Quasi-resonant circulation regimes and hemispheric synchronization of extreme weather in boreal summer
Quasi-resonant circulation regimes and hemispheric synchronization of extreme weather in boreal summer
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DOI：
10.1073/pnas.1412797111
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发表时间：
2014-08-26
影响因子：
11.1
通讯作者：
Schellnhuber, Hans Joachim
中科院分区：
文献类型：
--
作者：
Coumou, Dim;Petoukhov, Vladimir;Schellnhuber, Hans Joachim
近十年来，北方中纬度地区发生了异常多的夏季极端天气。许多这些事件与异常急流环流模式的特点是持续高振幅准定常Rossby波。最近提出了两种机制，可以激发这样的模式：（i）纬向平均射流的减弱和（ii）放大准定常波之间的共振自由和强迫波在中纬度波导。基于对流层中层风场的谱分析，我们表明持续的急流模式首先是由于纬向波数为6-8的准定常波的放大造成的。然而，我们也检测到在这些事件的纬向平均急流减弱，因此这两种机制似乎是重要的。此外，我们表明，异常的环流制度导致持续的地面天气条件，因此中纬度同步的极端高温和降雨事件的月时间尺度。最近发生的一系列共振事件导致7月和8月波数为7和8的高振幅准驻波的频率在统计上显著增加。我们表明，这是一个强大的发现，适用于不同的压力水平和再分析产品。我们认为，最近在北极地区的快速变暖和相关的纬向平均纬向风的变化创造了有利的条件，双急流形成的热带外，这促进了共振流制度的发展。
The recent decade has seen an exceptional number of high-impact summer extremes in the Northern Hemisphere midlatitudes. Many of these events were associated with anomalous jet stream circulation patterns characterized by persistent high-amplitude quasi-stationary Rossby waves. Two mechanisms have recently been proposed that could provoke such patterns: (i) a weakening of the zonal mean jets and (ii) an amplification of quasi-stationary waves by resonance between free and forced waves in midlatitude waveguides. Based upon spectral analysis of the midtroposphere wind field, we show that the persistent jet stream patterns were, in the first place, due to an amplification of quasi-stationary waves with zonal wave numbers 6-8. However, we also detect a weakening of the zonal mean jet during these events; thus both mechanisms appear to be important. Furthermore, we demonstrate that the anomalous circulation regimes lead to persistent surface weather conditions and therefore to midlatitude synchronization of extreme heat and rainfall events on monthly timescales. The recent cluster of resonance events has resulted in a statistically significant increase in the frequency of high-amplitude quasi-stationary waves of wave numbers 7 and 8 in July and August. We show that this is a robust finding that holds for different pressure levels and reanalysis products. We argue that recent rapid warming in the Arctic and associated changes in the zonal mean zonal wind have created favorable conditions for double jet formation in the extratropics, which promotes the development of resonant flow regimes.