The monsoons, with their annual reversal of winds, from the ocean to the land in summer and from the land to the ocean in winter, dominate the climate of the north Pacific on the west as far north as southern Japan and Korea.
Differences in the rate of heating and cooling between the oceans and the big continent of Asia account for the monsoon winds. On Asia in summer the pressure is low and in winter high, because land heats faster and cools quicker than water, and the pressure of the air is responsive to the heat balance. In summer, because the pressure is high over the cooler ocean and low over the hot land, the surface winds blow towards Asia. In the Indian Ocean and sometimes in the far-western Pacific, the intertropical front disappears or only small patches of it remain, and the southeast trade wind continues across the equator and joins other masses of air to form the summer monsoon.
North of the equator, the southeast trade wind, or monsoon, is deflected to the right and becomes a southwest monsoon in Borneo and the Philippines. On approaching Asia, however, it is deflected again and pours on to the continent towards the low pressure in the interior as a southeast wind. In winter the monsoons do not extend far out to sea, and the intertropical front persists in the Indian Ocean and into the Australian sector. At this season, December to March, Australia is hot when Asia is cool, and the winter monsoon continues across Indonesia as a northwest wind, bringing precipitation to northern Australia, New Guinea, and Indonesia from the tropical seas to the north. In these same land areas the monsoon from April to October comes in general from the east and is relatively dry. For several weeks in the spring and fall the northeast trades blow from the western Pacific sector southeast of Asia. The winds of these transition seasons are usually weak and variable, although local thunderstorms and typhoons .may occur that have winds of high velocity.
Some island groups towards the west in the central Pacific have hot temperatures and abundant precipitation with no particular rainy season, but in the regions dominated by the monsoons the maximum rainfall comes in summer. The exception to this generalization occurs when the winter monsoon has blown across a wide sea against a mountainous coast, resulting in winter rainfall. Examples are portions of the eastern and northern Philippines, the coast of Indo-China, and as mentioned previously Indonesia and New Guinea. Japan receives monsoonic rains in summer, but its winter rainfall is related mostly to cyclonic storms in the prevailing westerlies.