Annual Cycle of Rainfall in the Western North Pacific and East Asian Sector

Abstract

The annual cycle of precipitation over the western North Pacific and East Asian (WNP-EA) sector has five major periods: spring, the first and second wet periods, fall, and winter. In this study, processes that induce precipitation in each period are examined from a large-scale point of view. The wet phase over this sector has two distinct periods, which are dominated by the Asian summer monsoon circulation induced by the land-ocean contrast of net energy into the atmospheric column (F net ). In the first wet period, the pre-mei-yu/mei-yu rainband is directly associated with a moisture flux convergence caused by the southwesterly Asian summer monsoon flow and the southeasterly trade winds, and indirectly associated with a dynamic feedback induced by this horizontal moisture convergence. The tropical convection, in the meantime, is associated with a rising motion that is induced by positive F net . In the second wet period, the WNP summer monsoon gyre dominates the rainfall of this region, which is partially associated with warmer local sea surface temperature (SST) via positive F net . The land-sea contrast of F net and the atmosphere-ocean interaction also play an important role in establishing the monsoon gyre. The dry phase over the WNP-EA region is the winter period in which precipitation is associated with winter storm activities and large-scale lifting associated with a pressure surge. In the two transition phases, due to a difference in heat capacity, the atmosphere and ocean have distinct impacts on precipitation, albeit similar solar insolations during the two periods. In the spring period, the atmospheric condition is favorable for convection, while the ocean surface is relatively colder, so the horizontal moisture advection associated with the westward extent of the Pacific subtropical high, which is different from a typical winter frontal system, is a major source for the spring rain. In the fall period, however, the atmospheric conditions dominated by the Asian winter monsoon circulation suppress convection, while relatively warmer SST still maintains tropical convection over the southern part of the WNP-EA region. Over the northern part of the WNP-EA region, the fall precipitation is associated with frontal systems, similar to those in winter.

Dynamical Control of the Tibetan Plateau on the East Asian Summer Monsoon

“…The southerly winds blowing into East Asia act to supply water vapor to the EASM front (Chou et al, ; Figure a). The correlation between the meridional wind and precipitation is estimated to be as high as 0.84 from May to July (Figure b), indicating their strong physical connection.…”

Changes in precipitation frequency and intensity in the vicinity of Taiwan: typhoon versus non-typhoon events

“…In this study, we focus on rainfall in Taiwan, which is located in the western North Pacific-East Asian (WNP-EA) sector. Over the WNP-EA region, the Asian monsoon and typhoons are the two most important systems for producing precipitation (Chou et al 2009a, with the typhoon-induced rainfall accounting for 50-60% of the total rainfall between 18 • N and 26 • N along 125 • E during the July to October period (Kubota and Wang 2009). Taiwan is located in a region dominated by two prevailing typhoon tracks (Tu et al 2009), and the typhoon-related rainfall accounts for 47.5% of the total annual rainfall .…”

Characteristics of Summertime Circulation Patterns for Southern Taiwan’s Monsoon Rainfall from July to September

“…The concept of a natural season suggested by Wang and Lin (2002), Hsu (2005), Chou et al (2009), andChen et al (2011) provides a better method to define the summer season over East Asia. A natural season is defined based on the distinct weather/climate systems that hardly coincide with the four seasons based on the astronomical calendar.…”