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海水硝酸盐跃层深度计算方法研究
Quantification of nitracline depth in seawater
硝酸盐是海洋中浮游植物生命活动可利用的主要氮形态,其跃层深度(ZN)会直接影响硝酸盐垂向输送、海洋初级生产力以及海洋碳循环。随着海洋观测技术的不断发展,硝酸盐剖面数据的采集呈现多样化,包括船基CTD观测和生物地球化学浮标BGC-Argo自动观测等,且垂向采样分辨率差异较大(CTD较低,BGC-Argo较高)。针对不同采样数据,亟需对硝酸盐跃层深度计算方法进行系统且定量化的对比分析研究。本文利用西北太平洋历史船测CTD数据和BGC-Argo浮标数据,采用差值法、梯度法和阈值法分别计算对应硝酸盐跃层深度。研究结果表明:就单一硝酸盐剖面,基于BGC-Argo数据,差值法计算的ZN与目视解译的ZN相差仅为0.2 m,阈值法次之为20.0 m,梯度法相差最大为202.8 m;基于CTD数据,差值法计算的ZN与目视解译的ZN相差2.0 m,阈值法相差49.0 m,梯度法相差155.0 m。相较于梯度法和阈值法,差值法计算的ZN与目视解译的ZN相差最小。根据误差统计分析结果发现,基于BGC-Argo数据,三种方法计算得到的ZN与目视解译的ZN均呈现良好相关性,其中差值法计算结果误差最小(R2为0.77,RMSE为28.48 m),阈值法的R2为0.64,RMSE为34.85 m,梯度法的R2为0.52,RMSE为53.80 m;对于CTD数据,由于其垂向采样分辨率较低,三种方法计算得到的ZN与目视解译的ZN相差较大,但相比于梯度法和阈值法,差值法的误差仍最小(R2为0.81,RMSE为16.13 m),阈值法的R2为0.47,RMSE为27.65 m,梯度法的R2为0.42,RMSE为36.41 m。通过对比分析各方法的特点和差异性,初步探究了各方法的适用性,可为深入研究硝酸盐垂向分布特征和向上输运过程提供科学参考。
Nitrate is the main nitrogen form available for phytoplankton life activities in the ocean, and its nitracline depth (ZN) directly affects the vertical transport of nitrate and the ocean primary productivity, and then further influences the carbon cycle. With the advancement of ocean observation technologies, the profile data of nitrate have been collected in diversified ways, such as ship-based CTD observations and BGC-Argo automatic observations. The vertical sampling resolution of these techniques varies significantly (the vertical sampling resolution of CTD is lower than that of BGC-Argo). In view of different sampling data, it is urgent to conduct systematic and quantitative comparative analysis and study on the computing methods of ZN. In this study, three different methods: difference method, gradient method and threshold method, are adopted to compute the corresponding ZN by using the historical ship-based CTD data and BGC-Argo buoy data in the Northwest Pacific Ocean. The results show that in the case of single nitrate profile, based on BGC-Argo data, the difference between observed ZN and the ZN calculated by difference method is only 0.2 m, followed by threshold method is 20.0 m and gradient method is 202.8 m at most. Based on CTD data, the difference between observed ZN and ZN calculated by difference method is 2.0 m, the threshold method is 49.0 m, and the gradient method is 155.0 m. Compared with the gradient method and threshold method, the difference between the ZN calculated by the difference method and the observed ZN is the smallest. According to the results of statistical error analysis, it is found that the ZN calculated by the three methods based on BGC-Argo data show a good correlation with the observed ZN. Among them, the error of difference method is the smallest (R2=0.77, RMSE=28.48 m). The R2 and RMSE of threshold method are 0.64 and 34.85 m, and the R2 and RMSE of gradient method are 0.52 and 53.80 m. For CTD data, due to its low vertical sampling resolution, the ZN calculated by the three methods is quite different from the observed ZN. However, compared with the gradient method and threshold method, the error of the difference method is still the smallest (R2=0.81, RMSE =16.13 m). The R2 and RMSE of threshold method are 0.47 and 27.65 m, and the R2 and RMSE of gradient method are 0.42 and 36.41 m. The applicability of each method is preliminarily explored through comparing and analyzing the characteristics and differences of them so as to provide some scientific reference for the in-depth research on the vertical distribution characteristics and upward transport process of nitrate.
硝酸盐跃层深度 / 梯度法 / 差值法 / 阈值法 / 西北太平洋 / BGC-Argo / CTD
nitracline depth / gradient method / difference method / threshold method / Northwest Pacific / BGC-Argo / CTD
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. In most regions of the ocean, nitrate is depleted near the surface by phytoplankton consumption and increases with depth, exhibiting a strong vertical gradient in the pycnocline (here referred to as the nitracline). The vertical supply of nutrients to the surface euphotic zone is influenced by the vertical gradient (slope) of the nitracline and by the vertical separation (depth) of the nitracline from the sunlit surface layer. Hence it is important to understand the shape (slope and curvature) and depth of the oceanic nitracline. By using density coordinates to analyze nitrate profiles from autonomous Autonomous Profiling EXplorer floats with In-Situ Ultraviolet Spectrophotometers (APEX-ISUS) and ship-based platforms (World Ocean Atlas – WOA09; Hawaii Ocean Time-series – HOT; Bermuda Atlantic Time-series Study – BATS; and California Cooperative Oceanic Fisheries Investigations – CalCOFI), we are able to eliminate much of the spatial and temporal variability in the profiles and derive robust relationships between nitrate and density. This allows us to characterize the depth, slope and curvature of the nitracline in different regions of the world's oceans. The analysis reveals distinguishing patterns in the nitracline between subtropical gyres, upwelling regions and subpolar gyres. We propose a one-dimensional, mechanistic model that relates the shape of the nitracline to the relative depths of the surface mixed layer and euphotic layer. Though heuristic, the model accounts for some of the seasonal patterns and regional differences in the nitrate–density relationships seen in the data.\n
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. Subduction associated with mesoscale eddies is an important but difficult-to-observe process that can efficiently export carbon and oxygen to the\nmesopelagic zone (100–1000 dbar). Using a novel BGC-Argo dataset covering the\nwestern North Pacific (20–50∘ N, 120–180∘ E), we\nidentified imprints of episodic subduction using anomalies in dissolved\noxygen and spicity, a water mass marker. These subduction patches were\npresent in 4.0 % (288) of the total profiles (7120) between 2008 and\n2019, situated mainly in the Kuroshio Extension region between March and\nAugust (70.6 %). Roughly 31 % and 42 % of the subduction patches were\nidentified below the annual permanent pycnocline depth (300 m vs. 450 m) in the\nsubpolar and subtropical regions, respectively. Around half (52 %) of these episodic events injected oxygen-enriched waters below the maximum\nannual permanent thermocline depth (450 dbar), with >20 %\noccurring deeper than 600 dbar. Subduction patches were detected during winter\nand spring when mixed layers are deep. The oxygen inventory within these\nsubductions is estimated to be on the order of 64 to 152 g O2/m2.\nThese mesoscale events would markedly increase oxygen ventilation as well as\ncarbon removal in the region, both processes helping to support the nutritional and\nmetabolic demands of mesopelagic organisms. Climate-driven patterns of\nincreasing eddy kinetic energies in this region imply that the magnitude of\nthese processes will grow in the future, meaning that these unexpectedly\neffective small-scale subduction processes need to be better constrained in\nglobal climate and biogeochemical models.\n
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感谢美国国家环境信息中心(NCEI,
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