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More than 50 years of successful continuous temperature section measurements by the global expendable bathythermograph network, its integrability, societal benefits, and future
Full text linkThe first eXpendable BathyThermographs (XBTs) were deployed in the 1960s in the North Atlantic Ocean. In 1967 XBTs were deployed in operational mode to provide a continuous record of temperature profile data along repeated transects, now known as the Global XBT Network. The current network is designed to monitor ocean circulation and boundary current variability, basin-wide and trans-basin ocean heat transport, and global and regional heat content. The ability of the XBT Network to systematically map the upper ocean thermal field in multiple basins with repeated trans-basin sections at eddy-resolving scales remains unmatched today and cannot be reproduced at present by any other observing platform. Some repeated XBT transects have now been continuously occupied for more than 30 years, providing an unprecedented long-term climate record of temperature, and geostrophic velocity profiles that are used to understand variability in ocean heat content (OHC), sea level change, and meridional ocean heat transport. Here, we present key scientific advances in understanding the changing ocean and climate system supported by XBT observations. Improvement in XBT data quality and its impact on computations, particularly of OHC, are presented. Technology development for probes, launchers, and transmission techniques are also discussed. Finally, we offer new perspectives for the future of the Global XBT Network
Improved ocean analysis for the Indian Ocean
No full textThe National Centers for Environmental Prediction (NCEP) and the Indian National Centre for Ocean Information Services (INCOIS) produce global ocean analyses based on the Global Ocean Data Assimilation System (GODAS). This system uses a state of the art ocean general circulation model named moduler ocean model (MOM) and the 3D-Variational (3DVar) data assimilation technique. In this study we have evaluated the INCOIS-GODAS operational analysis products with an upgrade of the physical model from MOM4p0d to MOM4p1. Two experiments were performed with same atmospheric forcing fields:(i) using MOM4p0d (GODASp0), and (ii) using MOM4p1 (GODASp1). Observed temperature and salinity profiles were assimilated in both experiments. Validation with independent observations show improvement of sea surface temperature(SST), sea surface salinity (SSS) and surface currents in the new analysis GODASp1 as compared to the old analysis GODASp0
Role of Andaman and Nicobar Islands in eddy formation along western boundary of the Bay of Bengal
Full text linkEddies along western boundary of the Bay of Bengal (WBoB) play an important role in regulating regional climate and marine productivity of the north Indian Ocean. In this paper, role of Andaman and Nicobar islands (ANIs) in the formation of eddies along the WBoB is studied using an ocean general circulation model. Our analysis shows that, in the absence of ANIs, there is a significant reduction in the total number of mesoscale eddies in this region. The impact is particularly evident for the cyclonic eddies as a reduction of ~50% can be noticed in the absence of the islands. In contrast, influence of ANIs on anticyclonic eddies is not homogeneous in the WBoB; while absence of ANIs significantly increases anticyclonic eddies in the central part of the WBoB, a decrease can be noticed in the southern part. We further show that the reduction in number of cyclonic eddies along the WBoB is primarily driven by reduced baroclinic and barotropic instabilities. This process is more conspicuous during winter (October–January) season compared to summer (June–September) and spring (February–May) seasons
Dissolved boron in a brackish-water lagoon system (Chilika lagoon, India): Spatial distribution and coastal behavior
No full textBoron is a bio-essential metalloid and its concentrations as well as isotopic compositions are useful tracers for various environmental processes; however, its oceanic budget is not yet well-constrained. In this contribution, spatial and seasonal distribution of dissolved boron of the Chilika lagoon, India (Asia's largest brackish-water lagoon) and its possible source waters have been investigated to constrain its coastal behavior and chemical budget. Further, oxygen isotopic analyses of selected samples were carried out to quantify the evaporation process. The boron concentrations show significant spatial variations (0.6–246 μmol/kg; for monsoon (Aug., 2017) season), with the lower values being observed in the river-dominated northern sector of the lagoon. The area-weighted boron concentration of the Chilika during monsoon (128 μmol/kg) is found intermediate to that of the riverine (1.7 ± 0.8 μmol/kg), groundwater (25 ± 38 μmol/kg) and oceanic (406 ± 13 μmol/kg) water sources. In contrast to boron, the average δ18O value (2.2 ± 0.7‰) for the lagoon samples is significantly enriched than their source waters. Calculations based on the δ18O and salinity data estimate about 40% loss of surface water via evaporation. Co-variation between boron and salinity of the samples establishes conservative behavior during onset of the monsoon (June) and also, in the monsoon (Aug) seasons. The boron-salinity trend and boron/salinity ratios of pre-monsoon (May) samples, however, point to its non-conservative behavior with significant boron removal at low-saline regime through ion-exchange (adsorption) processes. Removal of boron is mostly limited to salinity <15 psu and the intensity (in %) of removal increases steadily with decrease in salinity. These adsorptive losses of boron during pre-monsoon period are mostly dependent on the water residence time; higher residence time allows efficient particulate-water interaction, which possibly intensifies the removal. Further, the boron concentrations show significant changes on diurnal and fortnightly timescales due to tide/ebb cycles. However, the coastal behavior of boron, despite of large concentration changes, remains invariant due to tidal forcing. Outcomes of this study underscore adsorptive removal of boron from coastal regimes and its importance in understanding authigenic boron distribution in clay-rich sedimentary archives from near-shore setting
Application of Singular Spectrum Analysis for Investigating Chaos in Sea Surface Temperature
No full textThe goal of this study is to explore the chaotic behavior of sea surface temperature (SST) in the Indian Ocean and in the equatorial Pacific Ocean. The SST time series is analyzed for Bay of Bengal, Arabian Sea and South Indian Ocean as well as for two extreme phenomena: El Niño and Indian Ocean Dipole (IOD). The analysis is based on Singular spectrum analysis, and singular value decomposition (SVD). Our analysis reveals that the dynamics of SST is chaotic in varying degrees in all the studied cases, since Lyapunov exponent, an indicator of chaoticity, is positive in each case. To study the degree of predictability of these SST series, we search for embedded periodic component(s) using two different approaches: Orthogonal functions extracted from the Singular spectrum analysis and Periodicity spectrum analysis based on SVD. Both the methods reveal presence of a strong periodic component(s) for the SST signals in the Arabian Sea, Bay of Bengal and South Indian Ocean, whereas no periodicity is found for El Niño and IOD. Therefore, it can be concluded that the dynamics of SST is more complex in the El Niño and IOD region compared to Bay of Bengal, Arabian Sea and South Indian Ocean; hence it is much more difficult to predict El Niño and IOD
Framework for mapping the drivers of coastal vulnerability and spatial decision making for climate-change adaptation: A case study from Maharashtra, India
No full textThe impacts of climate change are of particular concern to the coastal region of tropical countries like India, which are exposed to cyclones, floods, tsunami, seawater intrusion, etc. Climate-change adaptation presupposes comprehensive assessment of vulnerability status. Studies so far relied either on remote sensing-based spatial mapping of physical vulnerability or on certain socio-economic aspects with limited scope for upscaling or replication. The current study is an attempt to develop a holistic and robust framework to assess the vulnerability of coastal India at different levels. We propose and estimate cumulative vulnerability index (CVI) as a function of exposure, sensitivity and adaptive capacity, at the village level, using nationally comparable and credible datasets. The exposure index (EI) was determined at the village level by decomposing the spatial multi-hazard maps, while sensitivity (SI) and adaptive capacity indices (ACI) were estimated using 23 indicators, covering social and economic aspects. The indicators were identified through the literature review, expert consultations, opinion survey, and were further validated through statistical tests. The socio-economic vulnerability index (SEVI) was constructed as a function of sensitivity and adaptive capacity for planning grassroot-level interventions and adaptation strategies. The framework was piloted in Sindhudurg, a coastal district in Maharashtra, India. It comprises 317 villages, spread across three taluks viz., Devgad, Malvan and Vengurla. The villages in Sindhudurg were ranked based on this multi-criteria approach. Based on CVI values, 92 villages (30%) in Sindhudurg were identified as highly vulnerable. We propose a decision tool for identifying villages vulnerable to changing climate, based on their level of sensitivity and adaptive capacity in a two-dimensional matrix, thus aiding in planning location-specific interventions. Here, vulnerability indicators are classified and designated as ‘drivers’ (indicators with significantly high values and intervention priority) and ‘buffers’ (indicators with low-to-moderate values) at the village level. The framework provides for aggregation or decomposition of CVI and other sub-indices, in order to plan spatial contingency plans and enable swift action for climate adaptatio
Is there an effect of Bay of Bengal salinity on the northern Indian Ocean climatological rainfall?
No full textThe northern Bay of Bengal (BoB) receives a large amount of freshwater directly from monsoonal rains over the ocean, and indirectly through river runoffs. It has been proposed that the resulting strong salinity stratification inhibits vertical mixing of heat, thus contributing to maintain warm sea surface temperature and high climatological rainfall over the BoB. In the present paper, we explore this positive feedback loop by performing sensitivity experiments with a 25-km resolution regional coupled climate model, that captures the main BoB features reasonably well. We confirm that salinity stratification tends to stabilize the upper ocean, thereby increasing the mixed layer warming due to vertical mixing by ∼+0.5 °C.month−1 on annual average. Salinity however also induces a compensating cooling by altering the mixed layer heating rate by air-sea heat fluxes, so that the net effect on climatological surface temperature is negligible. During and shortly after the southwest monsoon, this compensation predominantly occurs through increased cooling by upward latent heat fluxes. During boreal winter, it occurs because salinity favours a thinner mixed layer, which is more efficiently cooled by negative air-sea heat fluxes. These compensations result in a negligible climatological surface temperature and rainfall change at all seasons. This weak influence of salinity stratification on climatological surface temperature and rainfall in our model is robust when applying a flux correction to alleviate model biases, when neglecting the solar absorption below the mixed layer and when using different atmospheric radiation and convective parameterizations
Present Status of the Sustainable Fishing Limits for Hilsa Shad in the northern Bay of Bengal, India
No full textThe hilsa, Tenualosa ilisha fishery in the northern Bay of Bengal (nBoB) is passing through a crisis manifested by the persistent decline of fish catch in spite of increasing efforts. During the period 2002–2015, the number of boats engaged in the fishery increased by 25% while the hilsa catch decreased by 13%. The exponential value (b) and condition factor (K) of hilsa has also decreased by 46% and 28% respectively. The value of fishing mortality (F = 2.34 year−1) has considerably exceeded the natural mortality (M = 0.56 year−1) during the study period. It is observed that in place of maximum exploitation rate (Emax) of 0.78, the current exploitation rate (E) of hilsa is 0.81 which is above the sustainable limit. It is a matter of serious concern that with 75% probability the first spawners of the population are being targeted by the present fishing practice. The present study observes that the hilsa population of nBoB are being significantly overexploited in the present level of fishing pressure. The maximum sustainable yield (MSY) limit for hilsa is estimated to be around 25,440 tons per year with the corresponding effort (fMSY) that may be deployed to achieve the above mentioned catch ranged from 3571 to 3987 (number of boats). It can be inferred that the hilsa fishery in the nBoB is being unsustainably exploited
Wyrtki Jets: Role of intraseasonal forcing
Full text linkDirect current measurements observed from the acoustic Doppler current profilers in the equatorial Indian Ocean (EIO) and solutions from an ocean general circulation model are investigated to understand the dynamics of the Wyrtki jet. These jets are usually described as semiannual direct wind forced zonal currents along the central and eastern EIO. We show that both, spring and fall, Wyrtki jets show predominant semiannual spectral peaks, but significant intraseasonal energy is evident during spring in the central and eastern EIO. We find that for the semiannual band, there is a strong spectral coherence between the overlying winds and the currents in the central EIO, but no coherency is observed in the eastern part of the EIO. Moreover, for the intraseasonal band, strong coherency between the winds and currents is evident. During spring, intraseasonal currents induced by the Madden–Julian oscillation (MJO) superimpose constructively with semiannual currents and thus intensify the strength of the spring Wyrtki jet. Also, the atmospheric intraseasonal variability accounts for the interannual variabilities observed in spring Wyrtki jet
INCOIS-Real time Automatic Weather Station(IRAWS) dataset - Quality control and significance of height correction
Full text linkThe INCOIS-Real time Automatic Weather Station(IRAWS) program was started in the year 2009 and was first installed onboard ORV Sagar Nidhi. Currently, there are 36 ships carrying IRAWS setup. Apart from storing one minute observations in the log onboard the ship, hourly averaged observations are reported through INSAT satellite communication. This report briefs about the hourly dataset of IRAWS and its quality control. In this report, QC results of SST and all meteorological
parameters except radiation parameters is discussed. Specific quality check was applied to wind speed (WS) and sea
surface temperature (SST) observations. The WS observations measured onboard few ships had a dimensional correction and SST was observed only on few ships. As SST observations are required to compute meteorological variables like DBT, RH, WS to standard height of 10 m, level-3 dataset of AVHRR SST was utilized in place of IRAWS SST wherever the data is found to be faulty. On similar terms bias correction could not be applied to IRAWS SST with the help of AVHRR SST as the error in SST observations are due to the failure of sensor. However all those IRAWS SST observations that passed the QC
check were observed to be of high quality and have a correlation coefficient of 0.5 with AVHRR SST and is significant at 95% significant level. Apart from SST and radiation observations, all other parameters observations are found out to be of good quality with 70 to 90 QC pass percentage . Apart from the details of QC check, significance of representing climate variable at a homogeneous standard height is also shown in this repor