中国科学院海洋研究所机构知识库

温度和盐度是海洋的两个基础性热力学变量，紧密参与了几乎所有的海洋物理现象。温度和盐度的空间分布表现出不均一性，从根本上决定了海洋的密度结构、海洋环流以及海-气相互作用方式等，并影响了海洋中物种的分布、生态系统的功能以及生物多样性格局。海洋的温盐变化也存在空间不均一性。大西洋的总体增暖快于太平洋和印度洋，对区域海平面变化和飓风活动产生影响，并加速北极海冰和格陵兰冰川融化，深刻影响全球气候变化。研究海洋温度和盐度空间不均一性及其变化为理解当前的气候变化提供了全新的视角，有助于认识海洋内部对气候变化的响应和反馈机理、提高气候变化预估的能力、推动生态系统演变研究的发展。本文采用多套海洋观测和再分析资料，结合CMIP6气候模式模拟结果，围绕着“全球海洋热盐不均一性变化”这一前沿问题进行了研究。

首先研究了自上世纪中叶以来全球海洋热盐不均一性的变化规律和机理，提出了用三维体积加权空间标准差（SSD）作为量化全球海洋空间不均一性的主要指标，给出了全球海洋温度和盐度不均一性（SSD_T和SSD_S）的变化。多套资料一致表明：全球海洋热盐不均一性自1960年以来呈现持续上升趋势，其中SSD_T增加了1.4±0.1%，SSD_S增长了1.5±0.1%；作为综合衡量海洋温度和盐度不均一性的THI指数则增加了2.4±0.1%，其中温度变化对THI指数变化的贡献远大于盐度变化。CMIP6历史气候模拟多模式集合平均（MMM）可以较好地模拟观测中的全球海洋SSD_T、SSD_S以及THI的增加，表明人类活动的外部强迫作用是海洋热盐不均一性的日益增加的主要原因。CMIP6 模式的SSP2-4.5（中等排放情景）未来预估结果显示全球海洋不均一性将进一步增加，2015-2100年变化比1960-2014年强2-3倍。通过量化不同区域对全球海洋不均一性变化的贡献，发现全球海洋不均一性的加强很大程度上反映了海温和盐度气候态空间差异的放大。较暖的中低纬度上层海洋的快速增暖，特别是大西洋，对全球海洋温度不均一性的增加贡献很大；盐度不均一性加强则反映了海洋三维盐度空间差异的放大，这可能是全球变暖之下全球水循环加强的结果。

进而研究了对全球海洋温度不均一性贡献较大的大西洋快速增暖现象，指出了大气环流和气溶胶变化通过改变海表面热通量的重要作用。1960-2014年期间，大西洋0-2000米平均增暖速率ΔT明显快于全球平均，大约是南大洋的两倍，是印度洋和太平洋的3倍。CMIP6 MMM较好地模拟了上述海盆间增暖差异，表明了人类活动强迫的主导性作用。在CMIP6的模拟结果之中，各大洋ΔT的差异主要由海表面热通量变化—即海洋热吸收（ΔT_uptake）所决定。ΔT_uptake在全球中纬度海洋较强，低纬度较弱。北大西洋中纬度的ΔT_uptake强于其他区域，是吸收全球变暖热量的重要窗口；而且，大西洋中纬度的面积占其总面积的比例明显大于太平洋和印度洋，有利于其平均温度的快速增暖。进一步分析表明，北大西洋海表面短波辐射（SWR）的增加和向大气的的湍流热通量释放（THF）的减少是其ΔT_uptake的主要贡献者。在亚极地北大西洋暖洞（NAWH）区域，大西洋经圈翻转环流（AMOC）减弱使得海表面温度（SST）发生冷却，同时西风急流北移导致表面大气风速减弱，两者共同导致THF释放减少，有利于海洋吸收热量。SST增暖和大气环流总体的向极移动造成北大西洋大气对流不稳定，下沉运动减弱，云量减少；同时人类活动排放的气溶胶浓度也在北大西洋显著减少，两者共同导致北大西洋表面SWR增加，也有利于其热量吸收。三种未来增暖情景SSP1-2.6、SSP2-4.5和SSP5-8.5下预估结果分析表明，未来排放越多，ΔT_uptake在海洋增暖中的贡献越大，但大洋间ΔT差异对排放情景并不敏感。

本文揭示了气候变化背景下海洋不均一性的加强趋势，这表明全球变暖下海洋的水文结构正在向一个新的，更多样化的平衡状态调整，对气候变化预测、海洋生态环境及生物多样性研究具有重要的启示。对于大西洋的快速增暖现象，以往研究普遍强调海洋环流变化（如AMOC减弱）的决定性作用，而本文则指出了大气环流和气溶胶变化的重要贡献。这个科学发现为观测中AMOC强度相对稳定但观测和模式中大西洋增暖明显快于其他大洋这一现象提供了合理的科学解释，增进了对海洋热量储存和气候变化机理的理解，有助于认识和评估气候模式未来预测结果的不确定性。

Temperature and salinity are two fundamental thermodynamical properties of ocean, showing close relationships with almost all ocean physical phenomena. The ocean is intrinsically inhomogeneous in temperature and salinity, fundamentally determining the ocean density structure, ocean circulation, and the ocean-atmosphere interaction, and ultimately influencing marine biodistribution, ecosystem functioning, and marine biodiversity. The changes of temperature and salinity also show spatial differences, where the Atlantic Ocean warming has been faster than the Pacific and Indian Ocean since the mid-20^th century. Dramatic Atlantic Ocean warming affects regional sea level changes, hurricane activity, and accelerates the loss of arctic sea ice and Greenland ice sheet that influence the global climate changes. Therefore, investigating the ocean inhomogeneity and its changes provides a new perspective for understanding climate changes, facilitates the understanding of the response and feedback of ocean internal process to climate change, improves the climate change projection, and promotes the development of ecosystem evolution studies. Based on multiple ocean observation and reanalysis datasets, combined with CMIP6 climate model simulations, our study systematically investigates global ocean thermohaline inhomogeneity and its changes under global warming.

The changes and potential mechanisms of global ocean inhomogeneity since the mid-20^th century are firstly studied in this paper. We quantify the temperature inhomogeneity (SSD_T) and salinity inhomogeneity (SSD_S) of the global ocean for the first time by using three-dimensional volume-weighted spatial standard deviation (SSD). Multiple observational and reanalysis datasets reach a consensus that global ocean thermohaline inhomogeneities exhibit a persistent upward trend since the 1960s. The ensemble mean of multiple datasets shows that the global ocean inhomogeneity has increased by 1.4±0.1% in SSD_T, 1.5±0.1% in SSD_S, and 2.4±0.1% in THI index since the 1960s. The contribution of temperature change to THI change is much greater than that of salinity. CMIP6 multi-model mean (MMM) can well simulate the observed increases in global ocean SSD_T, SSD_S, and THI index, suggesting that the observed ocean inhomogeneity increase is dominated by anthropogenic forcing. Under SSP2-4.5 scenarios, the global ocean inhomogeneity is projected to accelerate by 200-300% during 2015-2100 relative to the 1960-2014. The regional contributions to the global ocean inhomogeneity change show the global ocean inhomogeneity increase is largely due to the amplification of spatial contrast in temperature and salinity. Geographically, the upper ocean in mid-to-low latitudes with high temperature warms rapidly, especially the Atlantic Ocean, dominating the temperature inhomogeneity increase, while the increasing salinity inhomogeneity is mainly due to the amplified three-dimensional salinity contrast that may result from the enhanced global water cycle under global warming.

We then study the rapid Atlantic Ocean warming that shows large contributions to the increased global inhomogeneity, and suggests the important roles of the changes in atmospheric circulation and aerosols on the rapid warming of the Atlantic Ocean through changing the air-sea heat fluxes. The 0-2000 m Atlantic Ocean warming is obviously faster than global mean, which is about two-folds stronger than the Southern Ocean and about three-folds stronger than the Indo-Pacific Oceans during 1960-2014. CMIP6 MMM well captures the observed ocean-by-ocean warming difference, suggesting the rapid Atlantic warming is the result of anthropogenic forcing. In CMIP6 MMM, the ocean-by-ocean warming difference is determined by the changes of the sea surface heat fluxes, i.e., ocean heat uptake (ΔT_uptake). Besides, the large ΔT_uptake difference occurs in the mid-latitude ocean while small in the low latitudes. Further analysis suggests the dramatic T_uptake increases over the North Atlantic is due to large increases of surface shortwave radiation (SWR) over the North Atlantic and the loss of the upward turbulent heat flux (THF) over the subpolar North Atlantic, which dominates the stronger T_uptake increase over the whole Atlantic basin than other oceans. In the Subpolar North Atlantic warming hole (NAWH), the cooling SST due to the slowdown of AMOC and the weakening of surface atmospheric wind speed due to the northward movement of westerly jet, together led to the loss of the upward THF that is benefit for ocean heat uptake. In the North Atlantic Ocean, the warming SST and the polar shift of atmospheric circulation caused the instability of atmospheric convection that weakened the subsidence movement and reduced the cloud cover. At the same time, aerosol concentrations in the North Atlantic Ocean were significantly decreased, causing more SWR into the ocean and ocean heat uptake. From three future warming scenarios SSP1-2.6, SSP2-4.5 and SSP5-8.5, we find that although the higher the emission scenarios are, the more important ocean heat uptake plays in ocean warming, the ocean-by-ocean warming differences are independent on anthropogenic emission scenarios.

The increase in ocean inhomogeneity implies that ocean hydrological structures are modulated to a new and more diversity equilibrium, which poses important enlightenment for the future climate prediction, marine ecological environment and biodiversity. For the rapid Atlantic Ocean warming, existing studies have demonstrated the impacts of the slowdown of Atlantic meridional overturning circulation (AMOC), our work innovatively suggests an essential role of atmospheric circulation and aerosol changes in shaping this inter-ocean contrast of ocean heat uptake. These results provide one scientific explanation that can accounts for the stronger warming of the Atlantic than other oceans within a stable AMOC in observation, which is facilitated to our understanding of ocean heat distribution and climate changes, as well as the climate model modifications.

第1章 绪论... 1

1.1 研究意义... 1

1.2 相关内容的研究回顾与综述... 3

1.2.1 全球海洋温盐气候态分布及特征... 3

1.2.1 全球海洋温度长期变化的研究现状... 7

1.2.3 全球海洋盐度长期变化的研究现状... 14

1.3 科学问题和主要研究内容... 18

1.3.1 科学问题的提出... 18

1.3.2 主要研究内容... 19

第2 章 数据资料和研究方法... 21

2.1 本文所用数据... 21

2.1.1 观测资料... 21

2.1.2 再分析资料... 23

2.1.3 气候模式资料... 25

2.2 研究方法... 27

2.2.1 海洋不均一性的定义... 27

2.2.2 温度收支方程... 33

2.2.3 其他统计方法... 34

第3章 全球海洋热盐不均一性变化... 35

3.1 引言... 35

3.2 数据与方法... 36

3.3 全球海洋温度和盐度不均一性的变化... 38

3.4 全球海洋热盐不均一性加强的原因... 43

3.5 温度和盐度不均一性变化的地理分布... 47

3.6 本章小节... 52

第4章 大西洋快速增暖机理... 57

4.1 引言... 57

4.2 数据与方法... 60

4.3 各大洋平均海温增暖差异... 62

4.4 海洋热吸收在大西洋快速增暖中的重要性... 65

4.5 大气环流和气溶胶变化促进大西洋热吸收... 76

4.6 本章小结... 82

第5章 总结与展望... 89

5.1 对目前工作的总结... 89

5.1.1 全球海洋的热盐不均一性变化... 89

5.1.2 大西洋快速增暖机理... 91

5.2 本文特色与创新... 93

5.3 对未来工作的展望... 94

参考文献... 97

致 谢... 113

作者简历及攻读学位期间发表的学术论文与研究成果 115