Indian and African monsoons: Trajectories and interactions

Monsoons are summer large-scale circulations fuelled by the latent heat released by marine air masses, which spiral cyclonically inland from nearby tropical oceans. This work is about the interactions within the Indian–African monsoonal system. The two monsoons are dynamically interconnected in the upper atmosphere: the Indian monsoon exerts its influence on the African monsoon with its westward-propagating planetary waves, whilst the African monsoon exerts its influence on the Indian monsoon with its eastward-propagating Kelvin waves. In the lower atmosphere, the two monsoons are physically separated by the Somali mountains, a barrier limited in height and longitudinal extent. This barrier reflects the impinging waves from the west as long planetary waves and those impinging from the east as short planetary waves. When the Indian monsoon is strong, incoming Atlantic air particles can overcome these mountains by climbing over the smooth ramp provided by the reflected long waves. In contrast, when the African monsoon is strong, the short waves, the reflection of its Kelvin waves, pose a steep obstacle for Indian Ocean air particles, which can get around this barrier when carried by the Somali jet. If the mountains were not there, the low-level winds of the Saharan heat low would carry away marine air masses originating in the Gulf of Guinea into the Indian monsoonal basin, depriving the African monsoon of most of its fuel. Our analysis discusses the role played by the Sahara and by the Somali mountains in interrupting or favouring marine air-mass exchange between the two monsoonal basins. This on–off marine particle exchange can induce free oscillations and damped forced oscillations of the system at different time-scales. The monsoonal dynamics are simulated using Gill's model, the feedbacks are analysed in terms of Lotka–Volterra's theory, and the on–off interaction is analysed in terms of stochastic resonance theory.