南海北部次表层叶绿素最大值年际变化特征的数值模拟研究

Subsurface chlorophyll maximum (SCM) is one of the most common types in the vertical profiles of chlorophyll concentration. This phenomenon is frequently observed through the whole year in tropical and subtropical oceans, while it exists only during summer in temperate and high latitude oceanic zones. SCM is an important controlling factor for the distribution of marine primary production, which is helpful to deeply understand marine carbon cycle and climate change. SCM can be characterized by the thickness, depth, and intensity of SCM layer (SCML). These three characteristics are influenced by marine hydrometeorology, nutrient concentration, and phytoplankton species, etc, and they appear both regional differences and temporal variations, so that the contribution of SCM to primary production varies spatial-temporally. In large regions of the northern South China Sea (SCS), SCM occurs through the whole year. However, because SCM does not detected by remote sensing and the accumulation of long-term, large space-scale observation is relatively insufficient, the interannual variations of SCM in the northern SCS remain an open question, and other questions, such as, how about the interannual variations of SCM, whether or not it is profoundly affected by climate change, what the contribution of SCMs to primary production is, are all need further examining. In this project, a three-dimensional (3D) physical-biogeochemical coupled model will be used to reproduce the SCM during 1984-2013 in the northern South China Sea. Using modeling results, the interannual variations of SCM characteristics will be analyzed, and the influence of marine hydro-meteorological factor (e.g., the intensity of monsoon, the precipitation, and the depth of surface mixed layer, et al.) and nutrient distribution on the interannual variations of SCM characteristics will be examined. Then, the contribution of SCMs to primary production in the northern SCS will be estimated and studied quantitatively. This project will be scientifically helpful to accurately assess the roles of marine carbon cycles on climate change, and to further expand the applications of remote sensing data.

次表层叶绿素浓度最大值（SCM）是海洋中叶绿素垂直分布的常见形式，普遍存在于大洋和沿岸海域，通常用次表层叶绿素最大值层的深度、厚度和强度等参数表征。SCM不仅是决定海洋初级生产力分布的重要因子，也是深入理解海洋碳循环与气候变化的重要方面。受海洋水文气象环境、营养盐分布等因素控制，SCM特征参数呈现较大的区域差异和时间变化，对初级生产力的贡献也存在显著差异。南海北部大部分海域SCM现象常年存在，但由于SCM位于遥感可探测范围外，且长期大面积的船测资料积累相对不足，目前对SCM的年际变化特征尚不清楚，其是否受到了气候变化的明显影响，对初级生产力贡献如何，都是需要探讨的问题。本研究拟运用三维物理-生物耦合模式，模拟1984-2013南海北部SCM的时空变化，分析近30年来SCM特征参数的年际变化规律，揭示气候变化（季风强度、上混合层深度等）对其年际变化的影响，并定量研究SCM对初级生产力的贡献。

次表层叶绿素浓度最大值（SCM）是海洋中叶绿素垂直分布的常见形式，普遍存在于大洋和沿岸海域，通常用次表层叶绿素最大值层的深度、厚度和强度三个参数表征。SCM不仅是决定海洋初级生产力分布的重要因子，也是深入理解海洋碳循环与气候变化的重要方面。本项目结合观测、模拟及理论分析三种方法系统分析了南海北部SCMs三个特征因子的变化及其影响因素。观测数据统计显示，南海北部海域秋季SCMs多发生在50-75 m处，平均深度为55 m，SCML强度主要集中在0.25-0.35 mg/m3，SCML厚度变化范围较大（30-75 m），平均厚度为45 m。SCML强度大的站位，其SCML深度往往较浅，厚度也较小。物理-生物耦合模式结果显示：1）南海北部海域SCMs特征对台风过境有明显响应，台风过境前SCMs显著存在，过境后5-6天SCMs现象消失，之后又逐渐形成，且SCML强度较之前显著增加。2）在台风“达维”过境期间，营养盐向真光层的输运量约占全年的20%，初级生产力达到6.5 × 103 mg C m–2，约占全年初级生产力的14%，真光层内生物有机碳向深层水体输出1.96 × 103 mg C m–2，约占全年的3%。基于叶绿素垂直分布拟合函数，并结合一维浮游植物生长控制模型（N-P模型），建立了分层水体中SCMs特征因子及硝酸盐跃层与其影响因素的理论关系式。将此理论关系式应用到南海北部海域，结果显示：1）光衰减系数是影响南海北部SCML深度的关键因素，浮游植物最大生长率是SCML厚度的主要影响因素，浮游植物损耗率与混合层以下的垂直混合系数对SCML强度影响最大，其次是浮游植物最大生长率及上混合层深度，海表光强及光衰减系数对SCML强度也有重要影响；2）硝酸盐跃层发生的深度位于次表层叶绿素最大值层深度下方，最大值层厚度与硝酸盐跃层坡度呈反相关；硝酸盐跃层深度与光衰减系数反相关，随海表面光强增大呈指数变深，具有显著的季节变化，但与次表层的湍扩散系数无关；硝酸盐跃层越深，最大值层内的新生产速率越大，这一深度处的光强水平与新生产速率及硝酸盐通量成反比，可作为新生产力的一个重要指标。