Central Arabian Sea Research Articles

NWP perspective of the extreme precipitation and flood event in Kerala (India) during August 2018

In depth examination of the extreme precipitation over Kerala during 15–17th August 2018 – triggering major flood episode in the peninsular Indian state - have been made via the National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Model. A rare alignment of an intense low pressure area (LPA) over head Bay of Bengal (BoB) and a couple of typhoons over South China Sea and West Pacific, driven by anomalous sea surface conditions along the south-east Asian coastal belts, kept the low level jet meander from central Arabian Sea to West North Pacific (WNP) and remain relentless throughout the episode. Positive velocity potentials (VP, at 850 hPa) were seen prevailing for the region and some of these VP fringes unevenly crossed over to Kerala - suggestive of an uneven wind convergence – and associated rainfall necessarily not dictated by orography alone. A single system of negative stream functions (SF) suggested the prevalence of a conveyor belt like flow – termed, the ‘Remotely Aligned Intense Tropical Circulations’ (RAITC) – helping bring ‘an additional supply of moisture’ from WNP to Kerala; the cyclones over the WNP provided large amounts of moisture to the upper air and while some of these cyclones were weakening, a part of this moisture got injected to the south-westward flow and was then propelled further by the BoB LPA. While the NCUM global forecasts could capture the general circulation pattern for the intense precipitation over Kerala in the medium range, the spatial distribution of the heavy spells couldn’t be captured well in day-7 forecast; the strength and movement of the BoB LPA as well as the alignment and intensity modulation of the multiple tropical circulations showed some deviations. The convection permitting regional version of the model, NCUM-R (4 km grid length), exhibited lesser frequency for the light rainfall events, while over-predicting the frequency for the extreme precipitations.

Long‐Term Variations in Nearshore Wave Climate and Littoral Drift at Ganpatipule Coast, Eastern Arabian Sea and its Link to Pacific Climate Variability

AbstractNearshore wave characteristics and its variations at Ganpatipule coast, eastern Arabian Sea, for the past 4 decades (1979–2016) have been examined. The study is based on European Centre for Medium‐Range Weather Forecasts Re‐Analysis‐Interim wave data at ~50 m water depth and the nearshore wave parameters estimated using the numerical model Simulating WAves Nearshore, executed from the Delft3D dashboard considering shallow water wave propagation, wave setup, and breaking. The littoral drift is estimated based on bulk transport formula of the Coastal Engineering Research Centre with the input from the hydrodynamic module. The model shows good agreement with wave rider buoy measured wave parameters at 14‐m water depth. Anomalies of breaker height, period, and direction vary in the range of −0.35–0.4 m, −0.80–1 s, and − 16 to 12°, respectively during the study period. A detailed analysis of monthly variations of breaker height along the study region suggests a significant increasing trend during the Asian summer monsoon, whereas a decreasing trend is identified during the nonmonsoon period. The study region accounts for a northerly net transport of 3.9 × 105 m3 yr−1 with a significant increasing trend of 1,840 m3 yr−1. The strong wind field region along the central Arabian Sea (65–70°E and 15–20°N) gives a strengthened positive correlation (r > 0.7) with the littoral drift. Impacts of Pacific climate variability on interannual and decadal timescales are observed in the littoral drift rates.

Modelling nitrogen-oxygen dynamics in the central Arabian Sea: Large-scale meridional structure and seasonal variations

The oxygen minimum zone (OMZ) of the central Arabian Sea is investigated with a biogeochemical model that couples the marine nitrogen and oxygen cycles and includes euphotic, aphotic, aerobic and anaerobic processes. The model is used to quantify the strength, location and relative timing of governing processes, in particular, nitrite generation, nitrogen losses and oxygen consumption. In an idealized two-dimensional physical environment the model is able to reproduce the observed meridional-vertical structure of oxygen and nitrate, as well as the secondary nitrite maximum layer. Characteristic features of vertical profiles are well represented and the modelled nitrogen transformation rates are in good agreement with observed values. The model results show that (i) the OMZ is neither vertically nor horizontally homogeneous, (ii) minute differences in oxygen concentration determine the thickness of the nitrite layer, (iii) there is hardly any seasonal cycle in the extent of the OMZ but a pronounced seasonal cycle in nitrite, and (iv) the nitrogen loss in the OMZ is accomplished by a succession of denitrification and anammox, based on the seasonal supply of labile detritus. We conclude that the nitrogen loss in the Arabian Sea can only be understood by taking into account the diversity, vertical arrangement and temporal succession of microbial processes.

Spatial variability of microzooplankton in the central Arabian Sea during spring intermonsoon

AbstractSpatial variability of microzooplankton (MZP; organisms in size 20–200 μm) along with total heterotrophic bacteria (THB) and picophytoplankton in the upper 120‐m water column of the central Arabian Sea (AS) was studied during the spring intermonsoon period (March 2015). Six stations divided into three regions (northern, southern, and central) were sampled along 68°E transect, between 9°N and 21°N. A total of 25 species of MZP were recorded corresponding to loricate ciliates (11 species), aloricate ciliates (two species), and heterotrophic dinoflagellates (12 species). Along the transect, MZP total abundance that varied between 5 and 9 × 103 org./m3 was largely contributed by ciliates, especially loricate ciliates (30%–62%) followed by heterotrophic dinoflagellates (23%–26%). MZP counts were lower in the central region (SD ± 0.4, 7–8 × 103 org./m3) than in the northern and the southern regions (SD ± 0.7, 8–9 × 103 org./m3) of the Arabian Sea. Heterotrophic bacterial counts were higher in the northern (28–202 × 103 cells/ml) and southern (16–92 × 103 cells/ml) and lowest in the central (13–89 × 103 cells/ml) region. In contrast, picophytoplankton showed higher counts in central (3–54 × 103 cells/ml) and southern (4–113 × 103 cells/ml) regions and lowest in the north (1–45 × 103 cells/ml). Strong positive coupling of MZP–THB (0.05 > p > 0.01) than that of MZP–picophytoplankton density and MZP–chlorophyll a concentration suggests the spatial scale variability and functioning of microbial loop in the central Arabian Sea during the spring intermonsoon period.

The climatology of shamals in the Arabian Sea—Part 2: Surface waves

The wave conditions in the Arabian Sea are mainly controlled by the SW/NE monsoon winds, the swells from the South Indian Ocean, and to some extent by the regional/local wind systems. The shamal winds are one of the important regional wind systems, which contribute to the wave characteristics in the Arabian Sea considerably. Previous studies explored the existence of shamal swells in the Arabian Sea, based on measurements and modelling during selected shamal events. The present study investigates the climatological features of the shamal waves in the Arabian Sea by analysing ERA‐Interim significant wave heights (SWH) for the period 1987–2016. The ERA‐Interim SWHs were validated against altimeter observations and available measurements in the Arabian Sea. Monthly and annual climatology of shamal and total waves were derived, the trends were estimated and discussed. The results indicate that the annual and monthly shamal SWHs (mean and 99th percentile) are significantly decreasing in most part of the Arabian Sea, while they are increasing in the Gulf of Oman. Significant increase in the 99th percentile total SWHs were estimated in the central Arabian Sea. The makran wind‐induced waves were identified in the northern Arabian Sea for the first time.

Recent changes in the summer monsoon circulation and their impact on dynamics and thermodynamics of the Arabian Sea

The present study examines changes in the low-level summer monsoon circulation over the Arabian Sea and their impact on the ocean dynamics using reanalysis data. The study confirms intensification and northward migration of low-level jet during 1979 to 2015. Further during the study period, an increase in the Arabian Sea upper ocean heat content is found in spite of a decreasing trend in the net surface heat flux, indicating the possible role of ocean dynamics in the upper ocean warming. Increase in the anti-cyclonic wind stress curl associated with the change in the monsoon circulation induces downwelling over the central Arabian Sea, favoring upper ocean warming. The decreasing trend of southward Ekman transport, a mechanism transporting heat from the land-locked north Indian Ocean to southern latitudes, also supports increasing trend of the upper ocean heat content. To reinstate and quantify the role of changing monsoon circulation in increasing the heat content over the Arabian Sea, sensitivity experiment is carried out using ocean general circulation model. In this experiment, the model is forced by inter-annual momentum forcing while rest of the forcing is climatological. Experiment reveals that the changing monsoon circulation increases the upper ocean heat content, effectively by enhancing downwelling processes and reducing southward heat transport, which strongly endorses our hypothesis that changing ocean dynamics associated with low-level monsoon circulation is causing the increasing trend in the heat content of the Arabian Sea.

The Impact of High‐Frequency Weather Systems on SST and Surface Mixed Layer in the Central Arabian Sea

AbstractThe role of high‐frequency (subdaily time scales) weather systems in modulating the sea surface temperature (SST) and the mixed layer (ML) depth in the central Arabian Sea is investigated using one‐dimensional mixed‐layer models for different monsoon seasons. Simulations forced by subhourly sampled meteorological variables, including surface wind, air temperature, humidity, and cloud, are compared to simulations forced by daily‐averaged meteorological variables. It is found that including high‐frequency signals in the meteorological variables lowers the daily‐mean SST (by 0.8°C on average) and damps its variability (the standard deviation decreases by 1.0°C) but has little systematic effect on the SST diurnal variability. There is a small but consistent deepening of the ML depth associated with the slightly intensified wind stress and heat loss due to high‐frequency weather systems at this site. The cooling effect on the daily‐mean SST is found to be closely related to the ML depth on daily‐to‐seasonal time scales. The impact of high‐frequency weather systems is primarily driven by the high‐frequency wind via the turbulent heat and momentum fluxes.

Intensification and deepening of the Arabian Sea oxygen minimum zone in response to increase in Indian monsoon wind intensity

Abstract. The decline in oxygen supply to the ocean associated with global warming is expected to expand oxygen minimum zones (OMZs). This global trend can be attenuated or amplified by regional processes. In the Arabian Sea, the world's thickest OMZ is highly vulnerable to changes in the Indian monsoon wind. Evidence from paleo-records and future climate projections indicates strong variations of the Indian monsoon wind intensity over climatic timescales. Yet, the response of the OMZ to these wind changes remains poorly understood and its amplitude and timescale unexplored. Here, we investigate the impacts of perturbations in Indian monsoon wind intensity (from −50 to +50 %) on the size and intensity of the Arabian Sea OMZ, and examine the biogeochemical and ecological implications of these changes. To this end, we conducted a series of eddy-resolving simulations of the Arabian Sea using the Regional Ocean Modeling System (ROMS) coupled to a nitrogen-based nutrient–phytoplankton–zooplankton–detritus (NPZD) ecosystem model that includes a representation of the O2 cycle. We show that the Arabian Sea productivity increases and its OMZ expands and deepens in response to monsoon wind intensification. These responses are dominated by the perturbation of the summer monsoon wind, whereas the changes in the winter monsoon wind play a secondary role. While the productivity responds quickly and nearly linearly to wind increase (i.e., on a timescale of years), the OMZ response is much slower (i.e., a timescale of decades). Our analysis reveals that the OMZ expansion at depth is driven by increased oxygen biological consumption, whereas its surface weakening is induced by increased ventilation. The enhanced ventilation favors episodic intrusions of oxic waters in the lower epipelagic zone (100–200 m) of the western and central Arabian Sea, leading to intermittent expansions of marine habitats and a more frequent alternation of hypoxic and oxic conditions there. The increased productivity and deepening of the OMZ also lead to a strong intensification of denitrification at depth, resulting in a substantial amplification of fixed nitrogen depletion in the Arabian Sea. We conclude that changes in the Indian monsoon can affect, on longer timescales, the large-scale biogeochemical cycles of nitrogen and carbon, with a positive feedback on climate change in the case of stronger winds. Additional potential changes in large-scale ocean ventilation and stratification may affect the sensitivity of the Arabian Sea OMZ to monsoon intensification.

Diversity of culturable nitrate-reducing bacteria from the Arabian Sea oxygen minimum zone

The subsurface waters of the northern Arabian Sea display a pronounced oxygen minimum layer associated with high nitrite maxima which provide an ideal niche for organisms that can respire nitrate. Culture-based studies elaborate the physiological characteristics of the organisms and their metabolic activities in biogeochemical cycles. In this study, the bacterial diversity and nitrate utilizing activity of the culturable heterotrophic bacteria inhabiting the water column oxygen minimum zone of Arabian Sea were investigated. Nitrate-reducing bacteria were isolated from the water column in the central Arabian Sea. Genotypic characterization of the isolates using 16 S rDNA gene sequencing grouped them into three phylogenetic groups i.e. Proteobacteria, Firmicutes, and Actinobacteria. Out of the 56 isolated bacteria, 45 strains belonged to Proteobacteria, 6 to Firmicutes and 5 to Actinobacteria. The nitrate reducing ability of the isolates was tested using Griess test. Thirty-six species belonging to genera Alcanivorax, Alteromonas, Halomonas, Pseudoalteromonas, Marinobacter, Bacillus, and Vibrio were positive for NO2-/NO3- reduction. Our results imply that cultivable bacteria capable of utilizing NO3- available in the system are present in the Arabian Sea oxygen minimum zone and the conditions existing therein must be favorable for their growth and functionality.

POC export fluxes in the Arabian Sea and the Bay of Bengal: A simultaneous 234Th/238U and 210Po/210Pb study

Abstract Seasonal variation in POC export flux, from the upper ocean, was studied during post southwest monsoon (October–November 2013) and spring intermonsoon (May 2014) at selected locations in the central and southern Arabian Sea and during northeast monsoon (January 2014) and spring intermonsoon (March 2014) in the northern Bay of Bengal. POC fluxes estimated from two particle-reactive radionuclide flux proxies (234Th/238U and 210Po/210Pb) were compared. Spatial and temporal variations were observed in the POC/radionuclide ratios, net deficit flux of radionuclides and the estimated POC export fluxes in both the basins. During post southwest monsoon, the 234Th flux (1346 ± 101 dpm m− 2 d− 1), POC/234Th ratio (5.96 ± 0.29 μmol dpm− 1) and associated POC flux (8.0 ± 0.6 mmol m− 2 d− 1) were high in the southern Arabian Sea than those in the central Arabian Sea (450 ± 95 dpm m− 2 d− 1, 1.76 ± 0.07 μmol dpm− 1 and 0.8 ± 0.2 mmol m− 2 d− 1, respectively). However, during spring intermonsoon these trends were similar in both the central and southern Arabian Sea with mean 234Th fluxes, POC/234Th ratios and 234Th based POC export fluxes being 2762 ± 61 dpm m− 2 d− 1, 1.45 ± 0.05 μmol dpm− 1 and 4.0 ± 0.14 mmol m− 2 d− 1, respectively. On the other hand, the 210Po based POC export fluxes in the southern Arabian Sea (3.37 ± 0.32 and 6.73 ± 0.47 mmol m− 2 d− 1) were higher by a factor of 2–3 than those in the central Arabian Sea (2.08 ± 0.16 and 2.12 ± 0.28 mmol m− 2 d− 1) during post southwest monsoon and spring intermonsoon, respectively. In the northern Bay of Bengal, the 234Th based POC export fluxes were similar and low (0.6–1.2 mmol m− 2 d− 1) while the 210Po based POC export fluxes were much higher (3.75–8.48 mmol m− 2 d− 1) during both the northeast monsoon and spring intermonsoon seasons. One reason could be that more production and carbon export occurring in months prior to the period preceding sample collection due to weeks to several months difference in the mean-lives of the isotopes. The computed POC export fluxes and inferred efficiency of the biological carbon pump in the Arabian Sea and the northern Bay of Bengal are differentiated by this different span in half-lives of 234Th and 210Po (weeks to months, respectively). Also these values are distinct due to their biogeochemical behavior and preferred affinity for specific types of particles. This would require more joint application of each tracer in time and space for various biogeochemical regimes.

Variability of the Somali Current and eddies during the southwest monsoon regimes

The meso-scale eddies and currents in the Arabian Sea are analyzed using different satellite observations, Simple Oceanic Data Assimilation (SODA) reanalysis, and Ocean Reanalysis System 4 (ORAS4) from 1993 to 2016 to investigate the impacts of Southwest (SW) Monsoon strength on Somali Current (SC) mesoscale circulations such as the Great Whirl (GW), the Socotra Eddy (SE), the Southern Gyre (SG), and smaller eddies. Increased Ekman pumping during stronger SW monsoons strengthens coastal upwelling along the Somali coast. The Arabian Sea basin-wide anticyclonic circulation and presence of the GW form mesoscale circulation patterns favourable to advection of upwelled waters eastward into the central Arabian Sea. In September, after the SW monsoon winds reach peak strength in July and August, a higher number of discrete anticyclonic eddies with higher (>20cm) sea surface height anomalies develop in strong and normal intensity SW monsoon seasons than weaker SW monsoon seasons.

Paired N and O isotopic analysis of nitrate and nitrite in the Arabian Sea oxygen deficient zone

The Arabian Sea is home to one of the three main oceanic oxygen deficient zones (ODZs). We present paired nitrogen (N) and oxygen (O) isotope measurements of nitrate (NO3-) and nitrite (NO2-) from the central Arabian Sea in order to understand the effects of N biogeochemistry on the distribution of these species in the low oxygen waters. Within the ODZ, NO2- accumulated in a secondary NO2- maximum (SNM), though the shape and magnitude of the SNM, along with the isotopic composition of NO3- and NO2-, were highly dependent on the location within the ODZ. We also explored water mass mixing within the Arabian Sea as an explanatory factor in the distribution of NO2- in the SNM. The intrusion of Persian Gulf Water at depth may influence the shape of the NO2- peak by introducing small amounts of dissolved oxygen (O2), favoring NO2- oxidation. There was also evidence that vertical mixing may play a role in shaping the top of the SNM peak. Finally, we present evidence for NO2- oxidation and NO2- reduction co-occurring within the ODZ, as has been previously suggested in the Arabian Sea, as well as in other ODZs. The decoupling of the N and O isotopes of NO3-, deviating from the expected 1:1 ratio for dissimilatory NO3- reduction, indicates that NO2- oxidation has a significant influence on the isotopic composition of NO3-. Additionally, the N isotopes of NO2- were generally fit well by Rayleigh curves for NO2- oxidation. However, the removal of dissolved inorganic nitrogen (DIN) within the domain reflects the importance of NO2- reduction to N2.

MORPHOLOGICAL CHARACTER AND MOLECULAR IDENTIFICATION WITH COI GENE MARKER OF HORSESHOE CRABS (Tachypleus gigas) AT COASTAL WATERS OF NORTHERN JAVA ISLAND

The existence of horseshoe crabs Tachypleus gigas is currently on the threatened condition and its population status is also unclear. The objectives of this research were to asses the horseshoe crabs morphological characters and to identify based on molecular marker of Cytochrome Oxydase Subunit I (COI) gene. The samples were collected from five locations (Segara Menyan Subang, Ujung Kulon Banten, Tambak Lorok Semarang, Betah Walang Demak, and Kenjeran Surabaya) in October 2014 until June 2015. A total of 27 parameters for 62 individuals were used to determine connectivity and character identifier among populations. Data were tested using non-parametric analysis. While, the 5 individuals from each sites were used for molecular analysis including several stages of the DNA extraction, DNA electrophoresis, amplification and visualization of DNA fragments, and DNA sequencing. The results obtained based on morphological characters that T. gigas of Surabaya formed a unique group due to its body size (X1-X2 and X4-X16) was larger than the other 5 body parts (X22, X23, X26-X28) of Semarang, Demak, Subang, and Banten. According to molecular results, there were 650 bp conserve nucleotide sequences. Analysis using software MEGA 5.0 showed that there were 73 existence of specific nucleotide sites that can be used to distinguishes between T. gigas in-groups and out-groups (T. gigas from Central Arabian Sea, T. tridentatus from Sweden, and Carcinoscorpius rotundicauda from USA).Keywords: horseshoe crabs, Java coast, morphology, identification molecular, CO1 gene.

Natural isotopic composition of particulate organic nitrogen in the eastern and central Arabian Sea

We report measurements of isotopic composition of nitrogen (﷿d 15 N) of particulate organic nitrogen (PON) at eighteen sampling locations in the north-eastern and central Arabian Sea during the waning phase of the summer monsoon. This season is ideal to detect signatures of upwelling and denitrification in the Arabian Sea. Our observations indicate significant spatial variability in the﷿d 15 N as a result of the upper ocean response to the monsoonal forcing that triggers upwelling and thus supplies sub surface nitrate to the surface. An increasing (decreasing) trend has been found in the ﷿d 15 N (PON) from the upwelling zone to the open ocean, which tracks the progressive utilization of advected nitrate from the coast. Monsoondriven mixing and supply of nitrate and/or PON with lower﷿d 15 N from the west through advection are mainly confined to the upper 50 m, however, at some locations advection imprints its signature in the deeper layers as well. The results have implications to the interpretation of sedimentary﷿d 15 N record from this region and deciphering the role of monsoonal forcing over the nitrogen utilization by plankton.

A multiproxy approach to understanding the "enhanced" flux of organic matter through the oxygen-deficient waters of the Arabian Sea

Abstract. Free-drifting sediment net traps were deployed 14 times at depths between 80 and 500 m for 1–3 days each during the late monsoon–intermonsoon transition in the central Arabian Sea. Two locations (19.5 and 15.5° N) were within the permanently oxygen-deficient zone (ODZ), and a third (11° N) had a shallow and thin oxygen minimum. The secondary nitrite maximum, which serves as a tracer of the ODZ, thinned from ∼ 250 m thick at stations 19.5 and 15.5° N to ∼ 50 m thick at station 11° N. Overall, organic carbon fluxes ranged from 13.2 g m2 yr−1 at 80 m to a minimum of 1.1 g m2 yr−1 at 500 m. Fluxes at the more oxygenated 11° N station attenuate faster than within the permanent ODZ. Martin curve attenuation coefficients for 19.5 and 15.5° N are respectively 0.59 and 0.63 and for 11° N it is 0.98. At least six potential mechanisms might explain why particles sinking through the ODZ are more effectively transferred to depth: (M1) oxygen effects, (M2) microbial loop efficiencies and chemoautotrophy, (M3) changes in zooplankton dynamics, (M4) additions of ballast that might sorb and protect organic matter from decay (M4a) or change sinking speeds (M4b), (M5) inputs of refractory organic matter and (M6) temperature effects. These mechanisms are intertwined, and they were explored using a combination of mineral (XPS) and organic matter characterizations of the sinking material, shipboard incubation experiments, and evaluations of existing literature. Direct evidence was found supporting an oxygen effect and/or changes in the efficiency of the microbial loop including the addition of chemoautotrophic carbon to the sinking flux in the upper 500 m. Less direct evidence was found for the other potential mechanisms. A simple conceptual model consistent with our and other recent data suggests that the upper ODZ microbial community determines the initial flux attenuation, and that zooplankton and sinking speed become more important deeper in the water column. The exact interplay between the various mechanisms remains to be further evaluated.

Remotely Driven Anomalous Sea-Air Heat Flux Over the North Indian Ocean During the Summer Monsoon Season

The atmosphere typically receives heat from the open tropical ocean; an exception is the western and central Arabian Sea during the summer monsoon season. This article analyzes in situ observations from buoys and ships and examines mechanisms responsible for the reverse sea-air interfacial heat flux that have yet to be addressed. Key factors are the horizontal advection of heat and the heat flux derived via mixing and entrainment of air from above the atmospheric mixed layer in a Richardson number regime where shear-driven mixing would not normally be expected. This study invites a fresh look at the physics of entrainment and mixing in the atmospheric boundary layer. The reverse heat flux observations offer an opportunity to test the efficacy of physics encapsulated in boundary layer parameterization schemes for coupled models.

MODIS-Aqua detects Noctiluca scintillans and hotspots in the central Arabian Sea.

Northern Arabian Sea is considered as an ecologically sensitive area as it experiences a massive upwelling and long-lasting algal bloom, Noctiluca scintillans (green tide) during summer and spring-winter, respectively. Diatom bloom is also found to be co-located with N. scintillans and both have an impact on ecology of the basin. In-house technique of detecting species of these blooms from Moderate Resolution Imaging Spectroradiometer (MODIS)-Aqua data was used to generate a time-series of images revealing their spatial distribution. A study of spatial-temporal variability of these blooms using satellite data expressed a cyclic pattern of their spread over a period of 13 years. An average distribution of the blooms for January-March period revealed a peak in 2015 and minimum in 2013. Subsequently, a time-series of phytoplankton species images were generated for these 2 years to study their inter-annual variability and the associated factors. Species images during active phase of the bloom (February) in 2015 indicated development of N. scintillans and diatom in the central Arabian Sea also, up to 12° N. This observation was substantiated with relevant oceanic parameters measured from the ship as well as satellite data and the same is highlight of the paper. While oxygen depletion and release of ammonia associated with N. scintillans are detrimental for waters on the western side; it is relatively less extreme and supports the entire food chain on the eastern side. In view of these contrasting eco-sensitive events, it is a matter of concern to identify biologically active persistent areas, hot spots, in order to study their ecology in detail. An ecological index, persistence of the bloom, was derived from the time-series of species images and it is another highlight of our study.

Trends of wave height and period in the Central Arabian Sea from 1996 to 2012: A study based on satellite altimeter data

The variability of annual maximum and annual mean significant wave height (SWH) and wave period in the Central Arabian Sea is studied using satellite altimeter data from 1996 to 2012 at a deep water (water depth~3500m) buoy location (15.5°N, 69.28°E). Altimeter wave heights are validated with in-situ measurements and are intercompared. Extreme tropical storm/cyclone events that affect the wave conditions are studied using Joint Typhoon Warning Centre best-track data. The study shows a positive trend of 0.63cmyr−1 in the annual mean SWH. In contrast, a negative trend of 2.66cmyr−1 is found for the annual maximum SWH due to the decreasing trend of extreme tropical cyclone events. The annual mean and maximum wave period shows a decrease of 0.005syr−1 and 0.011syr−1, respectively.

Phytoplankton bloom phenomena in the North Atlantic Ocean and Arabian Sea

Abstract We review bio-optical and physical data from three mooring experiments, the Marine Light–Mixed Layers programme in spring 1989 and 1991 in the Iceland Basin (59°N/21°W), and the Forced Upper Ocean Dynamics Experiment in the central Arabian Sea from October 1994 to 1995 (15.5°N/61.5°E). In the Iceland Basin, from mid-April to mid-June in 1989, chlorophyll-a concentrations are sensitive to small changes in stratification, with intermittent increases early in the record. The spring increase occurs after 20 May, coincident with persistent water column stratification. In 1991, the bloom occurs 2 weeks earlier than in 1989, with a background of strong short-term and diurnal variability in mixed layer depth and minimal horizontal advection. In the Arabian Sea, the mixing response to the northeast and southwest monsoons, plus the response to mesoscale eddies, produces four blooms over the annual cycle. The mixed layer depth in the Arabian Sea never exceeds the euphotic zone, allowing interactions between phytoplankton and grazer populations to become important. For all three mooring experiments, change in water column stratification is key in producing phytoplankton blooms.

Biogeochemistry and ecology of Pyrosoma spinosum from the Central Arabian Sea.

A swarm of pelagic tunicate (Pyrosoma spinosum) was found in the surface open waters of the Arabian Sea during late southwest monsoon (September 2007). The swarm site was characterized by moderate southwesterly wind (approximately 7 m s-1), relatively low sea-surface temperature (approximately 26°C), shallow mixed layer (approximately 50 m), and substantial macro-nutrient concentrations (surface values: 2.5 μM nitrate, 0.3 μM phosphate, 0.9 μM silicate, and 5.0 μM ammonium). Despite adequate macronutrient availability, the swarm site was characterized by low diversity of phytoplankton (>5 μm) and mesozooplankton in the upper 200 m. Low chlorophyll a concentration (27.3 mg/m2 in the upper 120 m) at the swarm site was dominated (90% to 95% in the upper 40 m) by the Synechococcus (20 × 106 /ml). Silicate deficiency in surface waters upwelled or entrained from the thermocline may be a key factor for the dominance of smaller autotrophs (flagellates and cyanobacteria) that seems to offer favorable conditions for episodic occurrence of swarms of these filter feeders. Low carbon content (37% of total dry weight) and the lower molar(carbon-to-nitrogen) ratio (5) in P.spinosum suggestgrowth of these organisms is carbon-limited. Wedescribe various physicochemical and biological conditions at the P.spinosum swarmlocation and at two other nearby sites not affected by it. The biological factors predominantly high densities of Synechococcus and flagellates were best suited conditions for the proliferation of pyrosome biomass in the central Arabian Sea.