海洋环流与波动重点实验室知识库

太平洋北赤道逆流(NECC)是热带太平洋赤道流系中的一支重要海流，它将西太平洋暖池水向东输运到东太平洋，在厄尔尼诺南方涛动（ENSO）的发生、发展中起着重要作用。以往研究多关注了大洋内区NECC的结构特征及变异规律，而在NECC生成地因观测所限，人们对其三维结构及其变异机理的认知还非常有限。同时，由于NECC源区靠近西边界区域，是南、北太平洋水团交汇地，具有丰富的渔业资源，也是印尼贯穿流水源地，了解其三维结构及多尺度变异特征与机理对于认识局地海洋环流与水团变性及海气相互作用、生物生态效应，乃至全球海洋环流与气候变化都具有重要意义。

本文基于近几年在西太获取的现场观测资料，结合卫星遥感数据、再分析数据及高分辩率模式数据，对源区NECC不同时间尺度的变化特征及机制进行了系统的分析，重点揭示了源区NECC在强厄尔尼诺事件中的变异特征及机制，并探讨了其季节变异规律以及对区域海洋初级生产力的影响。

本文首先发展了一种三维的地转经验模态方法（MGEM），此方法引入月份作为额外参数，用于从压力逆式回声仪（PIES）观测估算赤道西太平洋不同纬度带的次表层温盐结构。与传统的GEM方法相比，MGEM可以捕捉到海洋内部更高比例的温盐变化。在500-1900 dbar之间的深海中，GEM方法只能捕捉到12%的温盐变化，而MGEM方法可以捕捉到44%的温盐变化。对于500 dbar以上的海洋， GEM方法只能捕捉62%的温盐变化，MGEM方法能捕捉到77%的温盐变化。基于MGEM方法，由2014年9月至2016年10月布放在沿130°E断面，4°N、6°N和8°N的3台PIES获取的观测数据，估算了连续25个月的源区NECC区域纬向体积输运时间序列。

基于长时序的PIES观测资料和现场水文观测，结合高度计数据、Argo数据和再分析数据，研究了2014-2016厄尔尼诺事件期间西太平洋源区NECC的结构和变化机制。研究表明，源区NECC在厄尔尼诺事件发展阶段增强，在拉尼娜事件发展阶段减弱。在2015/16年厄尔尼诺事件的发展阶段，NECC向南移动约1度，强度显著加强，输运超过40 Sv（1Sv=10⁶m³/s），几乎是气候态数值的两倍。通过与西太平洋137ºE断面历史观测进行对比分析发现，2015/16年的厄尔尼诺事件对源区NECC的影响与1997/98年超强厄尔尼诺事件相当，甚至完全掩盖了NECC的季节变异。基于2.5层约化重力模型，我们进一步分析发现，斜压不稳定是2015/16年厄尔尼诺事件期间源区NECC区域涡动能（EKE）的重要能量来源。在厄尔尼诺发展阶段，NECC与北赤道次流（NESC）之间的垂直切变增强和密度梯度减小，使得NECC-NESC系统在西太平洋处于斜压不稳定状态。这里的涡旋-平均流相互作用与厄尔尼诺南方涛动（ENSO）的不同模态有关。本项工作基于观测通过理论分析证明了源区NECC区域的斜压不稳定是2015/16超强厄尔尼诺期间中尺度涡动能的重要能量来源，改变了低纬海洋是由正压不稳定主导的传统认知。

接着，本文根据PIES观测结合卫星高度计数据，重构了源区NECC的季节循环。在其主轴区域，2月份输运最强，约为23 Sv，9月份输运最弱，约为11 Sv。与以往所认为的源区NECC季节变化不显著不同，本研究表明源区NECC具有较强的季节变化，由区域温跃层深度及其经向梯度的变化通过地转平衡控制。另外，受棉兰老涡（ME）和哈马黑拉涡（HE）影响，源区NECC流轴的南北位置和强度变化并不完全一致。涡旋变化的复杂特征以及温跃层深度、经向梯度变化的不一致性也导致源区NECC具有半年周期的变化特征。

最后，在上述研究基础上，本文对ENSO事件中源区NECC变化对区域海洋初级生产力的影响进行了初步探讨。研究发现，厄尔尼诺事件中热带西太平洋区域平流输运变化改变了区域水体分布，源区NECC的平流效应在区域海表面叶绿素的年际变化中扮演着重要角色。

The North Equatorial countercurrent (NECC) in the Pacific Ocean is an important current in the equatorial current system of the tropical Pacific. It transports water from the western Pacific warm pool eastward to the eastern Pacific and plays an important role in the occurrence and development of the El Niño Southern Oscillation (ENSO). Previous studies have paid more attention to the structural characteristics and variation of NECC in the inner region, but due to the limitation of observation, the understanding of its three-dimensional structure and its variation mechanism is still very limited. As the nascent NECC is close to the western boundary and is the confluence of water masses in the South and North Pacific Ocean, it is rich in fishery resources, and it is also the source of Indonesian Throughflow (ITF). Understanding its three-dimensional structure and multi-scale variation characteristics and mechanism is of great significance for understanding local ocean circulation and water mass variability, air-sea interaction, bio-ecological effects, and even global ocean circulation and climate change.

Based on the field observation data obtained in the Western Pacific in recent years, combined with satellite remote sensing data, reanalysis data, and high-resolution model data, this paper makes a systematic analysis on the variation and mechanism of nascent NECC in different time scales, focuses on the variation characteristics and energy conversion mechanism of NECC during strong El Niño events, its seasonal variation, and the impact of nascent NECC on regional marine primary productivity on the scale of interannual variation.

At first, a spatially varying non-stationary gravest empirical mode (GEM) field is constructed by incorporating the calendar month as an extra parameter to infer the subsurface thermohaline structure from the Pressure-sensor equipped Inverted Echo Sounder (PIES) observations in the western equatorial Pacific Ocean. This monthly GEM (MGEM) overcomes the application limitation of traditional GEM in the equatorial ocean due to the poor vertical coherence thermohaline structure there. The MGEM can capture 77% of the thermohaline variance for the upper 500 dbar, which is only 62% by the traditional GEM method. The most significant improvement of MGEM is for the deep ocean between 500-1900 dbar, where only 12% of the thermohaline variance can be captured by the GEM, but improved to 44% by the MGEM method. Based on the MGEM, a 25-month volume transport time series is estimated from observations by three PIESs deployed across the nascent NECC at 4ºN, 6ºN, and 8ºN along the 130ºE transect during 2014-2016.

Based on long-term PIES observation data and hydrological observations, combined with altimeter data, Argo data, and reanalysis data, we studied the structure and changes of nascent NECC of the Western Pacific from 2014 to 2016. The nascent NECC is strengthened during the developing phase of El Niños, and is weakened during La Niñas. The NECC shifts ~1 degree southward and intensifies significantly with its transport exceeding 40 Sv (1 Sv = 10⁶m³/s), nearly double its climatology value, during the developing phase of the 2015/16 El Niño event. A comparative analysis with the historical observations of the 137ºE section of the Western Pacific revealed that the 2015/16 El Niño exerts a much stronger impact on the nascent NECC compared to that of the 1997/98 event, which even obscures the NECC’s seasonality. Based on a 2.5-layer reduced gravity model, we further analyzed and found that baroclinic instability was an important energy source for eddy kinetic energy (EKE) in the nascent NECC region during the 2015/16 El Niño event. The enhanced vertical shear and the reduced density jump between the NECC layer and the subsurface North Equatorial Subsurface Current (NESC) layer renders the NECC–NESC system to be baroclinically unstable in the western Pacific Ocean during El Niño developing phase. During the 2015/16 El Niño, the NECC is a baroclinically unstable jet at its birthplace, and the baroclinic instability provides the main energy source for the mesoscale eddy field, which differs from the traditional understanding of the energy source of EKE in low latitudes ocean.

Next, this paper reconstructs the seasonal cycle of nascent NECC based on PIES observations combined with satellite altimeter data. The nascent NECC between 4ºN-6ºN was found to have quite strong seasonal variations with the strongest transport of 23 Sv occurring in February and the weakest transport of 11 Sv occurring in September. Under the influence of Mindanao eddy (ME) and Hamahera eddy (HE), the north-south position and intensity of the NECC flow axis in the source region are not completely consistent. The half-yearly periodic variation of nascent NECC is caused by the inconsistency between the complex characteristics of eddy variation and the variation of thermocline depth and thermocline depth gradient. The change of the depth of the regional thermocline and its meridional gradient controls the change of NECC through the adjustment of geostrophic balance.

 Finally, on the basis of the above research, this paper makes a preliminary discussion on the impact of NECC on regional marine primary productivity on the scale of interannual variation. It is found that the change of advection transport in the tropical western Pacific during the El Niño events changes the regional water mass distribution, and the advection effect of NECC plays an important role in the interannual variation of sea surface chlorophyll.