Articles written in Journal of Earth System Science

    • Altimeter data assimilation in the tropical Indian Ocean using water property conserving scheme

      Bhasha M Mankad Rashmi Sharma Sujit Basu P K Pal

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      Altimeter data have been assimilated in an ocean general circulation model using the water property conserving scheme. Two runs of the model have been conducted for the year 2004. In one of the runs, altimeter data have been assimilated sequentially, while in another run, assimilation has been suppressed. Assimilation has been restricted to the tropical Indian Ocean. An assessment of the strength of the scheme has been carried out by comparing the sea surface temperature (SST), simulated in the two runs, with in situ derived as well as remotely sensed observations of the same quantity. It has been found that the assimilation exhibits a significant positive impact on the simulation of SST. The subsurface effect of the assimilation could be judged by comparing the model simulated depth of the 20°C isotherm (hereafter referred to as D20), as a proxy of the thermocline depth, with the same quantity estimated from ARGO observations. In this case also, the impact is noteworthy. Effect on the dynamics has been judged by comparison of simulated surface current with observed current at a moored buoy location, and finally the impact on model sea level forecast in a free run after assimilation has been quantified in a representative example.

    • Evaluation of OSCAR ocean surface current product in the tropical Indian Ocean using in situ data

      Rajesh Sikhakolli Rashmi Sharma Sujit Basu B S Gohil Abhijit Sarkar K V S R Prasad

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      The OSCAR (ocean surface current analysis real-time),which is a product derived from various satellite observations,has been evaluated in the tropical Indian Ocean (TIO)in two di fferent ways.First,the OSCAR-derived monthly climatology has been compared with available drifter-derived climatology in the TIO.From the comparison of the two climatologies,one can infer that OSCAR product is able to capture the variabilities of the well-known surface current systems in the TIO reasonably well.Fourier analysis of the major current systems,as reproduced by OSCAR,shows that the dominant annual and semiannual periodicities,known to exist in these systems,have been faithfully picked up by OSCAR. Next,the evaluation has been carried out by comparing the OSCAR currents with currents measured by moored buoys.The zonal component of OSCAR-current is in good agreement with corresponding component of buoy-observed current with a correlation exceeding 0.7,while the match between the meridional components is poorer.The locations of the peaks of the mean and eddy kinetic energies are matching in both the climatologies,although the peak in the drifter climatology is stronger than the same in the OSCAR product.Finally,an important feature of Indian Ocean circulation,namely the reverse Wyrtki jet,occurring during anomalous dipole years,has been well-reproduced by OSCAR currents.

    • Investigation of rip current processes along Visakhapatnam beaches, east coast of India: A study based on GNSS drifters and dye experiments


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      Rip currents are known as one of the most dangerous hazards on sandy beaches worldwide. Indian beaches are least explored to study the variability of rip currents in space and time. The present study is the first attempt made to understand its dynamics by utilizing indigenously developed GNSS (Global Navigation Satellite System) drifters and Rhodamine-B dye experiments. The Rama Krishna (RK) and Rushikonda Beaches of Visakhapatnam are chosen for this study, due to a recorded number of rip current-related drowning cases observed during the past decade. Few experiments were conducted during pre- and post-monsoon seasons of 2018. Drifters work on the Lagrangian principle, where they measure the current velocities along their paths driven by the surface currents. Error analysis of drifter measurements showed that they are capable of resolving surf zone motions very accurately. Strong rip currents were observed in few locations in the study area, where at times current velocities reached ${\sim}$1 ms$^{-1}$. Also, Rhodamine-B dye was released into the rip current prone zones along with the drifters and observed that the dye patches also followed the drifters. From these experiments, it has been observed that the rip currents are relatively strong during the post-monsoon season, which could be due to the change in the beach morphology. Similar experiments with more number of drifters would help in understanding rip current dynamics and would help in reducing rip current drowning in the beaches.


      $\bullet$ GNSS based drifters have been designed, developed and field-tested to measure rip currents in the surf zone.

      $\bullet$ Post-Processing Kinematic (PPK) resulted in position estimates with centimeter level accuracy.

      $\bullet$ The drifters are capable to resolve the surf zone motions more accurately in the order of greater than 0.02 ms$^{-1}$.

      $\bullet$ Several drifter deployments and Rhodamine-B dye experiments were carried out at RK and Rushikonda Beaches to measure the dangerous rip currents and coastal currents.

    • Analysis of atmospheric and oceanic conditions during unusual occurrence of tropical cyclone Gulab and Shaheen in North Indian Ocean


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      During September 2021, a tropical cyclone (TC) named Gulab formed in Bay of Bengal (BoB) region of North Indian Ocean (NIO) and by nature, it was quite an unusual one as it crossed the Indian subcontinent and re-emerged as TC Shaheen in Arabian Sea (AS) which made landfall at the coast of Oman. These two cyclones were unique from two aspects: (i) formed in active southwest monsoon period in the month of September, which is a very rare event in NIO and (ii) formation of TCs in BoB and its reemergence in AS, in the form of a cyclonic storm after crossing the Indian continent is uncommon. Along with these two, another exception was landfall at Oman coast, which is again very rare. The different large scale atmospheric and oceanic parameters, during development of TC Gulab in BoB and its reemergence as TC Shaheen in AS are analysed using the reanalysis data and satellite derived products. The results suggest that vertical wind shear (VWS) during genesis of TC Gulab was unusually low and favourable for cyclonic storm development in BoB. The middle level relative humidity over central India was also high (positive anomaly), which supported remnants of TC Gulab to survive as a low-pressure weather system in land region. Later, it evolves into as TC Shaheen in AS, and due to favourable Sea Surface Temperature and Oceanic Heat Content it further intensifies to a very severe category cyclonic storm.


      $\bullet$ The best track data analysis in the north Indian Ocean reveals that TC Gulab and Shaheen were the unique systems in terms of their genesis, track and landfall.

      $\bullet$ During the September month due to high vertical wind shear in the BoB, in general the low- pressure systems do not attain the intensity of a tropical storm. However, TC Gulab could turn into a TC in the September month due to favourable SST and low vertical wind shear.

      $\bullet$ The atmospheric moisture content over the land was high due to active monsoon month, which helps the survival of remnant of TC Gulab, re-emerges and evolves as TC Shaheen in the Arabian Sea due to high ocean heat potential and sufficiently warm sea water, along with the favourable atmospheric conditions like low wind shear and high relative humidity.

      $\bullet$ The behaviour of TC Gulab and Shaheen may have some linkage to the climate change, which is influencing the occurrence of more intense cyclonic system in the Arabian Sea that is affecting the Middle East costal region.

      $\bullet$ The recent trend of rising SST signals the more frequent occurrence of such TCs in the Arabian Sea, creating an alarm situation for the cyclone vulnerability of the Arabian Sea basin countries.

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