Why Are Stratospheric Sudden Warmings Sudden (and Intermittent)?
Why Are Stratospheric Sudden Warmings Sudden (and Intermittent)?
复制标题
DOI：
10.1175/jas-d-19-0249.1
复制
发表时间：
2020-02
影响因子：
3.1
通讯作者：
N. Nakamura;J. Falk;Sandro W. Lubis
中科院分区：
文献类型：
--
作者：
N. Nakamura;J. Falk;Sandro W. Lubis
本文研究了波-平均流相互作用在平流层突然变暖（SSW）的发生和突然性中的作用。有证据表明，平均而言，SSW是有限振幅Rossby波的阈值行为，其产生于波浪活动A增加和纬向平均纬向风u '减少之间的竞争。这种竞争限制了静止Rossby波向上传播的波活动通量。一旦上升流波活动通量达到极限，或等效地，一旦u ′下降到uREF的某个分数以下，一个快速的、自发的涡旋破裂发生，uREF是一个无波的、参考状态的风，从分带准地转位涡反演而来。这个分数在理论上是0.5，在再分析中大约是0.3。我们建议r u <$/uREF作为接近涡破裂的局部瞬时测量（即，预处理）。在强涡旋冬季，r的比值通常保持在阈值以上，直到最后明显变暖，而在弱涡旋冬季，它在季节早期接近阈值，在隆冬时随着SSW的形成而急剧下降。阈值行为的本质是捕获的半经验一维模型SSWs，类似于“交通堵塞”模型的中村和黄大气阻塞。该模型预测了SSW的显著特征，包括快速涡旋破裂和波活动/纬向风异常的向下迁移，并给出了相应时间尺度的解析表达式。模型的各种瞬态波强迫和阻尼的响应进行了讨论。
This paper examines the role of wave–mean flow interaction in the onset and suddenness of stratospheric sudden warmings (SSWs). Evidence is presented that SSWs are, on average, a threshold behavior of finite-amplitude Rossby waves arising from the competition between an increasing wave activity A and a decreasing zonal-mean zonal wind u¯. The competition puts a limit to the wave activity flux that a stationary Rossby wave can transmit upward. A rapid, spontaneous vortex breakdown occurs once the upwelling wave activity flux reaches the limit, or equivalently, once u¯ drops below a certain fraction of uREF, a wave-free, reference-state wind inverted from the zonalized quasigeostrophic potential vorticity. This fraction is 0.5 in theory and about 0.3 in reanalyses. We propose r≡u¯/uREF as a local, instantaneous measure of the proximity to vortex breakdown (i.e., preconditioning). The ratio r generally stays above the threshold during strong-vortex winters until a pronounced final warming, whereas during weak-vortex winters it approaches the threshold early in the season, culminating in a precipitous drop in midwinter as SSWs form. The essence of the threshold behavior is captured by a semiempirical 1D model of SSWs, similar to the “traffic jam” model of Nakamura and Huang for atmospheric blocking. This model predicts salient features of SSWs including rapid vortex breakdown and downward migration of the wave activity/zonal wind anomalies, with analytical expressions for the respective time scales. The model’s response to a variety of transient wave forcing and damping is discussed.