Knowledge Management System Of Institute of Oceanology, Chinese Academy of Sciences
|Place of Conferral||中国科学院海洋研究所|
|Keyword||潜沉率 拉格朗日方法 亚南极模态水 Tasman溢流 示踪物逆时回溯方法|
然后，我们使用了上述改进的潜沉率计算方法，以长时间序列高分辨率的模式数据研究了亚南极模态水潜沉率在不同位置处的长期变化趋势。结果显示，在1958~2016年间，SAMW(Sub-Antarctic Mode Water)的潜沉量在南太平洋和南印度洋在长时间段上存在着相反的长期变化趋势，即在南太平洋增大，在南印度洋减少，这与已有研究结果相符。但进一步的分析发现，SAMW潜沉量的空间分布存在着明显的差异。在南印度洋，其北部潜沉区的潜沉率有很微弱的上升趋势，而位于南部潜沉区的的潜沉率则有明显的下降趋势。与此同时，在南太平洋中，其西部潜沉区的潜沉率趋势非常小，而东部潜沉区的潜沉率则有明显上升趋势。总体规律是，密度较大的SAMW潜沉水团比密度较小的潜沉水团表现出更显著的长期变化的趋势。其中，南部变化趋势明显的大密度潜沉水量大概占总潜沉水量的60%，可见SAMW的总体趋势更多地来自其南部密度更大的潜沉区的贡献。进一步的分析表明，SAMW潜沉区的混合层深度(MLD)的长期变化趋势与潜沉率的长期变化趋势之间存在较为一致的空间分布。其中，在南太平洋东侧潜沉区的MLD长期增大趋势，主要由于风应力增大的作用，而其西侧潜沉区的MLD长期减小趋势，则主要受到海表浮力强迫的控制；在南印度洋，南侧潜沉区的潜沉率长期减小趋势更多的是受到浮力强迫的影响，而西北部的潜沉率长期增加趋势则主要由风应力增强导致的。
The subduction rate is an important physical quantity in the ventilated thermocline theory and indicates the volume at which water crosses the bottom of mixed layer and enters the ocean interior. A more accurate estimate of the subduction rate would help us to better quantify the formation of certain water masses. In this paper, three aspects of subduction rates are investigated.
The first is the improvement of the traditional Lagrangian approach to the calculation of the subduction rate, and two factors are pointed out that may lead to deviations: first, there is a limit to the amount of water at the source of subduction, and in some areas of the ocean, due to the increased intensity of vertical pumping and the small fluctuations of the mixed layer, it is easy to occur where the subducted water column is below the local mixed layer in the second year, so that the deviation arises. By using more reliable observations for the Southern Hemisphere oceans, the deviation was confirmed to occur mainly in the lower latitudes of the Pacific and Atlantic, corresponding to the areas of tropical water mass formation. Second, the deviation is from the Stommel's assumption that effective subduction is only occurred in the deep winter season, therefore the effective subduction contributed by the other months was calculated in one year by tracking particles at the bottom of the mixed layer for each month, results show that the deviation is also distributed in the lower latitudes of the ocean. Taken together, the two deviations indicated that there is an overestimation of the subduction rate in the main subduction zone of the southern hemisphere oceans at low latitudes, while at mid-latitudes, the deviation in the subduction rate calculated by the traditional method is smaller. Based on the results, we give an improved formulation of the method. In addition, in the subduction area of subantarctic mode water in mid-latitude waters, the vertical velocity used by the conventional method is not accurate enough, which also leads to deviations in the calculation of the subduction rate, while the vertical velocity obtained by solving the QG omega equation maintains a high degree of similarity to the model output, and since the vertical velocity is often not available from the observed data, the use of this method for the calculation of the vertical velocity improves the accuracy of the subduction rate calculation.
In the following, we investigates the long-term trend of subantarctic mode water subduction rate at different locations with model data of long time coverage and high resolution using the improved method, and the results show that the subduction of SAMW(Sub-Antarctic Mode Water) in the South Pacific and the South Indian Ocean indicate an opposite trend in the time period during 1958-2016, i.e., it increases in the South Pacific and decreases in the South Indian Ocean, which is consistent with the results of existing studies. However, further analysis revealed that there were significant differences in the spatial distribution of SAMW subduction. In the South Indian Ocean, the subduction rate in the northern part of the subduction zone only shows a very weak increasing trend, while the subduction rate in the southern part of the subduction zone shows a significant decreasing trend. Meanwhile, in the South Pacific, the trend of subduction rate in the western subduction zone is very weak, while the water subduction in the eastern subduction zone has a significant increasing long-term trend. Overall, the denser SAMW subducted water masses show more significant long-term trends than the lighter subducted water masses. The subduction water with significant southern variability accounts for roughly 60% of the total subduction water, thus suggesting that the overall trend in SAMW comes more from the contribution of its denser southern subduction zone. Further analysis shows that there is a more consistent spatial distribution between the long-term trends in the mixed layer depth (MLD) of the SAMW subduction zone and the long-term trends in the subduction rate. In the South Pacific, the long-term increasing trend of the MLD in the eastern subduction zone is mainly due to increased wind stress, while the long-term decreasing trend of the MLD in the western subduction zone is mainly controlled by the buoyancy forcing at the sea surface; in the South Indian Ocean, the long-term decreasing trend of the subduction rate in the southern subduction zone is more influenced by buoyancy forcing, while the long-term increasing trend of the subduction rate in the northwest is mainly caused by increased wind stress in the south Indian Ocean.
It is known from the previous analysis that long-term trends in subduction rates reflect information about oceanic water masses and climate change, and their signals can be transmitted through the mid-deep overturning circulation (the oceanic conveyor belt). The nature of water in the South Pacific can be signaled to the Indian Ocean by inter-oceanic water exchange, and the Tasman leakage is an important way. The results of Lagrangian tracing experiment show that most of the particles released at the Tasman leakage location will be captured by the mixed layer in the SAMW subduction zone in the South Pacific, and the deeper the initial release location, the more easterly its longitude across the basin to reach and less likely to enter within the mixed layer, and still move in the deep subtropical circulation in the South Pacific even after the tracing experiment. While the JPL adjoint tracing experiments show two subduction source areas, where 62% of the water is captured by the mixed layer after 95 years of backtracking, with 46.4% of the fraction west of the dateline, while the fraction of the mixed layer traced to local area west of the dateline decreases to 26.7% when the starting point of backtracking is changed to the upstream of the Tasman leakage when it is not turned, and most of it is located near the New Zealand Island This confirms the conclusion that most of the Tasman leakage water is from the subduction of subantarctic mode water.
|MOST Discipline Catalogue||理学|
|Table of Contents|
|刘凯. 海洋潜沉率拉格朗日计算方法的误差分析及其相关应用[D]. 中国科学院海洋研究所. 中国科学院大学,2022.|
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