Seasonal variability of salinity budget and water exchange in the northern Indian Ocean from HYCOM assimilation

Based on HYbrid Coordinate Ocean Model (HYCOM) assimilation and observations, we analyzed seasonal variability of the salinity budget in the southeastern Arabian Sea (AS) and the southern part of the Bay of Bengal (BOB), as well as water exchange between the two basins. Results show that fresh water flux cannot explain salinity changes in salinity budget of both regions. Oceanic advection decreases salinity in the southeastern AS during the winter monsoon season and increases salinity in the southern BOB during the summer monsoon season. In winter, the Northeast Monsoon Current (NMC) carries fresher water from the BOB westward into the southern AS; this westward advection is confined to 4°-6°N and the upper 180 m south of the Indian peninsula. Part of the less saline water then turns northward, decreasing salinity in the southeastern AS. In summer, the Southwest Monsoon Current (SMC) advects high-salinity water from the AS eastward into the BOB, increasing salinity along its path. This eastward advection of high-salinity water south of the India Peninsula extends southward to 2°N, and the layer becomes shallower than in winter. In addition to the monsoon current, the salinity difference between the two basins is important for salinity advection.

末次冰期以来阿拉伯海底层水体氧含量变化及其驱动因素

末次冰期以来阿拉伯海水体氧含量变化在时空上具有显著的差异。目前对其空间变化规律及主导因素尚缺乏系统的研究,尤其缺乏对千年尺度上深层水氧含量变化过程及其控制因素的综合分析。本文基于阿拉伯海中部深水区WIND-CJ06-6与WIND-CJ06-13两个岩芯的XRF岩芯扫描结果,结合前人已发表的指示阿拉伯海水体氧含量变化数据,重建了末次冰期以来千年尺度阿拉伯海不同海域和深度的水体氧含量变化历史并分析了其驱动因素。阿拉伯海水深小于1500 m的水体在千年尺度上的氧含量变化受到表层初级生产力和中层水流通性的共同控制,但在不同时期主导因素不同;在B/A(Bolling-Allerod)到YD(Younger Dryas)期间,阿拉伯海西北部表层生产力显著高于同时期其他海域,导致了中层水体的氧含量在西北部降低而在其他海域增高的空间差异。阿拉伯海水深大于1500 m的水体氧含量在末次冰期以来整体上受北大西洋深层水(NADW)强弱的控制,在LGM(Last Glacial Maximum)到HS1(Heinrich stadial 1)阶段则受到南大洋通风增强的影响,水体氧含量显著升高。

Zooplankton Research in Indian Seas:A Review

To promote long-term studies on the distribution and diversity of marine zooplankton in Indian seas,a comprehensive review has been carried out based on the available literature.Zooplankton studies in Indian waters started in the early 1900 s,and a plethora of literature has accumulated dealing with various aspects of zooplankton,especially from the Bay of Bengal,Arabian Sea and their associated estuaries and backwaters.From this review,a comprehensive description is offered on the species composition and distribution of zooplankton in the Indian Seas.Emphasis is given to reflect the existing knowledge on the variations in zooplankton species composition in the Bay of Bengal and Arabian Sea.Copepods emerge as the most dominant component in all of these marine waters,as is the case worldwide.Copepods are more diverse in the Bay of Bengal than in Arabian Sea.

An Optical Model for the Remote-Sensing of Absorption Coefficients of Phytoplankton in Oceanic/Coastal Waters

A new model for the remote sensing of absorption coefficients of phytoplankton aph (λ) in oceanic and coastal waters is developed and tested with SeaWiFS and MODIS-Aqua data. The model is derived from a rela-tionship of the remote sensing reflectance ratio Rrs (670)/Rrs (490) and aph (490) and aph (670) (from large in-situ data sets). When compared with over 470 independent in-situ data sets, the model provides accurate retrievals of the aph (λ) across the visible spectrum, with mean relative error less than 8%, slope close to unity and R2 greater than 0.8. Further comparison of the SeaWiFS-derived aph (λ) with in-situ aph (λ) values gives similar and consistent results. The model when used for analysis of MODIS-Aqua imagery, provides more realistic values of the phytoplankton absorption coefficients capturing spatial structures of the massive algal blooms in surface waters of the Arabian Sea. These results demonstrate that the new algorithm works well for both the coastal and open ocean waters observed and suggest a potential of using remote sensing to provide knowledge on the shape of phytoplankton absorption spectra that are a requirement in many inverse models to estimate phytoplankton pigment concentrations and for input into bio-optical models that predict carbon fixation rates for the global ocean.

赤道东印度洋次表层高盐水的年际变化

本文利用2010—2019年东印度洋海洋学综合科学考察基金委共享航次数据、Argo(array for real-time geostrophic oceanography)和简单海洋再分析数据(simple ocean data assimilation,SODA),研究了赤道东印度洋次表层高盐水(subsurface high salinity water,SHSW)的年际变化,并探讨了其形成机制。仅限于春季的观测资料显示,来自阿拉伯海的高盐水位于东印度洋赤道断面次表层70~130m深度处,且具有显著的年际变化。基于月平均SODA资料的研究结果表明,不同时期SHSW盐度异常的变化趋势存在显著差异,2010—2015年趋势比较稳定,而2016—2019年则呈现出显著的上升趋势。通过对SHSW的回归分析表明,风场和次表层纬向流是控制该高盐水年际变化的主要因子。进一步的分析表明,赤道印度洋的东风异常导致水体向西堆积,产生东向压强梯度力,进而激发出次表层异常东向流,最终引起SHSW盐度异常升高。此动力关联在印度洋偶极子事件中尤为显著,这进一步反映了赤道东印度洋SHSW的年际变化受到印度洋偶极子的调制。

Spatiotemporal variation and mechanisms of temperature inversion in the Bay of Bengal and the eastern equatorial Indian Ocean

In the northern Bay of Bengal,the existence of intense temperature inversion during winter is a widely accepted phenomenon.However,occurrences of temperature inversion during other seasons and the spatial distribution within and adjacent to the Bay of Bengal are not well understood.In this study,a higher resolution spatiotemporal variation of temperature inversion and its mechanisms are examined with mixed layer heat and salt budget analysis utilizing long-term Argo(2004 to 2020)and RAMA(2007 to 2020)profiles data in the Bay of Bengal and eastern equatorial Indian Ocean(EEIO).Temperature inversion exists(17.5%of the total 39293 Argo and 51.6%of the 28894 RAMA profiles)throughout the year in the entire study area.It shows strong seasonal variation,with the highest occurrences in winter and the lowest in spring.Besides winter inversion in the northern Bay of Bengal,two other regions with frequent temperature inversion are identified in this study for the first time:the northeastern part of the Bay of Bengal and the eastern part of the EEIO during summer and autumn.Driving processes of temperature inversion for different subregions are revealed in the current study.Penetration of heat(mean~25 W/m;)below the haline-stratified shallow mixed layer leads to a relatively warmer subsurface layer along with the simultaneous cooling tendency in mixed layer,which controls more occurrence of temperature inversion in the northern Bay of Bengal throughout the year.Comparatively lower cooling tendency due to net surface heat loss and higher mixed layer salinity leaves the southern part of the bay less supportive to the formation of temperature inversion than the northern bay.In the EEIO,slightly cooling tendency in the mixed layer along with the subduction of warm-salty Arabian Sea water beneath the cold-fresher Bay of Bengal water,and downwelling of thermocline creates a favorable environment for forming temperature inversion mainly during summer and autumn.Deeper isothermal layer depth,and thicker barrier layer thickness intensify the temperature inversion both in the Bay of Bengal and EEIO.

阿拉伯海高盐水入侵孟加拉湾的季节形态及动力机制

本研究利用Argo温盐、Aquarius遥感盐度等资料,研究了阿拉伯海高盐水入侵孟加拉湾的主要路径及季节变化机制。分析显示阿拉伯海高盐水入侵孟加拉湾存在3种类型,即夏季型、冬季型和春季型。夏季型入侵发生在湾口西部,入侵时间为7—10月,净体积输送达1.53 Sv。冬季型(12月至次年1月)和春季型(3—5月)阿拉伯海高盐水入侵分别发生在湾口中部和东部,净输送量相对较弱,分别为0.39 Sv和0.41 Sv。夏季西南季风漂流分支的入侵是夏季型阿拉伯海高盐水进入孟加拉湾的主因。秋季赤道Wyrtki急流向东输运并在苏门答腊岛附近海域堆积,使得湾口南部海域海面东高西低,进而产生西向水平压强梯度力,地转平衡关系使得湾口南部高盐水向北入侵孟加拉湾,这一过程是造成冬季型阿拉伯海高盐水入侵孟加拉湾的主要机制。同理,春季Wyrtki急流是引起春季型阿拉伯海高盐水入侵孟加拉湾的主要因素。

A review of jellyfish aggregations, focusing on India’s coastal waters

A review of jellyfish aggregations focused on India’s coastal waters was conducted,with the aim to enhance understanding of conducive conditions and subsequent ecological impacts.Jellyfish swarming,as well as their beach strandings,have been reported from many areas of the world—including India’s coastal waters.A variety of natural(winds,tidal fronts,surface currents,water temperature,salinity,turbidity,dissolved oxygen)and anthropogenic(water quality deterioration,overfishing,translocation,habitat modification)factors play pivotal roles in triggering jellyfish aggregations.Jellyfish aggregation events in the forms of their swarming in coastal waters and beach strandings have resulted in ephemeral nuisances such as water quality deterioration,food chain alterations,hindrance in seawater uptake by power plants,clogging of nets during fishing operations,and tourism declines.Several well-known Indian tourist beaches(e.g.,Puri,Chennai,Goa,and Mumbai)have experienced beach strandings.Despite recurrence of such events,jellyfishes are relatively less scientifically investigated and monitored in Indian coastal waters.Therefore,it is important to determine the environmental conditions that trigger jellyfish swarming,in order to develop effective monitoring and prediction strategies.This study additionally proposes a conceptual framework towards development of a jellyfish monitoring system for Indian waters using satellite and model data.