Intraseasonal variability and atmospheric controls on daily dust occurrence frequency over the central and western Sahara during the boreal summer

Abstract

[1] We classify satellite-derived maps of daily dust occurrence frequency over the central and western Sahara (CWS) during the boreal summer in order to identify typical patterns using the neural network based system of self-organized maps. Resulting dust states vary in terms of the frequency of dust occurrence and its location. The most commonly occurring dust states are those of relatively low dust detection frequency. On days with relatively high dust occurrence, dust tends to favor either a location close to the Algeria-Mali-Niger border triple point (TP) or further to the northwest across the western half of the Mali-Algeria border (MAB). States in which dust is detected at both locations simultaneously are rare. There is a distinct intraseasonal progression in preferred dust location from the TP in the early season to the MAB later in the season. The evolution of dust states reveals a one-way transition from dust at the TP to dust at the MAB and then to reduced daily dust occurrence frequency. There is a distinct degree of interannual variability in the occurrence frequency of the different states, dominated by the extremes of high and low dust detection frequency. Analysis of climatological composites demonstrates that monsoon surges into the Saharan heat low are associated with days of high dust detection frequency, while a strong Harmattan into the CWS is linked to days with less frequent dust presence. The CWS atmospheric dust budget for June-August is thus strongly linked to the dynamics of the West African monsoon.Citation: Ashpole, I., and R. Washington (2013), Intraseasonal variability and atmospheric controls on daily dust occurrence frequency over the central and western Sahara during the boreal summer,

Dust detection from ground‐based observations in the summer global dust maximum: Results from Fennec 2011 and 2012 and implications for modeling and field observations

“…The photometer cannot take measurements at night; and cloud‐contaminated AOD measurements must be removed. Since a significant proportion, if not the majority of dust emission in the region occurs at night or is associated with cloud [ Marsham et al ., ; Allen et al ., ; Ashpole and Washington , ; Heinold et al ., ; Marsham et al ., ] AOD measurements are therefore not representative of the overall or average atmospheric dust loading. This is addressed further in the discussion.…”

“…African Easterly Waves, for example, which show significant interannual variability [ Thorncroft and Hodges , ], can promote cold pool emission and also transport dust aloft toward the Atlantic [ Jones et al ., ; Knippertz and Todd , ; Zuluaga et al ., ]. Interannual variability in the West African Monsoon [reviewed by Rodríguez‐Fonseca et al ., ] is also linked to variability in dust emission: Ashpole and Washington [] find that the dustiest summers are associated with a ≈3 ms −1 positive southwesterly anomaly in 925 hPa wind speed that extends from northern Mali into central Algeria. A strong moist monsoon inflow can lead to dust emission as an intrusive surge or by promoting cold pool activity [ Bou Karam et al ., ; Marsham et al ., ; Allen et al ., ; Marsham et al ., ]; strong low‐level jets (LLJs) are also frequently embedded in the moist monsoon inflow [ Parker et al ., ; Allen and Washington , ].…”

A 14‐Year Climatology of Saharan Dust Emission Mechanisms Inferred From Automatically Tracked Plumes

“…Seasonal displacement of the CPO signal is particularly pronounced around the triple point region (Figures b and d). Ashpole and Washington () found that the early season dust maximum around the Mali‐Niger‐Algeria triple point migrates northwestward along the Mali‐Algerian border as the summer progresses. Figures b and c indicate that this signal is dominated by the CPO dust response, lending credence to the importance of monsoon dynamics in this transition.…”

Harmattan, Saharan heat low, and West African monsoon circulation: modulations on the Saharan dust outflow towards the North Atlantic

“…Dust may also be deposited over the northern African continent. Airborne dust circulating over West Africa may become sucked into the SHL as described by Ashpole and Washington (2013) or form the ridge of an AEW (Jones et al, 2003). In both cases, dust may eventually be pushed towards the Atlantic and released as a dust plume once the SHL is in its western phase (Chauvin et al, 2010) or an AEW enters the Atlantic (Jones et al, 2003).…”