海洋环流与波动重点实验室知识库

气候变化背景下，海洋盐度发生了显著的长期变化，呈现出复杂的地理分布。大西洋、太平洋、和印度洋（三大洋）的盐度变化具有明显的差异，深刻影响着区域海平面、海洋层结变化、海洋生态系统、海洋物质能量循环以及周边国家的社会经济活动。因此，有必要对三大洋盐度长期变化的物理机制及气候效应进行研究。本文针对大西洋-太平洋的盐度变化差异、非绝热过程的作用、对区域海平面的影响三个关键科学问题开展了研究。

本文首先基于多套观测数据对三大洋盐度长期变化中最突出差异：大西洋盐度上升-太平洋盐度下降（the saltier Atlantic-fresher Pacific; SAFP）现象及其形成机制进行了分析。结果表明，SAFP现象主要发生在45°S-45°N之间的上层800 m海洋。1958-2018年，大西洋盐度上升了~0.02 psu，太平洋盐度下降~0.01 psu，两个海盆的盐度变化差异达~0.03 psu，调节了大西洋变暖更快的背景下两个海盆比容海平面的变化速率差，有利于抚平海盆之间海平面上升速率的差异。同时，使太平洋的层结进一步加强，造成太平洋缺氧严重。除了海表淡水通量之外，海洋表层变暖导致（heat-driven）的温跃层露头区向高纬度区域迁移和表面风场的变化驱动（wind-driven）的上层海洋辐合也显著加强了SAFP格局，强调了海洋过程对SAFP的重要性。由于盐度气候态分布的差异，这些过程导致了海盆尺度的盐度变化差异，即形成SAFP趋势。LICOM3海洋模式的敏感性实验验证了上述海洋过程对SAFP的重要作用。气候模式CMIP6可以模拟出SAFP现象，但由于盐度气候态分布的模式偏差，它们往往低估了大西洋盐度的增加，突出了海洋盐度对气候变化响应的海洋过程的本质，并为气候模型提供了基准。

盐度长期变化的物理机制研究中，一个主要的困难就是区分绝热再分配过程（由海洋环流变化引起的盐分空间位置改变）和非绝热过程（真实热量和淡水吸收及混合）的作用；而海洋的密度变化是所有海洋物理过程的关键因素。因此，为了更好地理解盐度变化背后的物理过程，我们引入了“示踪-温度百分位”方法诊断了非绝热过程对海洋密度变化的作用。通过海洋模式的敏感性实验和气候模式的异常强迫比较计划实验（FAFMIP）验证了该方法的有效性。分析结果表明，对于中低纬度（南北纬45度之间）的上层海洋（大约1000米以上）而言，非绝热的温度变化主导了非绝热的密度变化。CMIP6气候模式可以较好地模拟出海洋密度长期变化的全球尺度分布及非绝热过程的作用，但整体上高估了密度的变化。这种高估的主要来源于非绝热盐度变化。发现了非绝热过程对SAFP趋势的作用较小，证实了绝热的海洋再分配对SAFP的重要作用，实现了对SAFP机制的深入认识。

不同于大西洋和太平洋，印度洋的盐度变化呈现南北相反的趋势：北印度洋盐度上升，南印度洋尤其是东南印度洋（SEIO）盐度下降，并对区域海平面的变化产生了重要的影响。SEIO区域位于澳大利亚以西和印度尼西亚以南，其海平面上升对沿岸和岛屿区域产生了威胁。我们以SEIO为例研究了盐度长期变化对区域海平面的影响。1960-2018年平均SEIO比容海平面的上升速率为7.4±2.4 mm decade^-1，盐度和温度的贡献分别为~42%和~58%。盐度的作用主要来自400-1000 m的盐度减小，反应了亚南极模式水（SAMW）和南极中层水（AAIW）的性质变化。而自1990年以来，比容海平面的急剧加速上升主要是由上层400 m的温度变化引起的。这表明在快速变化的气候背景下区域海平面上升的复杂性，其中海洋盐度的作用至关重要且随时间变化。

本文围绕着三大洋盐度长期变化的物理机制和气候效应，以其最突出特征SAPF为研究对象，揭示了海洋过程对SAFP趋势的作用；进一步研究了以往认知薄弱的非绝热过程，加深了对盐度长期变化的物理机制的认识；最后以SEIO为例分析了盐度变化的气候效应，指明了盐度变化对区域海平面上升速率的重要影响。

Under climate change, dramatic ocean salinity changes have been observed and presents a noteworthy geographical difference. The salinity changes in the Atlantic, Pacific, and Indian oceans (the three oceans) have significant differences, which have a profound impact on regional sea level and ocean stratification changes, Marine ecosystems, as well as the socio-economic activities of surrounding countries. Therefore, it is necessary to systematically and quantitatively study the long-term change of salinity in the three oceans and to clarify the relevant processes. This paper focuses on three key scientific issues, the difference in salinity change between the Atlantic and Pacific Oceans, the role of diabatic processes, and the influence on regional sea level.

Based on observation data, we first analyze the most prominent differences in the long-term salinity changes of the three oceans: the saltier Atlantic-fresher Pacific (SAFP) and its formation mechanism. The results show that SAFP mainly occur in the upper 800 m ocean between 45°S-45°N. During 1958-2018, the salinity of the Atlantic Ocean (AO) increased by ~0.02 psu, while that of the Pacific Ocean (PO) decreased by ~0.01 psu, the difference of salinity between the two basins reached ~0.03 psu, which adjusted the difference of steric sea level (SSL) between the two basins with the faster warming of the Atlantic Ocean. It is helpful to smooth the difference of sea level change between basins. On the other hand, the stratification of the PO was further strengthened, resulting in severe hypoxia in the PO. In addition to the surface freshwater fluxes stressed by existing studies, heat- and wind-driven oceanic processes also play a vital role. Specifically, sea surface warming leads to poleward migration of outcrop areas of the thermocline, while surface wind changes drive upper-ocean convergence in the mid-latitudes. These processes give rise to basin-scale salt redistribution that involves substantial salinity increases in the upper AO but much weaker signatures in the PO, owing to their differences in salinity climatology. These conclusions are confirmed by experiments of an ocean-sea ice model. Although climate models can broadly replicate the SAFP pattern, they tend to underestimate the Atlantic salinity increase due to model biases in salinity climatology. This work highlights the essence of ocean processes in the response of ocean salinities to climate change and provides a benchmark for climate models.

A major difficulty in attributing salinity changes is to distinguish adiabatic redistribution processes (changes due to ocean redistribution) and diabatic processes (true heat and fresh water uptake; namely the role of pure heating and pure freshening), And the density of the ocean is a key factor in all the physical processes of the ocean. Therefore, in order to better understand the physical processes behind salinity changes, we introduced a "tracer-temperature percentile" approach to diagnose the role of diabatic processes in long-term density changes. The effectiveness of the proposed method was verified by the ocean model sensitivity experiment and the climate model Anomaly Forcing comparative Planning experiment (FAFMIP). Through our analysis, we find that diabatic temperature changes dominate diabatic density variations in the upper ocean (approximately above 1000 meters) within the mid-latitudes (between 45°S-45°N). The CMIP6 models successfully capture the global-scale distribution of long-term ocean density changes and the role of diabatic processes. However, these models tend to overestimate overall density changes, primarily due to the overestimation of diabatic salinity changes. Our findings indicate that diabatic processes have a relatively minor impact on the SAFP trends, confirming the significant role of adiabatic redistribution in influencing SAFP. These results contribute to a deeper understanding of the mechanisms underlying SAFP and its implications.

Different from the Atlantic and Pacific Oceans, the salinity changes in the Indian Ocean showed an opposite north-south trend: the salinity increased in the northern Indian Ocean and decreased in the southern Indian Ocean, especially the southeastern Indian Ocean (SEIO). SEIO is located in the west of Australia and the south of Indonesia. Its sea level rise has significant implications for coastal areas. We studied the effects of long-term salinity changes on regional sea level using SEIO as an example. The average steric sea level (SSL) rising rate from 1960 through 2018 was 7.4±2.4 mm decade^-1, and contributions of the halosteric and thermosteric components were ~42% and ~58%, respectively. The notable salinity effect arises primarily from a persistent subsurface freshening trend at 400-1000 m depths, reflecting property modifications of the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the southern Indian Ocean. However, since 1990, the acceleration of steric sea level rise is mainly caused by the temperature change in the upper 400 m. This study highlights the complexity of regional sea-level rise in a rapid-changing climate, in which the role of ocean salinity is vital and time-varying.

This paper focuses on the physical mechanisms and climate effects of long-term salinity changes in the three oceans, with a specific emphasis on the prominent feature, SAFP. It reveals the influence of ocean processes on SAFP trends, shedding light on the understanding of the physical mechanisms behind long-term salinity changes. Moreover, the study delves into the previously understudied diabatic processes, further enhancing our understanding of the physical mechanisms driving salinity changes. Furthermore, employing the SEIO as a case study, the paper explores the climate effects resulting from salinity changes, emphasizing the substantial impact such changes have on regional sea-level rise rates.

第1章 绪论... 1

1.1 研究背景及意义... 1

1.2 相关研究进展... 2

1.2.1 三大洋盐度的空间分布差异... 2

1.2.2 三大洋盐度的长期变化差异... 6

1.2.3 三大洋盐度长期变化差异的机制... 9

1.3 科学问题和主要研究内容... 14

1.3.1 科学问题的提出... 14

1.3.2 主要研究内容... 14

第2章 数据资料和研究方法... 17

2.1 数据资料... 17

2.1.1 观测数据... 17

2.1.2 再分析资料... 17

2.1.3 气候模式... 18

2.1.4 海洋模式... 19

2.2 研究方法... 22

2.2.1 盐度长期变化的计算... 22

2.2.2 盐度变化的分解... 22

2.2.3 盐度变化对海平面、层化影响的量化... 23

2.2.4 估算非绝热密度变化... 23

第3章 大西洋-太平洋盐度变化差异及相关过程... 27

3.1 海洋盐度的SAFP趋势... 27

3.2 SAFP趋势背后的海洋过程... 31

3.3 海洋模式实验的验证... 40

3.4 本章小结... 41

第4章 非绝热密度变化及其对SAFP的影响... 43

4.1 非绝热密度变化及验证... 43

4.2 CMIP6对相关过程的模拟评估... 50

4.3 非绝热密度变化对SAFP的影响... 52

4.4 本章小结... 55

第5章 东南印度洋盐度变化及对区域海平面上升的影响... 57

5.1 SEIO海平面的长期变化及盐度的作用... 57

5.2 不同深度温、盐变化对海平面的作用... 60

5.3 SEIO温、盐变化的机制... 63

5.4 本章小结... 72

第6章 总结与展望... 77

6.1 工作总结与讨论... 77

6.1.1 大西洋-太平洋盐度变化差异及相关海洋过程... 77

6.1.2 非绝热密度变化及其对SAFP的影响... 77

6.1.3 盐度长期变化对海平面的影响... 78

6.2 工作创新性... 78

6.3 工作展望... 79

参考文献... 81

致 谢... 93

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 95