Date: May 9, 2008 (Fri.) 10:30AM - 12:00
Venue:E207, 2nd floor of East Building, CSEAS
Title: Seasonal Changes in Rainfall in Southeast Asian Monsoon Region: Focusing on Multi-time Scale Phenomenon
Presentation:
Hiroshi Takahashi (Fronties Research Center for Global Change)
Given that the tropics experience warm temperatures year-round, the seasons are differentiated primarily by precipitation. The large-scale seasonal change in precipitation is explained by the North-South movement of the precipitation zone called the Intertropical Convergence Zone (ITCZ). The ITCZ moves with the seasonal change in solar radiation, and broadly stated, the precipitation zone resides in the northern hemisphere from June to August and in the southern hemisphere from December to February. However, observing seasonal changes from satellite images, there is no readily apparent ITCZ belt, but rather seemingly random cloud activity. In the mean field, although it appears that winds from the North and South are converging, in reality they are small-scale (several km to several thousand km) precipitation events resulting from tropical disturbances such as typhoon. What is important is that if one observes one location over time, it becomes apparent that this location experiences rain periods lasting several days to several weeks. Taking the Northern hemisphere summer monsoons in Southeast Asia as an example, the weather at the beginning and end of the rainy period differs. It is becoming apparent that this is largely the result of precipitation due to tropical disturbances. In the pre-monsoon months of May and June, before precipitation events and the westerly monsoons begin, there are thunderstorms accompanied by severe rains. In the Mainland Southeast Asia, for example, before the start of the monsoon season, rains start to occur intermittently. Then, after a period of no rain in late June, in the later stages of the monsoon period, the tropical disturbances become more active. In other words, in monsoon regions, the daily change in rain events are controlled by a variety of tropical disturbances over the period of several days or weeks. The traditional theory of monsoons, which the precipitation being caused by westerly winds, could not explain the rains in September. This new understanding suggests that they are the related to tropical disturbances. It is necessary to consider the time-scale of tropical disturbances from appearance to dissipation, which can be on the order of days to weeks.
Regarding the long-term decrease in precipitation levels in the Mainland Southeast Asia during the last few decades, there is a theory that this is related to the reduction in forested area. It might be possible to explain this decrease in terms of a change in the number of tropical disturbances, that is, a long-term change of the number of typhoons hitting the region.
Also, by using tools such as TRMM, it is possible to resolve the detailed daily change in the spatial distribution of precipitation. It has become apparent that the mountainous regions of the Mainland Southeast Asia experience high levels of precipitation during the night.
As described above, it is possible to understand more detailed weather mechanisms by evaluating precipitation on various time scales, enabling us to understand the local agricultural systems informed by such precipitation conditions.
(Masayuki Yanagisawa)