pure and applied geophysics

, Volume 115, Issue 5–6, pp 1463–1491 | Cite as

Mechanisms effecting the state, evolution and transition of the planetary scale monsoon

  • Peter J. Webster
  • Lang Chou
  • Ka Ming Lau


An attempt is made to identify the basic structure of the planetary scale monsoon and to define and describe the various mechanisms effecting or producing that structure and its variation on a variety of time scales. Both observational and theoretical descriptions are used to define the monsoon system or to point towards problems that require clarification.

The basic mean summer and winter monsoon regimes are described and a discussion presented on their degree of dependence on the distributions of orography, the form and distribution of land and ocean and the state of the interactive oceans adjacent to the major land masses. It is suggested that orography plays a critical role in determining the state of the mean summer and winter circulations behaving principally as an elevated heat source in summer and a mechanical perturber of the mean flow in winter. The spatial variation of sea-surface temperature and the distribution of land are shown to be theprimum mobile of the mean seasonal monsoon.

Various modes and time scales of the variable monsoon are defined. It is shown that the principal time scale variation or phasing of theinterseasonal transition is determined by the different response times of the oceans and land areas to variable solar heating. The reaction of the ocean is a strong function of latitude because of its ability to be heated by solar radiation and mixed mechanically by the atmosphere. The return heating distribution imposed on the atmosphere by the ocean strongly effects the circulation. The inclusion of a hydrology cycle adds a further time scale to the monsoon system allowing much more rapid variations to be superimposed on the modulations caused by radiative and sensible heating effects. The inclusion of clouds introduces a furtherintra-seasonal time-scale into the monsoon circulation causing ‘break-like’ variations of the precipitation pattern and circulation fields during the late summer.

Finally, a brief discussion is given on the various forms of interhemispheric interaction. It is suggested that contrary to earlier theoretical speculation, the equatorial latitudes may be considerably more ‘porous’ to mid-latitude disturbances than hitherto anticipated.

Key words

Monsoon evolution monsoon structure 


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Copyright information

© Birkhäuser Verlag 1977

Authors and Affiliations

  • Peter J. Webster
    • 1
    • 2
  • Lang Chou
    • 1
  • Ka Ming Lau
    • 1
  1. 1.Department of Atmospheric SciencesUniversity of WashingtonSeattleUSA
  2. 2.Division of Atmospheric PhysicsCSIROAspendaleAustralia

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