Elucidation of the formation of the Arctic Oscillation through the process of the Polar-night Jet Oscillation

通过对观测资料的分析和大气环流模式的模拟，研究了极夜急流振荡(PJO)诱发北极涛动(AO)的机制。结果表明，波的强迫作用产生了欧拉平均纬向平均经向环流和纬向平均纬向风，并通过经向环流产生了海面的声光信号。主要研究结果如下：1.由极温的温度结构定义的PJO的相态与AO的极性之间存在很强的统计依赖性。也就是说，当PJO处于环状阶段时，地表的AO指数明显倾向于具有正或负的极性。2.AO指数的持续时间强烈地依赖于PJO。特别是，当A0存在于PJO的较佳条件下时，与不满足该条件的A0相比，A0的寿命显著延长。3.PJO是通过缓慢演化的波强迫产生的。这些海浪强迫不仅驱动经向环流，还推动纬向风。经向环流与地面的相互作用在下降(上升)流区产生高(低)压异常。通过这种方式，波强迫的经向环流产生了声光信号。极冠区的下降、上升气流和地面压力变化尤为突出。4.纬向波数1波具有深厚的子午偶极子结构，对PJO的形成起着非常重要的作用。热力强迫对地面的影响几乎与对流层内部动量强迫的影响一样重要。5.平流层中的波强迫对地面声压的影响很大，与对流层中的影响相当。

We have investigated the mechanism of how the Polar-night Jet Oscillation (PJO) induces to create the Arctic Oscillation (AO) through the analysis of the observational data and the simulation of the general circulation model. It was found that the Eulerian mean zonal-mean meridional circulation as well as the zonal-mean zonal wind was created by the wave forcing, and the AO signal at the surface was created through the meridional circulation. Results clarified in this study were summarized as follows :1.There is a strong statistical dependence between the phase of the PJO which are defined by the temperature structures of the polar temperature, and the polarity of the AO. Namely when the PJO is in the phase of "annular stage", AO index at the surface significantly tend to have a polarity of positive or negative sign.2.Duration of the AO index strongly depends on the PJO. Especially, when the AO exists under the preferable condition of PJO as in 1., the lifetime of AO becomes significantly longer compared with that does not satisfy the condition.3.PJO is created through slowly evolving wave forcings. These wave forcings drive meridional circulation as well as force zonal wind. Interaction of the meridional circulation with the surface creates high (low) pressure anomaly at the area of descending (ascending) flow. In this way the AO signal was created by the wave-forced meridional circulation. Descending and ascending flow and the surface pressure change at the polar cap area is especially prominent. 4.For the formation of the PJO, thermal forcing from the wave of zonal wavenumber 1, which shows deep meridional dipole structure, is very important. The effect of the thermal forcing on the surface is nearly as important as that from the momentum forcing inside the troposphere. 5.The effect of wave forcing in the stratosphere to the surface AO is found to be very large, and it is as large as that in the troposphere.