THE EAST ASIA WINTER MONSOON
This chapter mainly focuses on the characteristics of the East Asia winter monsoon (EAWM). An examination of the climatology of the boreal winter in Asia shows that the EAWM results from the development of a cold-core high over the Siberia-Mongolia region. The movement of this cold air southward produces pressure surges and temperature drops across the Asian continent. Two types of such surges can be identified: the northerly surge (NS) and the easterly surge (ES). The initiation of the NS begins with the eastward passage of a polar jet streak west of Lake Balkhash. The eastward migration of this jet streak over the Siberia-Mongolia region intensifies a cold high there, which eventually leads to a southward outpour of the cold air in the lower troposphere. Such a push of the cold air then excites gravity waves that propagate across the South China Sea, which results in convection over the maritime continent. On the other hand, an ES is apparently the consequence of an initially eastward and then southeastward migration of a cold pool that splits off from a quasi-stationary high-pressure system over the Siberia-Mongolia region due to the passage of a 500-hPa ridge over the region. As the low-level anticyclone moves to the east coast of China, it initiates a southward surge of cool air and strong winds along the coast, resembling a coastal Kelvin wave. Its strength is usually much less than that of the NS. Other than these surges, a significant effect of the EAWM is the explosive development of low-pressure systems over the East China Sea as the cold air moves off the continent and over the warm water, which results from the strong baroclinity between the cold air from the continent and warm air over the ocean, and the subsequent potential instability, rising motion and latent heat release. The last section of the chapter discusses intraseasonal, interannual and interdecadal variations of the EAWM, which can be related to similar oscillations in other planetary-scale circulation features. These include the 10-20-day oscillation, the Madden-Julian Oscillation, the polar vortex, the El Niño/Southern Oscillation, sea-surface temperature anomalies in the North Pacific, the North Atlantic Oscillation, and the East Asia summer monsoon. Furthermore, “two-way” interactions between the EAWM and some of these oscillations have also been found.