The Asian monsoon is one of the most dramatic examples of annual cycle in the coupled ocean-atmosphere-land system. The annual cycle of solar forcing and the associated differential heating of land and ocean is considered the most important forcing factor controlling the northward migration of precipitation and the development of the monsoon regime. An interesting aspect of the monsoon onset is whether the land-atmosphere system is in local equilibrium with the “external” slowly-evolving forcing. The absence of equilibrium would imply that this singularity is a manifestation of “internal” sub-seasonal fluctuations of the land-atmosphere system.

This study makes use of a state-of-the-art atmospheric general circulation model (AGCM), the GFDL AM3, to conduct perpetual experiments aimed at investigating the existence of equilibrium states in the land-atmosphere system over the Indian subcontinent. The perpetual experiments are carried out at characteristics moments of the annual cycle: early spring (the transition between winter and pre-monsoon conditions), late spring (just before the monsoon onset), and mid-summer (when the monsoon is at full swing). Insolation and SSTs are held fixed at their monthly mean values in each experiment. As there is no annual cycle forcing in these integrations, if the land-atmosphere system is in equilibrium, no substantial advancement toward the climatological annual evolution of the system should be observed.

It is found that during May both precipitation and winds undergo a rapid northward migration followed by a local vigorous intensification in the perpetual run as compared to the control run. The equilibrium response bears a close similarity with the June climatological monsoon, that is the land-atmosphere system is far from being in equilibrium in the real system. It also follows that land-atmosphere processes are key players in driving the transition to the monsoon regime; the monsoon is able to advance by means of positive internal feedbacks even if the external forcing is fixed. The importance of hydrological processes, and specifically soil moisture, is highlighted. By converse, minor changes are found in March and, importantly, in July.

These findings reveal that land-atmosphere coupling plays a crucial role in the development of the South Asian monsoon. We also argue that land-surface processes are an important mediator of the aerosol impact over South Asia in late spring through aerosol-induced changes in the surface energy balance.