Chih-wen Hung

Symmetry and Asymmetry of the Asian and Australian Summer Monsoons

The rainfalls associated with the Asian summer monsoon have significant correlation with succeeding Aus- tralian summer monsoon rainfalls. This is partly due to the typical life cycle of the El Nino-Southern Oscillation (ENSO) phase locked with the annual cycle within an Asian monsoon year (May-April). This feature is referred to as the symmetric behavior of the Asian-Australian monsoon system. However, preceding Australian summer monsoon rainfalls have no significant impact on succeeding Asian summer monsoon rainfalls. Thus, there is a one-way (asymmetric) relationship between the two monsoons and a communication gapexists in boreal spring prior to the onset of the Asian summer monsoon. The annual cycles of precipitation associated with the intertropical convergence zone (ITCZ) and the sea surface temperature (SST) anomaly in the Indonesia sector (908-1208E) that links the two monsoon regions are compared. The continuous propagation of the ITCZ from the Asian to the Australian summer monsoon regions in Northern Hemisphere (NH) fall provides a one-way link between them. However, the SST anomaly shows a sudden change of sign in November, suggesting that the anomaly is mainly a response to the ITCZ movement rather than its cause. From January to May, the ITCZ is confined at the latitudes south of 58N due to the subsidence in oceanic regions adjacent to the southern coast of Asia. This subsidence prevents the ITCZ from propagating northward until the abrupt onset of the Asian summer monsoon with the reversal of meridional temperature gradient. The discontinuity of the ITCZ movement and the seasonal heating over the Asian continent result in the communication gap in NH spring.

Weakening of the Winter Monsoon and Abrupt Increase of Winter Rainfalls over Northern Taiwan and Southern China in the Early 1980s

Abstract The rainfall characteristic of the East Asian winter monsoon (EAWM) is less emphasized in previous works. This study reveals that the circulation of the EAWM weakened in recent decades, which results in a decrease of winter rainfall over several windward coastal areas over East Asia including the hills in northern Taiwan. In contrast, there is an abrupt increase of rainfall in southern China and the plains of northern Taiwan during the early 1980s. This is due to the increase in sea surface temperature and lower-troposphere moisture over the South China Sea and the anomalous northward flow that enhances the moisture transport to southern China. Because more moisture is provided for the frontal system that moves eastward, the fronts frequently come with abundant moisture and a well-developed rainband in winter. Therefore, the plains of northern Taiwan receive more rainfall after the 1980s.

The First Transition of the Asian Summer Monsoon, Intraseasonal Oscillation, and Taiwan Mei-yu

Abstract This study reveals the close relationship between the first transition of the Asian summer monsoon (ASM), the tropical intraseasonal oscillation (TISO), and the mei-yu in Taiwan, which occurs climatologically between mid-May and mid-June. For about half of the years in 1958–2002, the first transition of the Asian summer monsoon can be classified as a sharp onset, which is characterized by an abrupt reversal of the monsoon flow from northeasterly to southwesterly. The evolution of the large-scale monsoon circulation and convection in the sharp-onset years is characterized by an eastward-propagating TISO from eastern Africa and the western Indian Ocean to the Maritime Continent. Upon the arrival of the TISO in the Maritime Continent, a sharp onset of the ASM occurs, and a channel supplying moist air in the lower troposphere is well established across the Indian Ocean to the South China Sea (SCS). This channel consists of the Somali jet, transporting the moisture from the Southern Hemisphere to the ...

Madden–Julian Oscillation and the Winter Rainfall in Taiwan

AbstractThis study discusses major impacts of the Madden–Julian oscillation (MJO) on the winter (November–April) rainfall in Taiwan. The results show that Taiwan has more rainfall in MJO phases 3 and 4 (MJO convectively active phase in the Indian Ocean and the western part of the Maritime Continent), and less rainfall in phases 7 and 8 (the western Pacific warm pool area). Mechanisms associated with the MJO are suggested as follows. 1) The tropics to midlatitude wave train: when the MJO moves to the middle Indian Ocean, a Matsuno–Gill-type pattern is induced. The feature of this tropical atmospheric response to the MJO diabatic heating is a pair of upper-level anomalous anticyclones symmetric about the equator to the west of the heating. The northern anomalous anticyclone over the Arabian Sea and northern India induces a northeastward-propagating wave train to the midlatitudes. The wave pattern consists of a cyclonic anomaly centered at East Asia that enhances the winter rainfall in Taiwan. 2) Increase of...

Decadal Variation of the East Asian Winter Monsoon and Pacific Decadal Oscillation

This study finds that the winter (December - February) decadal variability of northerly winds in the East Asian Winter Monsoon (EAWM) over the northern part of the East Asia Coast were influenced by forcing from the middle latitudes from the 1950s to 2000s and related to the Pacific Decadal Oscillation (PDO). We propose that the decreased EAWM in recent decades is associated with the change in pressure gradient along the East Asia coast. This mechanism accounts for the change of westward sea-level pressure (SLP) gradient along the Northeast Asia coast, and is affected by the Aleutian Low location, which is associated with the PDO phases. As the Aleutian Low is influenced by the negative PDO phase and moves westward, the SLP gradient between the Siberian High and the Aleutian Low can increase and the northerly wind at 850 hPa will be enhanced. This change is associated with the increased EAWM near Sakhalin Island of Russia to Hokkaido of Japan.