• G S MICHAEL

      Articles written in Journal of Earth System Science

    • Satellite-tracked drifting buoy observations in the south equatorial current in the Indian Ocean

      S R Shetye G S Michael

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      Three satellite-tracked drifting buoys released in the south equatorial current in the Indian Ocean followed the path of the current moving westward approximately zonally in the vicinity of 10 S latitude. On nearing the east coast of Africa two buoys moved north and the third moved south. Over the open sea regime the buoys moved with a speed of approximately 30 cm/s at an angle of about 35° to the left of the wind. The overall tendencies seen in the buoy drift are similar to those observed elsewhere in the world oceans.

    • Observed variability of the West India Coastal Current on the continental slope from 2009–2018

      ANYA CHAUDHURI D SHANKAR S G APARNA P AMOL V FERNANDO A KANKONKAR G S MICHAEL N P SATELKAR S T KHALAP A P TARI M G GAONKAR S GHATKAR R R KHEDEKAR

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      We describe the variability of the West India Coastal Current (WICC) during October 2008 to October 2018 using data from ADCP (acoustic Doppler current profiler) moorings deployed on the continental slope off the west coast of India. The four moorings are deployed off Mumbai ($\sim 20^{0}\rm{N}$), Goa ($\sim 15^{0}\rm{N}$), Kollam ($\sim 9^{0}\rm{N}$), and Kanyakumari ($\sim 7^{0}\rm{N}$). This 10-year data set allows us to attach a statistical significance to the conclusions drawn by Amol et al. (2014) on the basis of four years (October 2008–October 2012) of ADCP data. The longer data set confirms the earlier finding that intraseasonal variability in the 30–90-day band dominates the variability of the WICC at all locations and that this intraseasonal variability peaks during the winter monsoon. The annual cycle (300–400 days) is strong and statistically significant at all locations. The phase propagates upward for the annual cycle and this phase difference is seen in the relative phases of both, the ADCP currents at 25 and 48 m as well as the 48 m ADCP and satellite-derived currents. The intra-annual (100–250 days) and intraseasonal currents show instances of both upward and downward phase propagation. The alongshore wavelet coherence is high on seasonal time scales between adjacent mooring locations and several instances of high coherence are seen even on intraseasonal time scales. Data gaps off Goa and Kanyakumari restrict the significant wavelet power to the ADCP records off Kollam and Mumbai, and the coherence analysis shows that the WICC off Kollam leads Mumbai on seasonal scales. The direction of the alongshore WICC is, however, largely determined by the direction of the significantly larger intraseasonal component. Though the climatological seasonal cycle over the whole record does show the canonical equator ward flow during the summer monsoon (June–September) and poleward flow during the winter monsoon (November–February), the scatter around the daily mean is very high.The data show that the WICC may flow in either direction on a given day of the year, with this unpredictability of direction being stronger off Kollam, where the $1-\sigma$ band of the daily mean alongshore WICC shows that it can flow in either direction in most months. The seasonality is stronger off Mumbai, where the width of the $1-\sigma$ band is less. The decade-long continuous record off Kollam and Mumbai shows that the sub-annual along shore WICC at both locations is significant and is comparable to or stronger than the annual component.The cross-shore sub-annual current is also strong off Kollam and is seen to be associated with eddy-like circulations.

    • Variation of salinity in the Sundarbans Estuarine System during the Equinoctial Spring tidal phase of March 2011

      MEENAKSHI CHATTERJEE D SHANKAR V VIJITH G K SEN D SUNDAR G S MICHAEL P AMOL ABHISEK CHATTERJEE P SANYAL SIDDHARTHA CHATTERJEE ANWESHA BASU SARANYA CHAKRABORTI SURJA KANTA MISHRA K SUPRIT DEBABRATA MUKHERJEE A MUKHERJEE SOUMYA MUKHOPADHYAY GOPAL MONDAL ARAVIND KALLA MADHUMITA DAS

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      The Sundarbans Estuarine System (SES), comprising the southernmost part of the Indian portion of the Ganga-Brahmaputra delta bordering the Bay of Bengal, is India’s largest monsoonal, macro-tidal, delta-front estuarine system. The Sundarbans Estuarine Programme (SEP), covering six semi-diurnal tidal cycles during 18–21 March 2011 (the Equinoctial Spring Phase), was the first comprehensive observational programme in the SES. The 30 observation stations, spread over more than 3600 km2km2, covered the seven inner estuaries of the SES: the Saptamukhi, Thakuran, Matla, Bidya, Gomdi, Harinbhanga, and Raimangal. At all stations or time-series locations (TSLs), the water level was measured every 15 min and water samples were collected every hour for estimating salinity. We report the observed spatio-temporal variations of salinity in this paper. The mean salinity over the six tidal cycles decreased upstream and the mean range of salinity over a tidal cycle increased upstream. In addition to this along-channel variation, the mean salinity also varied zonally across the SES. Salinity was lowest in the eastern SES, with the lowest value occurring at the TSLs on the Raimangal. Though higher than at the Raimangal TSLs, the mean salinity was also low at Mahendranagar, the westernmost TSL located on the West Gulley of the Saptamukhi. Salinity tended to be higher in the central part of the SES. CTD (conductivity–temperature–depth) measurements at three stations on the Matla show a well-mixed profile. Only the Raimangal has a freshwater source at its head. Therefore, the upstream decrease of salinity in the SES is likely to be the effect of the preceding summer monsoon, which would have freshened the estuary, and the ingress of salt from the seaward end due to the tide following the cessation of of the monsoon rains. The freshwater inflow from the Raimangal leads to the lowest salinities occurring in the eastern SES. The lower salinity in the western SES also suggests inflow from the Hoogly estuary, whose freshwater source is regulated via the Farakka Barrage. At 20 of the 30 TSLs, the salinity varied semi-diurnally, like the water level, and the maximum (minimum) salinity tended to occur at or around high (low) water. The temporal variation was more complex at the other 10 TSLs. Even at the TSLs at which a tidal stand exceeding 75 min was seen in the water level, the salinity oscillated with a semi-diurnal period. Thus, the salinity variation was unaffected by the stand of the tide that has been reported from the SES.

      $\bf{Highlights}$

      $\bullet$ Comprehensive description of salinity variability in the Sundarbans Estuarine System (SES)

      $\bullet$ Semi-diurnal variation seen at a majority of the stations and the estuaries are well-mixed

      $\bullet$ Mean salinity decreases upstream and is lower in the eastern and western SES

      $\bullet$ The upstream decrease is due to the preceding monsoon and the tidal ingress of salt

      $\bullet$ Direct (indirect) inflow from the Ganga (Hoogly) lowers salinity in the eastern (western) SES

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