Effect of latitudinally displaced gravity wave forcing in the lower stratosphere on the polar vortex stability
Effect of latitudinally displaced gravity wave forcing in the lower stratosphere on the polar vortex stability
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DOI：
10.5194/angeo-37-507-2019
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发表时间：
2019-01
影响因子：
1.9
通讯作者：
N. Samtleben;C. Jacobi;P. Pišoft;P. Šácha;A. Kuchař
中科院分区：
文献类型：
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作者：
N. Samtleben;C. Jacobi;P. Pišoft;P. Šácha;A. Kuchař
摘要。为了研究局地受限重力波热点对北半球冬季大气环流的影响，采用非线性、机械、一般环流模式进行了局地受限重力波热点的敏感性研究。为此，我们在东亚地区选择了一个固定的经度范围（120-170°E）和22.5到52.5°N的纬度范围（18到30公里）作为热点地区，然后按5个°的步骤向北移动。在最南端的热点地区，平流层上部和中间层下部的静止行星波（SPW）随着波数1 （SPW 1）的活跃度减小，即向上传播的SPW 1减少。这些GW热点导致负折射率，抑制了SPW在中纬度的传播。spw1活动减弱与低纬度纬向平均纬向风增加有关。这反过来又降低了中纬度向极地地区的经向位涡度梯度（qy）。反向的qy表明局地斜压不稳定，在极区产生波数为1的SPWs，在极区我们观察到强烈的正elassen - palm （EP）辐散。结果表明，向极平流层的EP通量增加（对应于spw1振幅的增强），其中波数为1的SPW断裂，纬向平均纬向风减小。因此，局地GW强迫导致极涡向低纬度移动。由反向qy表示的局地斜压不稳定也在中下层产生波数为1的SPWs。50°N以北的GW热点对中部大气动力学的影响可以忽略不计，因为极地地区大气的折射率是强烈的负折射率。因此，任何由局地GW强迫引起的SPW活动的变化都是无效的。
Abstract. In order to investigate the impact of a locally confined gravity wave (GW) hotspot, a sensitivity study based on simulations of the middle atmosphere circulation during northern winter was performed with a nonlinear, mechanistic, general circulation model. To this end, we selected a fixed longitude range in the East Asian region (120–170∘ E) and a latitude range from 22.5 to 52.5∘ N between 18 and 30 km for the hotspot region, which was then shifted northward in steps of 5∘. For the southernmost hotspots, we observe a decreased stationary planetary wave (SPW) with wave number 1 (SPW 1) activity in the upper stratosphere and lower mesosphere, i.e., fewer SPWs 1 are propagating upwards. These GW hotspots lead to a negative refractive index, inhibiting SPW propagation at midlatitudes. The decreased SPW 1 activity is connected to an increased zonal mean zonal wind at lower latitudes. This, in turn, decreases the meridional potential vorticity gradient (qy) from midlatitudes towards the polar region. A reversed qy indicates local baroclinic instability, which generates SPWs with wave number 1 in the polar region, where we observe a strong positive Eliassen–Palm (EP) divergence. As a result, the EP flux increases towards the polar stratosphere (corresponding to enhanced SPW 1 amplitudes), where the SPWs with wave number 1 break, and the zonal mean zonal wind decreases. Thus, the local GW forcing leads to a displacement of the polar vortex towards lower latitudes. The effect of the local baroclinic instability indicated by the reversed qy also produces SPWs with wave number 1 in the lower mesosphere. The effect on the dynamics in the middle atmosphere due to GW hotspots that are located northward of 50∘ N is negligible, as the refractive index of the atmosphere is strongly negative in the polar region. Thus, any changes in the SPW activity due to the local GW forcing are quite ineffective.