Impact of local gravity wave forcing in the lower stratosphere on the polar vortex stability: effect of longitudinal displacement
Impact of local gravity wave forcing in the lower stratosphere on the polar vortex stability: effect of longitudinal displacement
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DOI：
10.5194/angeo-38-95-2020
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发表时间：
2019-08
影响因子：
1.9
通讯作者：
N. Samtleben;A. Kuchař;P. Šácha;P. Pišoft;C. Jacobi
中科院分区：
文献类型：
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作者：
N. Samtleben;A. Kuchař;P. Šácha;P. Pišoft;C. Jacobi
摘要。利用简化的中大气环流模式，对平流层下重力波破裂热点的影响，特别是其纵向分布的作用进行了敏感性研究。对于当地GW热点的位置，我们首先选择了一个固定的纬度范围37.5到62.5°N，经度范围112.5到168.75°E，海拔范围18到30公里。然后将这个受限的GW热点在经度上移动45次，这样我们总共制造了8个人造的GW热点。局部GW强迫可能对模拟的spw1产生建设性或破坏性的干扰，这在很大程度上取决于各个GW热点相对于模型中产生的波数为1的平稳行星波（spw1）相位的位置。位于北美落基山脉附近的GW热点导致spw1振幅和EP通量的增加，而位于高加索、喜马拉雅或斯堪的纳维亚地区附近的热点导致这些参数的减少。因此，受盛行的SPW活动影响，极地涡旋减弱（高加索和喜马拉雅热点）或减弱（落基山脉热点）。由于局地GW强迫通常抑制中纬度地区的波传播，SPWs 1传播到极区，在极区，大多数人工GW热点的折射率变为正值。spw1的另一个来源可能是由极区经向位涡梯度的反转所表明的局地不稳定性，这与正EP散度有关。在大多数情况下，SPWs 1在极区破裂并保持减速，从而使极涡减弱。当GW热点位于北美上空时形成的SPWs 1通过极区传播到大气中，而在其余GW热点模拟中SPWs 1由于极区正折射率异常上方的负折射率而无法进一步向上传播。GW热点位于喜马拉雅山附近，影响中间层-低层热层区域，因为低层中间层可能存在局部不稳定，产生额外的SPWs 1，这些SPWs 1向上传播到中间层。
Abstract. The effects of gravity wave (GW) breaking hotspots in the lower stratosphere, especially the role of their longitudinal distribution, are evaluated through a sensitivity study by using a simplified middle atmosphere circulation model. For the position of the local GW hotspot, we first selected a fixed latitude range between 37.5 and 62.5∘ N and a longitude range from 112.5 to 168.75∘ E, as well as an altitude range between 18 and 30 km. This confined GW hotspot was then shifted in longitude by 45∘ steps, so that we created eight artificial GW hotspots in total. Strongly dependent on the location of the respective GW hotspot with regard to the phase of the stationary planetary wave of wavenumber 1 (SPW 1) generated in the model, the local GW forcing may interfere constructively or destructively with the modeled SPW 1. GW hotspots, which are located in North America near the Rocky Mountains, lead to an increase in the SPW 1 amplitude and EP flux, while hotspots located near the Caucasus, the Himalayas or the Scandinavian region lead to a decrease in these parameters. Thus, the polar vortex is less (Caucasus and Himalayan hotspots) or more weakened (Rocky Mountains hotspot) by the prevailing SPW activity. Because the local GW forcing generally suppresses wave propagation at midlatitudes, the SPWs 1 propagate into the polar region, where the refractive index turned to positive values for the majority of the artificial GW hotspots. An additional source of SPW 1 may be local instabilities indicated by the reversal in the meridional potential vorticity gradient in the polar region in connection with a positive EP divergence. In most cases, the SPWs 1 are breaking in the polar region and maintain the deceleration and, thus, the weakening of the polar vortex. While the SPWs 1 that form when the GW hotspots are located above North America propagate through the polar region into the middle atmosphere, the SPWs 1 in the remaining GW hotspot simulations were not able to propagate further upwards because of a negative refractive index above the positive refractive index anomaly in the polar region. GW hotspots, which are located near the Himalayas, influence the mesosphere–lower thermosphere region because of possible local instabilities in the lower mesosphere generating additional SPWs 1, which propagate upwards into the mesosphere.