The Role of Barrier Layer in Southeastern Arabian Sea During the Development of Positive Indian Ocean Dipole Events

Using data from Argo and simple ocean data assimilation (SODA), the role of the barrier layer (BL) in the southeastern Arabian Sea (SEAS: 60°E-75°E, 0°-10°N) is investigated during the development of positive Indian Ocean Dipole (IOD) events from 1960 to 2008. It is found that warmer sea surface temperature (SST) in the northern Indian Ocean appears in June in the SEAS. This warm SST accompanying anomalous southeastern wind persists for six months and a thicker BL and a corresponding thinner mixed layer in the SEAS contribute to the SST warming during the IOD formation period. The excessive precipitation during this period helps to form a thicker BL and a thinner mixed layer, resulting in a higher SST in the SEAS. Warm SST in the SEAS and cold SST to the southeast of the SEAS intensify the southeasterly anomaly in the tropical Indian Ocean, which transports more moisture to the SEAS, and then induces more precipitation there. The ocean-atmosphere interaction process among wind, precipitation, BL and SST is very important for the anomalous warming in the SEAS during the development of positive IOD events.

Phytoplankton community characteristics in the coastal waters of the southeastern Arabian Sea

Remote sensing applications are important in the fisheries sector and efforts were on to improve the predic-tions of potential fishing zones using ocean color. The present study was aimed to investigate the phyto-plankton dynamics and their absorption properties in the coastal waters of the southeastern Arabian Sea in different seasons during the year 2010 to 2011. The region exhibited 73 genera of phytoplankton from 19 orders and 41 families. The numerical abundance of phytoplankton varied from 14.235×10^3 to 55.075×10^6 cells/L. Centric diatoms dominated in the region and the largest family identified was Thalassiosiraceae with main genera as Skeletonema spp.,Planktionella spp.and Thalassiosira spp. Annual variations in abun-dance of phytoplankton showed a typical one-peak cycle, with the highest recorded during premonsoon season and the lowest during monsoon season. The species diversity index of phytoplankton exhibited low diversity during monsoon season. Phytoplankton with pigments Chlorophylla, Chlorophyllb, Chlorophyll c, peridinin, diadinoxanthin, fucoxanthin,β-carotene and phycoerythrobilin dominated in these waters. The knowledge on phytoplankton dynamics in coastal waters of the southeastern Arabian Sea forms a key parameter in bio-optical models of pigments and productivity and for the interpretation of remotely sensed ocean color data.

Characteristics and mechanisms of the intraseasonal variability of sea surface salinity in the southeastern Arabian Sea during 2015-2020

Based on Soil Moisture Active Passive sea surface salinity(SSS)data from April 2015 to August 2020,combined with Objectively Analyzed Air-Sea Heat Flux and other observational data and Hybrid Coordinate Ocean Model(HYCOM)data,this work explores the characteristics and mechanisms of the intraseasonal variability of SSS in the southeastern Arabian Sea(SEAS).The results show that the intraseasonal variability of SSS in the SEAS is very significant,especially the strongest intraseasonal signal in SSS,which is located along the northeast monsoon current(NMC)path south of the Indian Peninsula.There are remarkable seasonal differences in intraseasonal SSS variability,which is very weak in spring and summer and much stronger in autumn and winter.This strong intraseasonal variability in autumn and winter is closely related to the Madden-Julian Oscillation(MJO)event during this period.The northeast wind anomaly in the Bay of Bengal(BOB)associated with the active MJO phase strengthens the East India Coastal Current and NMC and consequently induces more BOB low-salinity water to enter the SEAS,causing strong SSS fluctuations.In addition,MJO-related precipitation further amplifies the intraseasonal variability of SSS in SEAS.Based on budget analysis of the mixed layer salinity using HYCOM data,it is shown that horizontal salinity advection(especially zonal advection)dominates the intraseasonal variability of mixed layer salinity and that surface freshwater flux has a secondary role.

A review on the parasitic isopod genus Notophryxus G.O.Sars,1883(Crustacea:Isopoda),and first report of Notophryxus globularis G.O.Sars,1885 from Lakshadweep Sea(Amini Island)

Isopod crustaceans of the family Dajidae are exclusively marine ectoparasites.The genus Notophryxus G.O.Sars,1883 currently includes nine nominal species,which are very rarely reported as the chances for encountering these specimens are unpredictable.Our comprehension of the taxonomy,biology,ecology,and life cycle of species belonging to the Notophryxus genus is notably lacking.All nominal species of Notophryxus genus are reviewed here to obtain a comprehensive and integrative taxonomic understanding of this genus,in an attempt to address the lacunae.This paper also documents the rediscovery of an ectoparasitic isopod from the genus Notophryxus in Indian waters,after 55 years.Adult isopod specimens obtained from the outer reefs of Amini Island in the Lakshadweep archipelago,Southeastern Arabian Sea,display morphological congruence to Notophryxus globularis G.O.Sars,1885.The only previous report on this species dates back to 1885 from G.O.Sars'description of N.globularis as an ectoparasite on the euphausiid Thysanoessa gregaria G.O.Sars,1883 in the North Pacific.This discovery represents a range extension of N.globularis from the North Pacific to the Arabian Sea.Two cryptoniscid larvae and three adult females of N.globularis(with dwarf males)were found attached to four adult mysid specimens of Siriella aequiremis Hansen,1910.The present study provides an improved diagnosis of N.globularis with the aid of light microscopy images and line drawings which was not given in the original G.O.Sars,1885 report.This research also provides a concise description of cryptoniscid larvae from the same sampling location and same host which is most likely to be N.globularis larvae.

阿拉伯海东南海域盐度收支的季节变化

采用SODA海洋同化产品的月平均资料分析了阿拉伯海东南海域表层盐度的季节变化特征,发现局地海面淡水通量不能解释盐度的变化。两个典型区域的表层海水盐度收支分析表明,海洋的平流输送是造成阿拉伯海东南海域盐度冬季降低、夏季升高的主要原因,而淡水通量仅在夏季印度西侧沿岸区域造成盐度降低。冬季,东北季风环流将孟加拉湾北部的低盐水向西输送到阿拉伯海,然后向北输送,使表层海水盐度降低;夏季,西南季风环流把阿拉伯海西北部的高盐水向南、向东输送,使阿拉伯海东南海域盐度升高。受地理位置因素的影响,阿拉伯海东南海域表层盐度的变化冬季明显强于夏季。

Coastal upwelling off the southwest coast of India:observations and simulations

Spatial hydrographic data collected from August 2007 through May 2008 and the Princeton Ocean Model(POM)are utilized to explain and document the upwelling in the south eastern Arabian Sea(SEAS).The decrease in the magnitude of winds towards the coast favors local anticyclonic vorticity,resulting in the formation of cyclonic eddy and reversal of coastal currents.The Ekman transport due to alongshore winds,Ekman pumping due to wind stress curl,cyclonic eddy and southward West Indian Coastal Currents play different roles in the SEAS upwelling.In the offshore,wind stress curl leads to the formation of meso-scale eddies,resulting in Ekman pumping at the center and consequent upwelling.The rapid depth variation on the western side of Quilon Mount intensifies upwelling in the offshore.The upwelling Kelvin waves generated at the equator terminates in the Bay of Bengal and do not contribute to the SEAS upwelling.The possible role of local and remote winds,especially around Sri Lanka on the upwelling dynamics of SEAS is described utilizing POM.The Kelvin waves generated off the coast of Sri Lanka during the active phase of the summer monsoon also contribute to the upwelling dynamics off the west coast of India.