中国科学院海洋研究所机构知识库

热带西太平洋是全球暖水水体最丰富的区域之一，对局地以及全球的气候系统都拥有十分重要的影响作用。而西太平洋中除了上表层流系外，次表层和中层流也是整个环流系统中的重要组成部分，与包括厄尔尼诺南方涛动（El Nino-Southern Oscillation, ENSO）以及太平洋涛动（Pacific Decadal Oscillation, PDO）等在内的大尺度现象都存在着紧密的联系。并且次表层和中层流能够驱动太平洋表层以下水体营养物质的东西向输送过程，促进东西太平洋间的碳循环及生物生态环境的交互。但现如今大洋表层以下深度洋流的观测手段都有着局限性，无法对次表层和中层流进行长期高质量的连续观测。而中国科学院海洋研究所建立与维护的热带西太平洋潜标科学观测网恰好弥补了这一短板，为针对次表层和中层流的研究提供了良好的观测基础。本文从潜标观测数据出发，对热带西太平洋的次表层和中层流的变异规律及其机制进行了研究。具体结果如下：

1）我们利用受南侧倾斜边界影响的赤道罗斯贝短波解释了位于热带西太平洋中层（1000 m以深）季节内变异（ISV，20至90天周期）的传播机制。自2014年九月至2015年十月期间，七套潜标布放在沿142°E 断面，0°至7.5°N之间。位于1200 m深度上的ISV能量在4.5°N最强。在全球环流模型中可以看到类似的现象。通过分析模式的结果，我们认为ISV是通过赤道罗斯贝短波的形式传播，其相速度向西，群速度水平方向向东南垂直方向向下。此外，赤道Rossby波前三斜压模态的线性叠加是ISV能量垂直传播的必要条件。通过进一步分析一层半浅水模型的结果我们认为罗斯贝波的第一经向模态能够解释此处ISV能量的经向分布特征。结果表明，正是由于位于潜标阵列南侧的巴布亚新几内亚倾斜海岸线的作用，ISV能量最强的位置才由3°N向北偏移到了4.5°N。因此，在解释ISV能量经向分布的问题上，我们需要考虑到南侧倾斜岸线的作用。最后，通过正压转化率的计算我们可以得知1000 m以深的ISV动能能够向平均纬向急流提供能量，这为海洋中层纬向急流的形成机制提供了另一种可能性。

2）从分析科学观测网布放在4.7°N，140°E处潜标观测到的连续六年时间长度的数据出发，我们研究了北赤道次表层流（NESC）的多时间尺度变异规律并发现了其对ENSO事件的响应。结果表明在2016年ENSO事件之后，NESC的半年周期变异强度明显减弱（减小了67%）。其季节内周期的变异由于2016年强涡旋事件的存在有所增强，并在ENSO事件之后也依然保持其强度。通过对再分析资料以及Mercator-Ocean模式和连续层化模型的结果进行分析，我们初步推测NESC的半年变异是由第二经向模态赤道罗斯贝长波控制，其波动位相向西向下传播。并且通过表层风场数据我们进一步验证了位于160°E附近处的敏感风区风场的半年周期变异能量在2016年ENSO事件之后也存在着明显的强度减弱现象。这一现象的发生与ENSO事件中的西风爆发事件相关。强ENSO事件发展期的西风强度更大，影响范围向东延伸更远，因此也更易影响到NESC半年变异的敏感风区。关于NESC的季节内变异，我们通过相关性分析结果得到的结论与上一章节中的类似，即季节内变异能量来源于西北方向的海洋上层。

3）我们通过理论推导得到了一种跨山脊的全新地形罗斯贝波（TRW）理论解。虽然前人对地形罗斯贝波已经进行过很多的研究，但都局限于单侧倾斜海底地形的条件。参照东西卡洛琳海盆之间的欧里皮克山脊区域的地形，我们利用一个简化的无层结f平面浅水模型，通过对位涡守恒方程中不同情形的讨论，得到了一个跨海底山脊地形的全新TRW形式，其振幅的纬向分布也在山脊的东西两侧有所差别。在相速度前进方向的右侧为指数型分布，在左侧则为波动形式分布。参考到欧里皮克山脊区域的各项地形参数可以发现，在此区域这种新型TRW的振动周期将会大于69天。

The Western Tropical Pacific Ocean (WTPO) is one of the most abundant areas with warming water and has a significant impact on both of local and global climate systems. Besides the current system in the upper layer, the subsurface and intermediate currents also play important roles in the entire circulation system, which are closely related to large-scale phenomena such as El Nino-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). Moreover, the subsurface and intermediate currents can drive the east-west transport process of nutrients below the surface layer of the Pacific Ocean, and promote the carbon cycle and biological-ecological environment interaction between the east and west Pacific Ocean. However, at present, the observation measurements of ocean currents at depths below the surface layer are limited and impossible to make long-term and high-quality continuous observation. The Scientific Observing Network of the Chinese Academy of Sciences (CASSON) in the WTPO, established and maintained by the Institute of Oceanology, Chinese Academy of Sciences (IOCAS), makes up for this shortcoming and provides a good observation basis for studies of the subsurface and intermediate currents. In this paper, based on the mooring observations, the variability and mechanism of subsurface and intermediate currents in the WTPO are studied. The specific results are as follows:

1) Intermediate-depth (below 1000 m) intraseasonal variability (ISV) at 20-90-day period in the western tropical Pacific can be interpreted in terms of equatorial short Rossby waves modified by the tilted southern boundary. Seven moorings were deployed between 0° and 7.5°N along 142°E from September 2014 to October 2015. The strongest ISV energy at 1200 m occurs at 4.5°N. Similar pattern is also seen in an eddy-resolving global circulation model. An analysis of the model output identifies that the ISV propagates as equatorial short Rossby waves with westward phase speed and southeastward and downward group velocity. Additionally, it is shown that a superposition of first three baroclinic modes is required to represent the ISV energy propagation. Further analysis using a 1.5-layer shallow water model suggests that the first meridional mode Rossby wave accounts for the specific meridional distribution of ISV in the western Pacific. The same model suggests that the tilted coastlines of Irian Jaya and Papua New Guinea, which lie to the south of the moorings, shift the location of the northern peak of ISV from 3°N to near 4.5°N. The tilt of this boundary with respect to a purely zonal alignment therefore needs to be taken into account to explain this meridional shift of the peak. Calculation of the barotropic conversion rate indicates that the intraseasonal kinetic energy below 1000 m can be transferred into the mean flows, suggesting a possible forcing mechanism for intermediate-depth zonal jets.

2) Based on the analysis of the continuous mooring observation data at 4.7°N and 140°E for six years, the multi time scale variability of North Equatorial Subsurface Current (NESC) and its response to ESNO event was studied. It was found that the intensity of the semiannual periodic variability of the NESC after 2016 ENSO event was significantly weakened (reduced by 67%). The ISV energy increased with the presence of strong eddy, and remained intensity thereafter. Based on the reanalysis data and the results of Mercator-Ocean model and Linearly Continuously Stratified (LCS) model, it is preliminarily speculated that the semiannual variability of NESC is dominated by the second meridional mode equatorial long Rossby wave, propagating westward and downward. Moreover, based on the surface wind field data, it is further verified that the energy of semiannual variability of wind stress in the sensitive area near 160°E also shows significant weakening after 2016 ENSO event. This phenomenon is related to the westerly wind burst event with ENSO. The intensity of westerly wind in strong ENSO event is stronger, and the range of westerly wind extends farther to the east. Therefore, the westerly wind in strong ENSO event is more likely to affect the wind field in sensitive area. The results of correlation analysis of ISV between NESC and seasurface currents suggest similar speculation to the previous chapter that ISV energy comes from the upper layer in the northwest.

3) A new topographic rossby wave (TRW) covering ridge is derived theoretically. Although there have been many previous studies on the TRW, they all have been limited to the situation of single-sided sloping topography. Considering the topography of Eauripik rise between west and east Caroline Basin, utilizing a simplified homogenous, f-plane, shallow water model, we got a new form of TRW through the conservation of potential vorticity equation. The zonal distributions of amplitude are different between two sides of rise. The exponential distribution is on the right side of direction of phase velocity and oscillation form is on the left side. In view of the parameters of Eauripik rise, it is found that the period of this new TRW is generally greater than 69 days.