Spatio-temporal variability of chlorophyll-<i>a</i> in response to coastal upwelling and mesoscale eddies in the South Eastern Arabian Sea

The influence of mesoscale dynamics on variability of phytoplankton biomass in terms of chlorophyll-a (chl-a) concentration was studied in the coastal waters of the South Eastern Arabian Sea (SEAS) using long-term satellite data. Satellite-derived chl-a, sea level anomaly, sea surface temperature, and sea surface wind data for the period 1998–2016 were compiled from various sources and analysed to investigate the chl-a variability associated with coastal upwelling and mesoscale eddies. The Empirical Orthogonal Function and Morlet wavelet analyses were performed to estimate the quantitative variability and the result showed strong seasonal and interannual modulation in chl-a concentration and associated environmental variables. The Okubo–Weiss criterion was applied for the identification of mesoscale eddies. The results indicated the presence of cyclonic (cold core) eddies during the summer monsoon season (June–September). The wind-induced upwelling and the cyclonic eddies were most intense during the summer monsoon season, causing higher values of chl-a compared with other season. It is revealed that the variability of chl-a, which might be attributed to seasonal and interannual differences in the surface and sub-surface nutrients, is caused either by coastal upwelling or cyclonic eddies. In particular, the wind-induced upwelling strongly controls the spatial and temporal variability of chl-a compared with mesoscale eddies along the SEAS. The regression model we adopted points out the dominant role played by the wind and its forcing bring about variability in chl-a. The occurrence of extreme climatic events such as El Niño, La Niña and Indian Ocean Dipole (IOD) was noticed during the study period and particularly taken into account to understand the interannual fluctuations in chl-a and associated environmental variables. The relative variability in chl-a concentration was prominent during strong El Niño, La Niña, and IOD. We have attempted to determine the relationship between chl-a with coastal upwelling and mesoscale eddies, the overall importance of such physical forcings, and their influence on bio-production in the SEAS.本研究利用长期卫星数据，针对阿拉伯海东南部（South Eastern Arabian Sea, SEAS）近岸海域的中尺度动力过程对以叶绿素-a（chlorophyll-a, chl-a）浓度表征的浮游植物生物量变异性的影响开展了研究。研究收集了1998–2016年的卫星反演叶绿素-a浓度、海平面异常、海表温度及海表风场数据，通过多源整合与分析，探究与近岸上升流及中尺度涡旋相关的叶绿素-a浓度变异性。本研究采用经验正交函数（Empirical Orthogonal Function）与Morlet小波（Morlet wavelet）分析方法定量估算变异性特征，结果显示叶绿素-a浓度及相关环境变量存在显著的季节与年际调制特性。本研究运用Okubo–Weiss准则（Okubo–Weiss criterion）识别中尺度涡旋，结果表明夏季风期（6–9月）存在气旋式（冷核）涡旋。风致上升流与气旋式涡旋在夏季风期最为强盛，相较其他季节，该时期叶绿素-a浓度水平更高。研究表明，叶绿素-a浓度变异性由近岸上升流或气旋式涡旋驱动，其成因可归结于表层与次表层营养盐的季节及年际差异。尤为值得注意的是，相较于阿拉伯海东南部海域的中尺度涡旋，风致上升流对叶绿素-a浓度的时空变异性具有更强的调控作用。本研究采用的回归模型证实，风场及其强迫效应对叶绿素-a浓度的变异性起到了主导作用。研究时段内发生了厄尔尼诺（El Niño）、拉尼娜（La Niña）及印度洋偶极子（Indian Ocean Dipole, IOD）等极端气候事件，本研究特别将这些事件纳入考量，以解析叶绿素-a浓度及相关环境变量的年际波动特征。在强厄尔尼诺、强拉尼娜及印度洋偶极子事件期间，叶绿素-a浓度的相对变异性尤为显著。本研究旨在明确叶绿素-a浓度与近岸上升流、中尺度涡旋之间的关联，阐明此类物理强迫过程的整体重要性，及其对阿拉伯海东南部海域生物生产过程的影响。