Long Term Observations of Currents Over the Off Southern Bay of Bengal

The study was carried out in the southern Bay of Bengal (BoB) which located in east of Sri Lanka. The southern BOB is important for understanding water masses of the Bay of Bengal, Arabian Sea (AS) and equatorial Indian Ocean, interaction with currents. Previous studies revealed the entire BoB undergoes a dramatic seasonal variation in circulation patterns. The seasonally reversing north-south currents transport low-salinity water from the BoB into the AS while the overall flows are equatorward. The reversing monsoon currents pass adjacent to the south of Sri Lanka and transport water between the southern BoB and AS. Freshwater inputs, salinity-controlled mixed layers are important factors and have a strong influence in the flushing of the BoB. The field activities were carried out under Air-Sea Interaction in the Northern Indian Ocean (ASIRI) project in the BoB. The program included deployment of deep ocean (~4000 m) Acoustic Doppler Current Profiler (ADCP) mooring. The deployment was done in December 2013 and recovery was done in August 2015. The purpose was to understand and quantify currents, hydrography, mixing, and fresh and salty exchanges between the Southeast Arabian Sea and the Bay of Bengal which leading to give new insights into deep currents and small scale, high-frequency variability, some of which had not been observed ever before. Long-term time series of oceanographic data were collected from subsurface mooring in the southern Bay of Bengal. It was observed that some of the moorings were entangled with commercial fishing nets and lines, which leads top subsurface buoys ended up with unexpectedly closer to the surface than planned. The results indicated that near-surface currents as large as 1.75 m/s in July 2014. Currents stronger than 0.5 m/s were confined to the upper 250 m. Observations of oceanic currents, temperature, and sea surface height (SSH) revealed eddy like features formed with positive and negative SSH anomalies (~20 cm) moving westward at speeds of about 0.1 m/s. Further, results indicated the time period of the deployment, root-mean square velocity fluctuations were about 0.1 m/s near the surface but decayed with depth and became nearly uniform (~0.03–0.06 m/s) below 100 m.Keywords: Currents, Bay of Bengal, Eddies, Intra-seasonal oscillations, Sea surface height negative SSH anomalie

Temporal Variations in the Abundance and Diversity of Surface Zooplankton in the South Coast of Sri Lanka

Zooplankton have extensively studied in the Indian Ocean, but very few systematic studies have been carried out in the marine waters around Sri Lanka. This study aimed to understand the temporal variations in the abundance and diversity of surface zooplankton in the south coast of Sri Lanka. Surface zooplankton samples were collected at five different locations in the south coast of Sri Lanka in November 2015 (second inter monsoon), December 2015 (Northeast monsoon, NE) and May 2016 (Southwest monsoon SW) using R/V Samuddrika. The vertical hauls of zooplankton were collected from 5 m depth at each station using WP-2 net with 180 μm mesh size and samples were immediately preserved in 5% buffered formalin. At the laboratory, zooplankton were identified to the lowest possible taxon using existing guides, species diversity, abundance (ind/m3), evenness and richness were estimated with respect to three different sampling phases. The data were subjected to hierarchical cluster analysis to identify the similarity between temporal variation and zooplankton abundance. A total of 152 zooplankton species were identified during this study. Species belonging to phylum Arthropoda appeared be the most abundant group and their standing crop (whole time period and sites) was 1906±115 ind/m3. Arthropods occupied nearly 86% of total zooplankton community. Among arthropods, order Poecilostomatoida showed the highest abundance (858±8 ind/m3) followed by phylum Protochordata (~3% and average of 74 ind/m3) chetognaths (2.1%) and mollusks (~1.9%). The highest species abundance was recorded in Novemver (1392±5 ind/m3) and lowest was in May (360±2 ind/m3). Species diversity was also highest in November and lowest in May. Cluster analysis revealed two clearly visible abundance clusters with respect to time. The results indicated that there are significant temporal variations in zooplankton abundance (ANOVA, P<0.05) in the South coast of Sri Lanka.Keywords: Zooplankton, Abundance, Diversity, Cluster analysi

ASIRI: An ocean-atmosphere initiative for bay of Bengal

An observation and modeling campaign in the Bay of Bengal is aimed at studying upper-ocean and lower-atmosphere processes and interactions in relation to Indian Ocean monsoons