有界大洋对纬向风强迫的响应及共振

为揭示北太平洋主、次要气候模态即太平洋年代际振荡(PDO)和北太平洋流涡振荡(NPGO)的形成机理及其振荡周期与大洋水平尺度之间的联系,采用中纬β通道中的约化重力准平衡线性大洋模型,解析求解了纬向风强迫下的大洋流场响应,讨论了其中的共振问题.1)有界大洋的响应形态分别类似于冬季PDO和NPGO的流场模.2)响应形态分别表现为在大洋西海岸以东,前者有一个椭圆状流涡,后者有南北两个旋转方向相反的流涡并构成流涡偶;在整个大洋,前者有一个洋盆尺度环流,后者在大洋南北分别有两个旋转方向相反的洋盆尺度环流;在中纬度西风急流异常位置偏北和偏南,则能分别强迫出以上的两种情况.3)大洋流场对纬向风场强迫的响应频率(周期)与纬向风强迫频率(周期)相同,但大洋响应要滞后于纬向风的强迫;而响应流场即流函数的强度则与纬向风强迫的大小成正比.当纬向风强迫频率(周期)与该大洋固有频率(周期)相同时,二者会有共振发生,此时大洋响应最为强烈;而二者频率(周期)相差较远时,响应则不大.摩擦越小共振就越强,共振的个数也越多.有界大洋东西向的长度对其固有频率(周期)即共振频率(周期)有明显影响,并起着决定作用;当该长度减小时,相邻两个共振周期的间隔会增大.海洋大气间的两两非线性相互作用,使得随机风场的振荡包含了从极低频到高频的各种成分;通过该共振,可从中挑选出与大洋固有频率相同或相近的共振频率,在该频率上流场对风场的响应最为强烈,从而也就锁定了PDO和NPGO的周期.最终结论为:非线性相互作用、风场对流场的强迫、共振是造成PDO和NPGO的三个关键因子;该解析解的性质为时变的共振Rossby波.

Response and resonance of bounded ocean under zonal wind forcing

To illustrate the formation mechanisms for the Pacific decadal oscillation (PDO) and the North Pacific gyre oscillation (NPGO) as the dominant and less dominant climate patterns of the North Pacific, and correlations between their periods of oscillation and the length of the ocean in the East-West direction, this paper adopts a mid-latitude β channel linear quasi-equilibrium ocean model with reduced gravity to seek the analytical solution of the ocean flow field response to zonal wind forcing, with a special focus on resonance. Main findings include that the response pattern of the bounded ocean resembles the PDO and NPGO modes during winter respectively; specifically, to the east of the west coast of the ocean, the former is characterized by a gyre in an oval shape and the latter by two gyres rotating in opposite directions in the north and the south, constituting a gyre couple; across the entire ocean, the former features basin-wide ocean general circulation, while the latter features basin-wide general circulation in the north and the south respectively, which rotate in opposite directions. The above situations can be forced by anomalous positions of mid-latitude westerlies to the north and the south respectively. The frequency (period) of ocean flow field response to zonal wind field forcing is identical to the frequency (period) of zonal wind forcing; the response is observed after zonal wind forcing while the flow field (stream function) of the response is proportional to the zonal wind in scale. When the frequency (period) of zonal wind forcing equals that of the natural frequency (period) of the ocean, resonance will happen, with the observation of the strongest ocean response; while when the two frequencies differ by wide margins, rather small response will be observed. Smaller frictions correlate with stronger resonance along with more resonance occurrences. The length of the bounded ocean in the East-West direction has an obvious effect on the natural frequency (period), namely, the frequency (period) of resonance, and plays a decisive role in determining such a frequency; the distance between two neighboring resonance periods increases as the length is reduced. Different non-linear air-sea interactions lead to the complexity of the oscillation frequencies of a random wind field, ranging from extremely low to extremely high frequencies; through the resonance, resonance period identical or similar to the natural frequency of the ocean can be identified, at which frequency the ocean flow response to wind fields is the strongest, thus determining the periods of the PDO and NPGO. The final conclusion is that such a non-linear interaction, the effect of wind field forcing on flow field, and resonance are three key factors leading to the PDO and NPGO; the analytical solution is in nature a time-varying resonant Rossby wave.