Department of Earth and Planetary Science, University of California, Berkeley, CA 94720.
Miller Institute for Basic Research in Science, University of California, Berkeley, CA 94720.
Proc Natl Acad Sci U S A. 2023 Mar 21;120(12):e2215278120. doi: 10.1073/pnas.2215278120. Epub 2023 Mar 14.
Heatwaves damage societies worldwide and are intensifying with global warming. Several mechanistic drivers of heatwaves, such as atmospheric blocking and soil moisture-atmosphere feedback, are well-known for their ability to raise surface air temperature. However, what limits the maximum surface air temperature in heatwaves remains unclear; this became evident during recent Northern Hemisphere heatwaves which achieved temperatures far beyond the upper tail of the observed statistical distribution. Here, we present evidence for the hypothesis that convective instability limits annual maximum surface air temperatures (TXx) over midlatitude land. We provide a theory for the corresponding upper bound of midlatitude temperatures, which accurately describes the observed relationship between temperatures at the surface and in the midtroposphere. We show that known heatwave drivers shift the position of the atmospheric state in the phase space described by the theory, changing its proximity to the upper bound. This theory suggests that the upper bound for midlatitude TXx should increase 1.9 times as fast as 500-hPa temperatures at the time and location of TXx occurrences. Using empirical 500-hPa warming, we project that the upper bound of TXx over Northern Hemisphere midlatitude land (40°N to 65°N) will increase about twice as fast as global mean surface air temperature, and TXx will increase faster than this bound over regions that dry on the hottest days.
热浪在全球范围内破坏社会，并且随着全球变暖而加剧。热浪的几个机械驱动因素，如大气阻塞和土壤湿度-大气反馈，以其提高地表气温的能力而闻名。然而，限制热浪中最高地表气温的因素尚不清楚；这在最近的北半球热浪中表现得很明显，这些热浪达到的温度远远超过了观测到的统计分布的尾部。在这里，我们提出了一个假设，即对流不稳定性限制了中纬度陆地上的年最高地表气温 (TXx)。我们提供了一个适用于中纬度温度的相应上限理论，该理论准确地描述了地表温度和中对流层温度之间的观测关系。我们表明，已知的热浪驱动因素会改变理论描述的大气状态在相空间中的位置，从而改变其接近上限的程度。该理论表明，TXx 的中纬度上限应随 TXx 发生时和发生地的 500hPa 温度以 1.9 倍的速度增加。利用经验性的 500hPa 变暖，我们预计北半球中纬度陆地（40°N 至 65°N）的 TXx 上限将比全球平均地表气温快两倍左右，并且在最热天气下干燥的地区，TXx 将比这一上限增加得更快。
