| 摘要: | 本研究使用越南北部氣象測站雨量及溫度資料，做為描繪此區域的夏季降雨和酷熱天氣的極端特徵。依據測站月雨量觀測資料統計顯示，越南紅河三角洲主要降雨發生在晚夏的7 – 9月間，並具有明顯的年際變化情形。在1983 – 2015年間，以季節總雨量的 ± 0.8個標準差門檻得以分別篩選出7個相對濕年及6個乾年。除強降雨日貢獻了70.5%季節總雨量外，濕年及乾年強降雨累積雨量的顯著差異，基本上已建造了兩個極端濕、乾年的特徵。進一步分析降雨的變異度發現，主要係受到熱帶氣旋、7–24- 和30–60-天季內振盪影響；特別在濕(乾)年時，影響紅河三角洲的熱帶氣旋數量及所引致的雨量增加(減少)，且季內振盪的振幅也明顯的增強(減弱)。多數強降雨日係由熱帶氣旋與季內振盪合成共伴效應影響，但仍有些強降雨事件則僅由季內振盪所主導引起的。水氣收支分析得知在濕(乾)年時，一距平氣旋(反氣旋)盤據在中南半島上空，促使多(少)量水氣被傳輸到紅河三角洲，再藉由水氣通量的輻合(輻散)環流得以維持此區域過多(不足)的降雨。然而，聖嬰南方振盪對於紅河三角洲雨量的年際變化則僅有微量的影響。
越南北部除了強降雨外，夏季也是酷熱的季節。在31個夏季(1985–2015)中共計331酷熱天數，其中102酷熱天與熱帶氣旋有關連。統計分析顯示，在夏季前半時期5 – 6月中酷熱天的發生沒有受到熱帶氣旋的影響，而在晚夏的7 – 8 期間則明顯受到熱帶氣旋活動的影響。因此，夏季季風發展及熱帶氣旋活動成為引發北越夏季酷熱天的兩個主要天氣系統。根據水氣收支分析得知，由於東亞夏季季風的梅雨帶在夏季前半期持續往北遷徙，促使北越地區由豐沛降雨轉為相對偏乾環境，遂引致酷熱現象發生；而活躍的熱帶氣旋東北風分量穿越北越山嶺促成過山的焚風效應，以致北越成為偏乾且熱的環境。經局部位溫趨勢分析，不論在5 – 6月或7 – 8月的酷熱天都是藉由沿著背風下沉的乾空氣透過乾絕熱加壓過程增溫；由於熱帶氣旋的過山下沉氣流相對微弱些，遂而致使晚下地面增溫稍微緩和。另，在5 – 6 月的酷熱天數，則濕年比乾年顯著的多。
;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. |
