 700 hPa.
The Bodele contains large exposures of very friable di-
atomite (a silicious, low-density deposit) and so dust emis-
sion is transport, not supply, limited. Based on MODIS
surface reflectance data, the exposed diatomite has been
estimated to cover
10 800 km
2
(Warren
et al.
, 2007).
The largest extant areas of diatomite are those that were
deposited in the deepest basins of Palaeolake Mega, Chad,
which, some 6000 years ago, was the largest lake on
earth, bigger than the present Caspian Sea (Drake and
Bristow, 2006). It is diatomite, not highly weathered ma-
terial, that is the source of by far the greatest propor-
tion of the dust from the Bodel´e(Warren
et al.
, 2007)
∼
