中国科学院海洋研究所机构知识库

大洋深层环流，特别是经向翻转环流，在全球气候系统中扮演着重要角色，它在全球经向和垂向热量再分配上起到重要作用。相比于观测资料丰富的上层环流，深层环流因观测手段的限制，我们对其认知仍十分有限。近些年，中国科学院成功构建了西太平洋科学观测网，以潜标阵列为主对全水层水文动力过程开展了长期连续观测，为该海域深层环流的研究打开了一个突破口。本论文主要利用潜标观测数据，并结合航次大面观测数据、卫星观测数据、高分辨率海洋模式数据等，对西太平洋，主要是雅浦-马里亚纳海沟连接区的深层经向翻转环流的结构特征和变异规律进行分析和探讨。相比于深层经向翻转环流的大西洋分支，以往大家对其太平洋分支的研究不多，但北太平洋拥有全球最大的海域面积，具有全球最大的热量和碳存储能力，在全球变暖越来越多的热量向深海转移的背景下，太平洋深层海洋越来越得到大家的重视和关注。

本论文首先基于六套全球高分辨率海洋模式资料初步分析了热带西太平洋深层环流的基本特征。研究发现西太平洋深层环流于1000-3000米水层以东西向交替变化的纬向射流为主，而3000米以深因为地形的阻挡形成若干独立的海盆，深层环流以海盆内环流为主，但不同海盆之间可通过关键深水通道进行海水交换。这些关键深水通道的输运通量存在显著的季节变化，甚至不同季节流向也不一致，而海盆内的环流形态受制于位涡收支积分约束，进而也表征出季节变化特征，即不同季节海盆内的环流主要旋转形态不同。模型结果的分析为随后的西太平洋深层环流研究奠定了基础。

雅浦-马里亚纳连接处（YMJ）深水通道是深层经向翻转环流进入西太平洋的关键深水通道，我们在这一区域开展了密集的观测研究。首先，我们利用位于YMJ深水通道的东西两侧2017年和西侧1997年的潜标观测结果，对该局地区域深层经向翻转环流上、下两个分支（Lower and Upper branches of the Pacific Meridional Overturning Circulation，分别简称为L-PMOC和U-PMOC）的结构和变化规律进行了探究。研究发现L-PMOC和U-PMOC主要以季节变化为主。具体来讲，在YMJ通道西侧3800米以深北向的L-PMOC存在两个显著季节位相：从每年12月至次年5月，流速较强，在4200米深度上其量值可达45 cm s^-1；从6月至11月，流速较弱。在YMJ通道东侧4000米以深为南向的L-PMOC回流，其具有和L-PMOC一样的季节位相但强度相对较弱，而在较浅的3000-3800米层，南向的U-PMOC也具有显著的季节变化，但其位相与L-PMOC相反，即从每年12月至次年5月，流速较弱，从6月至11月，流速较强。结合同步观测的温盐和溶解氧数据可分析得到，该通道L-PMOC向北输运下层绕极深层水（Lower Circumpolar Water，以下简称LCPW），U-MPOC则向南输运上层绕极深层水（Upper Circumpolar Water，以下简称UCPW），L-PMOC和U-PMOC的季节变化分别伴随着LCPW和UCPW的季节性入侵。借助于大面观测、数值模式结果等，我们发现LCPW和UCPW的季节性入侵导致通道内密度结构发生变化，通过地转调整形成了L-PMOC和U-PMOC反位相的季节变化特征、L-PMOC的东侧回流结构、U-PMOC的南向输运等。

除显著的季节变化外，基于YMJ深水通道的潜标观测数据，我们进一步发现深层经向翻转环流还存在着显著的30-90天季节内变化特征，而且季节内变化强度随深度的增加而增加。在4200米深度上观测到的等温线垂向振幅可达到600米。在季节内时间尺度上，我们分析发现深层流速和温度变化振幅的垂向结构特征符合地形罗斯贝波（Topographic Rossby Waves, 以下简称TRW）的特性，即振幅随深度的增加而增强且垂向位相一致，这表明季节内深层强化现象是由TRW引起的。结合能有效模拟TRW现象的模式数据，我们探讨了YMJ处TRW的产生机制，主要包括两个过程，一是表层的强涡旋通过位涡调整能够产生TRW，二是大尺度平均流的正压和斜压不稳定可以对TRW的产生做出贡献。由于深水通道具有丰富的地形坡度且较大的流速剪切，因此更易产生TRW，而TRW能够将深层变异信号较为快速的向远处传播，进而可能影响着深层大洋或海盆之间的物质能量交换。

深层经向翻转环流在YMJ深水通道的季节和季节内变化特征得到了1990和2010年代间隔20年两批观测数据的共同证实。我们进一步基于这两批观测，讨论了深层经向翻转环流的年代际变化。通过对比通道西侧深度积分的流量估计，表明L-PMOC流量约减小了20%，和南太平洋萨摩亚通道27%的减小幅度基本相当，而其输运的LCPW温度变化范围没能超过1990年代观测设备的准确度（±0.03°C），说明北半球西太平洋LCPW没有显著的温度变化，和萨摩亚通道深层水显著变暖的情况不同。

除在YMJ区域外，我们还在其东北部的麦哲伦海山区对PMOC进行了大面投放式温盐和海流观测研究，讨论了L-PMOC进入YMJ区域前的路径。麦哲伦海山区主要包括两种深层水团，分别是位于3800米以深的LCPW和位于1800-3800米的UCPW。大面海流直接观测结果表明该海域1800米以深的深层流基本为南向流，同时麦哲伦海山区3800米以深溶解氧浓度比YMJ深水通道处的高，因此我们认为L-PMOC会先向北到达麦哲伦海山区而后向西南回传进入YMJ区域。另外，我们还对麦哲伦海山区域的混合特征做了大致估算，发现近海山站位的混合在1500米以深显著增强，湍扩散系数的量级可达10^-4，比远离海山站位大1-2个量级。

The meridional overtuning circulation plays an important role in global climate system as vast amounts of heat are redistributed meridionally and vertically. In contrast to upper ocean circluations, measurements in deep ocean are still sparse and synoptic, and hence there remains large gaps in our knowledge of deep ocean circulations. Recently, Chinese Academy of Sciences has built a scientific observing network in the western Pacific, comprising several subsurface moorings to continuously measure the full-depth ocean parameters. These data provide us a good opportunity to study the deep ocean circulations in this region. In this study, using the mooring observation data, observed conductive-temperature-depth (CTD) - lowered acoustic Doppler current profiler (LADCP) data, satellite data, and multiple sets of global ocean model outputs, we aim to study the structure and variability of Pacific Meridional Overturning Circulation (PMOC). The PMOC is less observed compared with the main limb of the global meridional overturning circulation in the Atlantic. However, the North Pacific possesses a long residence time and enormous volume as storage of heat and carbon, attracting more attentions than before from oceanography communite.

We first study the characteristics of deep ocean circulations in the tropical western Pacific based on six ocean model outputs. The deep currents between 1000 and 3000 m are dominated by alternating westward and eastward zonal jets. At depth below 3000 m, deep currents become discontinuous in the zonal direction due to topograohic barriers, and the watermass can be exchanged through the choke channels. Both the transport across these deep channels and current patter within deep basins show seasonal variation. Above analyses on the model outputs provide a reference for the following observational studies.

The deep channel at the Yap-Mariana Junction (YMJ) is the major gateway for the PMOC flowing into the western Pacific, and thus we have made extentive observations in this region. We first present the spatial and seasonal variations of the lower and upper branches of PMOC (L-PMOC and U-PMOC) at the YMJ channel. On the western side of YMJ channel, mooring observations in 2017 and 1997 both revealed seasonal phase of L-PMOC at depths of 3800-4400 m: strong northward flow with speed exceeding 45 cm s^-1 at 4200 m and lasting 6 months from December to next May, and weak flow during the following 6 months. On the eastern side of the channel, mooring observation during 2014-2017 revealed two southward deep flows with broadly seasonal phases: one is the return flow of L-PMOC at deeper layer below ~4000 m with the same phase of L-PMOC but reduced magnitude, and the other is the U-PMOC at shallower depth of 3000-3800 m with opposite phase of L-PMOC. Seasonal variations of the L-PMOC and U-PMOC are accompanied by the seasonal intrusions of the LCPW and UCPW in lower and upper deep layers, respectively. With the aid of the CTD/LADCP profiles and the model outputs, we find the seasonal intrusions of the LCPW and UCPW can change the isopycnal structure in the channel, and further influence the deep currents through geostrophic adjustment.

Besides the seasonality, the intraseasonal variability of the PMOC is also present in the mooring observations in the YMJ channel. This intraseasonal variability is characterized as the intensified fluctuations with depth at 30-90-day period. Observed isotherm displacement can reach ~600 m at 4200 m. Vertical profiles of observed currents reasonably conform to topographic Rossby wave (TRW) features of hyperbolic intensification with depth and highly vertical coherence in phase. A reanalysis product well reproduces observed TRW and is used to study their energy source. Besides the stong surface eddies which can induce the TRW in the deep layer, the energy transferred from the mean flow through barotropic and baroclinic instabilities are deemed to contribute to the generation of the TRW. Considering the favorable conditions of deep channel for TRW, and the relatively rapid teleconnection between Southern Ocean and bottom water warming in the North Pacific through TRW, we expect that TRW may have an important role in the inter-ocean and/or inter-basin exchanges.

Two separate mooring observations in 1990s and 2010s show remarkably similar features of PMOC on seasonal and intraseaonal timescales. Using the two separate mooring observations, we further discussed the interdecadal variation of the L-PMOC. The time-mean of depth-integrated L-PMOC on the western side was reduced by 20% compared to the measurement two decades ago, which is slightly smaller but generally comparable to 27% on the western side of the Samoan Passage. No significant warming trend was found in the deep ocean of the western Pacific as compared to the 1990s, which is different from the situation in the Samoan Passage with significant warming.

Based on the CTD and LADCP profiles data around the Magellan seamounts, which are located northeast of the YMJ channel, we discussed the path of the PMOC before it flows into the YMJ channel. Two kinds of deep watermass, including UCPW and LCPW, were observed in this region with separate depth at 3800 m. The deep flow around the Magellan seamounts is mainly southward. At the depth deeper than 3800 m, the value of the dissolve oxygen around the seamounts is larger than that in YMJ channel. These suggest that the Antarctic origined LCPW first arrive at the Magellan seamounts region and then returns southwestward into the YMJ channel. The mixing features around the Magellan semounts are also studied using the CTD profiles. The value of eddy difusivity enhances below 1500 m, and can reach 10^-4 m² s^-1 in the layer of 500-1000 m above the bottom. The difusivity is relatively larger near the seamount than that on the farther flank of the seamount.

第一章 绪论... 1

1.1 研究意义... 1

1.2 研究现状... 2

1.3 研究内容... 9

第二章 深层环流模式结果对比分析... 11

2.1 研究背景... 11

2.2 数据与方法... 13

2.2.1 六套海洋模式资料... 13

2.2.2 ROMS模式简介... 13

2.2.3 WOA01和WOA13. 14

2.2.4 温盐数据集GDEMv3. 14

2.2.5 EN4-Levitus温盐资料集... 14

2.2.6 Argo数据... 15

2.2.7 输运通量和位涡通量... 15

2.2.8 谐波分析... 15

2.3 结果... 16

2.3.1 温盐分析... 16

2.3.2 流场分析... 23

2.3.3 季节变化... 26

2.4 小结与讨论... 30

2.4.1 不同初始温盐场的差异... 30

2.4.2 潮汐对热带西太平洋深层环流的影响... 31

2.4.3 影响热带西太平洋深层环流数值模拟的其它要素... 33

第三章 深层经向翻转环流的结构和季节变化特征... 35

3.1 研究背景... 35

3.2 数据和方法... 38

3.2.1 潜标观测... 38

3.2.2 CTD/LADCP. 39

3.2.3 模式再分析数据... 40

3.3 结果... 40

3.3.1 水团特性... 40

3.3.2 流场结构... 42

3.3.3 季节变化... 45

3.3.4 动力过程... 50

3.4 小结与讨论... 58

3.4.1 年代际变化... 58

3.4.2 讨论... 59

第四章 地形罗斯贝波引起的深层季节内变化增强的现象... 61

4.1 背景... 61

4.2 数据与方法... 63

4.2.1 潜标观测... 63

4.2.2 模式再分析数据... 64

4.2.3 AVISO数据... 64

4.2.4 中尺度涡旋识别及追踪... 64

4.2.5 频域经验正交函数分解（FDEOF）... 65

4.2.6 相干分析... 66

4.3 结果... 67

4.3.1 深层强化现象... 67

4.3.2 TRW性质... 71

4.3.3 TRW的能量来源... 77

4.4 小结与讨论... 82

第五章 麦哲伦海山区深层环流的特征... 83

5.1 研究背景... 83

5.2 数据和方法... 85

5.2.1 CTD/LADCP. 85

5.2.2 细尺度参数化方法... 85

5.3 结果... 86

5.3.1 水团特性... 86

5.3.2 流速特性及输运通量... 90

5.3.3 LCPW的路径... 93

5.3.4 混合特征... 94

5.4 小结与讨论... 96

第六章 结论与展望... 97

6.1 结论... 97

6.1.1 基于模式的深层环流的基本特征... 97

6.1.2 深层环流的季节变异特征... 97

6.1.3 地形罗斯贝引起的季节内深层强化现象及其机制... 98

6.1.4 麦哲伦海山区的深层环流... 99

6.2 创新点... 99

6.3 未来工作展望... 100

参考文献... 101

附录... 114

附录A：地形罗斯贝波（TRW）... 114

附录B：论文使用的缩写（按首字母顺序排序）... 117

致 谢... 119

作者简历及攻读学位期间发表的学术论文与出海经历... 121

作者简历... 121

在读期间学术论文发表情况... 121

在读期间的出海调查经历 121