The Role of Planetary-scale Eddies on the Recent Isentropic Slope Trend during Boreal Winter
The Role of Planetary-scale Eddies on the Recent Isentropic Slope Trend during Boreal Winter
复制标题
DOI：
10.1175/jas-d-20-0348.1
复制
发表时间：
2021-06
影响因子：
3.1
通讯作者：
Min-Cheoi Park;Sukyoung Lee
中科院分区：
文献类型：
--
作者：
Min-Cheoi Park;Sukyoung Lee
根据斜压调整理论，斜压不稳定的等熵斜率维持在其边缘状态。然而，最近北极变暖的趋势提出了一种可能性，即在赤道外等熵斜率可能有一个系统的变化。在这项研究中，全球再分析数据被用来调查这种可能性。结果表明，近25年冬季，50°N以北的对流层等熵线明显变平。这种趋势模式在季节内时间尺度上波动。时间演变的检查表明，它是行星尺度（纬向波数1-3）涡动热通量，而不是天气尺度涡动热通量，平坦的等熵线;天气尺度涡动热通量，而不是响应于随后的变化等熵斜率。热带外行星尺度波的增长伴随着热带加热和向极波活动矢量纬向非对称性的增强，并利用一个数值模式来检验观测到的潜热加热是否能产生观测到的等熵斜率异常。结果表明，热带加热确实对等熵斜率趋势有贡献。如果在强迫中也包括大气层外潜热加热，则模式解与观测之间的一致性大大提高。模式的温度响应显示出类似于暖北极冷大陆模式的模式。结果表明，50°N以北等熵斜坡近期的平坦化趋势主要是由潜热加热产生的行星尺度涡动活动引起的，这种变化伴随着贯穿整个对流层的暖北极冷大陆型。
According to baroclinic adjustment theory, the isentropic slope maintains its marginal state for baroclinic instability. However, the recent trend of Arctic warming raises the possibility that there could have been a systematic change in the extratropical isentropic slope. In this study, global reanalysis data is used to investigate this possibility. The result shows that tropospheric isentropes north of 50°N have been flattening significantly for the recent 25-yr winters. This trend pattern fluctuates at intraseasonal time scales. An examination of the temporal evolution indicates that it is the planetary-scale (zonal wavenumber 1-3) eddy heat fluxes, not the synoptic-scale eddy heat fluxes, that flatten the isentropes; synoptic-scale eddy heat fluxes instead respond to the subsequent changes in isentropic slope. This extratropical planetary scale wave growth is preceded by an enhanced zonal asymmetry of tropical heating and poleward wave activity vectors.A numerical model is used to test if the observed latent heating can generate the observed isentropic slope anomalies. The result shows that the tropical heating indeed contributes to the isentropic slope trend. The agreement between the model solution and the observation improves substantially if extratropical latent heating is also included in the forcing. The model temperature response shows a pattern resembling the warm-Arctic-cold-continent pattern. From these results, it is concluded that the recent flattening trend of isentropic slope north of 50°N is mostly caused by planetary scale eddy activities generated from latent heating, and that this change is accompanied by a warm-Arctic-cold-continent pattern that permeates the entire troposphere.