海洋环流与波动重点实验室知识库

太平洋北赤道流（NEC）处于全球最强的年际异常信号—厄尔尼诺与南方涛动（ENSO）事件的发源地，大气强迫过程对该区域上层海洋环流变化具有显著影响。同时，由于该海域异常活跃的海-气相互作用，大尺度海洋环流的变化也会对大气环流和一些气候事件产生重要的影响。NEC连接着太平洋热带及副热带环流，它在到达太平洋西边界时会发生重要的分叉现象，直接决定了其携带的水体在热带和副热带环流系统中的质量、热量和营养盐的分配，在区域以及全球海洋环流中发挥着至关重要的调控作用。因此，了解NEC及其分叉点的多尺度时空变化对理解大洋环流动力学机制，探究ENSO等重要气候事件的海气耦合机制具有重要意义。以往对于NEC的研究多集中在西太平洋海域，但鉴于NEC是一支横跨太平洋海盆，流幅超过10个纬度的超大海流，了解它在太平洋整个海盆尺度的结构和变化特征，对于我们更全面理解其动力机制及与气候变化的关系具有重要意义。以往缺乏海盆尺度的同步观测数据，不具备解决该问题的观测基础。而今，全球海洋观测计划（Argo）的实施，开创了一个海洋观测新时代，为更真实地刻画太平洋海盆尺度NEC的时空变化特征、探究其动力学机制提供了重要的数据支撑。

因此，本文基于Argo观测数据，结合卫星高度计数据、再分析数据以及HYCOM模式数据，对NEC及其分叉点的多尺度时空变化及动力机制进行了较系统的研究，重点探讨了三个问题：1.整个太平洋海盆中NEC的季节及年际变化规律及动力机制是什么？2.太平洋年代际涛动（PDO）能否对NEC年际变化产生调制？3.NEC分叉点的季节内振荡规律及潜在的影响因素是什么？主要成果如下：

揭示了Sverdrup理论估算的NEC热带和副热带环流分支的流量与观测存在较大差异。Sverdrup理论可以较好地模拟气候态NEC的空间结构，但高估了NEC流轴附近的输送。不同风场数据估算的Sverdrup输送在NEC主轴南、北两侧，即热带和副热带环流分支与观测值存在巨大差异，表明风场误差会对强迫模式中北太平洋热带-副热带环流水体、能量分配产生显著影响。

基于Argo数据刻画了NEC在整个太平洋海盆中的季节变化特征及其机理。在季节尺度上，以145°E及145°W为界，NEC的季节变化在太平洋西、中、东三个海盆中截然不同。在西部太平洋中，NEC在上半年增强并南移，下半年减弱并北移；在中部太平洋中，北赤道流的输送异常与西太相反，且流轴存在明显的西传信号；在东部太平洋，北赤道流的流轴及强度季节异常的对应关系与西太相反，即北赤道流增强对应着流轴北移，而减弱对应着南移。这一差异可以归结于北赤道流海域温跃层季节异常极值的位置在三个海域中存在差异，更加靠北的极值位置导致西太平洋经向梯度的变化与中东部相反，从而使北赤道流的变化相反。进一步利用线性第一斜压罗斯贝波模型对不同海域的北赤道流流域Rossby波传播和Ekman抽吸控制试验揭示了西部及东部太平洋NEC主要受局地风的Ekman抽吸作用，而中部太平洋受局地Ekman抽吸及Rossby波共同控制。这项工作从观测角度揭示了NEC季节变化在不同海域的显著差异，并指出前人所提出的NEC强度季节变化与该区域温跃层深度变化的关系不适用于西太平洋，从而进一步完善了对太平洋NEC季节变化的认知。

在年际尺度上，揭示了太平洋海盆中NEC对ENSO事件的响应有着复杂的空间结构。ENSO事件期间，NEC在日界线两侧以及南北海域的变化均不一致，直接原因归结于海面高度(SSH)经向梯度的正/负异常的增强和向北扩张。另外，Rossby波的调节在西太中高纬度海域及东部太平洋有更重要的作用，而在低纬太平洋，局地风引起的Ekman抽吸和远场风激发的Rossby波在太平洋年际变化中的调节作用中都非常重要。并指出PDO不同位相对NEC的年际变化具有显著调制作用。PDO冷位相与ENSO事件相似的结构增强了西太平洋ENSO事件的影响，而PDO暖位相与ENSO事件相反的结构则减弱了西太平洋ENSO事件的影响。同时，PDO暖/冷位相中海气相互作用增强了EP/ CP型ENSO事件，导致PDO冷位相中NEC输运和流轴对ENSO事件的响应与对CP型ENSO事件的响应相似，暖位相中则与CP型ENSO事件相似。PDO对NEC显著的年际调节也提示我们在研究年际变化时应考虑PDO的位相，从而避免出现偏差。

揭示了NEC分叉的季节内变化特征与机理，填补了对该问题研究的空白。利用卫星高度计数据和Argo数据，对于NEC分叉的季节内变化的研究表明，其周期主要集中于50-140天，影响其季节内变化的主要因素为Madden-Julian振荡（MJO）以及中尺度涡。分析表明，NEC分叉季节内变化与其所在区域附近的海表高度异常表现为负相关关系，即海表高度异常升高时，NEC分叉会偏南，反之亦然。MJO主要通过作用于分叉海域的SSH来控制NEC分叉的季节内变化。另外，统计分析显示，中尺度涡旋对NEC分叉的季节内变化主要表现为气旋涡/反气旋涡会使分叉表现出北/南移的趋势。同时，中尺度涡旋对NEC季节内变化的影响还受其年际信号的调制：在厄尔尼诺年，该海域垂直和水平剪切的增强能够导致活跃的涡旋活动，进而对分叉点的移动产生更显著的影响。本文还通过一个被气旋式涡旋捕获的Argo浮标观测分析证明气旋式涡旋造成分叉点显著北向移动的事实，并指出中尺度涡旋对于NEC分叉点的移动在季节内以及更长时间尺度上都有不可忽视的贡献。同时，通过比较分析发现1997-98 和2015-16两次超强厄尔尼诺事件期间，NEC分叉附近涡动能（EKE）强度显示出非常巨大的差异，且2015-16超强厄尔尼诺期间该区域垂向和水平剪切明显强于1997-98事件，表明不同类型的厄尔尼诺事件对NEC分叉海域EKE具有显著影响。

The Pacific North Equatorial Current (NEC) in the Pacific Ocean is the birthplace of El Niño-Southern Oscillation (ENSO) events, which is the strongest interannual signal in the world. And the atmospheric forcing has a significant effect on the variation of upper ocean circulation in this region. Similarly, the space-time variation of large-scale currents also plays an important role in atmospheric circulation and some climate events through sea-air interaction. The NEC connects the Pacific tropical and subtropical gyres and has an important bifurcation process when it reaches the western boundary of the Pacific Ocean, which directly determines the mass, heat and nutrient distribution in the tropical and subtropical gyres and plays a crucial role in regional and global ocean circulation. The multi-scale space-time variation of NEC and its bifurcation process is of great significance for understanding the dynamics of ocean circulation and exploring the air-sea coupling mechanism of ENSO and other important climate events. Previous studies on NEC mainly focused on the western Pacific Ocean. However, given that NEC is a large current across the Pacific basin with a current amplitude of more than 10 latitudes, it is of great significance to understand its structure and variation characteristics at the scale of the entire Pacific basin for a more comprehensive understanding of its dynamic mechanism and its relationship with climate change. In the past, there was no observation basis to solve these problems due to the lack of synchronous observation data of ocean basin scale. Recently, the Global Ocean Observation Program (Argo) has ushered in a new era of ocean observation, providing important data support for describing the space-time variations of NEC at the Pacific Ocean basin scale and exploring its dynamics mechanism.

Therefore, based on Argo data, satellite altimeter data, reanalysis data and HYCOM model data, the multi-scale space-time variation and dynamic mechanism of NEC and its bifurcation are systematically studied in this paper. Three questions are mainly discussed: 1. Seasonal and interannual variations of NEC and its dynamic mechanism in the whole Pacific Ocean; 2. Modulation of the Pacific decadal Oscillation (PDO) on interannual variation of NEC; 3. Intraseasonal variability of the NEC bifurcation and potential influencing factors. Main conclusions are as follows:

Studies reveal that the tropical and subtropical branches of the NEC estimated by Sverdrup theory is quite different from the observation. Sverdrup theory can well describe the spatial structure of NEC but overestimate the transport near the current axis. The Sverdrup transport estimated from different wind data is significantly different from the observation on the north and south sides of the current axis, namely the tropical and subtropical branches, indicating that the wind field error in the forced model has a significant impact on the mass and energy distribution between the tropical and subtropical gyre in the Pacific Ocean.

Based on Argo data, the seasonal variation characteristics and mechanism of NEC in the entire Pacific Ocean are revealed. At the seasonal scale, the NEC variation shows distinct modes in the Pacific western, interior and eastern basins bounded by 145°E and 145°W. In the western Pacific Ocean, the NEC shifts southward/northward when it is intensified/weakened in the first/latter half of the year. In the interior Pacific Ocean, the transport shows antiphase fluctuations with that in the western Pacific Ocean and with obvious westward propagations of the meridional shift of its axis. In the eastern Pacific Ocean, the phase relationship between the axis and transport is opposite to that in the western Pacific Ocean with the strengthening /weakening of the NEC being accompanied by a northward/southward shift of its axis. Distinct seasonal variations are due to the different locations of the NEC seasonal thermocline departure maxima/minima in these three basins. More northerly locations cause the change of the meridional gradient in the western Pacific to be opposite to that in the east-central Pacific and thus causes the opposite seasonal variations of the NEC. Further, 1.5-layer linear Rossby model experiments by blocking the baroclinic Rossby wave propagations and Ekman pumping in different basins reveal the dynamic mechanism of NEC seasonal variation in different basins. Model experiments by blocking the baroclinic Rossby wave propagations and Ekman pumping in different basins suggest that the seasonal variations of the NEC in the western Pacific Ocean and eastern Pacific Ocean are mainly controlled by the Ekman pumping, while those in the interior Pacific Ocean are controlled by the joint effect of the local Ekman pumping and the Rossby waves. Based on the observations, this study reveals significant differences of NEC seasonal variation in different basins. It is pointed out that the relationship between the seasonal variation of NEC intensity and the variation of thermocline depth proposed by predecessors is not applicable to the western Pacific Ocean, which further improves the cognition of the seasonal variation of NEC in the Pacific Ocean.

On the interannual scale, it reveals that the response of NEC to ENSO events in the Pacific Ocean has a complex spatial structure. During ENSO events, the changes of NEC on west/east side of the dateline and the north/south side of the current axis are inconsistent, which is directly attributed to the enhancement and northward movement of the positive / negative anomaly of the meridional gradient of sea surface height (SSH). Moreover, the Rossby waves plays a more important role in the mid-high latitudes of the western Pacific Ocean and the eastern Pacific Ocean, while in the low latitudes of the Pacific Ocean, both the local wind-induced Ekman pumping and the far wind-induced Rossby waves play an important role in the interannual variability of the Pacific.

Our study also points that the interannual variation of NEC is significantly modulated by PDO. The similar structure of PDO with ENSO events in the PDO cold phase further strengthens the influence of ENSO events, while the opposite structure of PDO during the warm phase weakens the influence of ENSO event in the western Pacific Ocean. Furthermore, air-sea interaction in the warm/cold phases of PDO enhances the modes of EP/ CP ENSO events. The evolutions of the NEC transport and current axis in the ENSOs during the PDO cold phase exhibit similar features with that in the CP ENSOs and the same relationship also found between the PDO warm phase and EP ENSOs. The significant interannual modulation of the PDO also suggests that the two phases of PDO should be covered to avoid the divergence when the interannual variation is studied.

The characteristics and mechanism of intra-seasonal variation of NEC bifurcation latitude are revealed, filling in the blank of this research. Based on the satellite altimeter data and Argo data, studies show that its period mainly concentrates in around 50–140 days, and both the Madden-Julian oscillation and mesoscale eddies have a significant impact. Analysis shows that, the relationship between the NEC bifurcation latitude and sea surface height anomalies near the bifurcation area is negative, that is, negative anomalies correspond to NEC bifurcation latitude moving northward, and vice versa. And MJO mainly controls the intraseasonal variability of bifurcation latitude through sea surface height. Statistical analysis shows that, the cyclonic/anticyclonic eddy causes the northward/southward movement of the bifurcation latitude. Meanwhile, the influence of mesoscale eddies on the intraseasonal variability of bifurcation latitude is highly modulated by its interannual variations. During the El Niño years, enhanced vertical and horizontal shear induces active eddy activities and further have a large influence on the movement of bifurcation latitude. Observations of Argo float captured by a cyclonic eddy show that cyclonic eddy can cause the bifurcation shifting northward and indicate that mesoscale eddies play a significant role in the movement of NEC bifurcation in both seasonal and longer time scales. Moreover, comparative analysis also shows that the eddy kinetic energy around the bifurcation shows quite diverse characters for the 1997–1998 and the 2015–2016 extreme El Niño events, with the latter have much stronger vertical and horizontal shears. The results indicate that different types of El Nino events have significant influence on EKE in NEC bifurcation area.