Mechanisms of Monsoon Low-Frequency Variability: Surface Hydrological Effects

Observations indicate that monsoon systems are characterized by orderly large-scale and low-frequency variations. With a time scale of two weeks and sometimes longer, regions of ascending motion are observed to form to the north of the equator and propagate slowly northward across southeast Asia. The propagation appears to be associated with the “active-break sequence” of the summer monsoon which acts as a modulator on the activity of the synoptic-scale disturbances. A zonally symmetric non-linear two-layer model containing an interactive ocean and a “continent” poleward of 18°N is used to investigate the mechanisms which produce the observed low-frequency variability. Only when a full hydrology cycle is considered does the model product variations which resemble the observed structures. Mechanisms are traced to include the interaction of the components of the total heating function. The sensible heat input in the boundary layer, although considerably smaller than the other heating components, destabilizes the atmosphere ahead of the ascending zone allowing the moist convective heating component to move northward slightly ahead of the band of precipitation. The poleward encroachment of these components of the heating forces the vertical velocity, which is proportional to the total heating, to move poleward also. The poleward movement is aided by the evaporative cooling of the precipitation moistened ground on the equatorial side of the rising motion which reduces the sensible heat input and effectively stabilizes the troposphere and thus reduces the convective heating in that sector while at the same time reducing the latent heat flux. The time scale of the event is determined by the rate of evaporative drying behind the ascent and the formation of a new zone of ascent in the vicinity of the coastal margin. A schematic representation of heating intercomponent interaction and dynamic feedback is given and the generality of the mechanism to other observed situations is considered. The hypotheses developed and tested in this study underline the importance of the role of ground hydrology related processes in large-scale atmospheric dynamics.