Characteristics and attributions of a Rossby normal mode in the South China Sea
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摘要: 南海是一个准封闭海盆，其本征值问题是理解南海动力学的重要内容。本文利用一种新的泛函工具—多尺度子空间变换，从卫星观测资料中分离得到一个南海本征模态，即罗斯贝标准模态的近似场。发现该模态的周期为6个月左右、波长近250 km，在深水海盆向西传播，这与寿命为3个月左右的南海中尺度涡群体活动特征相一致。在此基础上，本文通过Liang-Kleeman信息流这一严格建立在第一性原理上的定量因果分析工具，探究南海两个最重要外部强迫，即黑潮入侵与南海季风对该罗斯贝标准模态的影响。结果表明二者与该模态均有较强因果关系，但分别影响模态的不同阶段：黑潮入侵主要影响其1/2π和3/2π位相，季风的作用则体现在3/4π位相。二者共同作用，调制该模态在近一个周期内的变化。进一步研究发现，黑潮入侵的过渡态是影响该模态的关键，这时黑潮在南海的分支与流套强度相当，有利于吕宋海峡西部形成不同极性涡旋的排列，从而影响南海内部罗斯贝标准模态。对南海季风而言，冬季风与夏季风的成熟阶段是影响该模态的重要时期，但并非整个南海的季风都发挥作用，泰国湾是季风改变南海罗斯贝标准模态的关键区域，这表明局地的强迫对激发全局模态起作用。
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关键词:
- 南海罗斯贝标准模态 /
- 中尺度涡 /
- 多尺度子空间变换 /
- Liang-Kleeman信息流 /
- 定量因果分析
Abstract: Rossby normal mode as a solution of the boundary problem is a key part of the dynamics in the South China Sea. With the usage of multiscale window transform, a recently developed decomposition tool for multiscale processes, mesoscale-eddy fields analogous to Rossby normal modes are separated from the satellite observation of the South China Sea. The eddies are dominated by a group with lifetime of 3 months, which belongs to an oscillating mode with period of 6 months and wavelength of 250 km. To explore the external influence on this mode, we apply an information flow-based causality analysis that has been rigorously established from first principles in physics. It is found that both the Kuroshio intrusion and monsoon, believed to be two primary forcings of the South China Sea circulations, are causal to the Rossby normal mode within one period. Specifically, the Kuroshio intrusion mainly affects the 1/2π and 3/2π phases, while the monsoon the 3/4π phase. Further analysis reveals that the transition between penetration and leaping of the Kuroshio at the Luzon Strait is key to the mode development. Under this circumstance, the leaking and looping branches are almost of equal intensity, facilitating the generation of cyclonic and anticyclonic eddies west of the Luzon Strait, and hence the Rossby normal mode. The monsoon over the South China Sea, on the other hand, shows the greatest influence at its mature phases. The Gulf of Thailand is a key region for the monsoon to affect the Rossby normal mode in the South China Sea, indicating that local forcing plays an important role in exciting a global mode.1)1 ①面板数据是由两个或两个以上不连续时间段内的观测组成的数据。这里计算因果所用的时间序列是由多个时间段内的数据组合而成，故属于面板数据。 -
图 2 使用MWT得到的 1996年4月12日（a）、1995年6月6日（b）与 2002年2月2日（c）南海中尺度子空间海面高度与相应地转速度的分布
Fig. 2 Snapshots of the mesoscale-window sea surface height and geostrophic velocity on April 4, 1996 (a), June 6, 1995 (b), and February 2, 2002 (c) obtained from the MWT
图 3 使用MWT得到的1996年7月22日（a）、1995年9月5日（b）与2002年5月3日（c）南海中尺度子空间海面高度与相应地转速度的分布
Fig. 3 Snapshots of the mesoscale-window sea surface height and geostrophic velocity on July 22, 1996 (a), September 5, 1995 (b) and May 3, 2002 (c) obtained from the MWT
图 4 南海罗斯贝标准模态RNM6I随时间的变化
Fig. 4 Daily time series of the Rossby normal mode RNM6I in the South China Sea
图 5 中尺度子空间海面高度沿18°N（a）与12°N（b）的时间−经度演变图
Fig. 5 Time-longitude plots of the mesoscale-window sea surface height along 18°N (a) and 12°N (b)
图 7 RNM6I的不同位相合成（a）与LST到不同位相的信息流绝对值（b）
Fig. 7 Composite of the RNM6I (a) and the absolute phase-dependent information flow from LST (b)
图 8 RNM6I波峰前1周的海面高度与地转速度合成图
Fig. 8 Composite map of the sea surface height and geostrophic velocity a week before the RNM6I peaks
图 9 SCSMI超前不同时间到RNM6I的信息流 （a）与超前82日到不同位相的信息流绝对值（b）
Fig. 9 Time-delayed information flow from SCSMI to RNM6I (a) and phase-dependent absolute information flow from 82-day-led SCSMI to RNM6I (b)
图 10 RNM6I到达
$3/4{\text{π}} + 2n{\text{π}} $ 位相前82日南海10 m风场及其旋度在夏季与冬季的合成图Fig. 10 Composite maps of the 10 m wind and its vorticity 82 days before the
$3/4{\text{π}} + 2n{\text{π}} $ phase of the RNM6I in summer and winter -
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