Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer
Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer
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DOI：
10.1073/pnas.1812312116
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发表时间：
2019-03-05
影响因子：
11.1
通讯作者：
O'Gorman, Paul A.
中科院分区：
文献类型：
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作者：
Gertler, Charles G.;O'Gorman, Paul A.
近几十年来，北方温带对流层的环流发生了变化，夏季温带气旋活动和涡动动能（EKE）显着减少，而所有季节的对流降水比例增加。夏季EKE的降低部分是由于纬向温度梯度的减弱，但垂直温度梯度的变化和湿度的增加也会影响平均可用位能（MAPE），MAPE是热带气旋吸收的能量库。此外，人们对平均热结构和湿度变化与对流变化之间的关系知之甚少。在这里，我们计算了1979-2017年夏季北方热带外地区的MAPE趋势，并将MAPE分解为对流和非对流分量。在此期间，非对流MAPE减少，一致的EKE和热带气旋活动的减少，但对流MAPE增加，这意味着可用的对流能量增加。理想大气的计算表明，非对流和对流的MAPE都随平均地表温度的增加而增加，随纬向地表温度梯度的减小而减小，但对流的MAPE对平均地表温度的增加相对更敏感。这些结果将大气平均状态的变化与大尺度环流和对流环流的变化联系起来，它们表明，即使与之相关的对流变得更加活跃，热带气旋也会减弱。
The circulation of the Northern Hemisphere extratropical troposphere has changed over recent decades, with marked decreases in extratropical cyclone activity and eddy kinetic energy (EKE) in summer and increases in the fraction of precipitation that is convective in all seasons. Decreasing EKE in summer is partly explained by a weakening meridional temperature gradient, but changes in vertical temperature gradients and increasing moisture also affect the mean available potential energy (MAPE), which is the energetic reservoir from which extratropical cyclones draw. Furthermore, the relation of changes in mean thermal structure and moisture to changes in convection associated with extratropical cyclones is poorly understood. Here we calculate trends in MAPE for the Northern extratropics in summer over the years 1979-2017, and we decompose MAPE into both convective and nonconvective components. Nonconvective MAPE decreased over this period, consistent with decreases in EKE and extratropical cyclone activity, but convective MAPE increased, implying an increase in the energy available to convection. Calculations with idealized atmospheres indicate that nonconvective and convective MAPE both increase with increasing mean surface temperature and decrease with decreasing meridional surface temperature gradient, but convective MAPE is relatively more sensitive to the increase in mean surface temperature. These results connect changes in the atmospheric mean state with changes in both large-scale and convective circulations, and they suggest that extratropical cyclones can weaken even as their associated convection becomes more energetic.