The effect of land-surface feedbacks on the monsoon circulation

The effects of feedbacks from land-surface forcing on intraseasonal monsoon activity are studied by performing idealized sensitivity experiments with a general circulation model. In agreement with observations, the simulated intraseasonal monsoon activity is mainly described by irregular alternations of active spells and break spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between a continental and an oceanic regime. In the model, the spatial characteristic of the intraseasonal monsoon variability is a robust feature which is primarily related to an internal mode of variability of the system, rather than to a response to land-surface feedbacks. Experimentation indicates that the simulation of northward propagating events, related to transitions in the regime, does not require the inclusion of interactive surface hydrological processes. This suggests that the transitions are also mainly related to internal atmospheric dynamics. The temporal characteristics of the fluctuations between the two TCZ regimes, however, are influenced by an interactive surface. The low-frequency intraseasonal monsoon variability is enhanced by hydrological surface feedbacks. When the surface interacts with the atmosphere, the active and break regimes of the monsoon are equally likely. In the absence of surface feedbacks, the probability distribution is modified and the changes depend on the land-surface conditions imposed. The results show that the probability of a monsoon break exceeds that of an active phase when the imposed land-surface conditions are based on climatological values for July. This asymmetry in the probability distribution affects intraseasonal monsoon variability. In turn, the time-mean monsoon circulation, depending as it does on the statistics of the intraseasonal oscillations (such as frequency of occurrence and mean amplitude), is also modified by the surface feedbacks. It follows that the surface conditions play a role in the interannual predictability of the time-mean monsoon.