The East Asian monsoon is a monsoonal flow that carries moist air from the Indian Ocean and Pacific Ocean to East Asia. It affects approximately one-third of the global population, influencing the climate of Japan (including Okinawa), the Koreas, Taiwan, Hong Kong, Macau, the Philippines, Indo-China, and much of mainland China. It is driven by temperature differences between the Asian continent and the Pacific Ocean. The East Asian monsoon is divided into a warm and wet summer monsoon and a cold and dry winter monsoon. This cold and dry winter monsoon is responsible for the aeolian dust deposition and pedogenesis that resulted in the creation of the Loess Plateau.
In most years, the monsoonal flow shifts in a very predictable pattern, with winds being southeasterly in late June, bringing significant rainfall to the Korean peninsula and Japan (in Taiwan and Okinawa this flow starts in May). This leads to a reliable precipitation spike in July and August. However, this pattern occasionally fails, leading to drought and crop failure. In the winter, the winds are northeasterly and the monsoonal precipitation bands move back to the south, and intense precipitation occurs over southern China and Taiwan.
The East Asian monsoon is known as jangma (장마) in Korea. In Japan the monsoon boundary is referred to as the bai-u (梅雨?) as it advances northward during the spring, while it is referred to as the shurin when the boundary retreats back southward during the autumn months. Over Japan and Korea, the monsoon boundary typically takes the form of a quasi-stationary front separating the cooler air mass associated with the Okhotsk High to the north from the hot, humid air mass associated with the subtropical ridge to the south. After the monsoon boundary passes north of a given location, it is not uncommon for daytime temperatures to exceed 32 °C (90 °F) with dewpoints of 24 °C (75 °F) or higher.