Reanalysed (depth and temperature consistent) surface ocean CO₂ atlas (SOCAT) version 2023

Note: The authors recommend the use of the ESA CCI-SST version of this dataset.

The Surface Ocean CO₂ Atlas (SOCAT) version 2023 dataset (Bakker et al., 2016; https://doi.org/10.25921/r7xa-bt92) is a quality-controlled dataset containing 35.6 million surface ocean gaseous CO₂ measurements collated from thousands of individual submissions. These gaseous CO₂ measurements are typically collected at many different depths (of the order of several metres below the surface) using many different systems, and the sampling depth varies dependent upon the sampling platform and/or setup. Different platforms (e.g. ships of opportunity, research vessels) and systems will collect water samples at different depths, and the sampling depth can even vary dependent upon sea state. Therefore, the collated SOCAT dataset contains high quality data, but these data are all valid for different and inconsistent depths. Therefore, the SOCAT provided individual gaseous CO₂ measurements and gridded data are sub-optimal for calculating global or regional atmosphere-ocean gas exchange (and the resultant net CO₂ sinks) and sub-optimal for verifying gas fluxes from (or assimilation into) numerical models.

Accurate calculations of CO₂ flux between the atmosphere and oceans require CO₂ concentrations at the top and bottom of the mass boundary layer, the ~100 μm deep layer that forms the interface between the ocean and the atmosphere (Woolf et al., 2016). Ignoring vertical temperature gradients across this very small layer can result in significant biases in the concentration differences and the resulting gas fluxes (e.g. ~5 to 29% underestimate in global net CO₂ sink values; Watson et al., 2020; Woolf et al., 2016). It is currently impossible to measure the CO₂ concentrations either side of this very thin layer, but it is possible to calculate the concentrations either side of this layer using the SOCAT data, satellite observations and knowledge of the carbonate system.

Therefore to enable the SOCAT data to be optimal for an accurate atmosphere-ocean gas flux calculation, a reanalysis methodology was developed to enable the calculation of the fugacity of CO₂ (fCO₂) for the bottom of the mass boundary layer (termed sub-skin value). The theoretical basis and justification for this is described in detail within Woolf et al., (2016) and the re-analysis methodology is described in detail in Goddijn-Murphy et al. (2015). The re-analysis calculation exploits paired in situ temperature and fCO₂ measurements in the SOCAT dataset, and uses an Earth observation dataset to provide a depth-consistent (sub-skin) temperature field to which all fugacity data are reanalysed. The outputs provide paired fCO₂ (and partial pressure of CO₂) and temperature data that correspond to a consistent sub-skin layer temperature. These can then be used to accurately calculate concentration differences and atmosphere-ocean CO₂ gas fluxes.

This data submission contains a reanalysis of the fugacity of CO₂ (fCO₂) from the SOCAT version 2023 dataset to a consistent sub-skin temperature field. The reanalysis was performed using a tool that is distributed within the FluxEngine open source software toolkit (https://github.com/oceanflux-ghg/FluxEngine) (Holding et al., 2019; Shutler et al., 2016). All data processing and driver scripts are available from the FluxEngine ancillary tools repository https://github.com/oceanflux-ghg/FluxEngineAncillaryTools. The reanalysis dataset was produced for two climate quality and depth consistent temperature datasets: (1) The ESA SST-CCI sea surface temperature product (Merchant et al., 2019) and (2) The NOAA Optimum Interpolation Sea Surface Temperature (OISST) dataset (Banzon et al., 2016; Huang et al., 2021; Reynolds et al., 2007).

For both datasets, the original daily data were first resampled to provide monthly mean values on a 1º by 1º degree grid. These data were then used as the temperature input for the reanalysis. The resulting reanalysed data are provided as a tab-separated value file (individual data points) and as netCDF-5 file (gridded monthly means). These are the same file formats as provided by SOCAT and analogous to the SOCAT single data point and gridded data. Each row in the tab-separated value file corresponds to a row in the original SOCAT version 2023 dataset.

The original SOCAT version 2023 data are included in full, with four additional columns containing the reanalysed data:

* T_reynolds - The temperature (in degrees C) taken from the consistent temperature field for the corresponding time and location.

* fCO2_reanalysed - The fugacity of CO₂ (in μatm) reanalysed to the consistent surface temperature indicated by T_reynolds.

* pCO2_SST - The partial pressure of CO₂ (in μatm) corresponding to the in situ (measured) temperature.

* pCO2_reanalysed - The partial pressure of CO₂ (in μatm) reanalysed to the consistent surface temperature indicated by T_reynolds.

The netCDF gridded version of the reanalysed dataset contains monthly mean data, binned into a 1º by 1º grid and uses the same units, missing value indicators and time and space resolution as the original SOCAT gridded product to maximise compatibility. The gridding is performed using the SOCAT gridding methodology (Sabine et al., 2013). The implementation of the gridding has been verified by performing the gridding on the original (non-reanalysed) SOCAT data and all results were identical to 8 decimal places. The result of gridding the original SOCAT data are included within these netCDF data, along with additional variables containing the equivalent results for the reanalysed SOCAT data. Statistical sample mean, minimum, maximum, standard deviation and count data for each grid cell are included, with unweighted and cruise-weighted versions (following the convention used by SOCAT). Full meta data are included within the file.

Comments

1. Due to the temporal range of the OISST and CCI-SST datasets the reanalysed values are only available from 1981 onwards. Pre-1981 rows contain "NaN" (not-a-number) in the reanalysis columns.

2. This submission contains four files contained within a single zip file: SOCATv2023with_header.tsv, SOCATv2023.nc, SOCATv2023with_header_ESACCI.tsv and SOCATv2023_ESACCI.nc. The first two files correspond to the OISST version, and the second two the ESA SST-CCI version. The .tsv files are the ungridded data, and the .nc files are the gridded data for the corresponding temperature datasets.

3. Please contact Daniel J. Ford ([e-mail]) if there are any questions on the dataset.

How to cite these data

Please cite the DOI of this dataset, the theory (Woolf et al., 2016), the reanalysis methodology (Goddijn-Murphy et al., 2015), the FluxEngine toolbox which was used to perform the reanalysis (Holding et al., 2019; Shutler et al., 2016) and the original SOCAT dataset (Bakker et al., 2016) and/or gridded equivalent (Sabine et al., 2013).

Previous versions

v2019: https://doi.org/10.1594/PANGAEA.905316

v2020: https://doi.org/10.18160/vmt4-4563

v2021: https://doi.org/10.1594/PANGAEA.939233

v2022: https://doi.org/10.5281/zenodo.8228585

Acknowledgements

The Surface Ocean CO₂ Atlas (SOCAT) is an international effort, endorsed by the International Ocean Carbon Coordination Project (IOCCP), the Surface Ocean Lower Atmosphere Study (SOLAS) and the Integrated Marine Biosphere Research (IMBeR) program, to deliver a uniformly quality-controlled surface ocean CO₂ database. The many researchers and funding agencies responsible for the collection of data and quality control are thanked for their contributions to SOCAT.

These data were produced with funding from the Ocean ICU project (https://ocean-icu.eu/) and the Convex Seascape Survey (https://convexseascapesurvey.com/). The UK part of the Horizon Europe OceanICU project is funded by UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee [grant number 10063673].

注意：作者推荐使用本数据集的ESA CCI-SST版本。

海洋表面二氧化碳图集（Surface Ocean CO₂ Atlas, SOCAT）2023版数据集（Bakker等，2016；https://doi.org/10.25921/r7xa-bt92）是一套经过质量管控的数据集，收录了来自数千份独立提交数据的3560万条海洋表面气态二氧化碳测量值。这类气态二氧化碳测量通常依托多种不同系统，在距海面数米的不同深度开展，采样深度会随采样平台及设置的不同而变化。不同平台（如志愿观测船、科研考察船）和测量系统的采样水深各不相同，甚至会因海况发生变动。因此，尽管整合后的SOCAT数据集数据质量优异，但所有测量值对应的采样深度均不一致，这使得SOCAT提供的单条气态二氧化碳测量数据与网格化数据，在计算全球或区域海-气气体交换（及由此得到的净二氧化碳汇）、验证数值模型的气体通量（或向模型同化气体通量）时并非最优选择。

准确计算海-气间二氧化碳通量，需要获取质量边界层上下的二氧化碳浓度。质量边界层是形成于海气界面、厚度约100μm的薄层（Woolf等，2016）。忽略该薄层内的垂直温度梯度，会导致浓度差及由此计算的气体通量出现显著偏差（例如全球净二氧化碳汇估值被低估5%~29%；Watson等，2020；Woolf等，2016）。目前无法直接测量该极薄层两侧的二氧化碳浓度，但可依托SOCAT数据、卫星观测结果及碳酸盐系统相关知识，计算该薄层两侧的浓度。

为使SOCAT数据能够最优适配精准的海-气气体通量计算，研究人员开发了一套再分析方法，用于计算质量边界层底部的二氧化碳逸度（fCO₂），即次表皮值（sub-skin value）。相关理论基础与依据详见Woolf等（2016），再分析方法的细节则见于Goddijn-Murphy等（2015）。该再分析计算利用了SOCAT数据中配对的原位温度与fCO₂测量值，并结合地球观测数据集生成深度一致的次表皮温度场，将所有逸度数据重新分析至该温度场中。最终输出为配对的fCO₂（及二氧化碳分压）与温度数据，对应一致的次表皮层温度，可用于精准计算浓度差与海-气二氧化碳通量。

本数据提交包包含针对SOCAT 2023版数据集的再分析数据，将fCO₂转换至一致的次表皮温度场。本次再分析依托开源软件工具包FluxEngine（https://github.com/oceanflux-ghg/FluxEngine）中的工具完成（Holding等，2019；Shutler等，2016）。所有数据处理与驱动脚本可从FluxEngine辅助工具仓库https://github.com/oceanflux-ghg/FluxEngineAncillaryTools获取。本次再分析数据集基于两套符合气候质量标准且深度一致的温度数据集生成：(1) 欧洲空间局海表温度气候数据记录（ESA SST-CCI）产品（Merchant等，2019）；(2) 美国国家海洋和大气管理局最优插值海表温度（NOAA Optimum Interpolation Sea Surface Temperature, OISST）数据集（Banzon等，2016；Huang等，2021；Reynolds等，2007）。

针对两套温度数据集，原始日度数据首先被重采样为1°×1°网格下的月均值，以此作为再分析的温度输入。最终得到的再分析数据以制表符分隔值（TSV）文件（单条数据点）与netCDF-5文件（网格化月均值）两种格式提供，两种格式均与SOCAT提供的文件格式一致，对应SOCAT的单数据点与网格化数据。制表符分隔值文件中的每一行均对应原始SOCAT 2023版数据集中的一行。

原始SOCAT 2023版数据被完整保留，并新增四列再分析数据：
* T_reynolds：对应时间与位置的一致温度场中的温度（单位：摄氏度）。
* fCO2_reanalysed：经再分析至T_reynolds所指示的一致表层温度的二氧化碳逸度（单位：μatm）。
* pCO2_SST：对应原位（实测）温度的二氧化碳分压（单位：μatm）。
* pCO2_reanalysed：经再分析至T_reynolds所指示的一致表层温度的二氧化碳分压（单位：μatm）。

再分析数据集的网格化netCDF版本包含月均值数据，被分箱至1°×1°网格中，采用与原始SOCAT网格化产品一致的单位、缺失值标识、时间与空间分辨率，以最大化兼容性。网格化流程采用SOCAT网格化方法（Sabine等，2013）。通过对原始（未再分析的）SOCAT数据执行网格化操作验证了该网格化实现的正确性，所有结果的精度均达到8位小数。原始SOCAT数据的网格化结果已包含在该netCDF文件中，同时还包含再分析SOCAT数据的等效结果变量。每个网格单元的统计样本均值、最小值、最大值、标准差与计数数据均已提供，包含未加权与按航次加权两种版本（遵循SOCAT使用的惯例）。文件中包含完整元数据。

注释
1. 受OISST与CCI-SST数据集的时间范围限制，再分析值仅可获取1981年及以后的数据。1981年之前的行在再分析列中会显示"NaN"（非数值）。
2. 本提交包包含单个压缩文件内的四个文件：SOCATv2023with_header.tsv、SOCATv2023.nc、SOCATv2023with_header_ESACCI.tsv与SOCATv2023_ESACCI.nc。前两个文件对应OISST版本，后两个对应ESA SST-CCI版本。.tsv文件为非网格化数据，.nc文件为对应温度数据集的网格化数据。
3. 若对本数据集有任何疑问，请联系Daniel J. Ford（[e-mail]）。

引用说明
请引用本数据集的DOI、相关理论（Woolf等，2016）、再分析方法（Goddijn-Murphy等，2015）、用于执行再分析的FluxEngine工具箱（Holding等，2019；Shutler等，2016）以及原始SOCAT数据集（Bakker等，2016）和/或其网格化版本（Sabine等，2013）。

历史版本
v2019: https://doi.org/10.1594/PANGAEA.905316
v2020: https://doi.org/10.18160/vmt4-4563
v2021: https://doi.org/10.1594/PANGAEA.939233
v2022: https://doi.org/10.5281/zenodo.8228585

致谢
海洋表面二氧化碳图集（Surface Ocean CO₂ Atlas, SOCAT）是一项国际合作项目，获得国际海洋碳协调计划（International Ocean Carbon Coordination Project, IOCCP）、表层海洋-低层大气研究计划（Surface Ocean Lower Atmosphere Study, SOLAS）与集成海洋生物圈研究计划（Integrated Marine Biosphere Research, IMBeR）的认可，旨在提供一套统一经过质量管控的海洋表面二氧化碳数据库。谨向所有参与数据收集与质量管控工作的研究人员及资助机构致谢。
本数据集的生成获得了Ocean ICU项目（https://ocean-icu.eu/）与Convex Seascape Survey项目（https://convexseascapesurvey.com/）的资助。欧洲地平线计划Ocean ICU项目的英国部分由英国研究与创新署（UKRI）依据英国政府的欧洲地平线资助担保提供资助[项目编号：10063673]。