Marine air particle trajectories into the Asia-Africa monsoonal system

Monsoons are composed by a lower cyclone coupled to an upper anticyclone by an intense mid-tropospheric updraft, surrounded by a larger region of weaker downdraft. In this work, the dynamics of the South Asia monsoon (SAM) and of the West African monsoon (WAM) are analyzed using Gill's tropospheric model (GTM). 

Since their dynamics are fueled by the latent heat released by the marine air masses, which cyclonically spiral inland from nearby tropical oceans in the warm season, the focus of this work is on the impact of the presence and in the absence of an Ekman frictional layer (EFL), of the Somali mountains and of the desert heal lows (DHLs) on these trajectories.

Results show that the addition of a lower Ekman frictional layer (EFL) to the GTM brakes the antisymmetry between the upper and the lower layer, making the lower cyclone deeper and more compact than the upper anticyclone; hence, the marine air particles reach the monsoons with tighter spirals. In addition, the presence of Ekman pumping weakens the low level subsidence about the monsoons, widening the monsoonal updraft region. 

In the absence of the East Africa mountains, particles from the Atlantic ocean can reach SAM, and particles from the Indian ocean can reach WAM. The Somali mountains separate WAM's catch basin of marine air from SAM's catch basin in the EFL; thus, the Atlantic and the Mediterranean air particles reach WAM, whilst the Indian ocean air particles reach SAM; while, the two monsoons are still coupled in the upper levels.

The Atlantic air particles sink over the Azores, the Mediterranean air particle sink over the Eastern Atlantic. The particles originating from the Indian ocean sink over the Arabian and Persian deserts. 

The Saharan-Arabian desert heat lows (DHL) deform the air particle trajectories and the intertropical front (ITF) over Africa; the Western Sahara heat low displaces the ITF northwards; whilst, the eastern Saharan-Arabian heat low displaces it southwards.