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Asian Monsoon

Introduction....................................................................................................39

Major Features of Asian Monsoon Climate and Its Two Sub-Systems: South Asian Monsoon and East Asian Monsoon

Sub-Systems: South Asian Monsoon and East Asian Monsoon.........39

Variability of Asian Monsoons.....................................................................40

Congbin Fu

Institute of Atmospheric Physics, Chinese Academy of Sciences

Introduction

Human-Monsoon Interaction......................................................................41

Global Monsoon.............................................................................................42

References........................................................................................................42

Bibliography....................................................................................................43

Asian monsoon is a component of the global climate system. The name “monsoon” is derived from the Arabic word “mausim” meaning “season,” large-scale seasonal reversals of dominant wind direction and distinct rainy and dry seasons of climate over the Indian subcontinent, which was named “Indian monsoon.” Later on, it became clear that the monsoon is a much larger and complex system that affects the weather and climate over a large domain of tropical and subtropical Asia. A more general term “Asian monsoon” is widely used in climatology.

Classical monsoon theory believes that the seasonal change of land-ocean thermal contrast forced by the seasonal change of solar radiation is the major cause of the formation of a monsoon system. In summer, land is warmer than the ocean and a thermal depression forms over land. The airflow, called summer monsoon, brings the moist air from the ocean into the continent where there are more clouds, higher humidity, and more rainfall; while in winter, land is cooler than the ocean and a high-pressure system forms over land. The airflow, called winter monsoon, brings cold and dry continent air-mass from higher latitudes to the tropical and subtropical continent where there are relatively fewer clouds, low humidity, and less rainfall. Research also points out the role of dynamic and thermal dynamic effects of the Tibetan Plateau in the formation of Asian monsoon. However, the term “monsoon” is very often used to refer to its rainy phase only, or the summer monsoon.

Major Features of Asian Monsoon Climate and Its Two Sub- Systems: South Asian Monsoon and East Asian Monsoon

The Asian monsoon has been classified into two subsystems: the South Asian monsoon and the East Asian monsoon. These two subsystems are linked to each other, but with different features. The South Asian monsoon, or Indian monsoon, is a tropical system affecting the Indian subcontinent and surrounding regions. In summer, the southwest monsoon comes from the Indian Ocean and Arabian Sea. The onset of southwest monsoon usually occurs at the end of May or early June in southern

India and then advances rapidly northward. In mid-July, it spreads over the whole Indian Peninsula. In early September, the southwest monsoon begins to retreat southeastward. The dominant period of southwest monsoon, from June through September, is the rainy season of India when the rainfall is 75% of the annual total. Around September, with the sun retreating south, the northern land mass of the Indian subcontinent begins to cool oft' rapidly. When a high-pressure system begins to build over northern India, the Indian Ocean and its surrounding region remain warm. This causes the cold wind to sweep down from the Himalayas and the Indo-Gangetic Plain toward the vast expanse of the Indian Ocean and south of the Deccan Peninsula. This is known as the northeast monsoon. From November to April, the India subcontinent enters into a dry season when the northeast monsoon dominates.

The East Asian monsoon is a tropical and subtropical system that affects large parts of Indo-China, the Philippines, China, Korea, and Japan. The East Asian summer monsoon comes from tropical and subtropical oceans. The onset of the East Asian summer monsoon has been generally recognized to start in mid-May over the South China Sea and then moves northward and eastward. It reaches the Yangtze River basin in mid-June and further across the Yellow River basin in late July to early August when the summer monsoon enters into its peak period. The East Asian summer monsoon begins to retreat in September. The beginning and ending of the rainy season in East Asia are closely linked with the advance and retreat of the summer monsoon.

The rain occurs in a concentrated belt that stretches east-west over China and tilts east-northeast over Korea and Japan. Such seasonal rain is known as Meiyu in China, Changma in Korea, and Bai-u in Japan. Different from the South Asia monsoon, the East Asian winter monsoon is stronger than the summer monsoon. It originates from the cold and dry continent air-mass of the Siberian high pressure system with dominant northwest or northeast flows. The East Asian winter monsoon is often accompanied by cold ocean waves, snow, and strong wind. Figure 6.1 presents the long-term mean precipitation (mm/day) and wind (m/s at 850 hPa) in monsoon Asia region in January and July.11,21

 
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