海洋环流与波动重点实验室知识库

热带不稳定波（Tropical instability wave，TIW）是赤道太平洋地区强烈而典型的波动之一，其诱导的湍流混合将直接影响当地的水文、生物化学和大气环境。目前，对TIW的相关研究大多集中在某一深度层次的水平结构或者某一固定观测站点的不同深度，其完整的三维结构尚未有研究直观给出。此外，有关其基本结构和混合效应的研究一般都是基于不同模态叠加得到的总的TIW，而针对其单个模态的研究较少。因此，本研究将首先利用高分辨率模式数据，重点聚焦于太平洋赤道附近的赤道模态热带不稳定波（equatorial mode TIW，eTIW），给出其流场以及温度、密度等相关的基本性质的3–D结构。研究发现，上200 m，eTIW的温度和密度的位相均以合成中心位置（0°，0°）为对称中心呈现出中心对称结构。其中，上70 m，正（负）温度位于流的顺时针分量以南（北）和逆时针分量以北（南），正（负）密度主要位于流的顺时针分量以北（南）和逆时针分量以南（北）。温度和密度的垂直位相转变发生在大约70 m深度附近。到70 m以下，温度和密度的位相与上层完全相反，这表明eTIW是Yanai波引发的不稳定波的第一斜压模态。在海表至70 m深度，盐度分布较为规律，呈反对称结构。垂向上，与温度与密度不同，盐度不存在明显的反位相转变过程。紧接着，通过分析eTIW的不稳定特征，我们首次揭示了eTIW的混合效应。eTIW的层结与约化剪切平方（the reduced shear squared，RSS）也同样呈现出关于合成中心位置（0°，0°）对称的中心对称结构。在上层约60–90 m深度范围内，eTIW的层结（–4倍）的正值分别分布在其流场的顺时针分量以北和逆时针分量以南。上90 m，eTIW正的RSS分量主要分布在顺时针区域以北和逆时针区域以南，主要由同一位置的层结所导致，意味着eTIW可能会导致更多不稳定。随着深度增加至90–110 m，层结和RSS也具有垂直位相反转的过程。此外，对不同深度的层结、剪切和RSS的分析表明，eTIW的层结在其RSS中起主要调制作用，这与过去普遍认为剪切在混合中更重要的观念有所不同。此外，eTIW对总流场的混合也有影响，即eTIW诱导产生的负的RSS会使总流场更加稳定，正的RSS使总流场趋于发生不稳定，从而产生混合。这为研究eTIW引起的混合机理提供了新的思路。

在前人研究中，热带太平洋的TIW一般具有类似于第一斜压模态的表层强化结构。我们研究发现，赤道附近普遍存在着次表层强化的流速异常，通过对（140°W，0°）三个典型时间段的观测流速数据进行5–30天的带通滤波以及功率谱分析可以看出，流速存在很强的表层强化特征，也进一步证实流速具有东北–西南向振荡模态。但与此同时，次表层也存在着强化特征，流速的纬向分量在次表层存在极值中心，而经向分量没有。这明显不同于传统认识下的eTIW。我们将其称为次表层模态热带不稳定波（Subsurface mode Tropical Instability Wave，subTIW），并且具体给出了其波动特征：流速的振幅约为10–20 cm·s^-1，周期约为5–20天，纬向振荡速度峰值位于70–90 m深度。同时通过拓展时间尺度，发现了subTIW具有显著的季节变异特征，在TIW存在的季节表现最为显著。它们多数受正压不稳定激发生成，一般来说持续时间为3–7个月。它们通过与平均流之间的相互作用，在赤道潜流（Equatorial Undercurrent，EUC）流核上方约50 m深度处引起强烈的反位相剪切，并进一步导致复杂的跨等密度面混合结构。

TIW的流速引起的垂向剪切通过与赤道流系相互作用，可以发生不稳定过程。从而导致混合层和温跃层的小尺度湍流混合发生地更为频繁。在本研究的最后，我们以赤道110°W为例，分析赤道东太平洋的不稳定特征，包括其空间分布以及季节变异。分析发现，不稳定过程在赤道上具有明显的季节变化特征：在夏季6、7月份，冬季12、1月份较强，不稳定的发生深度也较深；在春季3、4月份，秋季9、10月份较弱，发生不稳定的层次也相对较浅。除此之外，不稳定具有显著的垂直分布特征。在40 m处，不稳定发生得最为频繁，温跃层以上区域及温跃层底基本上很少有不稳定发生。这些时空变异特征相关的物理机制有待进一步研究。

The tropical instability wave (TIW) is one of the most intense and typical waves in the equatorial Pacific. The turbulent mixing induced by TIW will directly affect the local hydrologic, biochemical and atmospheric environment. At present, most of the relevant studies on TIW focus on the horizontal structures of a certain depth or different depths of a fixed in-situ observational site, and its complete three–dimensional structure has not been intuitively given. In addition, the research on its basic structures and mixing effect is generally based on the total TIW obtained by superposition of different modes, while the research on its single mode is relatively rare. Therefore, in our research, we focus on equatorial mode TIW (eTIW) firstly and presents the 3–D structures of its flow field, temperature, density and other basic properties by using high–resolution model data. It is found that the phases of eTIW’s temperature and density above 200 m show a centrosymmetric structure with (0°, 0°) as the symmetric center. Above 70 m, the positive (negative) temperature is located in the south (north) of the clockwise component and the north (south) of the anticlockwise component of the flow, and the positive (negative) density is mainly located in the north (south) of the clockwise component and the south (north) of the anticlockwise component of the flow. Vertical phase transitions in temperature and density occur at depths of about 70 m. Below 70 m, the phase of temperature and density is completely opposite to that of the upper layer, indicating that the eTIW is the first baroclinic mode of Yanai wave-initiated instability wave. Above 70 m depth, the distribution of salinity is relatively regular and presents an antisymmetric structure. In vertical direction, unlike temperature and density, salinity has no obvious phase reversal process. Subsequently, by analyzing the instability characteristics of eTIW, we reveal the mixing effect of eTIW for the first time. The stratification and the reduced shear squared (RSS) of eTIW also show a centrosymmetric structure with respect to (0°, 0°). The positive values of stratification (–4 times) are distributed in the north of the clockwise components and south of the anticlockwise components of the flow field respectively in the upper layer of about 60–90 m. At 90 m, the positive RSS components of eTIW are mainly distributed in the north of the clockwise region and the south of the anticlockwise region, which are mainly caused by stratification at the same location, suggesting that eTIW can lead to more instability. As the depth increases to 90–110 m, the stratification and RSS also have the process of vertical reversal. In addition, analysis of stratification, shear squared and RSS at different depths shows that eTIW’s stratification plays a major modulation role in RSS, contrary to the prevailing view that the velocity shear is more important in mixing. In addition, eTIW also has an impact on the mixing in total flow field. In other words, the negative RSS induced by eTIW will make the total flow field more stable, while the positive RSS will make the total flow field tend to be instability, thus resulting in mixing. This provides a new idea for studying the mixing mechanism caused by eTIW.

In previous studies, TIW in the tropical Pacific generally have a surface-intensified structure similar to the first baroclinic mode. In our research, we find that the equator generally exists subsurface velocity anomalies. The 5–30 days band-pass filter and the power spectrum density (PSD) analysis are performed on the observational data of velocity in three typical periods of at the (140°W, 0°). It can be seen that surface–intensified characteristics of velocity is very strong, also further confirmed that the velocity of the northeast–southwest oscillation. But at the same time, there is also an intense feature in the subsurface. The zonal component of velocity has an extreme value center in the subsurface, while the meridional component does not. This is obviously different from eTIW under traditional understanding. It is called the subsurface mode tropical instability wave (subTIW), and the wave characteristics of subTIW are given in detail: the velocity amplitude is about 10–20 cms^-1, and the zonal oscillation velocity peaks at a depth of 70–90 m with a period of 5–20 days. At the same time, by extending the time scale, it is found that the subTIW have significant seasonal variation, which is most significant in TIW season. Most of them are stimulated by barotropic instability and generally last for 3 to 7 months. By interacting with the mean flow, they cause strong opposite shear at a depth of about 50 m above the Equatorial Undercurrent (EUC) core, and further lead to complex diapycnal mixing structures.

The vertical shear caused by the velocity of TIW can induce the instability processes by interacting with equatorial flow systems. Hence, the mixing in the mixing layer and the thermocline layer is more frequent. In the last of this study, we take the equatorial 110°W as an example to analyze the instability characteristics of the equatorial eastern Pacific, including its spatial distribution and seasonal variation. It is found that the instability process has a prominent seasonal variation characteristic at the equator: it is stronger in June and July in summer, and stronger in December and January in winter. At the same time, the occurrence depth of instability is also deeper. It is weaker in March and April in spring and September and October in autumn, and the depth of occurrence of instability is relatively shallow. In addition, instability has a significant vertical distribution. At the depth of 40 m, the most frequent instability occurred, and in the region above and below the thermocline, instability rarely occurs. The physical mechanism of these spatio-temporal variation characteristics needs to be further studied.