Waters Of Arabian Sea Research Articles

The Role of Turbulence in Redistributing Upper-Ocean Heat, Freshwater, and Momentum in Response to the MJO in the Equatorial Indian Ocean

AbstractThe role of turbulent mixing in regulating the ocean’s response to the Madden–Julian oscillation (MJO) is assessed from measurements of surface forcing, acoustic, and microstructure profiles during October–early December 2011 at 0°, 80.5°E in the Indian Ocean. During the active phase of the MJO, the surface mixed layer was cooled from above by air–sea fluxes and from below by turbulent mixing, in roughly equal proportions. During the suppressed and disturbed phases, the mixed layer temperature increased, primarily because of the vertical divergence between net surface warming and turbulent cooling. Despite heavy precipitation during the active phase, subsurface mixing was sufficient to increase the mixed layer salinity by entraining salty Arabian Sea Water from the pycnocline. The turbulent salt flux across the mixed layer base was, on average, 2 times as large as the surface salt flux. Wind stress accelerated the Yoshida–Wyrtki jet, while the turbulent stress was primarily responsible for decelerating the jet through the active phase, during which the mean turbulent stress was roughly 65% of the mean surface wind stress. These turbulent processes may account for systematic errors in numerical models of MJO evolution.

Feeding habits of milk shark, Rhizoprionodon acutus (Ruppell, 1837) in the Gujarat coastal waters of north-eastern Arabian Sea

The feeding habit of milk shark Rhizoprionodon acutus was investigated in 684 specimens collected along Gujarat coast (India) of north-eastern Arabian Sea from January 2013 to December 2014. The length range of females was 32–89.6 cm and males 33.5–89 cm. The shark foraged on diversified prey items which were pooled into four distinct groups i.e., teleosts, crustaceans, mollusks and annelids. Teleosts (Dietary coefficient, %QI=83.05 and index of relative importance, %IRI=78.40) were found to be the preferred food items followed by crustaceans (%QI=16.21; %IRI=19.78), which formed the secondary food item group. Mollusks (%QI=0.74; %IRI=1.69) and annelids (%QI=0.01; %IRI=0.14) constituted the accidental or accessory food items. The species, though is a pelagic predator, probably performs vertical movements in search of prey items. The shark also showed some sorts of preference and selectivity for clupeids, engraulids and carangids. Females though showed significantly lower index of relative fullness (IRF) (P≤0.5) and comparatively lower vacuity index and lower mean number of preys per stomach compared to the males, the preference for prey items was not found to be significantly different between the females and males. Juveniles were found to have significantly higher IRF (P≤0.5) and comparatively higher vacuity index than that of the adults, whereas the mean preys per stomach was found to be lower than that of the adults. Moreover, the prey preference was also significantly different between the juveniles and adults. The study provides necessary baseline information about the feeding habits of the shark in the region which will be helpful in understanding the trophodynamics of the species under the influence of overfishing and climate change.

Increased CO2 availability promotes growth of a tropical coastal diatom assemblage (Goa coast, Arabian Sea, India)

Responses of tropical coastal phytoplankton assemblages to increasing CO2 levels are poorly known. The Arabian Sea (AS), the western part of the north Indian Ocean, is an upwelling-induced, highly productive region, but there are virtually no studies from this area documenting the responses of the natural phytoplankton communities to increasing CO2 concentrations. CO2-induced growth responses of a diatom-dominated phytoplankton community from the coastal water of Arabian Sea (Dona Paula, Goa) were investigated in a set of three experiments (one long-term and two short-term) in nutrient-replete, semi-continuous batches. Significant increases in photosynthetic rate, growth rate, carbohydrate content and carbohydrate to protein ratio were observed in the cells grown under elevated CO2. Trace amounts of zinc (Zn) addition to low CO2 grown cells also resulted in a marked increase in growth rate, carbohydrate content and carbohydrate to protein ratio. Zn addition at elevated CO2 only induced increased cell division. These results show that the experimental diatom-dominated phytoplankton community, (1) responded positively to CO2 enrichment, (2) probably possesses a carbon-concentration mechanism (CCM) and (3) uses a Zn-carbonic anhydrase in carbon acquisition. It is likely that the experimental diatom communities downregulated CCM activity with increased CO2 but grew faster. Future increases in coastal water CO2 concentrations may significantly impact phytoplankton primary production and biochemical composition and thus have large biogeochemical consequences.

Evaluation of the Corrosion Rate of Different Types of Marine Ship Steel using Potentiostatic Test Method in Arabian Gulf Sea Water

The effect of flow velocity and chemical composition of Arab gulf sea water on the corrosion behavior of three different types of steel (DNV/EH40, AB/FH32 and ABS DQ47) used in building of marine ship hull structures investigated by using electrochemical Potentiostatic test method was. The results indicate that the general corrosion rate under flow condition is larger than that obtained under static case. Increasing the velocity of sea water leads to increase the corrosion rates for certain limit and then began to decrease. NV/EH40 steel exhibited a significantly lower values of corrosion rate, ABS DQ47 steel has larger values and AB/FH32 marine steel plates has moderate values.

Time-series analysis of chlorophyll-a, sea surface temperature, and sea surface height anomalies during 2003–2014 with special reference to El Niño, La Niña, and Indian Ocean Dipole (IOD) Years

ABSTRACTThe present study was carried out to find the variability of chlorophyll-a (chl-a) concentration, sea surface temperature (SST), and sea surface height anomalies (SSHa) during 2003–2014, covering the Bay of Bengal (BoB) and Arabian Sea (AS) waters. These parameters were linked with El Niño, La Niña, and Indian Ocean Dipole (IOD) years. The observed results during 2003–2014 were evaluated and it was found that the monthly mean value for 12-year data ranged as follows: chl-a (0.11–0.46 mg m−3), SST (27–31 °C), and SSHa (−0.2 to 20 cm). The annual mean range of chl-a for 12-year data was 0.1–0.23 mg m−3, the SST range was 27–28 °C, and the SSHa range was 2.14–13.91 cm. It has been observed that with the SST range of 27–28 °C and the SSHa range of 7–9 cm, the chl-a concentration enhanced to 0.20–0.23 mg m−3. With a higher SST range of 28–29 °C and with a positive SSHa range of 11–14 cm, the chl-a concentration appeared to be low (0.17–0.18 mgm−3). During normal years, SSHa was positive with the >5 to <10 cm range during the months of April–June, which coincided with an increase in SST, >2 to <4 °C. During the normal years, SSHa (>−0.2 to <−10 cm) was observed to be negative during October–December, with a decrease in SST (<3 °C) observed. The high monthly mean chl-a concentration (>0.3 to <0.5 mg m−3) was noticed during December–February in the BoB and AS. Compared to the BoB chl-a range (<0.4 mg m−3), a high chl-a concentration was observed in AS (>0.4 mg m−3). However, during the phenomenon years, the study area had experienced low chl-a (<0.2 mg m−3), high SST (>5 °C), and more positive SSHa (>10 to <20 cm) during January–March and October–December in AS and BoB. The present study infers that a positive IOD leads to low chl-a concentration (<2 mg m−3) and low primary productivity in AS. El Niño caused the down-welling process, it results in a low chl-a concentration (<1 mg m−3) in BoB and AS. La Niña caused the upwelling process, and it results in a high chl-a concentration (>2.0 mg m−3) in BoB and AS. In the recent past years (2003–2014), the intensity and frequency of El Niño, La Niña, and IOD have been increasing, evidenced with few studies, and have impacts on the Indian Ocean climate. Therefore, the influences of the relative changes of these phenomena on the BoB and AS need to be understood for productivity assessment and ocean state monitoring.

Scales and drivers of seasonal pCO2 dynamics and net ecosystem exchange along the coastal waters of southeastern Arabian Sea

The impact of seasonal coastal upwelling on the dynamics of dissolved inorganic carbon (DIC) and sea-air fluxes of CO2 along the coastal waters of Kochi was investigated during 2015, as a part of Ecosystem Modelling Project. The surface water pCO2 varied from 396 to 630μatm during the study period. Significant inter-seasonal variations were found in the distribution of physico-chemical variables and surface pCO2. An increase of 102.1μatm of pCO2 was noticed over a two-decade period with a rate of 5.3μatmy−1. There was an agreement between the fluxes of CO2 and net ecosystem production (NEP) with respect to the trophic status while NEP was higher than CO2 fluxes by a factor of 3.9. The annual net ecosystem exchange (NEE) was estimated to be 15.02mmolCm−2d−1 indicating that the coastal waters of Kochi are highly heterotrophic in nature.

A Model for Deriving the Spectral Backscattering Properties of Particles in Inland and Marine Waters From &lt;italic&gt;In Situ&lt;/italic&gt; and Remote Sensing Data

A model is developed for deriving the backscattering properties of phytoplankton [ $b_{\mathrm{ bph}}(\boldsymbol {\lambda })$ ] and nonalgal particles [ $b_{\mathrm{ bNAP}}(\boldsymbol {\lambda })$ ] from the total particulate backscattering coefficient [ $b_{\mathrm{ bp}}(\boldsymbol {\lambda })$ ] in turbid and productive coastal and inland water environments. In situ spectral particulate backscattering data acquired from several field campaigns on the turbid and productive coastal regions of southern India are used to develop a partitioning method and establish robust relationships between the partitioned $b_{\mathrm{ bph}}(\boldsymbol {\lambda })$ and $b_{\mathrm{ bNAP}}(\boldsymbol {\lambda })$ versus chlorophyll and turbidity. The performance of this method is assessed using independent data sets from marine and inland productive waters. The $b_{\mathrm{ bph}}(\boldsymbol {\lambda })$ and $b_{\mathrm{ bNAP}}(\boldsymbol {\lambda })$ products derived from this method are found in good agreement with in situ data, with the overall percentile error of a few percent which is well within the benchmark for a validated uncertainty of ±35% endorsed for the chlorophyll-a retrieval in oceanic waters. Further comparison with other inversion methods demonstrated the relative performance of the new model, especially in productive and algal bloom-dominated waters. To illustrate the use of the new model for remote sensing applications, it was applied to both multispectral Moderate Resolution Imaging Spectroradiometer-Aqua and hyperspectral (Hyperspectral Imager for the Coastal Ocean) images from the algal bloom-dominated waters of the Arabian Sea and turbid productive lagoon waters on the coastal region of the Bay of Bengal. The results were interesting, i.e., the existence of strong spectral features and inflections in the phytoplankton backscattering spectra in bloom-dominated waters and the nearly featureless NAP backscattering spectra varying approximately according to a power law in sediment-laden waters. Corroborating the previous experimental and theoretical studies, our results for algal bloom waters show a steep slope in the blue region and enhanced backscattering in the green and near-infrared regions due to the influence of phytoplankton absorption and backscattering. These results further suggest that living algal matter in productive waters may be a source of significant backscattering. Under typical nonbloom conditions in turbid coastal waters, our analyses confirm that the minerogenic and detrital particles are likely the most important because of their high abundance relative to living organic particles and their spectral dependences have profound implications for our understanding of the relationship between the backscattering to scattering ratio and particle size distribution and related properties. The new model clearly represents as an important tool permitting the analysis of the spectral backscattering coefficients associated with phytoplankton and nonliving components in sediment-laden and algal-bloom-dominated productive waters within coastal environments.

The Potential Socio-Economic Impact of China Pakistan Economic Corridor

Road and transport corridor play a significant role in the development of the country. This study examines the socio economic impact of China Pakistan Economic Corridor. Pakistan and China has great relations from the beginning. China Pakistan economic corridor is very significant for both countries as it provide access of China to the warm water of Arabian Sea through a short and safe route which will save billions of shipping charges and time and will cover the current energy crises in Pakistan and provide employment and trade opportunities for the local people which will minimize unemployment and poverty in Pakistan and will improve the living standard and boost the economy of Pakistan.

Evaluation of model simulated and MODIS-Aqua retrieved sea surface chlorophyll in the eastern Arabian Sea

In this study we assess the accuracy of sea surface Chlorophyll-a (Chla) retrieved from satellite (MODIS-Aqua), using standard OC3M algorithm, and from a Regional Ocean Modelling System (ROMS) biophysical model against in situ data, measured in surface waters of the eastern Arabian Sea, from April 2009 to December 2012. MODIS-Aqua OC3M Chla concentrations showed a high correlation with the in situ data with slope close to unity and low root mean square error. In comparison, the ROMS model underestimated Chla, though the correlation was significant indicating that the model is capable of reproducing the trend in in situ Chla. Time Series trends in Chla were examined against wind driven Upwelling Indices (UIW) from April 2009 to December 2012 in north-eastern (Gujarat) and south-eastern (Kochi) coastal waters of the Arabian Sea. The annual peak in Chla along the Kochi coast during the summer monsoon was adequately captured by the model. It is well known that the peak in surface Chla along the Kochi and Gujarat coasts during the summer monsoon is the result of coastal upwelling, which the ROMS model was able to reproduce accurately. The maximum surface Chla along the Gujarat coast during the winter monsoon is due to convective mixing, which was also significantly captured by ROMS biophysical model. There was a lag of approximately one week between the maximum surface Chla and the peak in the Upwelling Index.

Ambient Noise Analysis and Modeling in Shallow water of Arabian Sea

work presents a shallow water ambient noise analysis and modeling effort in the Arabian Sea. Real ambient noise recording at very shallow depths has been analyzed for two environmentally unique periods when the sound velocity profile and the wind conditions present distinct characteristics. Ambient noise data were collected for January and March in the shallow water of Arabian Sea between the wind speed 0.0 m/s to 4.11 m/s. The relative spectral energy distribution of sea noise is presented for a number of wind speeds. Linear relationship between the sea noise spectrum levels and the wind speed were found for the entire frequency range, but the slope were frequency dependent. The results of empirical fitting based on the analysis were used for noise level prediction and the model predictions were compared with the measured noise level. Such study post extensive validation has the potential to develop predictive models for ambient noise estimation based on recorded data. dependent coefficient. The ambient noise was also shown to be site specific in shallow water where speed of the sound and the bottom properties vary substantially with the location. Gayer & Wille (1984) in their work made measurements in North Sea and Baltic Sea and reported that the influence of propagation loss on the wind dependent shallow water noise appears to be marginal even at extremely different area. It was observed that the contradicting behavior of the summer and winter noise levels was due to wind stress at sea surface. Similar work by Ingenito (1989) on site dependence of wind- dominated ambient noise in shallow water reported lower noise level of around 2 dB in the silt bottom at wind speed of 7 m/s when compared to sand bottom at wind speed 3-5 m/s and variability of 10 dB in the average noise spectrum level was observed in the essential fraction of the entire range of the spectrum. The large spread of wind generated noise levels measured for the same wind speed and wave height in the absence of other noise sources is essentially attributed to the site dependence and the primary site dependent factor that influence the noise level being acoustic propagation which in turn are dependent on the season, ocean depth and the bottom composition . Piggott (1964), has reported a logarithm relation for the ambient noise and the wind speed based on his study at two depths in very shallow water (36 m and 51 m), where the non wind dependent factors were reported to be not significant and the contribution of the wind dominated source was observe to be dominant at nearly all wind speeds and all frequencies. The relation has been extensively validated by numerous researchers and adapted for varying sites. The relation is presented below for reference:

Wave hindcast studies using SWAN nested in WAVEWATCH III - comparison with measured nearshore buoy data off Karwar, eastern Arabian Sea

Waves in the nearshore waters (~15m water-depth) of eastern Arabian Sea were simulated using SWAN nested in WAVEWATCH III (WW3) for the year 2014. The sensitivity of the numerical wave model WW3 towards different source term (ST) packages was tested. The performance of WW3 was evaluated with the measured deep water buoy data at 15.0000° N, 69.0000° E. Overall, the WW3 wave hindcast results using ST4 physics in deep water show a reasonable match (r=0.97 and SI=0.16) with the measured significant wave height (Hs). In deep water, a large overestimation (~23.7%) of mean wave period was observed using ST2 compared to ~8.2% using ST4. Komen et al. (1984) whitecapping formulation was found suitable based on the sensitivity studies of SWAN. Simulated Hs using nested SWAN model shows good correlation (r=0.96) with the nearshore measured data. During high sea state (Hs>2m), an overestimation (~19.8% for ST2 and ~21.4% for ST4) and during low sea state (Hs<1m), an underestimation (~16.1% for ST2 and ~31.5% for ST4) in mean wave period was observed in the nearshore. WW3 model forced by modified wind field (speed increased by 5%) during June and July resulted in a reduction in underestimation of Hs from 8.6% to 2.2%.

Ecological measurements of phytoplankton communities in the Arabian Sea water off Kerala and Karnataka coasts (South India)

Phytoplankton diversity, abundance and distribution were studied in the Arabian Sea water off Kerala and Karnataka coasts. The study was carried out from November 2009 to October 2011 on a seasonal basis and the relationship between physico-chemical characteristics of coastal waters and phytoplankton communities were analyzed statistically at sixteen sampling sites. During the study, Shannon-Weiner diversity index (J') varied from 3.271 to 4.139, Pileou's evenness (J') ranged from 0.935 to 0.980 and Margalef's richness index (d) varied from 6.511 to 12.021. The seasonal variation of H' showed high diversity during post-monsoon season followed by pre-monsoon and monsoon seasons, respectively. Based on NMDS analysis, the similarity of the phytoplankton of the sampling sites belonging to the same shoreline exhibited high species similarity. The phytoplankton abundance was positively related to pH, temperature, salinity, chlorophyll-a and nutrients and negatively correlated with dissolved oxygen, turbidity and silicate. Out of the 185 species of phytoplankton identified, the diatoms, namely Cymbella aspera, Licmophora abbreviata and Skeletonema costatum were found to be dominant while Oscillatoria limosa was the prevailing cyanobacterium.

Passive Acoustic Recognition of Fishing Vessel Activity in the Shallow Waters of the Arabian Sea: A Statistical Approach

The ambient noise system consisting of a vertical linear hydrophone array (VLA) with 12 elements was deployed in the waters of the Arabian Sea at a depth of 16[Formula: see text]m, off Goa, India, for extracting the ambient noise during the fishing season (March, April and May, 2013 before the onset of the south west monsoon). This study focuses on fishing vessel activity by finding the domination of the vessel and wind noise at two 12 hourly periodic cycles that start at midnight and noon, using the statistical analysis. It is performed using statistical parameters, like the mean, median, and standard deviation, skewness, percentile and spread of data. It is observed that the vessel noise dominates during the 12[Formula: see text]h period that starts at midnight and is an indication of the activity of fishing vessels while the wind generated noise is more during the 12[Formula: see text]h period that starts at noon, which is a sign of the domination of the sea breeze effects. This is the first time that a statistical analysis has been carried out to study the ambient noise data collected off Goa, in order to find the fishing vessel activity during the pre-monsoon season. The results are verified with the fishing information from the Directorate of Fisheries, Goa, ship traffic data from the Mormugao Port Trust and wind speed measurements.

Evaluation of the influence of monsoon climatology on dispersal and sequestration of continental flux over the southeastern Arabian Sea

Associated with the coupled land–ocean heating–cooling, seasonal reversal of monsoon winds regulates hydrography, precipitation, upwelling, fluvial influx and a host of biogeochemical processes. We evaluate the role of the monsoon winds, fluvial discharge and currents on continental flux dynamics from advection magnitude of clays over an anomalous high precipitation regime of the southeastern Arabian Sea. Coupled with the intense rains and a high fluvial discharge, we archived an elevated content of detrital clays (>41mgl−1) during the southwest monsoon over the <40m water depth. The deeper waters (>40m), however, had much reduced detrital clays (<6mgl−1) year round. Therefore, two distinct environments (high and low detrital clays) prevailed over the shelf. Derived from the variations in the clays in the local riverine discharge and in the seawater loads, we found a dominant role of the southwest monsoon winds and that of the winter and the southwest monsoon (summer) coastal currents over the dispersal and sequestration processes. Over the inner shelf, an alongshore (equatorward) advection and merging of several, small, fluvial plumes by the southwest monsoon winds had sequestered most of the local fluvial discharge over the shallow region only. The outer-shelf received detritus mostly as a result of (a) the high salinity Arabian Sea Water during the southwest monsoon, (b) through the Bay of Bengal Water and (c) by way of aeolian supply from the Arabia and the Somalia.

Long-term patterns of net phytoplankton and hydrography in coastal SE Arabian Sea: What can be inferred from genus level data?

Based on 21 years of phytoplankton and environmental monitoring data from the surface water of coastal southeast Arabian Sea, we demonstrate a shift in phytoplankton community towards higher sample genus richness and diatom abundance during the two decades of observations. Analyses were based on 587 water samples collected between 1990 and 2010 and additionally revealed marked long-term changes in sea surface temperature (SST), sea surface salinity (SSS) and turbidity (Secchi depth). The abundances of dinoflagellate genera were positively correlated to SST, particularly during and after the 1997–1998 El Niño Southern Oscillation event. Several diatom genera increased in abundances with decreasing turbidity around and after 1995–2000, mainly the large celled genera Coscinodiscus spp, Odontella spp and Ditylum spp. In 1996–2000, sample genus richness increased from mean of 8–14, and decreased to 11 thereafter. The increase in sample genus richness was linked to the more frequent presence of most taxa during 1996–2000. Increasing Secchi depth and abundance of frequently encountered large-celled diatoms concurrent with decreasing abundances of Trichodesmium spp might signal the alleviation of the nitrogen limitation in the region, favouring diatoms.

ARC. GIS Based Interpretation of Ground Water Pollution: A Case Study of Coastal Area District Badin, Sindh, Pakistan

The border line of Arabian Sea, near district Badin, Sindh, Pakistan was selected as a case study for the investigation of heavy metal concentration in ground water. Ground water is used for human consumption as well as agricultural purposes due to shortage of fresh water in the area. Badin Coastal area is poverty stricken, socio-economically backward, politically ignored, frequently affected and hit by natural disasters. Area is known by stormy cyclones, monsoon heavy rains, floods, tidal sea water intrusion and sea erosion. Study area has not been highlighted properly yet. Highly polluted waste water of Mega project Left Bank Outfall Drain (LBOD) which carries complexes of pollutants of agriculture run off, industrial and municipal effluents of different districts, are being disposed into Arabian Sea, passing through coastal areas of district Badin, contaminating lakes, lagoons, ground water and soils. Tidal sea water of Arabian Sea also pollutes ground water through seepage. The attempts were made to understand the effects on ground drinking water and results were interpreted by Arc GIS maps, to meet the current trends of research. The objective was to investigate ground water pollution extent in the area due to drains and tidal sea water. Sixteen (16) samples of ground water were collected from different villages near coastal area. The average data of various parameters of ground water near coastal were: pH 7.40 ± 0.3758, E.C(ms/cm) 7.45±6.8166, TDS(mg/L) 4044±3876.8, Cu 0.1821±0.486, Zn 0.1399±0.205,Co (mg/L) 0.0900±0.094, Cd(mg/L) 0.0244±0.010, Fe(mg/L) 0.3260±0.591,Ni(mg/L) 0.0055±0.012, Pb(µg/L) 0.1312±0.144. The evaluated results indicated that Left Bank Outfall Drain and sea water intrusion are major contributor for ground water pollution and ground water was found to be unfit for drinking purposes.

SEASONAL ASSESSMENT OF HYDROGRAPHIC VARIABLES AND PHYTOPLANKTON COMMUNITY IN THE ARABIAN SEA WATERS OF KERALA, SOUTHWEST COAST OF INDIA

The seasonal variation of the hydrographic variables and phytoplankton species in the Arabian Sea waters of the Kerala coast, Southern India was investigated during different seasons. The variables such as pH, temperature, salinity, turbidity and chlorophyll-a contents of water were found to be high during pre-monsoon season and the dissolved oxygen content was minimal. The concentration of nutrients viz., nitrate, phosphate, silicate varied independently. In the study a total of 53 species of phytoplankton were recorded. Their density was higher during the post-monsoon season than during other seasons and the diatoms were found to be the dominant species. The major phytoplankton in terms of frequency and abundance were the species namely, Biddulphia mobiliensis, Chaetoceros curvisetus, Licmophora abbreviata, Skeletonema costatum, Prorocentrum micans and Oscillatoria sp. They showed significant positive correlation with pH, temperature, salinity, nitrate, phosphate and chlorophyll-a contents, whereas turbidity, dissolved oxygen and silicate exhibited significant negative correlation. The Principal Component Analysis (PCA) developed two principal components with 84.74% of total variability in the water quality which separated pre- and post-monsoon periods from the monsoon season on axis I, and pre-monsoon and monsoon periods from post-monsoon on axis II.

Wind induced ambient noise modelling and comparison with field measurements in Arabian Sea

The present work focuses on modelling of wind induced ambient noise and comparison of noise properties with field measurements. Time series ambient noise data were taken from shallow waters of Arabian Sea, using a noise recording system consisting of a vertical array with twelve hydrophones. From the field measurements vertical coherence and noise spectral level were estimated and compared with the predictions from Harrison’s formulas for coherence and noise level based on ray theory. Considering that the noise generated by the action of wind dominates in mid frequency range, this study concentrates on the band 0.2–5kHz. Modelled results of both coherence and noise level compare well with field measurements. Moreover the simulated fluctuations in noise level with wind speed are in accordance with measured data. This is an initial attempt made to simulate noise properties in Arabian Sea and the model can be further used for geoacoustic inversion for extracting seabed parameters of the site.

Impact of water mass mixing and dust deposition on Nd concentration and εNd of the Arabian Sea water column

The concentration and isotopic composition (εNd) of dissolved Nd have been measured in the sub-oxic/denitrifying water column of the eastern Arabian Sea to explore the impact of water mass mixing and release from particles. Dissolved Nd in the north-eastern Arabian Sea is more radiogenic (εNd: −7 to −10) compared to its south-eastern part (εNd: −11 to −15) suggesting contribution of Nd from the Bay of Bengal (BoB). The vertical profile of Nd typically show higher values in surface waters (15.8–27.8pmol/kg), followed by minima (12–18pmol/kg) in the subsurface waters (200–300m) and a gradual increase with depth thereafter. The Nd concentration does not seem to show any changes in the sub-oxic layers suggesting no impact of the sub-oxic/denitrifying conditions in this oceanic region on the biogeochemistry of Nd. The enrichment of Nd in surface waters can be explained either by supply from high Nd low salinity waters from the BoB to the Arabian Sea via the East India Coastal Current (EICC) or, alternatively the Nd surface excess can be result of release from aeolian dust depositing over the sea surface. Inverse modeling computations suggest that in addition to Nd contributed from water mass mixing, some additional excess Nd (Ndxs) is required to balance the Nd inventory in the water column. There is significant Ndxs in the surface waters of the north-eastern Arabian Sea with εNd∼−6, similar to that of dust depositing over the Arabian Sea. The fractional solubility of Nd released from the aeolian dust was estimated to be varying from ∼3% to 17% (for excess Nd inventory per unit area (Ndxs∗) of 20μgm−2 with τNd ranging from 1 to 3years). This study highlights the significance of aeolian dust deposition in controlling the abundance and distribution of Nd in the Arabian Sea, the western arm of the northern Indian Ocean, lying in the proximity to the arid landmass and characterized by high lithogenic aeolian dust deposition.

Decadal Variations in Oceanic Properties of the Arabian Sea Water Column since Geosecs

This article reports temporal changes in the measured oceanic geochemical properties of the Arabian Sea and the equatorial Indian Ocean by reoccupying six stations investigated during the GEOSECS expedition in 1977 and 1978. Observed differences are interpreted in terms of plausible changes in the environment and climate that have occurred in response to natural or anthropogenic processes. The depth profiles of major parameters such as dissolved oxygen, ΣCO2, major nutrients (silicates, nitrates, and phosphates), and radiocarbon in dissolved inorganic carbon were measured during the cruises between 1994 and 1998 along with temperature and salinity. Most stations in the Arabian Sea show an increase in salinity by ∼0.2 to 0.3 salinity units in the top 400 m, whereas one station in the equatorial Indian Ocean showed a decrease in salinity by ∼0.1 units, indicating a likely change in the evaporation-precipitation (E-P) balance. The ΣCO2 increased by an average of 8 μM within the top 1200 m of the Arabian Sea. The depth profiles of nitrates and dissolved oxygen for the central Arabian Sea stations show significant variations, while only marginal changes are seen for silicates and phosphates relative to the GEOSECS data. The decrease in Δ14C of surface waters is due to the steady decrease in atmospheric 14C concentration since GEOSECS, and the Δ14C increase in subsurface waters is attributed to the downward vertical diffusion of bomb 14C interpreted in terms of atmosphere to ocean transfer and lateral advection of water masses.