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 activit y 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 char acteristic of the intraseasonal monsoon variability is a robust feature whi ch is primarily related to an internal mode of variability of the system, r ather than to a response to land-surface feedbacks. Experimentation indicat es that the simulation of northward propagating events, related to transiti ons in the regime, does not require the inclusion of interactive surface hy drological processes. This suggests that the transitions are also mainly re lated to internal atmospheric dynamics. The temporal characteristics of the fluctuations between the two TCZ regime s, however, are influenced by an interactive surface. The low-frequency int raseasonal monsoon variability is enhanced by hydrological surface feedback s. When the surface interacts with the atmosphere, the active and break reg imes of the monsoon are equally likely. In the absence of surface feedbacks , the probability distribution is modified and the changes depend on the la nd-surface conditions imposed. The results show that the probability of a m onsoon break exceeds that of an active phase when the imposed land-surface conditions are based on climatological values for July. This asymmetry in t he probability distribution affects intraseasonal monsoon variability. In t urn, the time-mean monsoon circulation, depending as it does on the statist ics 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 th e time-mean monsoon.