The severe heatwaves and hot spells in Vietnam were observed more frequently in intensity and duration due to global warming and climate change impacts. The hot days and extreme summer events make the weather harsh and significantly affect human health and the environment. This study presents the spatiotemporal distribution of the number of hot days (NHDs) in Vietnam. The variability of NHD in seven climate subregions is also examined in association with the large-scale drivers. The European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis version 5 (ERA5) data for the period 1981–2020 are used. Principal component analysis is also applied to the observed monthly NHD to obtain spatial patterns and time series. The results show that the NHD in the Highland and South subregions from March to May is linked with the subtropical high associated with decreased 500hPa-level vertical velocity (VV500). From May to June, the North and Central subregions seem related to deepening the Asiatic low and enhancement of southwest flows across the Indochina Peninsula. Finally, both increased southwest flows and decreased VV500 can partly contribute to the intensification of NHD in the North and Central subregions during July and August. The long trends of NHD are also examined. The results reveal that the increasing trends in NHD occur in most subregions, except for the Central Highland, and changing trends of NHD in June greatly contribute to the annual trend of NHD. Finally, the examinations with the El Niño-Southern Oscillation events show that NHD is significantly higher in El Niño events than in La Niña events in March and April for the Northwest, South Central, Central Highlands, and South, in May and June for all subregions, and in July and August for only the Red River Delta subregion. This suggests that ENSO can provide the potential for improving seasonal climate forecasts and mitigating natural disaster risks for the community.

中文翻译：

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越南炎热天气的时空变化与大规模大气驱动因素的关系

由于全球变暖和气候变化的影响，越南严重热浪和炎热天气的强度和持续时间更加频繁。炎热的天气和极端的夏季事件使天气恶劣，严重影响人类健康和环境。本研究展示了越南炎热天数（NHD）的时空分布。七个气候次区域 NHD 的变异性也与大尺度驱动因素相关联。使用欧洲中期天气预报中心 (ECMWF) 再分析版本 5 (ERA5) 1981-2020 年期间的数据。主成分分析也适用于观测到的每月 NHD，以获得空间模式和时间序列。结果表明，3-5月高原和南部次区域的NHD与500hPa垂直速度（VV500）下降的副热带高压有关。5月至6月，北部和中部次区域似乎与亚洲低压加深和中南半岛西南气流增强有关。最后，西南流量的增加和 VV500 的减少在一定程度上导致了 7 月和 8 月北部和中部次区域 NHD 的加剧。还研究了 NHD 的长期趋势。结果表明，除中部高地外，大多数次区域的NHD均呈上升趋势，且6月份NHD的变化趋势对NHD的年度趋势贡献较大。最后，对厄尔尼诺-南方涛动事件的检验表明，西北、中南、中部高地和南部3月和4月厄尔尼诺事件中的NHD显着高于拉尼娜事件，所有次区域5月和6月的NHD显着高于拉尼娜事件，七月和八月仅针对红河三角洲次区域。这表明 ENSO 可以为改善季节性气候预报和减轻社区自然灾害风险提供潜力。