Nordic Institute for Theoretical Physics, 106 91 Stockholm, Sweden.
Department of Environmental Atmospheric Sciences, Pukyong National University, 48513 Pusan, South Korea.
Proc Natl Acad Sci U S A. 2022 Sep 20;119(38):e2200890119. doi: 10.1073/pnas.2200890119. Epub 2022 Sep 12.
Recent studies have argued that global warming is responsible for a wavier jet stream, thereby driving midlatitude extreme flooding and drought. Polar amplification-the relative enhancement of high-latitude temperatures under global warming-is argued to be the principal climate state driving midlatitude extremes. Namely, the decreased meridional temperature gradient suppresses the mean zonal winds, leading to wavier midlatitude jets. However, although observations are consistent with such a linkage, a detailed dynamical mechanism is still debated. Here, we argue that the Northern Hemisphere land-sea thermal forcing contrast that underlies zonally asymmetric forcing drives a response in the planetary geostrophic motion, which provides balanced mean fields for synoptic eddies in midlatitudes and thus for wavier jet streams. We show that when the barotropic zonal mean wind is smaller than a threshold, proportional to the -plane effect and dry static stability, the flow field exhibits a dramatic transition from a response confined near the surface to one reaching the upper atmosphere. As global warming enhances polar amplification, the midlatitude jet stream intensity is suppressed. The confluence of these effects leads to wavier jet streams.
最近的研究认为，全球变暖导致急流变得更加不稳定，从而引发了中纬度地区的极端洪水和干旱。极地放大效应——全球变暖下高纬度地区温度的相对增强——被认为是驱动中纬度极端天气的主要气候状态。也就是说，纬向温度梯度的减小抑制了平均纬向风，导致中纬度急流变得更加不稳定。然而，尽管观测结果与这种关联一致，但详细的动力学机制仍存在争议。在这里，我们认为，北半球海陆热力强迫对比是导致行星地转运动响应的原因，这种响应为中纬度天气尺度气旋提供了平衡的平均场，从而导致急流变得更加不稳定。我们表明，当正压纬向平均风速小于一个阈值时，该阈值与 -平面效应和干燥静力稳定度成比例，流场会从一个局限于地表的响应剧烈转变为一个到达高层大气的响应。随着全球变暖增强极地放大效应，中纬度急流强度会受到抑制。这些效应的共同作用导致了急流变得更加不稳定。
