Arabian Sea Region Research Articles

Hydrological Cycle in the Arabian Sea Region from GRACE/GRACE-FO Missions and ERA5 Data

Hydrological Cycle in the Arabian Sea Region from GRACE/GRACE-FO Missions and ERA5 Data

Recent two decades witness an uptick in monsoon depressions over the northern Arabian Sea

The analysis reveals that the occurrence of summer Monsoon Depressions (MDs) over the North Arabian Sea is doubling during 2001–2022 compared to the 1981–2000 period. This increase stems from changes in the region’s dynamic and thermodynamic conditions. The heightened genesis potential parameter with sea surface temperature and moisture flux transport and its convergence over the North Arabian Sea inducing MDs formation, contrasting to the Bay of Bengal. The dynamic processes involved in its formation, a combination of barotropic and dynamical instability, are leading to increased rainfall over northwestern India. Strong East Asian jet variability, with an anomalous anticyclone in the north and weak cyclonic anomalies in the south, induces prevailing easterly wind anomalies along the monsoon trough. This leads to a poleward shift (~1.13°) in the low-level jet, significantly altering dynamic and thermodynamic parameters in the northern Arabian Sea region leading to a notable increase in MDs.

Performance of a Ship-Based Cupronickel Alloy in Exposure Conditions of Arabian Seawater-A Comparative Study.

Cupronickel-based alloys are widely known for their excellent resistance against aqueous corrosion, however, they can be susceptible to corrosion at accelerated rates and premature failure when exposed to a polluted or brackish seawater medium, even for short-term exposure durations. This unfamiliar corrosion behavior may be a result of the formation of an unprotected corrosion film during the early exposure durations. The paper investigates the corrosion phenomenon in cupronickel 90/10 alloy, by exposing the coupons in two different seawater compositions in the Arabian Sea region. Corrosion losses were investigated on the experimental coupons in a submerged position, for a maximum exposure duration of 150 days, using the conventional weight loss method and a new dimensional metrology-based measurement technique. Additionally, in this research the tubes of a marine heat exchanger having similar material that failed prematurely during operation in the Arabian Sea were also investigated for corrosion losses, followed by the characterization of the corrosion deposits using following analytical techniques: SEM, EDS, XRD and Raman Scattering. The experimental results showed significantly higher corrosion losses on coupons exposed to seawater site rich in pollutants and nutrients including dissolved inorganic nitrogenous compounds, compared to those subjected to a natural seawater solution in corrosion tanks maintained in a controlled environment.

In-situ observations of cloud microphysics over Arabian Sea during dust transport events

The unique in situ measurements of clouds and precipitation within the shallow and deep cumulus over the north-eastern Arabian Sea region during the Indian monsoon are illustrated in this study with a focus on droplet spectral parameters. The observational period showed a significant incursion of Arabian dust and the presence of giant cloud condensation nuclei (GCCN), modifying the cloud and precipitation spectral properties. Warm rain microphysics supported the mixed-phase development in these clouds and exhibited hydrometeors of snow, graupel and large aggregates as part of ice processes. Cloud base droplet number concentration is about 142 ± 79 cm−3 which is one third of the cloud condensation nuclei (CCN) number concentration at 0.2% supersaturation. A rapid broadening of droplet size distribution (DSD) near to the cloud base was noted in contrast to polluted continental clouds. Relationship between the relative dispersion ( ε; the ratio of DSD spectral width ( σ ) to mean radius ( rm )) and liquid water adiabatic fraction (AF) indicates that the entrainment effect has increased relative dispersion significantly (2–3 times larger) in these clouds. Effective radius ( reff ) is found to be proportional to mean volume radius ( rv ) with a proportionality constant ( β ) that varies between 1.0–1.6, depending on the spectral dispersion parameter. Drop size distributions for the small cloud droplets with size range 2–50 μ m and the large drizzle drops (or ice hydrometeors) with size range 100–6400 μ m are parameterized using the gamma function distributions useful for large-scale cloud models.

Prediction of sea-surface salinity using MODIS and Sentinel-3 data

ABSTRACT Precise estimation of Sea Surface Salinity (SSS) is of prime importance to know marine physical and biochemical processes. In this manuscript we have utilized the Random Forest (RF) and Support Vector Regression (SVR) to estimate the SSS concentration. Particularly, we have utilized the global SSS values provided by the Copernicus Marine Services. We have also extracted the Aqua MODIS and Sentinel-3 data from the Google Earth Engine platform. The complete range of SSS varies approximately from 6 psu to 38 psu which enables the global application of the dataset. We have also studied the importance of different wavelength bands and their significance to SSS using the RF model. It is found that wavelengths around 400 nm have their highest significance due to the sensitiveness to Chromophoric Dissolved Organic Matter (CDOM) and other related constituents. For the regression analysis we have also utilized the Laplacian kernel in the SVR. It is found that the SVR with Laplace kernel outperforms the RF and SVR with Radial Basis Function (RBF) kernel. The trained algorithms are used to estimate the SSS content over the Bay of Bengal and the Arabian Sea regions which show interesting variations in SSS content due to diverse climatological conditions.

Characterization of internal solitary waves in the Andaman Sea and Arabian Sea using EOS-04 and sentinel observations

ABSTRACT In this study, we synergistically used Synthetic Aperture Radar (SAR) and optical observations to characterize the Internal Solitary Waves (ISW) in the Andaman Sea and Arabian Sea region. Total 25 scenes combined from EOS-04, Sentinel-1&2 satellites are considered for the year 2022. Various approaches, including Fast Fourier Transform (FFT), Multiple Image Comparison (MIC), Tidal Time Period (TTP) and Korteweg-de Vries (KdV) equations, were employed to characterize the ISW, and its spatial and seasonal variations are also analysed. The dominant wavelength of ISW over the Andaman Sea in June 2022 is found to be 6500 m travelling towards the Andaman coast. The overall phase speed of ISW in the Andaman Sea lies in the range of 1.87–2.60 ms−1. It shows a seasonal variation of around 16% between winter and summer months, where a significant variation in the seasonal mixed layer depth was noticed. The phase speed also decreased as the ISW travelled from deep water to shallower regions. In contrast, the dominant wavelength of ISW in the continental shelf region of the west coast of India (near Goa) was much smaller (~400 m) compared to the Andaman Sea. Here, the ISW phase speed estimated using MIC and TTP methods was ~ 0.56 ms−1 in February 2022. The KdV equation yielded lower estimates of phase speed compared to the other two methods. The use of multiple satellite images and various approaches possess an advantage in comprehending the spatial and temporal variations in the characteristic features of ISW.

Constraints from GPS measurements on plate coupling within the Makran subduction zone and tsunami scenarios in the western Indian Ocean

SUMMARY Plate-coupling estimates and previous seismicity indicate that portions of the Makran megathrust of southern Pakistan and Iran are partially coupled and have the potential to produce future magnitude 7+ earthquakes. However, the GPS observations needed to constrain coupling models are sparse and lead to an incomplete understanding of regional earthquake and tsunami hazard. In this study, we assess GPS velocities for plate coupling of the Makran subduction zone with specific attention to model resolution and the accretionary prism rheology. We use finite element model-derived Green's functions to invert for the interseismic slip deficit under both elastic and viscoelastic Earth assumptions. We use the model resolution matrix to characterize plate-coupling scenarios that are consistent with the limited spatial resolution afforded by GPS observations. We then forward model the corresponding tsunami responses at major coastal cities within the western Indian Ocean basin. Our plate-coupling results show potential segmentation of the megathrust with varying coupling from west to east, but do not rule out a scenario where the entire length of the megathrust could rupture in a single earthquake. The full subduction zone rupture scenarios suggest that the Makran may be able to produce earthquakes up to Mw 9.2. The corresponding tsunami model from the largest earthquake event (Mw 9.2) estimates maximum wave heights reaching 2–5 m at major port cities in the northern Arabian Sea region. Cities on the west coast of India are less affected (1–2 m). Coastlines bounding eastern Africa, and the Strait of Hormuz, are the least affected (<1 m).

Distribution of microplastics in seafloor sediments and their differential assimilation in nearshore benthic molluscs along the south-west coast of India

Spatial and temporal distribution of microplastics (MPs) in the nearshore seafloor sediments along the Southwest coast of India and their patterns of accumulation in selected infaunal and epibenthic molluscs with diverse feeding strategies were investigated. Along the 300-km coastal stretch, which is one of the most productive and biodiversity rich regions of the eastern Arabian Sea, notable levels of MP contamination in both sediment (617.7 items/kg dry weight) and molluscs (5.39 items/g) was recorded. The concentration of MPs in sediments also varied seasonally, with a higher prevalence during the post-monsoon season. Among the four molluscan groups studied, the highest MP abundance was recorded among scavenging gastropod Pseudominolia biangulosa (9.13 items/g), followed by microcarnivore scaphopod Tesseracme quadrapicalis (5.96 items/g). In comparison, the suspension feeding bivalve, Anadara hankeyana and deposit feeding clam Jitlada philippinarum had lesser accumulation of MPs (2.98 items/g and 3.50 items/g respectively). The majority of MPs in sediments and within molluscs were less than 250 μm in size (89.14%) and were predominantly fibres and fragments. Chemical characterisation of MPs revealed eleven types of polymers dominated by polyethylene (PE) and polypropylene (PP). Present study identified positive correlations between ingested MP polymers and the feeding strategies of molluscs. Higher values for the ecological risk assessment indices (PHI, PLI and PERI) in most of the stations indicated the severity of plastic pollution in the region. Molluscs being a major contributor to the benthic food web is also a connecting link to higher trophic levels. Hence understanding the specificity in the MPs accumulation pattern within this group has far reaching significance in utilizing them as potential bioindicators for pollution studies in marine ecosystems.

Diapycnal mixing induced by salt finger and internal tides on the northwest coast of India

Microstructure measurements of velocity shear from the continental slope of the northwest coast of India (NWCI) in the eastern Arabian Sea are used to quantify the relative importance of double diffusion and internal tides induced diapycnal mixing in the different depth layers. It is found that the hydrographic conditions in the NWCI are conducive to the formation of moderately strong salt fingering (Turner angle between 55° and 72°). However, salt finger-induced vertical mixing dominates only in the upper 180 m of the water column, below which intense shear-driven turbulent mixing due to internal tide reduces its significance. As a result of this, the staircase structures, a measure of salt finger dominance in the water column, are frequent, and the mean temperature change across the interface (DTIH; 0.33 °C) is relatively larger in the upper 180 m compared to sporadic occurrence of steps with a small magnitude of DTIH (0.18 °C) below 180 m. It is also found that in the upper 180 m of the water column in the NWCI, mean diapycnal diffusivity (Kρ) is approximately a factor of eight larger (8.3 ± 1.3 × 10−5 m2s−1) than the estimation in the open ocean region of the eastern Arabian sea (5.4 ± 1.1 × 10−6 m2 s−1). However, due to internal tides, the magnitude of Kρ reaches as large as O (10−2) m2 s−1 below 180 m in the NWCI. The mean downward heat flux estimated in the salt finger-dominated (upper 180 m) layers is ∼ -6.1 Wm-2, and the shear-driven mixing-dominated layers (below 180 m) is ∼ -10.2 Wm-2.

Millennial‐Scale Changes in Atmospheric Nitrous Oxide During the Holocene

AbstractNitrous oxide (N2O) is an important greenhouse gas which destroys the ozone in the stratosphere. Primary sources of atmospheric N2O are nitrification and denitrification in terrestrial soils and the ocean, and the main sink is photolysis in the stratosphere. Studies have mostly focused on the climate‐related response of N2O during glacial‐interglacial periods. However, its mechanism of variation during the Holocene remains unclear. We present a high‐resolution N2O record from the South Pole Ice (SPICE) core covering the Holocene epoch. The millennial‐scale N2O trend agrees with existing records. We constructed a Holocene composite consisting of the new N2O measurements in SPICE and existing records from other ice core sites. The N2O composite reveals four distinct periods of N2O variation during 11.5–10.0 ka, 10.0–6.2 ka, 6.2–2.2 ka, and 2.2–1.4 ka, including two maxima in 11.0–10.0 ka and 3.0–2.2 ka and minima in 8.8–6.2 ka and approximately 1.4 ka. Apart from these, our new high‐resolution record from SPICE shows a short‐term N2O decrease around 2.8 ka which is not observed in other records possibly due to lower sample resolution and/or higher age smoothing. Comparison of our new Holocene N2O composite with the paleo‐proxy records suggests the plausible linkage of major monsoon (Asian, North African, South and North American, and Australian‐Indonesian monsoon) and upwelling (Arabian Sea and Eastern Tropical South Pacific) regions in regulating the atmospheric N2O during the Holocene.

Spatio-temporal patterns of the multi-day fishing in the Indian Ocean off Sri Lanka, and their importance for fisheries monitoring, control and surveillance

In the present study, an attempt was made to investigate temporal and spatial patterns of multi-day fishing in the Indian Ocean off Sri Lanka to elucidate how such patterns related to fishing methods of multi-day fishing vessels targeting different pelagic fish species. In the analysis, 269,564 fishing operations from 37,349 fishing trips of multi-day fishing vessels during 2019 and 2020 were used. Based on the standardized fishing effort of the three major fishing methods (i.e., longlining, drift gillnetting and ring netting), species-wise catch per unit effort (CPUE) was determined for each fishing operation.The CPUE values of the three fishing methods were grouped for fishing locations in three major basins of the Indian Ocean, and according to three monsoonal seasons having varying influences of cyclones. Accordingly, 269,564 fishing operations were summarized into 77 datasets during the study period. The mean CPUE values (kg per fishing operation) were 257.3 for longlining, 163.3 for drift gillnetting and 1642.7 for ring netting. The patterns of the data matrix were investigated using the self-organizing map routine of the Artificial Neural Network. Accordingly, based on the occurrence of target species, data were classified into six clusters from which spatio-temporal patterns of fishing operations could be visualized. Ring netting was predominantly concentrated in the equatorial region and to a lesser extent in the Bay of Bengal region, with a temporal occurrence during all three seasons. Longline fishing was also predominant in the equatorial Indian Ocean region with the majority registered in the non-cyclone period. Most drift gillnet fishing operations were concentrated in the equatorial region and the Arabian Sea region, especially during the northern Indian Ocean cyclone period. These results would provide important pieces of information for the management of pelagic fish species through monitoring, control and surveillance (MCS) of the fishery.

Chinese Investments in Maritime Sectors of Oman and UAE

This paper reviews Chinese investments in the maritime sectors of Oman and the United Arab Emirates (UAE), focusing on the potential challenges and opportunities that lie ahead. The study highlights China’s increasing influence in the Middle East (ME) and Africa, with a focus on the Belt and Road Initiative (BRI) and the development of key ports and infrastructure projects at Duqm and Khalifa ports. It looks into the strategic importance of Oman and UAE as the key regional players and their relationship with China in the current global context. China’s expanding naval footprints in the Arabian Seas Region (ASR) pose potential challenges to US interests. China, however, has not pronounced any of its military objectives and has extensively but strategically invested in developing key infrastructure in littoral states to avail significant advantages. In the backdrop of the evolving dynamics of great power competition and maritime security in the Arabian Sea region, this study indicates towards possible ways in which the US, China and the regional states need to take and address the challenges posed to each of them. This may help them strengthen their partnerships and balancing their strategic interests in the region.

Foreseeing the spatio-temporal offshore wind energy potential of India using a differential weighted ensemble created using CMIP6 datasets

Offshore wind energy assessments help in identifying suitable locations for offshore wind farms. Its importance is further amplified in the context of climate change as wind power potential is susceptible to it. The present study aimed to assess the offshore wind potential of India and its sensitivity to climate change with the help of two different ensemble variants developed using nine CMIP6 Global Climate Models (GCMs). First ensemble is created with equal emphasis on all GCMs, while differential weights derived using Shannon entropy technique is used to develop the other ensemble. Created ensembles are further compared with ERA5 data. Comparative results suggest that differential weighted ensemble is superior to uniform weights in terms of bias. Owing to this, weighted ensemble is further used to study the impact of climate change on wind power density (WPD) for the near (2021–2045) and far-future (2075–2099) periods under two shared socioeconomic pathways (SSP) scenarios, i.e., SSP2-4.5 and SSP5-8.5, the most widely used and probable scenarios. Findings suggest that WPD variation in the study area ranges between ＋10% and −20%. These variations are examined to study further the impact of climate change on geographical variations of WPD distributions. With the regions in Arabian Sea as an exception, WPD appears to increase in future scenarios. WPD varies more in far-future scenarios compared to near-future scenarios. The future variations of the WPD across study areas are prominent in the case of SSP5 - 8.5 compared to the variations noted in the case of SSP2-4.5. Findings of this study help stakeholders to understand the impact of climate change on offshore wind potential. Moreover, plots showing the variation of WPD for near and far-future scenarios complemented with additional studies help in choosing an appropriate location to tap the offshore wind potential in India.

Marine Proxies Of Paleomonsoon: Emphasis On Indian Monsoon System

Marine proxies are the most prominent tools in the history of the monsoon. Palaeomonsoon is the seasonalmovement of wind belts and differential heating of land and water caused by the interaction between theocean and atmosphere. The Indian monsoon is distinct from other monsoon systems in terms of its focalpoints, associated air masses, and mechanism for precipitation. It has two distinct phases: the southwestmonsoon (June to September), which brings the majority of the precipitation to India and the northeastmonsoon (November to February). The tropical monsoon has been proposed as a major modulator of thenorthern hemisphere climate and a major player at the end of the last ice age. Monsoon rainfall is recorded insediments deposited on the ocean floor from the Arabian Sea region. Humidity and temperature are measuredthrough physical and chemical parameters, and isotopic compositions are used to understand temperature,atmospheric composition, and ocean salinity. Microfossil assemblages and abundances of species are alsoimportant indicators of how physical and chemical factors affect biota.

Exploring biophysical links of catch rate from mid-water trawl fishery in the north-west coast of India, Eastern Arabian Sea: A remote sensing approach

Fish frequently shift their distribution ranges as a result of changes in preferred environmental factors. Knowledge on distribution of fish in relation to their environmental optima is crucial for improving the understanding of fishing grounds and planning sustainable exploitation. This study investigated the monthly variability in environmental factors impacting the catch rate and the spatio-temporal distribution patterns of fish along northwest coast of India (NWCI) from 2017 to 2019. The time series images of sea surface temperature (SST), sea surface height anomaly (SSHa), chlorophyll-a concentration (CHL), and euphotic depth (Zeu) indicate close coupling between bio-physical variables in the region. Generalized Additive Model (GAM) applied to the catch rate of mid-water trawlers found that CHL and Zeu were the most influential variables for ribbonfish distribution, while CHL and SST influence squid distribution and SST and SSHa influence cuttlefish distribution. The total deviation explained 37.70 %, 10.70 %, and 22.28 % in the final model for simulating the spatio-temporal distribution of ribbonfish, squid, and cuttlefish, respectively. Environmental factors were significant in the final GAM model (p < 0.05). Spatio-temporal distribution patterns of fishery resources from mid-water trawlers showed that they were related to biophysical changes in the northwest coast of India. The high catch rate was observed in the northern latitudes during the post-monsoon (October), the major fishing season in the region. However, it shrank and shifted to southern latitudes during the winter (January). This study could be adapted to nearby countries in the Arabian Sea region for an effective and useful tool to monitor and manage the fishery resources. Furthermore, it could aid in decision-making for policymakers and resource managers to formulate strategies for holistic marine fishery management and sustainable resource exploitation.

Trace metals as a redox proxy in Arabian Sea sediments in and below the oxygen minimum zone

Sedimentary trace metals are widely used to reconstruct past bottom water redox conditions. The calibration of trace metals as a redox proxy for oxygen minimum zones (OMZs) can still be improved. Here, we combine pore water and solid phase Mo, U, Re and V profiles with Fe and Mn data for ten sites along a bottom water oxygen (2 to 83 μmol L−1 O2) and water depth gradient (885 to 3010 m) in and below the perennial OMZ in the northern Arabian Sea (Murray Ridge). Trends in sedimentary Mo, U and Re contents generally follow ambient bottom water redox conditions, with the highest enrichments in OMZ sediments, supporting the validity of these trace metals as redox proxies. Vanadium and Fe are exclusively enriched in the sediments of the most anoxic OMZ site and do not capture further redox changes in and below the OMZ. We attribute the absence of a redox trend in sedimentary Fe content to the mildly reducing conditions in the sediments, with little FeS formation and benthic release of Fe. Manganese, in contrast, is depleted in the OMZ sediments and enriched in sediments below the OMZ, in accordance with loss from OMZ sediments and transfer of Mn to deeper sites (“Mn shuttling”). Manganese oxides are likely a key carrier of Mo and V to the sediments, especially below the OMZ, while diffusion across the sediment-water interface supplies U. Sedimentary Mo, U and V contents in the present-day Arabian Sea OMZ are generally lower than observed in other perennial OMZs. This may be related to a lower input of organic matter in this part of the Arabian Sea when compared to other OMZs, and hence, less anaerobic degradation of organic matter and less authigenic fixation of metals, even at the same bottom water oxygen concentrations. Our results have implications for the detection of OMZs in the geological record, implying that thresholds in trace metal concentrations for perennial OMZs may be lower than previously considered. Comparison of our Mo, U, Re and V data to trace metal records from 15 to 200 ka for the same Arabian Sea region suggests that the OMZ was periodically wider and more reducing in the past.

Techno-economic assessment of wind farm for sustainable power generation in Northern coastal region of Arabian sea

Northern coastal region of Arabian sea has an abundant wind potential available throughout the year which can be utilized for meeting the energy demands in this region. This study presents a techno-economic evaluation of a 100MW wind farm at Gwadar region of Pakistan which is ideally located at the shores of Arabian sea and holds great strategic and economic significance. Several feasibility studies have been undertaken on this location; however, this analysis is carried out using on-site wind data on an hourly basis as well as the most recent financial factors along with optimized wind farm design configuration. Empirical relationships are presented for analyzing the long-term economic feasibility of wind farm. Results indicate that annual energy production of 29,7859 MWh is possible with an optimized design of wind farm that will help to reduce 12,648 equivalent tons of CO2 emissions. Moreover, a positive net present value, levelized cost of electricity around 9.08 ¢/kWh, and payback period of approximately 4 years justify the economic viability of the proposed wind farm design at Gwadar region. This study will serve as a valuable reference for similar wind profile site locations in India and Iran. Moreover, this study presents long-term economic feasibility with site specific analysis which can provide prospects for investment in sustainable energy development and increase its utilization in developing countries.

Polycyclic aromatic hydrocarbons (PAHs) and their alkylated, nitrated and oxygenated derivatives in the atmosphere over the Mediterranean and Middle East seas

Abstract. Polycyclic aromatic hydrocarbons (PAHs) and their alkylated (RPAHs), nitrated (NPAHs) and oxygenated (OPAHs) derivatives are air pollutants. Many of these substances are long-lived, can undergo long-range atmospheric transport and adversely affect human health upon exposure. However, the occurrence and fate of these air pollutants have hardly been studied in the marine atmosphere. In this study, we report the atmospheric concentrations over the Mediterranean Sea, the Red Sea, the Arabian Sea, the Gulf of Oman and the Arabian Gulf, determined during the AQABA (Air Quality and Climate Change in the Arabian Basin) project, a comprehensive ship-borne campaign in summer 2017. The average concentrations of ∑26PAHs, ∑19RPAHs, ∑11OPAHs and ∑17NPAHs, in the gas and particulate phases, were 2.99 ± 3.35 ng m−3, 0.83 ± 0.87 ng m−3, 0.24 ± 0.25 ng m−3 and 4.34 ± 7.37 pg m−3, respectively. The Arabian Sea region was the cleanest for all substance classes, with concentrations among the lowest ever reported. Over the Mediterranean Sea, we found the highest average burden of ∑26PAHs and ∑11OPAHs, while the ∑17NPAHs were most abundant over the Arabian Gulf (known also as the Persian Gulf). 1,4-Naphthoquinone (1,4-O2NAP) followed by 9-fluorenone and 9,10-anthraquinone were the most abundant studied OPAHs in most samples. The NPAH composition pattern varied significantly across the regions, with 2-nitronaphthalene (2-NNAP) being the most abundant NPAH. According to source apportionment investigations, the main sources of PAH derivatives in the region were ship exhaust emissions, residual oil combustion and continental pollution. All OPAHs and NPAHs except 2-nitrofluoranthene (2-NFLT), which were frequently detected during the campaign, showed elevated concentrations in fresh shipping emissions. In contrast, 2-NFLT and 2-nitropyrene (2-NPYR) were highly abundant in aged shipping emissions due to secondary formation. Apart from 2-NFLT and 2-NPYR, benz(a)anthracene-7,12-dione and 2-NNAP also had significant photochemical sources. Another finding was that the highest concentrations of PAHs, OPAHs and NPAHs were found in the sub-micrometre fraction of particulate matter (PM1).

Distinct environmental parameters influence the abundance of living benthic foraminifera morphogroups in the southeastern Arabian Sea.

The ambient environmental parameters have a great bearing on the morphology of living flora and fauna. In this study, we tested this hypothesis on one of the most dominant groups of living unicellular marine microorganism, benthic foraminifera, in the dynamic region of the southeastern Arabian Sea. The living benthic foraminifera from 43 surface samples collected between 25 and 2980m of water depth were segregated into eight morphogroups (tapered/cylindrical, flattened-ovoid, biconvex, planoconvex, flattened-tapered, spherical, rounded-trochospiral, and rounded-planispiral). We report that the high organic carbon availability is combined with deficiency of oxygen results in benthic foraminifera with low surface area to volume ratio and mostly consists of tapered/cylindrical, flattened-ovoid forms, with a preference for infaunal habitat. However, the tests of the living benthic foraminifera thriving in the oxygen-rich bottom waters have a high surface area to volume ratio, commonly reported as epifaunal, consisting of biconvex and planoconvex forms. Additionally, we also report that the abundance of other morphogroups, namely flattened-tapered, spherical, rounded-trochospiral, and rounded-planispiral, is also controlled by the distinct environmental parameters. We suggest that the living benthic foraminifera are an excellent indicator of the ambient environmental parameters and can be used to reconstruct paleoenvironments.

Double Diffusion in the Arabian Sea during Winter and Spring

Abstract Microstructure measurements from two cruises during winter and spring 2019 documented the importance of double-diffusion processes for small-scale mixing in the upper 400 m of the open-ocean region of the eastern Arabian Sea (EAS) below the mixed layer. The data indicated that shear-driven mixing rates are weak, contributing diapycnal diffusivity (Kρ) of not more than 5.4 × 10−6 m2 s−1 in the EAS. Instead, signatures of double diffusion were strong, with the water column favorable for salt fingers in 70% of the region and favorable for diffusive convection in 2%–3% of the region. Well-defined thermohaline staircases were present in all the profiles in these regions that occupied 20% of the water column. Strong diffusive convection favorable regime occurred in ∼45% of data in the barrier layer region of the southern EAS (SEAS). Comparison of different parameterizations of double diffusion with the measurements of vertical heat diffusivity (KT) found that the Radko and Smith salt fingering scheme and the Kelley diffusive convection scheme best match with the observations. The estimates based on flux law show that the combination of downward heat flux of approximately −3 W m−2 associated with salt fingering in the thermocline region of the EAS and the upward heat flux of ∼5 W m−2 due to diffusive convection in the barrier layer region of the SEAS cools the thermocline.