Energetics of the African Easterly Waves Coupled to Saharan Dust Radiative Heating: A New Look to Variability of Tropical Atlantic Storm Tracks

Saharan Desert in North Africa is the largest source of dust in the world. On average, 182 million tons of dust transport each year across the Atlantic Ocean. Dust particles are lifted by atmospheric flow and horizontally transported within the Saharan Air Layer (SAL) across the Atlantic Ocean within 5 to 6 days and reach the Caribbean Sea, Gulf of Mexico and the United States (e.g. Liu et al., 2008; Creamean et al., 2013). Previous studies showed that African dust plumes have robust influences on regional and global climate through their impacts on radiation, cloud properties, hydrological cycle and atmospheric circulations (e.g. Ramanathan et al., 2001; Colarco et al., 2003; Lau et al., 2009; Wilcox et al., 2010; Kim et al., 2010). Because of the importance of mineral dust aerosols in Earth’s climate system, considerable efforts have been made to model dust distributions and properties in chemical transport and global climate models (Colarco et al., 2003; Nowottnick et al., 2010).

Tropical Atlantic easterly waves, also known as African Easterly Waves (AEWs), are synoptic-scale atmospheric waves over West Africa and the eastern tropical Atlantic Ocean that often serve as seeding the tropical Atlantic cyclogenesis. While the characteristics of the AEWs have been addressed in numerous previous studies (e.g. Hsieh and Cook, 2004; Kiladis et al., 2005; Diaz and Aiyyer, 2013), the mechanisms by which dust particles impact the wave systems are poorly understood and not well quantified. Understanding of the nonlinear interactions between dust evolution and development of the AEWs is crucial to better represent the dynamics of the West Africa and the eastern Atlantic Ocean in the global climate models. Wilcox et al. (2010) showed that African dust coincides with meridional shift of the African Easterly Jet (AEJ). Following Wilcox et al. (2010), we showed that the enhanced dust concentration has robust impacts on variability of the AEJ along with anomalous changes in dynamics and thermodynamics of the AEWs from a climatological point of view (Hosseinpour and Wilcox, 2014). Our recent findings establish a bridge between the knowledge of dust physics and dynamics of the AEJ-AEW system (Hosseinpour, 2017). We gained novel insights into mechanistic influences of dust particles embedded into the jet-wave system during boreal summer season, when the eddy activity of the AEWs peaks.

This study is focused on relationships between variability of dust radiative heating and wave activity across the tropical Atlantic storm tracks (Hosseinpour et al., 2018). Using advanced diagnostic tools applied to extensive NASA observations, reanalysis and models, we have found that dust radiative forcing in the elevated SAL is both modulated by the variability of tropical Atlantic storm tracks and has the capability to modify the kinetic energy of the jet-wave systems. We showed that Saharan dust radiative heating may act as an additional source of heat to modify the eddy available potential energy. Additionally, enhanced dust radiative properties are correlated with a strengthening or weakening of the kinetic energy the AEWs, depending on the time-scale of the wave, as well as the baroclinic and barotropic instability of the region. This study has suggested mechanisms of coupling the eddy kinetic energy of waves and dust radiative forcing that motivate variations in tropical Atlantic storm track dynamics.