Taiwan is located at the western stretch of the North Pacific high pressure (NP high) ridge in boreal summer, and its climate is highly sensitive to the NP high. By grouping years of anomalously high and low summer precipitation in Taiwan, this study investigated the large-scale atmospheric circulation and the land-sea temperature contrast during these two groups of years and identified the control of summer precipitation in Taiwan. It is found that in years when summer precipitation in Taiwan is anomalously high, the western stretch of the NP high weakens. Weakening of the western stretch of the NP high induces strengthened southerly wind and enhanced vertical motion in East Asia and the western NP (EA-WNP) region, which is essentially an invigorated summer monsoon circulation. Corresponding to the invigorated circulation, precipitation increases in the southern section of the EA-WNP but decreases in the midlatitude section of the EA-WNP. It is further found that in those wet years, the land-sea temperature contrast between Asia and the surrounding seas is anomalously large and that the westerly wind in the tropical Indian Ocean and the southerly wind in the South China Sea and the subtropical East Asia are strengthened, which is an accelerated cyclonic circulation surrounding South and Southeast Asia. Coincident with the invigorated monsoon circulation in the EA-WNP region is a weakened Asian high pressure (Asian high). This is in variance with the expectation that the invigorated monsoon circulation in the EA-WNP region is related to a strengthened Asian high. The weakened Asian high is related to the weakened monsoon circulation in South and Southeast Asia. It is suggested that these unexpected results might be due to the interannual time scale of this study as opposed to either climatological or decadal scales of previous studies.

Monsoon onset in July over the subtropical western North Pacific (SWNP, 15–25°N; 130–150°E) is investigated by focusing on the westward movement of cloudy region east of the SWNP, which is associated with high vorticity in the upper troposphere. At least in seven of the 22 years between 1985 and 2006, the arrival of clouds at the SWNP from the east coincides with a significant change in the upper tropospheric circulation and a rapid northeastward extension of strong convections from the tropical western North Pacific, which essentially is monsoon onset. Before the monsoon onset, the sea surface temperature (SST) increases, but winds remain divergent over the SWNP. Right after the monsoon onset, winds turn convergent and convections enhance, leading to a rapid decrease of surface heating and SST. It is suggested that westward‐moving upper‐level disturbances might trigger onset of monsoon in July when low‐level atmospheric conditions favor development of deep convections.

No abstract

No abstract

No abstract