No abstract

Mechanistic relationships exist between variability of dust in the oceanic Saharan air layer (OSAL) and transient changes in the dynamics of Western Africa and the tropical Atlantic Ocean. This study provides evidence of possible interactions between dust in the OSAL region and African easterly jet-African easterly wave (AEJ-AEW) system in the climatology of boreal summer, when easterly wave activity peaks. Synoptic-scale changes in instability and precipitation in the African/Atlantic intertropical convergence zone are correlated with enhanced aerosol optical depth (AOD) in the OSAL region in response to anomalous 3D overturning circulations and upstream/downstream thermal anomalies at above and below the mean-AEJ level. Upstream and downstream anomalies are referred to the daily thermal/dynamical changes over the West African monsoon region and the Eastern Atlantic Ocean, respectively. Our hypothesis is that AOD in the OSAL is positively correlated with the downstream AEWs and negatively correlated with the upstream waves from climatological perspective. The similarity between the 3D pattern of thermal/dynamical anomalies correlated with dust outbreaks and those of AEWs provides a mechanism for dust radiative heating in the atmosphere to reinforce AEW activity. We proposed that the interactions of OSAL dust with regional climate mainly occur through coupling of dust with the AEWs.

Abstract. Saharan dust aerosols are often embedded in tropical easterly waves, also known as African easterly waves, and are transported thousands of kilometers across the tropical Atlantic Oceans, reaching the Caribbean Sea, Amazon Basin, and the eastern U.S. However, due to the complexity of the African and Atlantic climate dynamics, there is still a lack of understanding of how dust particles may influence the development of African easterly waves, which are coupled to deep convective systems over the tropical Atlantic Ocean and in some cases may seed the growth of tropical cyclones. Here we apply 22 years of daily satellite observations and reanalysis data to explore the relationships between dust in the Saharan air layer and the development of African easterly waves. Our findings show that dust aerosols are not merely transported by the African easterly jet and the African easterly waves system across the tropical Atlantic Ocean, but also contribute to the changes in the eddy energetics of the African easterly waves. The radiative forcing efficiency of dust in the atmosphere is estimated to be a warming of approximately 20 Wm-2 over the ocean and 35 Wm-2 over land. This diabatic heating of dust aerosols in the Saharan Air Layer acts as an additional energy source to increase the growth of the waves. The enhanced diabatic heating of dust leads to the increase in meridional temperature gradients in the baroclinic zone, where eddies extract available potential energy from the mean-flow and convert it to eddy kinetic energy. This suggests that diabatic heating of dust aerosols can increase the eddy kinetic energy of the African easterly waves and enhance the baroclinicity of the region. Our findings also show that dust outbreaks over the tropical Atlantic Ocean precede the development of baroclinic waves downstream of the African easterly jet, which suggests that the dust radiative forcing has the capability to trigger the generation of the zonal and meridional transient eddies in the system comprising the African Easterly Jet and African easterly waves.