| dc.description.abstract | The intraseasonal oscillations (ISOs) are an important component of the monsoon variability, which play a significant role in modulating the rainy season/summer monsoon onset date, active/break cycles of summer monsoon, and heavy rainfall event in the Southeast Asia. The ISOs exhibit different characteristics between summer subseasons due to the influence of the distinct background flows. Located in the easternmost Southeast Asia, central Vietnam could be divided into two climatic subregions including the Central Highlands (CH) and central coast of Vietnam (CCV). The CH contributes up to 90% of the country’s total coffee production and 25% of its total hydropower potential. By contrast, heavy rainfall and flood event often appear during the rainy season in the CCV, resulting in great damage on human lives and properties. Thus, understanding about the ISOs of rainfall in central Vietnam during rainy subseasons is essential to improve the capability of prediction for weather and subseasonal climate, which may enhance coffee yields and hydropower production as well as reduce the damage caused by heavy rainfall and flood events. Hence, the main goal of this dissertation is to investigate the evolution of ISOs and their influence on rainfall variability during the rainy subseasons in central Vietnam by analysing high-resolution gridded rainfall and reanalysis data.
Firstly, a second rainy season (SRS) is observed in late July in the CH, which is distinct from the conventional rainy season that occurs in late April – early May. Therefore, the first part examines the climatology of and internnual variation in the SRS onset date (SRSOD) during 1983-2010 period as well as the role of the ISOs in the SRSOD. The average SRSOD for 28 years is July 28, with a standard deviation of 17 days. The SRSOD over the CH is synchronized with the developing phases of the 10-20-day and 20-60-day modes for each year. Surprisingly, the pronounced interannual variation in the SRS onset date has led to three apparent regimes: an early (late) SRS with a 1 month longer (shorter) rainfall period occurring in early July (until mid-August) and a normal SRS starting in late July. Almost all the early SRS years occur during El Niño developing phases, particularly during the Niño3.4 sea surface temperature (SST) increase from January through December. Water vapor budget analyses reveal that the interannual variation in the divergent water vapor flux is in response to the warmer July tropical Pacific SST anomalies, resulting in rainfall enhancement over the CH and eventually inducing early SRS onset.
Secondly, Southeast Asian summer monsoon (SEAM), which owns one longest rainy season in the Asian monsoon, can be divided into early (13 May-31 July) and late summer (1 August-17 October) based on SEAM index and the Indochina Peninsula rainfall index. The southwesterly prevails over the entire Southeast Asia in early summer while the monsoon trough controls this region in late summer, leading to two distinct rainy seasons in the Central Highlands (CH) of Vietnam. Consequently, the quasi-biweekly variation (QBV) of rainfall in the CH shows different characteristics between two subseasons. Compared to early summer, the QBV in late summer strengthens and plays more important role in modulating interannual variation of rainfall mean over the CH, especially during 1997-2010. The anomalous convergence (divergence) of water vapor flux over the tropical western Pacific enhances (reduces) the QBV intensity in the strong-and-wet (weak-and-dry) late summers, thus inducing more (less) rainfall over the CH.
Finally, in Southeast Asia, the autumn (Sepember-November) could be divided into early (Sepember-17 October) and late (18 October-November) autumn with the distinct background flow. Monsoon trough is the critical factor that controls Southeast Asia in early autumn. In contrast, the northeasterly wind accompanied with pressure surge, which is attributed to the intensification of the Siberian High, plays the key role in late autumn. The QBV (12-24-day variation) is the most significant ISO of rainfall over the CCV in the autumn, followed by quasi-9day variation (Q9V, 7-11-day). Thus, the tropical-extratropical interaction associated with the QBV and Q9V of rainfall in the CCV is then investigated during both early and late autumn by performing 8-phase composite analysis to bandpass-filtered precipitation and reanalysis data during 1983-2010 period. For the QBV, in early autumn, the QBV of rainfall is primarily induced by the northwestward-moving tropical depression disturbances (TDDs) originated from the equatorial western Pacific, which is then enhanced by pressure surge from northeast Russia. On the contrary in late autumn, the QBV of rainfall is regulated by the collaborative effects of pressure surge from the Siberian High, the easterly wind from the tropical western Pacific, and topography. For the Q9V, in early autumn, the wet phase of Q9V is first caused by the interplay of a cyclonic TDD moving from the East Sea to the CCV and an anticyclonic TDD propagating from the Philippine Sea to southern China, which is then strengthened by the southwestward extension of the high pressure anomaly from the Japan Sea. However, in late autumn, the Q9V of rainfall is governed by the interaction of the westward-migrating of TDDs from the tropical western Pacific, pressure surge from the Siberian High, and topography. | en_US |