Monsoon and general circulation system

Abstract

Listen

Translate article

The instrumental period of meteorological record is too short to capture the full variability of the climate system and, in particular to visualize the type of climate that is predicted to occur over the next decades and centuries. It is therefore important to reconstruct past climates and understand past climatic variations. Among them, those related to the monsoon appear to be more and more important for Society. The economy, culture and rhythms of life of a large part of humanity are critically influenced by the evolution and variability of the monsoon. In particular, the Asian monsoon is one of the most spectacular occurrences in the climate system, strongly influenced by the combination of the thermal contrast between the Eurasian continent and the Indo-Pacific Ocean and the presence the Tibetan Plateau. Climatologically, the monsoon regions as the most convectively active areas, are situated in the Intertropical Convergence Zone and account for the majority of global atmospheric heat and moisture transport [1] . The need to better understand the monsoon leads inevitably to the close inspection of its worldwide activity during the geological times to provide a long-term perspective from which any future change may be more effectively assessed. It is in that framework that the substantial paper by Wang Pinxian in this issue [2] is more than welcome. It is an authoritative review, and probably also the first in which the monsoon issues are reviewed in a global scale through a so long geological history. Wang’s main points are fundamental, and hence important. First, it is to emphasize the role of the tropics as an extremely important source of changes in the global climate system. Second, it is to regard the monsoons as a global feature through which the tropical forcing exerts its climatic role worldwide. I totally agree with these arguments, and share Wang’s concern that the climate and paleoclimate community has, since too long, viewed the monsoons as local phenomena, without trying enough to relate their behavior together. But the problem being extremely complex, let me take the opportunity to put it in its broad context. The reader will find an interesting brief introduction to monsoon, its origin, history and localization in http://en.wikipedia.org/wiki/Monsoon. Etymologically, monsoon comes from the Arabic word mawsim, which means season, explaining why the original definition includes only major wind systems that change direction seasonally. The most spectacular occurrence of such seasonally varying circulation is undoubtedly in Southern and Eastern Asia. It is the associated heavy rainfall there, which has allowed by extension the other regions of the world to qualify as monsoon regions. With time, the term monsoon has been broadened to include almost all of the phenomena associated with the seasonal weather cycle within tropical and subtropical land regions of the Earth, showing immediately the relevance of Wang’s argumentation to deal with monsoon as a global tropical system. Giving stronger emphasis to the tropical forcing does not mean that we can isolate the tropical climate from the rest of the world, as it is part of the Earth’s climate system. The response of the climate system to the energy that we receive from the Sun (both at the seasonal and geological time scales) involves all latitudes and seasons and depends upon the feedback mechanisms inherent in the climate system itself. Among them, the water vapor