Eastern Arabian Sea Research Articles

Application of remotely sensed sea surface temperature for assessment of recurrent coral bleaching (2014–2019) impact on a marginal coral ecosystem

The 2014–2016 El Niño Southern Oscillation (ENSO) caused a prolonged marine heatwave that led to widespread coral bleaching and mortality across the Indo-pacific coral reefs. Prediction of coral bleaching and assessment of bleaching impact on corals is vital for reef ecosystem functioning, services and management. Wherein, advanced satellite remote sensing approach to determine and quantify the thermal stress on corals can assist as an alternative and convenient tool for reef monitoring programs. The present study examines the impact of consecutive coral bleaching episodes on shallow-water marginalized patch reef ecosystems on the Eastern Arabian Sea. Advanced Very High-Resolution Radiometer (AVHRR) satellite data from the NOAA Coral Reef Watch’s (NOAA-CRW) platform, known as CoralTemp, were used to analyze the thermal stress on the coral reefs. Coral bleaching indices like Bleaching Threshold (BT), Positive Anomaly (PA), and Degree Heating Weeks (DHW) were calculated. Ground-truthing revealed that detected thermal stress from satellite-derived Sea Surface Temperature (SST) data over this region well corroborate with the mass coral bleaching events, and found reliable for detecting coral bleaching episodes in the marginalized turbid coral habitats. This study signifies the potential benefit of incorporating remote sensed SST data in coral bleaching monitoring program, which may guide to undertake targeted coral surveys and aid in decision-making and conservation of the vulnerable coral reef ecosystems.

Atmospheric river linked to extreme rainfall events over Kerala in August 2018

A multi-day atmospheric river (AR) in the second week of August 2018 was pivotal in causing extreme precipitation over south-west coast of India that eventually lead to record-breaking floods in the state of Kerala. Integrated water vapor transport (IVT) analysis depicted an intense and long-duration CAT 5 atmospheric river stretching from the Arabian Sea across South India into the Bay of Bengal. A high-pressure ridge over the eastern Arabian Sea and western India and a trough over the Bay of Bengal forming a subsection of monsoon wave-train was observed ahead of the flood event. The monsoon trough was exacerbated by the synchronous effect of a polar westerly jet trough that created an anomalous low-pressure region covering central and eastern India and a quasi-stationary depression over the Bay of Bengal. The system favored continuous supply of moisture from the Indian Ocean into Kerala. To locate major sources of moisture, the air parcels that rained out to generate extreme precipitation are tracked backwards using the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT). The backward trajectory analyses reveal that on 13th August more than 60% of the moisture was contributed from the Central-Eastern Indian Ocean, with some intermittent contributions from the Arabian Sea; while on 14th–16th August, about 25–30% was contributed from the Arabian Sea

Total organic carbon and its role in oxygen utilization in the eastern Arabian Sea

We report seasonal and temporal variation of total organic carbon (TOC) in the eastern Arabian Sea (AS). In comparison to the deep, TOC in the top 100 m showed spatial variation with higher concentrations towards northern AS during North east monsoon (NEM) and South west monsoon (SWM). A comparison with the US-JGOFS data (1995) shows warmer temperatures, enhanced TOC and low chlorophyll in the recent years. High TOC is associated with Arabian Sea high saline waters (ASHSW), advected from the Arabian Gulf, might have resulted in an enhancement of TOC in the eastern AS. This excess TOC supports a high abundance of bacteria despite the low primary productivity. TOC oxidation accounted for 14.3% and 22.5% of oxygen consumption for waters with potential density between 24.5 and 27.3 kg/m3. This study attains great significance considering the missing links with respect to the role of transport processes in ocean deoxygenation under ongoing warming scenarios.

Phytoplankton community structure in a contrasting physico-chemical regime along the eastern Arabian Sea during the winter monsoon

Abstract The paper describes the community composition of phytoplankton in the eastern Arabian Sea (EAS) during winter monsoon (WM, January–February 2018), encompassing the entire eastern part of the Arabian Sea basin (~6–22°N and 77–67°E) with high-resolution sampling (7 transects, 56 sites), from coastal to offshore regions. The phytoplankton pigment concentrations estimated by high-performance liquid chromatography were interpreted using ‘CHEMTAX’ (Mackey et al., 1996) to determine relative abundance and contribution of phytoplankton functional groups at the class level. A distinct spatial heterogeneity was observed in the distribution of phytoplankton groups in the coastal and offshore regions of the northern as well as southern part of the EAS. The basin-scale column integrated chlorophyll a (Chl a) concentration in the upper 100 m was higher (16–118 mg m−2) in the northeastern Arabian Sea (NEAS) as against 9–79 mg m−2 in the southeastern Arabian Sea (SEAS). The chromatographic analysis revealed the dominance of diatoms in the coastal waters of the EAS basin. A closer evaluation of the hydrodynamics and nutrient chemistry of the offshore regions revealed that entrainment of sub-halocline nutrients into the sunlit water column through convective mixing supported the dominant cyanobacterial population in the NEAS. In the warm oligotrophic waters of the SEAS, the substantially high NH4+ concentration indicates that the dominant prochlorophytes were sustained by the regenerated production. The dominance of large phytoplankton like diatoms indicates a herbivorous control in the food chain in coastal waters of the EAS. Offshore waters are characterised by large proportions of pico-nano plankton along with considerable micro phytoplankton, which suggests a mixotrophic ecosystem. The statistical analysis showed that the silicate (Si/N ratio) concentration played a major role in controlling the diatom abundance in the coastal areas of the north (south) EAS while temperature and salinity together with nutrients structured the phytoplankton assemblage in the offshore waters during WM.

Seismic stratigraphy and the sedimentation history in the Laxmi Basin of the eastern Arabian Sea: Constraints from IODP Expedition 355

Detailed interpretation of seismic stratigraphic sequences in the Laxmi Basin of the eastern Arabian Sea are presented in this study using closely spaced high resolution multi-channel seismic (MCS) data. Our stratigraphic interpretation is further corroborated using recent drilling results in the Laxmi Basin, derived from the long sediment cores collected during International Ocean Discovery Program (IODP) Expedition 355. Integrated core-log interpretation discussed in the present study, offer important insights about the lithostratigraphic variations in this region. Analyses of multi-channel seismic reflection data reveal five depositional sequences (ranging from Paleocene to Recent) that led to the development of this marginal basin since the Cenozoic period. Regional igneous basement is successfully imaged, which was also validated by deep sea coring during the IODP Expedition 355. In the present study, we primarily focus on the post-rift sedimentation in the Laxmi Basin and its possible mechanisms. Our detailed interpretation in the prevailing tectonic framework of the basin suggests that near-shelf oldest volcaniclastic sedimentation immediately overlying the acoustic basement is linked to the onset of India–Madagascar and India–Seychelles rifting activities during the Late Cretaceous period. Eventually, during the Early-Mid to Late Miocene, the basin received maximum sedimentation dominantly through an extensive mass transport mechanism implying possible large-scale deformation on the Indian shelf. Subsequent sediment input to the basin appears to have been fed variably via the Indus Fan as well as coastal discharge from the Indian mainland. The total sediment thickness in the Laxmi Basin ranges from 1.1 to 3.5 km. New stratigraphic information and sediment isopach maps presented here provide vital information about syn- and post-rift sedimentation pattern in the region and their long term tectonic implications.

Generation, propagation and dissipation of internal tides on the continental shelf and slope off the west coast of India

The present study focuses on the dynamics of M2 internal tides on the shelf and slope off the west coast of India (WCI) using a high-resolution (1/48∘) numerical ocean circulation model configured for the eastern Arabian Sea. The WCI is characterized by the presence of large internal tides where most of the internal tides generated on shelf-slope regions propagate offshore as beams. Baroclinic energy budget estimates show that about 2.4 GW of barotropic tides gets converted to baroclinic tides over the region. The regions of shelf breaks and upper slope around 16–18.5∘ N and the northern part of the Chagos-Laccadive Ridge (8–14∘ N) are the two most important regions of internal tide generation along the WCI, which together contribute to more than 50% of total area-integrated M2 conversion over the domain. The diapycnal diffusivity for internal tide dissipation exceeds 10−3m2/s in these regions. The central and northern slope regions off the WCI are dominated by locally generated internal tides, whereas part of internal tides in the southern region is characterized by the presence of remotely-generated internal tides coming from the Chagos-Laccadive Islands. Internal tides experience significant seasonal variation on the shelf owing to the variation in stratification, while their variations are negligible on the slope, where the seasonal variation in stratification is not significant.

Middle Miocene Intensification of South Asian Monsoonal Rainfall

AbstractDuring the middle Miocene, Earth's climate changed from a global warm period (Miocene Climatic Optimum) into a colder one with the expansion of the Antarctic ice sheet. This prominent climate transition was also a period of drastic changes in global atmospheric circulation. The development of the South Asian monsoon is not well understood and mainly derived from proxy records of wind strength. Data for middle Miocene changes in rainfall are virtually non‐existent for India and the Arabian Sea prior to 11 Ma. This study presents planktic foraminiferal trace element (Mg/Ca and Ba/Ca) and stable oxygen isotope records from NGHP‐01 Site 01A off the coast of West India in the Eastern Arabian Sea (EAS) to reconstruct the regional surface hydrography and hydroclimate in the South Asian monsoon (SAM) region during the middle Miocene. The Ba/Ca and local seawater δ18O (δ18Osw) changes reveal a notable gradual increase in SAM rainfall intensity during the middle Miocene. Additionally to this long‐term increase in precipitation, the seawater δ18O is punctuated by a prominent decrease, i.e. freshening, at ~14 Ma contemporary with Antarctic glaciation. This suggests that Southern Ocean Intermediate Waters (SOIW) transmitted Antarctic salinity changes into the Arabian Sea via an “oceanic tunnel” mechanism. The middle Miocene increase in SAM rainfall is consistent with climate model simulations of an overall strengthening Asian monsoon from the Eocene to the middle/late Miocene with a further acceleration after the middle Miocene climate transition.

First recorded bloom of Akashiwo sanguinea (Dinophyceae) from the Cochin backwaters, a tropical estuarine system along the South Eastern Arabian Sea

Abstract Akashiwo sanguinea, a cosmopolitan red tide-forming dinoflagellate, is reported for the first time as a bloom from the Cochin estuary along the South Eastern Arabian Sea. Monthly sampling showed the continuous presence of A. sanguinea with occasional blooms. Blooms of A. sanguinea with the highest abundance of 2.8 × 105 cells l−1 were observed during the monsoon season (June 2019) with chlorophyll a of 17.2 mg m−3. The bloom period was characterized by higher concentrations of nitrate (26 μmol l−1) and phosphate (3.8 μmol l−1). Routine monitoring of the bloom region showed the survival of the Akashiwo sanguinea population in the temperature (26 to 30°C) and salinity (22 to 24 PSU) range indicating its eurythermal and euryhaline characteristics.

New distributional record of Acroporids along the eastern Arabian Sea

The scleractinian family Acroporidae is highly diverse in the Indo-Pacific region, and dominates the coral species composition on many reefs. However, live Acroporids are seldom observed in the patch reefs along the eastern Arabian Sea, due to the marginal environmental condition that prevents the establishment of the sensitive ramose corals. A recent survey reveals the presence of two species of Acropora, Acropora cf. polystoma and Acropora efflorescens, in the reef areas of Netrani Island. This is the first report on the occurrence of live and healthy Acropora, in the patch reefs along the central west coast of India, and the first record on the occurrence of A. polystoma in the patch coral reefs along the Indian peninsula and the eastern Arabian Sea.

First report on an unusual bloom of the potentially toxic epibenthic dinoflagellate Prorocentrum rhathymum from Bangaram Lagoon of the Lakshadweep archipelago: Arabian Sea

Benthic dinoflagellates contribute significantly to the primary production and thereby the sustenance of shallow marine environments. However, some of them are considered potentially toxic or harmful species posing a serious threat to this marine ecosystem. The paper describes an extensive bloom of the dinoflagellate Prorocentrum rhathymum for the first time from the Lakshadweep waters in the eastern Arabian Sea. The dinoflagellates were observed as an intense bloom in the Bangaram Lagoon of Lakshadweep archipelago. The cells of P. rhathymum (7.6 x105 cells L−1) were observed in mucilaginous aggregations with macroalgal debris and filaments of the cyanobacterium, Trichodesmium erythraeum. The bloom area was devoid of other microalgal species, and P. rhathymum was also observed to be attached to fresh macroalgal thalli in the nearshore areas (4.67 x104cells g−1 wet weight of macroalgae). These observations suggest that the debris of macroalgae washed into the water column act as a substrate for the transport of P. rhathymum towards the water column. Warm sea surface temperature (SST, 29.7 °C) and stable conditions of the water column favoured the bloom of P. rhathymum in the Bangaram Lagoon during the spring inter-monsoon season. Coral reef ecosystems along the Indian EEZ are the least surveyed regarding the prevalence of harmful or toxic species, and most of the harmful algal bloom (HAB) events in these systems remain overlooked. Routine monitoring and meticulous bloom physiology studies can provide insights into the prevalence of HAB and prediction of such events in these diverse ecosystems.

Sagar Kanya Bathymetric High Complex: An extinct giant submarine volcanic caldera in the Eastern Arabian Sea?

The marine geophysical investigations carried out in the Eastern Arabian Sea revealed the presence of a bathymetric high feature, referred to as the Sagar Kanya Seamount. The preliminary morphology and geophysical characteristics of this feature were studied along a single transect by earlier researchers, however, a detailed geophysical mapping over the entire extent of this feature is still awaited. The present study aims for such a detailed investigation on the Sagar Kanya Seamount and its adjacent regions using newly acquired high-resolution multibeam bathymetry data, complemented with sea surface magnetic and gravity data. Bathymetric map reveals the presence of well-defined bathymetric high features constituting three seamounts connected with structural high/ridge-like features, together representing a large nearly elliptical bathymetric high complex surrounding a region of nearly flat seafloor, referred to as the Sagar Kanya Bathymetric High Complex (SKBHC). The overall morphology of this feature closely resembles with the shape of a submarine volcanic caldera. The gravity anomalies over this feature are mostly correlatable with topography; elliptical-shaped gravity highs are associated with bathymetric highs and gravity lows are associated with the enclosed flat seafloor. Some magnetic anomalies observed over the SKBHC are correlatable with the topographic highs, while others are observed over the flat seafloor. The magnetic anomalies are interpreted as features of post-caldera volcanism. In view of the proximity of this feature to the Réunion hotspot track, and the tectonic framework of the region, the genesis of the SKBHC is attributed to the Réunion hotspot volcanism.

A novel species Spinonema gracilispiculum sp. n. (Nematoda: Desmodoridae) from the oxygen minimum zone of Eastern Arabian Sea margin.

A new species of marine free living nematode Spinonema gracilispiculum sp. n. is described from the oxygen minimum zone of the eastern Arabian Sea (216m) margin. This new nematode species was found exclusively in the station with sandy texture and dissolved oxygen level 0.35 ml/l and are represented by only few individuals from a single location, despite analysing 288 core samples from an extensive survey along the eastern Arabian Sea OMZ region covering from 06°57'19″N to 21°30'639″N. Spinonema gracilispiculum sp. n. differs from other known species of the genus in the body length, position of the dorsal spine, contour and span of lateral alae, size and shape of spicules, number of pre cloacal setae and sexual dimorphism in presence of dorsal spine and amphideal fovea shape. This is the first record of the genus Spinonema from the Indian Ocean. A pictorial and dichotomous keys to the species of Spinonema are also provided, supplemented with comparative morphometric details of all valid species based on published information.

Dynamics and forcing mechanisms of upwelling along the south eastern Arabian sea during south west monsoon

The study was carried out to understand the upwelling and its forcing mechanisms along South Eastern Arabian Sea (SEAS) during the southwest monsoon. Ship observations onboard FORV Sagar Sampada during summer monsoon (August 2017) and satellite data during the period were used to delineate these mechanisms. Here, sea level anomaly (SLA), Ekman transport, Ekman pumping velocity and Brunt–Vaisala frequency are considered as indices as well as proxies for upwelling. Also, the zonal and meridional variations of hydrographic parameters along the SEAS during summer monsoon are studied in detail using in situ observations. Coastal variations showed maximum upwelling intensity off Cape whereas offshore variation showed maximum intensity off Kochi leading to maximum offshore Ekman transport. These variations are investigated by studying the differences in D26, D24 isotherms and local temperature anomaly. The alongshore component of wind is observed to be strong from southern transects extending from Trivandrum to Mangalore where the upwelling signals are dominant. Intense upwelling off Cape is observed to be due to cross-shore component of wind. Ekman transport due to cross-shore component of wind during the period is observed to be maximum at 6–8oN and decreases towards north but eventually increases on reaching to Goa. Ekman transport due to alongshore component of wind and pumping velocity are maximum at 8–10oN when compared to southern region (6–8oN) and also reduces further north (10–15.2oN). The offshore Ekman transport, pumping velocity, sea level anomaly and mixing of watermasses play a major role in the variation of upwelling intensities in the SEAS.

Ocean–atmosphere coupled processes in the tropical Indian Ocean region prior to Indian summer monsoon onset over Kerala

The present study examines atmosphere–ocean interaction before MOK using various observational data sets during the last 35 years (1982–2016). The analyses suggest a new mechanism for the evolution of MOK involving the coupling of radiation, sea surface temperature (SST), wind, evaporation, SST gradient, wind stress, total precipitable water (TPW), and convection processes. During the pre-monsoon period, Arabian Sea (ArS) starts warming and reaches a maximum just four pentads before MOK. Hence, a meridional SST gradient develops that pulls air from the colder southern Indian Ocean, which leads to enhanced wind speed and thus wind stress towards the warmer region. This is followed by the cooling of SST over southwestern ArS due to the wind-induced evaporation and upwelling along Somali coast in response to the enhanced wind stress curl. The low-level anticyclonic circulation over north ArS also aids this Somali coast upwelling, which builds up a zonal SST gradient between East and West ArS. In response, surface wind speed enhances further. The north-eastward directed gradient drives the low-level wind north-eastward towards the eastern ArS. The increase in evaporation enhances moisture content of the air which is carried by low-level wind and accumulates in a column of high TPW near the Kerala coast and adjoining south-east ArS. That moist air column fosters rainfall and thus monsoon onset over Kerala. In short, ocean warming by short-wave radiation during pre-monsoon sets the background, then the ocean surface provides the necessary energy to the atmosphere in the form of latent heat flux in response to wind driven by the developing SST gradient. Significant inter-annual correlations are found between wind stress, TPW and precipitation over the North Indian Ocean during this period, supporting the interpretation detailed above.

Description on variability of shelf-edge hydrography and current structure of the South Eastern Arabian Sea during summer and winter monsoons.

The paper describes the hydrography and vertical current structure along the shelf edge of South East Arabian Sea (SEAS) during summer and winter monsoons based on current profiles from moving Acoustic Doppler Current Profiler (ADCP). During summer monsoon, SEAS was characterized by upwelling with low saline water at the surface along the southern sector (8° N to 11° N). During winter, thermal structure was vertically homogeneous in the upper 80 m, and intrusion of low saline Bay of Bengal waters were found up to 14° N. In the southern sector, turbidity was more than the northern sector during winter and summer seasons. ADCP-derived current profiles during summer along 200-m isobath show dominant northward flow in the south, and southeasterly in the north as part of the West India Coastal Current (WICC). A comparison between ADCP current profiles and Ekman currents during summer indicates dominance of remote forcing (coastal Kelvin waves) over the local wind forcing in the 8-9° N sector whereas a combined influence of both remote forcing and wind in the 9-15° N sector. During winter, the direction of surface current reversed and was poleward generally except at the southern sector (7-8° N) where the flow was southwestward. Sector-wise comparison of ADCP and Ekman current showed less influence of wind on current fields throughout the sector except at south; wind has a major role in the current generation, whereas along the 8-15° N sector, the remote forcing dominates over the wind.

Aerobic microbial communities in the sediments of a marine oxygen minimum zone.

ABSTRACTThe ecology of aerobic microorganisms is never explored in marine oxygen minimum zone (OMZ) sediments. Here we reveal aerobic bacterial communities along ∼3 m sediment-horizons of the eastern Arabian Sea OMZ. Sulfide-containing sediment-cores retrieved from 530 mbsl (meters beneath the sea-level) and 580 mbsl were explored at 15–30 cm intervals, using metagenomics, pure-culture-isolation, genomics and metatranscriptomics. Genes for aerobic respiration, and oxidation of methane/ammonia/alcohols/thiosulfate/sulfite/organosulfur-compounds, were detected in the metagenomes from all 25 sediment-samples explored. Most probable numbers for aerobic chemolithoautotrophs and chemoorganoheterotrophs at individual sample-sites were up to 1.1 × 107 (g sediment)-1. The sediment-sample collected from 275 cmbsf (centimeters beneath the seafloor) of the 530-mbsl-core yielded many such obligately aerobic isolates belonging to Cereibacter, Guyparkeria, Halomonas, Methylophaga, Pseudomonas and Sulfitobacter which died upon anaerobic incubation, despite being provided with all possible electron acceptors and fermentative substrates. High percentages of metatranscriptomic reads from the 275 cmbsf sediment-sample, and metagenomic reads from all 25 sediment-samples, matched the isolates’ genomic sequences including those for aerobic metabolisms, genetic/environmental information processing and cell division, thereby illustrating the bacteria's in-situ activity, and ubiquity across the sediment-horizons, respectively. The findings hold critical implications for organic carbon sequestration/remineralization, and inorganic compounds oxidation, within the sediment realm of global marine OMZs.

PH variability off Goa (eastern Arabian Sea) and the response of sea urchin to ocean acidification scenarios

AbstractThe increasing atmospheric CO2 concentration in the last few decades has resulted in a decrease in oceanic pH. In this study, we assessed the natural variability of pH in coastal waters off Goa, eastern Arabian Sea. pHT showed large variability (7.6–8.1) with low pH conditions during south‐west monsoon (SWM), and the variability is found to be associated with upwelling rather than freshwater runoff. Considering that marine biota inhabiting dynamic coastal waters off Goa are exposed to such wide range of natural fluctuations of pH, an acidification experiment was carried out. We studied the impact of low pH on the local population of sea urchin Stomopneustes variolaris (Lamarck, 1816). Sea urchins were exposed for 210 days to three treatments of pHT: 7.96, 7.76 and 7.46. Our results showed that S. variolaris at pHT 7.96 and 7.76 were not affected, whereas the ones at pHT 7.46 showed adverse effects after 120 days and 50% mortality by 210 days. However, even after exposure to low pH for 210 days, 50% organisms survived. Under low pH conditions (pHT 7.46), the elemental composition of sea urchin spines exhibited deposition of excess Sr2+ as compared to Mg2+ ions. We conclude that although the sea urchins would be affected in future high CO2 waters, at present they are not at risk even during the south‐west monsoon when low pH waters reside on the shelf.

Influence of environmental variables on the distribution and community structure of mesozooplankton in the coastal waters of the eastern Arabian Sea

The spatio-temporal variation of mesozooplankton in response to environmental variables was studied for the coastal waters of southeastern Arabian Sea during the spring inter-monsoon (SIM) and fall inter-monsoon (FIM) of 2014. Overall 84 mesozooplankton taxa belonging to 17 taxonomic groups were identified. The copepod group dominated species richness (58 species) of the mesozooplankton community. Spatial variation of mesozooplankton biomass and total abundance changed significantly in relation to the distribution of sub-surface water (0.5 m) dissolved organic carbon (DOC). Carnivorous and omnivorous mesozooplankton abundance increased substantially with an increasing DOC level. Factor analysis and Mantel test revealed that the distribution of mesozooplankton community, biomass and abundance had significant positive correlation with the distribution of chlorophyll a (Chl-a) and DOC during both the seasons. Distribution of Chl-a had a positive influence on herbivorous mesozooplankton abundance, while DOC had a positive relationship with omnivorous and carnivorous mesozooplankton abundance. Our study indicates that the distribution of Chl-a and DOC influences the spatial variation of mesozooplankton community and abundance in the coastal waters of eastern Arabian Sea during the inter-monsoon seasons (SIM and FIM).

Distinct atmosphere-ocean coupling processes on the onset phase of Indian summer monsoon during 2017 and 2018 as revealed through SCATSAT-1 and its comparison with CFSv2

ABSTRACT Distinct atmosphere-ocean coupled processes determine the inter-annual variability in the onset processes of Indian Summer Monsoon. In this work, we have analysed the distinct characteristics of onset process during 2017 (MOK17) and 2018 (MOK18) using SCATSAT-1 observations. Two years of dedicated SCATSAT-1 analysed winds and other flux datasets are used to understand distinct-coupled processes and further compared with coupled model forecasts from National Centres for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2). Air-sea coupled mechanism during 2017 and 2018 suggests that there is a delayed response (about one pentad) of atmosphere-ocean coupling over North Indian Ocean on the monsoon onset and its propagation during 2018. It is found that the influence of cyclone Mekunu formed over Western Arabian Sea disturbed the onset vortex formed over Eastern Arabian Sea during MOK18. The presence of cyclone Mekunu results in a sudden spike of East-west Sea Surface Temperature gradient, Latent Heat Flux and wind stress over Arabian Sea during one pentad before monsoon onset over Kerala. This created an onset like situation in the next pentad, and later the conditions are not favoured for the further progress of the monsoon. It leads to a weak onset followed by a sluggish and halted northward progression of monsoon during 2018. In the absence of a cyclonic system during the onset phase of MOK17, conventional onset and its further progress took place during 2017. Further comparison of SCATSAT-1 and other analysed data sets with the CFSv2 model show that the CFSv2 model is able to capture the atmosphere-ocean coupled processes during the onset phase up to four pentads in advance during MOK17. However, due to the presence of Mekunu cyclone before the monsoon onset limited the forecast skill of CFSv2 only up to two pentads in advance during MOK18. We found that the presence of strong convective instability due to strong synoptic-scale system can influence the skill of coupled model forecast in the extended range. The new SCATSAT-1 observations provide high-quality observations that can be used to quantify the errors and identify common biases in the coupled models.

Massive coral bleaching in the patchy reef of Grande Island, along the eastern Arabian Sea during the 2015/16 global bleaching event

The 2014–2017 mass bleaching event is considered the third massive coral global bleaching event in recorded history. Based on the National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Watch (CRW) product western India was under an alert level 2 (May–June 2015) with ten-Degree Heating Weeks (DHW) followed by level 1 (six-DHW) from October–November 2015. In succession to our regular surveys, we conducted intensive bleaching surveys following the bleaching alerts issued by NOAA to quantify the coral bleaching in Grande Island along the eastern Arabian sea during October–November2015. The in-situ observations of massive bleaching (∼50%) during the predicted bleaching period by NOAA CRW for the study area contrasted with a lower prevalence of bleaching before (6%) and after (8%) the event. Bleaching severity was highest in Porites spp. corals and the least in foliose Turbinaria mesenterina. The long-term effects of bleaching in this area remain to be seen. This study is the first record of massive bleaching and quantitative assessment in patchy reefs along the eastern Arabian Sea and would act as a baseline data for further reef studies.