| dc.description.abstract | The rainfall and maximum temperature datasets at operational stations in northern Vietnam are used to depict the characteristics of summer rainfall and hot days in this region. Based on station monthly observational statistics, the major rainfall in the Red River Delta (RRD) of Vietnam occurs in late summer (July–September) with conspicuous year-to-year variation. Using ± 0.8 of the seasonal total rainfall standard deviation as a criterion, seven wet and six dry years were identified over the period 1983–2015. In addition to the 70.5% of the seasonal total rainfall contributed by heavy rainfall days, the distinct difference in heavy rainfall accumulation between wet and dry years seems to fundamentally establish these two separate extreme wet and dry groups. As revealed by further analyses, the large variability in rainfall is attributed to the influence of tropical cyclones (TCs) and 7–24- and 30–60-day intraseasonal oscillations (ISOs); in particular, the number of TCs affecting the RRD and rainfall produced by TCs increase (decrease) during wet (dry) years, and the amplitudes of ISOs also increase (decrease). In many cases, heavy rainfall days are induced by the combined effect of both ISOs and TCs, while some heavy rainfall events are triggered mainly by ISOs. Water vapor budget analyses reveal that an anomalous cyclone (anticyclone) dominates the Indochina Peninsula in wet (dry) years, resulting in more (less) water vapor being transported to the RRD, whereas the anomalous convergence (divergence) of water vapor flux leads to the maintenance of excessive (insufficient) rainfall across the RRD. However, the El Niño–Southern Oscillation (ENSO) forcing has minor effects on the interannual variation in rainfall in the RRD.
In addition to heavy rainfall, summer is also a hot day season in northern Vietnam. There are 331 hot days found during 31 summers (1985–2015), 102 of which are related to tropical cyclones. Based on the analyzed statistics, hot days mainly occur without tropical cyclone effects during the first half of the summer (May and June), whereas hot days are strongly related to typhoon activities in the late summer (July and August). Summer monsoon development and tropical cyclone activities are the two major weather systems that induce hot days in northern Vietnam during summer. According to the water vapor transport analysis, the hot days without typhoon cases demonstrate that the East Asian rainband, rather the Mei-Yu rainband, shifts further northward, allowing the originally abundant rainfall area relatively to dry, including in northern Vietnam, to induce hot day phenomena during the first half of the summer. In late summer, the northeasterly winds from active tropical cyclones flowing through the mountain range north of northern Vietnam induce the Foehn effect on the leeward side of the mountain range, causing a dry and hot situation in northern Vietnam. Furthermore, based on the local potential temperature tendency analysis, the descending dry air along the leeside downslope essentially induces surface warming via an adiabatic process during the May–June and July–August hot days, except for the relatively weaker descending motions under the typhoon effect, which led to mild temperature warming in late summer. Regarding the extreme weather climate in northern Vietnam, the significant statistics indicate that more hot days may occur during May–June for dry years than for wet years. | en_US |