Chlorophyll-a Monthly Deviation

Satellite-based assessments of ocean color began in 1978 with the launch of the Coastal Zone Color Scanner (CZCS) aboard the NASA Nimbus 7 satellite. Following a decade-long break in observations, there have been continuous satellite ocean color measurements since 1997 with Sea-Viewing Wide Field-of-View Sensor (SeaWiFS), followed by Medium Resolution Imaging Spectrometer (MERIS), Moderate Resolution Imaging Spectroradiometer (MODIS),Visible Infrared Imaging Radiometer Suite (VIIRS),Ocean Land Color Instrument (OLCI), Second Generation Global Imager (SGLI) and Ocean Color and Temperature Scanner (OCTS) with additional sensors in development. Data gaps from individual sensors are common due to revisit cycles, cloud cover, and spurious retrievals resulting from a host of confounding atmospheric and aquatic conditions. Some of these issues have been addressed by combining data from multiple sensors and creating a consistent, merged ocean color product (e.g., surface ocean chlorophyll-a concentration). The ESA Ocean Color CCI (OC-CCI) project, led by Plymouth Marine Laboratory (PML), has produced a consistent, merged chlorophyll-a product from SeaWiFS, MODIS, MERIS and VIIRS, spanning the years 1997 to 2019 (Sathyendranath et al., 2018). The merged multi-sensor product will be updated in both time and with data from additional sensors (e.g., OLCI) as part of the Copernicus Climate Change Service (C3S) and the Copernicus Marine Service (CMEMS) that will continue the time series on an operational basis. Future OC-CCI releases will also include algorithmic improvements developed under the CCI+ initiative. Chlorophyll-a concentrations for this indicator are obtained from the global ocean, 4 km spatial data from multiple sensors and creating a consistent, merged ocean color product for each pixel within a country’s EEZ. For purposes of this sub-indicator, reporting year values are compared to a baseline of years 2000 to 2004. The baseline climatology was calculated as the mean of the 5 years of each month by pixel (e.g., mean of 5 years of January) resulting in a 5 year mean of each calendar month over the period 2000 to 2004. The processing steps are outlined below. 1. Calculate the positive percent difference of monthly pixel values from the baseline monthly pixel values. Using the monthly baseline averages, the percentage difference of the pixel value for the reporting period from the baseline value will be calculated as follows: Percent difference of pixel value from baseline = [ ( γ - β ) / β ] x 100 values >0 Where β = the average monthly pixel chlorophyll-a concentration for years 2000 to 2004 Where γ = the average monthly pixel chlorophyll-a concentration for the reporting year 2. Identify pixels with deviations. Pixels with differences from the baseline that are in the 90th percentile of values >0 across the cumulative global EEZ area as defined in World EEZ v116. For more information see the Global Manual of Ocean Statistics Available on the UNEP Document Repository (pages 19-24).基于卫星的海洋颜色评估始于1978年，搭载于美国国家航空航天局（NASA）雨云7号（Nimbus 7）卫星的海岸带颜色扫描仪（Coastal Zone Color Scanner, CZCS）发射升空后，正式开启了相关观测。在经历长达十年的观测中断后，自1997年起，借助海视宽视场传感器（Sea-Viewing Wide Field-of-View Sensor, SeaWiFS），卫星海洋颜色测量实现了连续不间断的记录，后续又陆续投入使用中分辨率成像光谱仪（Medium Resolution Imaging Spectrometer, MERIS）、中等分辨率成像光谱辐射计（Moderate Resolution Imaging Spectroradiometer, MODIS）、可见光红外成像辐射计套件（Visible Infrared Imaging Radiometer Suite, VIIRS）、海洋陆地颜色仪器（Ocean Land Color Instrument, OLCI）、第二代全球成像仪（Second Generation Global Imager, SGLI）以及海洋颜色与温度扫描仪（Ocean Color and Temperature Scanner, OCTS），另有多款传感器尚处于研发阶段。由于重访周期、云层覆盖以及多种复杂大气与海洋水环境导致的虚假反演结果，单颗传感器的数据间隙问题十分常见。通过融合多传感器数据并构建统一的合并式海洋颜色产品（如表层海洋叶绿素-a浓度产品），部分此类问题已得到解决。由普利茅斯海洋实验室（Plymouth Marine Laboratory, PML）牵头的欧洲空间局海洋颜色气候变化倡议（ESA Ocean Color CCI, OC-CCI）项目，已基于SeaWiFS、MODIS、MERIS及VIIRS传感器数据生成了1997年至2019年期间的统一合并式叶绿素-a产品（Sathyendranath等，2018）。作为哥白尼气候变化服务（Copernicus Climate Change Service, C3S）与哥白尼海洋环境服务（Copernicus Marine Service, CMEMS）的组成部分，这款多传感器合并产品将在时间维度上持续更新，并纳入更多传感器（如OLCI）的数据，以业务化方式延续该时间序列。后续的OC-CCI版本还将纳入气候变化倡议+（CCI+）计划中开发的算法改进成果。本指标的叶绿素-a浓度数据取自全球海洋的4千米分辨率多传感器合并数据，针对一国专属经济区（Exclusive Economic Zone, EEZ）内的每个像素点生成统一的合并式海洋颜色产品。就本次子指标而言，报告年度的数值将与2000年至2004年的基准期进行对比。基准期气候态通过逐像素按月计算5年平均值得到（例如1月的5年平均），最终得到2000年至2004年期间每个日历月的5年平均结果。处理步骤如下所述：1. 计算报告月度像素值相较于基准月度像素值的正向百分比差异。基于月度基准平均值，报告期像素值相较于基准值的百分比差异计算公式如下：像素值相较于基准的百分比差异 = [(γ - β) / β] × 100，仅针对数值>0的情况其中，β代表2000年至2004年期间逐像素的月度平均叶绿素-a浓度，γ代表报告年度逐像素的月度平均叶绿素-a浓度。2. 识别存在偏差的像素点。选取全球专属经济区v116（World EEZ v116）划定的累计专属经济区范围内，正向百分比差异处于90th百分位的像素点。更多详细信息可参阅联合国环境规划署文件库中的《全球海洋统计手册》（第19-24页）。