There is evidence suggesting that fire activity in southeast Australia has increased in the past two decades. There is also anecdotal evidence that pyrocumulonimbus events have become more common, although the observed record is short. We explore the extent to which possible changes in fire and pyrocumulonimbus occurrence in southeast Australia can be explained by changes in the number of extreme (above the 95th percentile) days per year of two fire-weather indices, the Forest Fire Danger Index (FFDI) and the continuous Haines index (C-Haines). For the period 1958 through 2020, we show that there is a dependence between the number of extreme FFDI days per year and burned area for forested parts of southeast Australia. To a lesser extent, there is a relationship between the FFDI and pyrocumulonimbus occurrences. We find an increase in the occurrence of extreme FFDI days from the late 1990s, with up to 24 more extreme days per year. In the southwest of the state of Victoria, there are concomitant increases in temperature and wind speed, and decreases in relative humidity. We show that the structure of C-Haines has not changed, but extreme days more frequently coincide with extreme FFDI days. This suggests that possible observed increases in pyrocumulonimbus occurrences may partly be explained as a consequence of the increases in co-occurring extreme FFDI and C-Haines days, rather than due to changes in the atmospheric stability. We also find that changes in the atmospheric circulation are consistent with those seen in the fire weather indices. Increased surface and mid-tropospheric pressure since 1998 are consistent with a poleward shift of the storm track. In particular, an increase in anticyclonic anomalies in the Tasman Sea is indicative of enhanced blocking activity. Although blocking frequency in these longitudes has decreased overall, the proportion of extreme FFDI days that are considered blocked has increased.

有证据表明，澳大利亚东南部的火灾活动在过去二十年有所增加。还有轶事证据表明，虽然观察到的记录很短，但积雨云事件已经变得更加普遍。我们探讨了澳大利亚东南部火灾和积雨云发生的可能变化在多大程度上可以通过两个火灾天气指数森林火灾危险指数 (FFDI) 每年极端（高于第 95 个百分位数）天数的变化来解释和连续海恩斯指数（C-Haines）。在 1958 年到 2020 年期间，我们表明每年极端 FFDI 天数与澳大利亚东南部森林地区的燃烧面积之间存在依赖性。在较小程度上，FFDI 与积雨云发生之间存在关系。我们发现从 1990 年代后期开始，极端 FFDI 天数出现增加，每年增加多达 24 个极端天数。在维多利亚州西南部，温度和风速随之升高，相对湿度降低。我们表明 C-Haines 的结构没有改变，但极端日更频繁地与极端 FFDI 日重合。这表明观察到的焦积雨云发生率可能增加的部分原因可能是同时发生的极端 FFDI 和 C-Haines 天数的增加，而不是由于大气稳定性的变化。我们还发现大气环流的变化与火灾天气指数中的变化一致。自 1998 年以来增加的地表和对流层中压与风暴轨迹向极地移动一致。特别是，塔斯曼海反气旋异常的增加表明阻塞活动增强。尽管这些经度的阻塞频率总体上有所下降，但被视为阻塞的极端 FFDI 天数的比例有所增加。