Abstract
Global distribution of monsoons, in the sense of seasonal reversal of wind direction between summer and winter, is investigated with modern data. From NCEP Reanalysis 2 data of 1979-1993, four-times daily winds at the lowest model level in 192×94 grid points are taken. Monsoons is defined by two conditions: (1) the angle between mean wind vectors in June-August and December-February is larger than 120 degrees, and (2) the ratio between the square of vector mean wind speed to the average of square of wind speed is larger than 0.25 in at least one of the seasons. In the tropics, besides the well-known monsoon region in west Africa-south Asia-wetern Pacific and Indonesia-northern Australia, there are narrow monsoon zones in the eastern Pacific and to the east of Andes mountains. Around 30 degree North and South, alternation of wind direction with easterly in summer is found zonally, but its steadiness is low over most part of oceans, thus the area of monsoon is discontinuous. The largest of them is found around east Asia.
