Interactions within the Asia-Africa monsoonal system and with the Saharan desert

The Indian and African monsoons are large scale circulations fueled by the latent heat released by the marine air masses, which cyclonically spiral inland from nearby tropical Atlantic and Indian oceans. The catch basin of the Indian monsoon is in the Indian ocean, whilst the catch basin of the African monsoon is in the Atlantic.

The two monsoons are dynamically inter connected in the upper atmosphere, where the Indian monsoon exerts its influence on the African monsoon with its westwards propagating long planetary waves, whilst the African monsoon exerts its influence on the Indian monsoon with its eastwards propagating Kelvin waves. In the lower atmosphere, the Somali mountains physically separate the catch basin of marine air masses of the African monsoon from that of the Indian monsoon. However, since these mountains are a barrier of limited height and limited longitudinal length, they are partially permeable to the marine air masses transported eastwards by the African easterly jet, when the Saharan heat low is strong.

Thus, in his work we show the results of an analysis of the role played by the African desert and of the Somali mountains in interrupting or favoring air mass exchange between the two basins. This on-off marine particle exchange can induce free oscillations and damped forced oscillations of the monsoon-desert system at intra seasonal time scales.

Results show that the monsoon-desert does not stay in its average climatological state, because this state is metastable. When the Saharan desert is at its maximum and retreating, the African monsoon grows from its average climatological position; and when the monsoon retreats from its average climatological position, the Saharan desert grows form its minimum.

Results also show that oscillations of the monsoon-desert system can be triggered by a small external perturbation.

Thus, in the presence of weather perturbations introduced in parametric form as stochastic noise, these transitions become quasi-periodic, even when the interaction coefficient are weak, because of induced stochastic resonance.

In this work, the monsoonal dynamics are simulated using Gill's theory, whilst the monsoon-desert feedbacks are analyzed in terms of Lotka-Volterra's theory.