海洋环流与波动重点实验室知识库

      热带西太平洋有着极其复杂的三维海洋环流系统，北赤道流作为其中热带和副热带环流的边界以及低纬度西边界流的源头，在西太平洋经向（子午向）热盐交换、水团输运和能量收支平衡上起着关键作用，对区域和全球气候也具有重要影响。因此，准确认识北赤道流以及位于其下方的北赤道潜流的变化规律对深入理解热带西太平洋多尺度变化动力过程具有重要意义。

     “西北太平洋海洋环流与气候实验（NPOCE）”在130°E断面上布放了5套深海锚系潜标，获取了2014年至2021年间长时间序列的北赤道流和北赤道潜流的海流数据。本文基于这5套潜标观测数据、卫星高度计和数值模式等数据，对北赤道流和北赤道潜流的年际变化进行了研究，发现130°E断面上海流的年际变化存在经向的相位滞后以及垂直结构差异，并通过1.5层线性约化重力模式和线性连续层化模式进行了解释，进而讨论了北赤道流和北赤道潜流年际变化与厄尔尼诺-南方涛动事件之间的关系。

      基于上900米的潜标连续观测，本文首先探究了不同纬度上北赤道流和北赤道潜流的基本变化特征。结果显示，向西流动的北赤道流在观测期间稳定地存在于8°N至18°N之间，向东流动的北赤道潜流具有多核结构，在大部分纬度上间断性出现。北赤道流和北赤道潜流上900米的纬向流速在年际时间尺度上有显著的变化。当下层北赤道潜流减弱时，上层的北赤道流增强。以8.5°N为例，北赤道潜流的分支在厄尔尼诺的成熟期增强，在厄尔尼诺的衰退期达到最大速度，局地生成的信号和远程强迫产生的西传信号对130°E断面上的北赤道潜流年际变化的调节都很重要。而该纬度上的北赤道流的纬向流速在厄尔尼诺年减弱，主要是受北赤道流的流轴摆动的影响。基于多个海洋模式数据的流量诊断分析表明，厄尔尼诺事件期间北赤道流的总流量增强，流轴往北迁移，导致8.5°N北赤道流纬向流速减弱；北赤道潜流的南部急流在厄尔尼诺年增强，中部急流滞后厄尔尼诺事件一年左右增强，北部急流在厄尔尼诺年减弱，拉尼娜年则相反。

       进一步，潜标ADCP观测发现，130°E断面上海流纬向流速的年际变化并不是一致的，而是随纬度升高存在相位滞后，15°N处的年际变化信号滞后于8.5°N处的一年左右。该相位滞后的现象同样被卫星高度计捕捉到，并反映在海表面高度异常变化上。基于1.5层线性约化重力模式的研究结果显示，中太平洋的风场对130°E断面上海流年际变化的贡献约为55%，表明该年际变化主要受中太平洋风应力强迫产生的向西传播的斜压罗斯贝波的控制，而斜压罗斯贝波在不同纬度不同的传播速度解释了该年际信号的经向相位滞后。

       潜标观测还发现，130°E断面上的北赤道流和北赤道潜流在年际时间尺度上垂直结构的变化存在经向差异。纬向流速的年际变化在8.5°N处表现出次表层强化的特征，最强信号出现在300-500米之间，在17.5°N处表现出表层强化的特征。通过经验正交函数分解和垂直模态分解方法对北赤道流和北赤道潜流垂直结构的年际变化特征进行诊断分析，结果表明，北部的北赤道流和北赤道潜流的纬向流速垂直结构的年际变化以受第一斜压模主导的表层强化信号为主，而南部则以受第一和第二斜压模共同作用的次表层强化信号为主。基于线性连续层化模式的研究结果表明，第二斜压模在北赤道流和北赤道潜流南部海区被风应力强迫所激发的程度与第一斜压模相当，而在北部海区被风应力强迫所激发的程度明显小于第一斜压模。斜压模态对大尺度风场低频信号的不同响应解释了潜标观测到的北赤道流和北赤道潜流速度垂向结构的年际变化的经向差异。

    The western tropical Pacific Ocean has a complex three-dimensional ocean circulation system. The North Equatorial Current (NEC), as the boundary of the tropical and subtropical gyres and the origin of the low latitudes western boundary currents, play crucial roles in meridional heat and salt exchange, water mass transport and energy budget balance in the western tropical Pacific Ocean, and have important impacts on regional and global climate. An accurate understanding of the variations of NEC and North Equatorial Undercurrent (NEUC) is of particular significance for deepening our understanding of the multi-scale variations of the ocean dynamic process in the western Pacific Ocean. 

  "Northwestern Pacific Ocean Circulation and Climate Experiment (NPOCE)" program had deployed five subsurface mooring arrays along 130°E. The long time series of velocity of the NEC and NEUC were obtained during 2014-2021. In this paper, based on the five subsurface mooring observations, combined with satellite altimetry measurements and numerical models, the interannual variations of the NEC and NEUC were investigated. The interannual phase-lagged phenomenon of the current and the meridional difference of vertical structure between the interannual variations of the zonal velocity along 130°E were observed by mooring measurements and explained by a 1.5 layer linear reduced gravity model and a linear continuously stratified ocean circulation model, respectively. Further, the relationship of interannual variations of the NEC and NEUC with El Niño-Southern Oscillation events was discussed.

    Based on the continuous mooring ADCP observations in the upper 900 m, the basic characteristics of the variations of NEC and NEUC at different latitudes were firstly investigated in this study. The results show that the westward-flowing NEC exists stably between 8°N-18°N during the whole observation period, while the eastward-flowing NEUC with a multi-core structure appears intermittently at most of the latitudes. The zonal velocity of NEC and NEUC in the upper 900 m exhibits significant variations on the interannual time scale. The NEC strengthens when the underlying NEUC weakens, and the NEUC branch at 8.5°N is intensified during the mature phase of El Niño and reaches the maximum velocity during the decay phase of El Niño. Both the locally generated and westward propagating signals induced by remote forcing are important for the interannual modulation of NEUC at 130°E. The zonal velocity of NEC branch at 8.5°N weakens, which is mainly attributed to the impact of the NEC migration.The diagnostic analysis based on multiple ocean models show that the NEC transport increases in El Niño, and the axis of NEC migrates to the north, which induces the decrease of NEC velocity at 8.5°N. While the transport of the southern jet of the NEUC increases in El Niño, the enhancement of the middle jet of the NEUC lags the El Niño event by about one year, and the northern jet of the NEUC decreases in El Niño, but the interannual phase is almost reversed in La Niña.

    Further, mooring ADCP observations show that the phase of the interannual variations of zonal velocity of currents along 130°E is not consistent, but delays with the increasing latitudes, with the signal at 15°N lagging that at 8.5°N by about one year. The phase-lagged features are also captured by satellite altimetry measurements and reflected in the variations of the sea surface height anomalies. Based on a 1.5 layer linear reduced gravity model, the wind forcing in the central Pacific Ocean was computed to explain 55% of the total interannual variations of the currents at 130°E, indicating that the interannual variation is controlled mainly by the westward propagating baroclinic Rossby wave induced by the wind stress curl forcing in the central Pacific Ocean. Different propagating speed of the baroclinic Rossby wave at different latitudes explains the meridional phase lag of the interannual signal.

    In addition, the meridional difference of vertical structure of NEC and NEUC along 130°E on the interannual time scale was also observed by the mooring measurements. The interannual variations of zonal velocity exhibit subsurface-intensified features at 8.5°N with the strongest signal appearing between 300-500 m, and the signal is surface-intensified at 17.5°N. Utilizing empirical orthogonal function and vertical mode decomposition analysis, the vertical structure of interannual features of NEC and NEUC was diagnosed. It is suggested that the interannual variations of NEC and NEUC velocity in the northern part are dominated by surface-intensified signals with a vertical structure of the first baroclinic mode, while that in the southern part is dominated by subsurface-intensified signals which are associated with the combination of the first two baroclinic modes. Based on a linear continuously stratified model, the excitation of second baroclinic mode in response to wind stress forcing is comparable to that of the first baroclinic mode in the southern part of NEC and NEUC, but is obviously smaller than the first baroclinic mode in the northern part. The low-order mode baroclinic response of the ocean to the wind forcing accounts for the meridional difference of vertical structure of the interannual fluctuation of NEC and NEUC velocity observed by mooring measurements.

                                                  目  录                                                         

第 1 章 绪论.........................................................................................................................1

    1.1 研究背景及意义......................................................................................................1

    1.2 研究现状...................................................................................................................2

        1.2.1 热带西太平洋环流特征................................................................................2

        1.2.2 北赤道流..........................................................................................................5

        1.2.3 北赤道潜流......................................................................................................8

    1.3 本文的研究内容及章节安排...............................................................................11

第 2 章 资料和方法介绍..................................................................................................13

    2.1 数据简介..................................................................................................................13

        2.1.1 潜标观测数据................................................................................................13

        2.1.2 卫星高度计数据............................................................................................15

        2.1.3 风场数据.........................................................................................................15

        2.1.4 Argo 数据......................................................................................................16

        2.1.5 WOA18 数据集...........................................................................................16

        2.1.6 Niño3.4 指数...............................................................................................16

        2.1.7 其他数据........................................................................................................17

    2.2 研究方法.................................................................................................................18

        2.2.1 低通滤波........................................................................................................18

        2.2.2 相关性分析....................................................................................................18

        2.2.3 1.5 层线性约化重力模式...........................................................................19

        2.2.4 经验正交函数分解.......................................................................................20

        2.2.5 垂直模态分解................................................................................................21

        2.2.6 线性连续层化模式.......................................................................................22

第 3 章 北赤道流/潜流的年际变化特征......................................................................25

    3.1 潜标观测的平均结构............................................................................................25

    3.2 潜标观测的季节内和季节变化..........................................................................30

    3.3 潜标观测的年际变化...........................................................................................34

    3.4 小结.........................................................................................................................37

第 4 章 年际变化的经向滞后机制................................................................................38

    4.1 高度计数据验证....................................................................................................38

    4.2 1.5 层线性约化重力模式分析...........................................................................40

    4.3 不同区域风场的贡献...........................................................................................44

    4.4 小结.........................................................................................................................48

第 5 章 北赤道流/潜流垂向结构年际变化的经向差异分析..................................50

    5.1 垂向结构诊断分析...............................................................................................50

    5.2 动力学机制分析...................................................................................................53

    5.3 小结........................................................................................................................63

第 6 章 北赤道流/潜流的年际变化与 ENSO 的关系及相关机制......................64

    6.1 潜标观测的年际变化与 ENSO 的关系.........................................................64

    6.2 北赤道流的流速、流量和流轴变化之间的关系及相关机制分析...........65

    6.3 北赤道潜流的年际变化机制............................................................................72

    6.4 基于多模式诊断的北赤道潜流的年际变化..................................................73

    6.5 小结......................................................................................................................79

第 7 章 总结与展望.......................................................................................................81

    7.1 本文主要结论.....................................................................................................81

    7.2 本文创新点.........................................................................................................83

    7.3 对未来工作的展望............................................................................................83

参考文献......................................................................................................................... 85

致谢..................................................................................................................................95

作者简历及攻读学位期间发表的学术论文与其他相关学术成果.....................97