海水硝酸盐跃层深度计算方法研究

doi:10.3969/j.issn.1001-909X.2023.03.001

摘要/Abstract

摘要：

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(Z_N)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Z_N与目视解译的Z_N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Z_N与目视解译的Z_N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Z_N与目视解译的Z_N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Z_N与目视解译的Z_N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Z_N与目视解译的Z_N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。

关键词: 硝酸盐跃层深度, 梯度法, 差值法, 阈值法, 西北太平洋, BGC-Argo, CTD

Abstract:

Nitrate is the main nitrogen form available for phytoplankton life activities in the ocean, and its nitracline depth (Z_N) 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 Z_N. In this study, three different methods: difference method, gradient method and threshold method, are adopted to compute the corresponding Z_N 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 Z_N and the Z_N 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 Z_N and Z_N 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 Z_N calculated by the difference method and the observed Z_N is the smallest. According to the results of statistical error analysis, it is found that the Z_N calculated by the three methods based on BGC-Argo data show a good correlation with the observed Z_N. 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 Z_N calculated by the three methods is quite different from the observed Z_N. 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.

Key words: nitracline depth, gradient method, difference method, threshold method, Northwest Pacific, BGC-Argo, CTD

中图分类号:

P734.44

孟宇, 陈双玲. 海水硝酸盐跃层深度计算方法研究[J]. 海洋学研究, 2023, 41(3): 1-13.

MENG Yu, CHEN Shuangling. Quantification of nitracline depth in seawater[J]. Journal of Marine Sciences, 2023, 41(3): 1-13.

图/表 6

图1 实测数据采样站点空间分布图

Fig.1 Spatial distribution of the sampling stations of in-situ data

表1 BGC-Argo和CTD站位数据统计

Tab.1 Data statistics of BGC-Argo and CTD sampling sites

数据类型	时间范围	总站位数/个	预处理后站位数/个	数据利用率/%
CTD	2002-01-18—2019-08-02	3 526	383	10.86
BGC-Argo	2013-03-01—2017-02-24	427	159	37.24

图2 三种硝酸盐跃层深度的计算方法示意图 (cMLD为剖面中混合层深度的硝酸盐浓度,ci为剖面上第i个数据点的硝酸盐浓度,ci-1为剖面上第i-1个数据点的硝酸盐浓度,Δc1为ci-1与cMLD的差值,Δc2为ci与cMLD的差值,d2c/d z i + 1 2为硝酸盐浓度的二阶导数剖面上第i+1个点对应数值,σi和 σ i - 1分别为剖面上第i个和第i-1个点的位势密度。)

Fig.2 Schematic diagrams of three nitracline depth calculation methods (cMLD represents the nitrate concentration of the MLD point of the profile, ci represents the nitrate concentration of the ith data point of the profile, ci-1 is the nitrate concentration of the (i-1)th data point, Δc1 is the difference between ci-1 and cMLD, Δc2 is the difference between ci and cMLD, d2c/d z i + 1 2 is the (i+1)th data point on the second derivative profile of nitrate concentration, σi and σi-1 represent the potential density of the ith and (i-1)th point, respective.)

图3 针对单一剖面三种方法计算得到的硝酸盐跃层深度

Fig.3 The nitracline depth calculated by the three methods for a single profile

图4 三种方法计算的硝酸盐跃层深度与目视解译硝酸盐跃层深度的比较 (N为数据量,R2为决定系数,RMSE为均方根偏差。)

Fig.4 Comparison of the ZN calculated by the three methods with the observed ZN (N is the amount of data, R2 is the coefficient of determination, and RMSE is the root mean square difference.)

表2 JOO模型计算的硝酸盐跃层深度与目视解译以及其他三种方法计算结果的对比统计

Tab.2 Comparison of the ZN calculated by the JOO model with the results of observed and other three methods calculated

统计参数	目视解译法	差值法	阈值法	梯度法
R²	0.82	0.78	0.71	0.68
RMSE/m	14.06	15.12	19.05	20.06

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