In East Asia, there are two major meteorological periods called as rainy and dry periods during spring. The rainy period in China is from the end of February to the middle of May and it in Korea is from the beginning of April to the middle of May. The dry period in China and Korea starts at the middle of May and ends at the middle of June.

The most important cause made a distinction between two periods is the strength and location of Northwestern Pacific High (NPH) at the low level. Regarding seasonal progress of the low level NPH, many studies have focused on the relationship with South China Sea summer monsoon (SCSSM) that is expressed as distinctive wind components affected by its location. Compared with these, the mechanism and the march of NPH on subtropical region (20°-30°N) have not well known as well as the effects of it.

NPH appears over the vicinity of Philippine in the middle of February at first, and subsequently develops and moves to higher latitudes until the end of April. From mid-May, NPH rapidly retreats eastwards with convection belt placed on its western edge.

The rainy period is caused by the moist southerly along western ridge of NPH when it extends to East Asia, the dry period is caused by decreasing moist air when it retreats to the Pacific.

The variations of NPH as time passes are as follows.

Firstly, NPH is developed by heating process on the eastern region of Tibet during April. The increased heat on land makes a large temperature gradient against ocean, leads the increase of positive relative vorticity on land and negative vorticity on ocean. From these progresses, NPH is strengthened and extended to the continent.

Secondly, in mid-May, NPH is weakened along cooling process on the eastern region of Tibet. It is caused by the increased amount of latent heat flux and convection coming from Bay of Bengal and northern part of Indochina peninsula. Also, positive (negative) relative vorticity on land (ocean) is weakened at the same time.

The variation of NPH on inter-annual is investigated, which is closely related to ENSO during winter. This result suggests that spring precipitation in Korea affected by extension of NPH can be predicted in advance.