Rainfall variability and extreme precipitation events have a large socioeconomic relevance in Vietnam. The availability of freshwater is essential for the population by access to drinking water and for the rainfed agriculture. However, extreme precipitation events bear the risk of loss of life and economic losses in connection with flooding and landslides. Vietnam is located completely in the tropics and is characterized by a multitude of landscapes, that considerably influence the weather and climate of the country, and also affect the regional impacts of extreme events. Climatologically, more than 80% of total annual rainfall is observed in most regions of Vietnam during the rainy season, which lasts from May to October and is dominated by the southwest monsoon. On the contrary, the dry season from November to April is characterized by mainly northeasterly winds of the East Asian winter monsoon. During this season, substantial rainfall is predominantly limited to the narrow coastal plains on the eastern side of the Annamese Cordillera in central Vietnam. In these regions, rainfall is mainly related to orographic lifting and enhanced convection that is associated with periodical amplifications of the northeasterly flow.
The aim of this thesis is to investigate the role of tropical and extratropical waves in rainfall variability and extreme precipitation events in Vietnam in both seasons. In the first part, regional phases of enhanced or suppressed moist convection of large-scale tropical wave modes are determined for the rainy season in southern Vietnam. Based on these phases, the influence of the Madden–Julian oscillation (MJO) and convectively coupled equatorial waves (CCEWs) on the modulation of daily rainfall is evaluated and quantified for the period 1979–2007. In the second part, synoptic and dynamic causes of early dry-season rainfall events in the Central Highlands region, which is Vietnam's main coffee-growing region, are analyzed. From a station rainfall time series spanning the period 1981–2007, four dynamically different rainfall cases were selected for an in-depth investigation. The third part of this thesis consists of an analysis of the causes and predictability of a recent extreme rainfall event in northeastern Vietnam. The causes of the extreme event in July/August 2015 and its predictability are investigated using ground- and space-based observations, European Centre for Medium-Range Weather Forecasts (ECMWF) (re)analyses, and forecasts from the ECMWF ensemble prediction system.
Rainfall in Vietnam south of 16°N is significantly modulated by the MJO, and by convectively coupled Kelvin and equatorial Rossby (ER) waves. The MJO and ER waves exhibit the most coherent signals during the rainy season. However, the strongest ER signals occur in central Vietnam, and a pronounced influence of Kelvin waves is only discernible in the southernmost parts. The analysis also reveals that all three waves enhance the frequency of intense rainfall during wet phases, and that anomalies are significantly enhanced when wet or dry phases of the MJO occur concurrently with the respective phases of Kelvin or ER waves. In terms of thermodynamic causes of the observed rainfall modulation, at least for the MJO the depth of the moist monsoon layer and vertical wind shear are enhanced during convectively active phases. The latter provides favorable conditions for organized convection.
Although the influence of tropical waves in Vietnam is most pronounced during the rainy season, tropical waves can also cause rainfall during the early dry season in the Vietnamese Central Highlands. In two of the investigated cases, tropical waves and their interaction were the causes of substantial rainfall. Nonetheless, extratropically forced events, as in the other two cases, are more common during the northeast monsoon. Overall, the synoptic-dynamic analysis of the selected cases reveals that the spatiotemporal development of the events highly depends on interactions between large-scale tropical and extratropical wave forcing, synoptic forcing, and orographic effects.
The 2015 extreme precipitation event in northeastern Vietnam was caused by a tropical–extratropical interaction that was not documented for this region and season before. A surface low over the Gulf of Tonkin and northern Vietnam was related to a subtropical upper-level trough, and caused persistent moisture flux convergence and convection over the coast of the Quang Ninh province. Strong moisture flux into the region was caused by an exceptionally strong and persistent monsoon depression over the Bay of Bengal. In terms of ECMWF ensemble forecasts, predictability of the event emerged in 72-hour lead-time forecasts, and was strongly related to the correct forecasts of the intensity and location of the upper-level trough. To conclude, this link between predictability and large-scale synoptic forcing, along with improved seasonal and subseasonal forecasts of the MJO and CCEWs suggests opportunities for submonthly forecasts of dry and wet spells in Vietnam for both seasons
