• Aparna S G

      Articles written in Journal of Earth System Science

    • Seasonal variability of sea-surface temperature fronts associated with large marine ecosystems in the north Indian Ocean

      Kankan Sarkar Aparna S G Shrikant Dora Shankar D

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      We use 14 years of satellite-derived sea-surface temperature (SST) data to compute a monthly frontal probability index (FPI) to determine the existence of a front in a pixel. A persistent SST front is deemed to exist if the FPI in a narrow region exceeds that in the surrounding ocean. We describe the seasonal variability of 17 persistent SST fronts (eight associated with the shelf-slope boundary and five with the mixing between different water masses) in the north Indian Ocean. Only weak fronts exist during a few months in the strong upwelling regimes off Somalia and Oman.

    • Observed variability of the West India Coastal Current on the continental shelf from 2010–2017

      ANYA CHAUDHURI AMOL P SHANKAR D MUKHOPADHYAY S APARNA S G FERNANDO V A KANKONKAR

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      We describe the variability of the West India Coastal Current (WICC) during 2009–2017 using data from acoustic Doppler current profilers (ADCPs) attached to moorings deployed on the outer shelf at a water-column depth of 100–200 m at three locations: off Kollam (9$^{\circ}$N), Bhatkal (13.9$^{\circ}$N), and Goa (15$^{\circ}$N). Our study shows that the characteristics of the WICC on the outer shelf are similar to those observed on the slope except for the occasional decorrelation observed between them. Both shelf and slope WICC have a weak mean flow and a strong annual cycle. As seen on the slope, the depth of the undercurrent on the shelf changes with season because of the strong upward phase propagation associated with the annual cycle. Though the currents at all three shelf locations exhibit a strong seasonal cycle, there are prominent differences between the currents off the southwest coast of India (Kollam) and the central west coast of India (Bhatkal and Goa). The seasonal cycle off Kollam is often punctuated by strong intraseasonal bursts, which cause the shelf WICC off Kollam to be highly unpredictable; this unpredictability implies that on a given day of the year, one cannot expect a poleward (equatorward) current during the winter (summer) monsoon. On the central west coast, the poleward (equatorward) direction during the winter monsoon (summer monsoon) is prominent. Comparison with the available datasets shows that the WICC is occasionally coherent along and across the shelf, and the coherence is stronger for the seasonal cycle.

      $\bf{Highlights}$

      $\bullet$ Characteristics of the WICC on the outer shelf are similar to those observed on the slope.

      $\bullet$ Shelf WICC has a weak mean flow and a strong annual cycle.

      $\bullet$ Differences observed between currents off central west coast and southwest coast of India.

      $\bullet$ Seasonal cycle off Kollam is often punctuated by strong intraseasonal bursts.

    • Two decades of current observations in the equatorial Indian Ocean

      VINEET JAIN AMOL P APARNA S G FERNANDO V KANKONKAR A MURTY V S N ALMEIDA A M AREEF A SARDAR KHALAP S T SATELKAR N P GHATKAR S TARI P A GAONKAR M G KHEDEKAR R

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      Deep-sea moorings in the equatorial Indian Ocean were first deployed by India in the year 2000, and currents were measured at three locations (77$^{\circ}$E, 83$^{\circ}$E, and 93$^{\circ}$E) on the equator. In this paper, we present two decades of current observations from these moorings and discuss how the moorings have evolved with time. The observations show that the 180-day (90-day) period dominates the surface and mid-depth (bottom) circulation. Though the Wyrtki Jets are strong, the near-surface currents do not show any clear semi-annual reversals. The reversals become evident only below 100 m.

      $\bf{Highlights}$

      $\bullet$ Twenty years of ocean current data collected at the equatorial Indian Ocean

      $\bullet$ Bottom current data show strong intraseasonal variability

      $\bullet$ Upward phase propagation associated with the semi-annual cycle more prominent for the mid-depth currents

      $\bullet$ The semi-annual reversals not evident every year near the surface

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