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<p>To classify and understand the causes of heat waves in western Japan, this study investigated atmospheric circulation patterns during August from 1992–2021, using reanalysis data. First, we defined heat-wave days using daily surface air temperature and identified 108 days out of 930, each having a spatial scale larger than a few hundred kilometers. We then applied empirical orthogonal function (EOF) analysis to the 850-hPa geopotential height for those 108 heat-wave days. The leading three EOF modes explained over 60% of the total variance. Because positive and negative principal components (score time-series) of the EOF modes can be associated with physically different phenomena, we did composite analyses separately for the EOF1-positive and EOF1-negative cases. Expectedly, westward extension of the subtropical high, considered the major cause of heat waves in western Japan, was observed. Additionally, slightly different patterns, such as a southwestward extension, produce regional differences across East Asia. In contrast, other causes were also observed under retreated-subtropical-high conditions. These other causes include a tropical cyclone approaching along the southern coast of western Japan and a response to convective activity around the Philippines (Pacific–Japan-like teleconnection pattern), accounting for 24% and 12% of events, respectively, indicating that these circulation types are non-negligible contributors to heat waves in the region. Notably, the tropical-cyclone–related categories induce a relatively unique "moist heat wave." We also examined the long-term frequency variability of each category. Our research shows that exploring the causes of heat waves in climate projections improves understanding of future heat waves.</p>

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Atmospheric circulation patterns associated with August heat waves in western Japan: tropical and mid-latitude influences

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年4月9日

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  • Hiroshi G. Takahashi,
  • Hirokazu Endo,
  • Yuhei Takaya,
  • Tomoaki Ose,
  • Nozomi Kamizawa,
  • Toshiyuki Nakaegawa

摘要

To classify and understand the causes of heat waves in western Japan, this study investigated atmospheric circulation patterns during August from 1992–2021, using reanalysis data. First, we defined heat-wave days using daily surface air temperature and identified 108 days out of 930, each having a spatial scale larger than a few hundred kilometers. We then applied empirical orthogonal function (EOF) analysis to the 850-hPa geopotential height for those 108 heat-wave days. The leading three EOF modes explained over 60% of the total variance. Because positive and negative principal components (score time-series) of the EOF modes can be associated with physically different phenomena, we did composite analyses separately for the EOF1-positive and EOF1-negative cases. Expectedly, westward extension of the subtropical high, considered the major cause of heat waves in western Japan, was observed. Additionally, slightly different patterns, such as a southwestward extension, produce regional differences across East Asia. In contrast, other causes were also observed under retreated-subtropical-high conditions. These other causes include a tropical cyclone approaching along the southern coast of western Japan and a response to convective activity around the Philippines (Pacific–Japan-like teleconnection pattern), accounting for 24% and 12% of events, respectively, indicating that these circulation types are non-negligible contributors to heat waves in the region. Notably, the tropical-cyclone–related categories induce a relatively unique "moist heat wave." We also examined the long-term frequency variability of each category. Our research shows that exploring the causes of heat waves in climate projections improves understanding of future heat waves.