大气环流指数低频振荡机制的流体物理实验研究

使用旋转的环形水槽,用热力驱动的斜压流体作模拟实验,研究地形对斜压波流动状态的影响。使用对称分布的双峰正弦波地形,测量流体中层的流场,计算流函数并作谐波分析,对比无地形及有地形强迫的大尺度流动状态。实验表明,大尺度地形的作用是,对斜压波在时间和空间进行调制,使流动在时间和空间变得不均匀,使斜压波产生127转台旋转周期的振荡。量纲分析表明,这一周期相当于地球自转的26d。这使我们认为,大气环流指数的低频振荡的原因,可能是由于近似对称分布的亚洲及美洲山系大尺度地形的作用,造成大气动量随时间的变化,即对大气流动在时间和空间进行调制造成的。

AN EXPERIMENTAL STUDY OF THE MECHANISM OF THE LOW FREQUENCY OSCILLATION IN THE INDEX CYCLE OF ATMOSPHERIC CIRCULATION

The present work using a rotating annulus with a thermally driven baroclinic fluid made a modeling investigation on the mechanism of low frequency oscillation in the index cycle of atmospheric circulation. By comparison of the experiments with and without topography, the role of topography in formation of the flow regimes is discussed. The topography used in the experiments consists of two ridges and two valleys around the annulus. Two sets of experiments have been done. One is the experiment with topography, and the other is the control experiment with no topography. The experiments show that the traveling waves strongly vacillate with time due to the topographic effects. Otherwise, in the experiments under the same imposed external conditions but with no topography, the baroclinic waves with wavenumber 6 would travel regularly with almost no vacillation. It was found that a prominent feature of experiments with topography is large amplitude vacillation of the waves with a period of approximately 127 annulus rotations, which is equivalent to approximatelly 26 days low frequency oscillation period in the Earth atmosphere. The role of topography in the experiment is to modulate the unstable baroclinic waves both in space and time. The reason that the vacillation exists in the experimental fluid is due to the forcing of the two sinusoidal wave-shape topography to the slowly moving wave flow.