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Abstract Submission No. | ABS-2022-04-0350 |
Title of Abstract | Observations of the dynamical ocean state on a continental slope in the north-west Bay of Bengal |
Authors | N. Jithendra Raju*, J. Lijin, N. Suresh Kumar, V. P. Thangaprakash, B. Praveen Kumar, Manikandan Mathur |
Organisation | Indian Institude of Technology Madras |
Address | Department of Aerospace Engineering and Geophysical Flows Lab, Indian Institute Chennai, Tamil Nadu, India Pincode: 600036 Mobile: 9002320211 E-mail: jithuraju1290@gmail.com |
Country | India |
Presentation | Oral |
Abstract | Satellite altimetry and numerical ocean modeling reveal strong internal tide activity in the north-west Bay of Bengal (BoB), the source of which is likely to be the continental shelf break region in its neighbourhood. This region is also dynamically unique owing to the large freshwater input from the riverine systems along the northern boundary of BoB. In this work, we report on an integrated set of measurements in the shelf break region off Paradip performed onboard ORV Sagar Nidhi during 3-6 October, 2019. Specifically, vertical profile measurements of conductivity-temperature-depth (CTD), currents, and turbulence microstructure (small-scale shear) were made at different locations along a 20 km transect across the continental slope. Strong changes of around 8 psu in the near-surface salinity and 1 deg C in the near-surface temperature are observed during a tidal cycle at a fixed location. CTD profiles from various instruments are then visualized on the (T, S) plane, and they sometimes reveal noticeable temperature excursions at around 40 m depth, with fine-scale lateral gradients in temperature observable up to around 150 m depth. The aforementioned features are then understood in terms of advective transport by barotropic tides and geostrophic currents, which are obtained from satellite and reanalysis products. A prominent double pycnocline structure is observed in vertical stratification profiles, with a staircase-like density structure present all the way up to 250 m. With underway CTD (uCTD) measurements, lateral density gradients are also measured, using which we comment on how far the upper ocean is from geostrophic adjustment. Vertical microprofiler (VMP) measurements are then used to reveal that enhanced turbulence occurs up to around 200m depth along the transect, Correspondingly, lowered ADCP (LADCP) measurements reveal alternating current directions with depth up to 200 m. These features in the small-scale turbulence and horizontal currents are likely due to internal tide interactions with the strongly stratified upper ocean, a hypothesis motivated by internal tide ray paths and weak wind conditions. The internal tide hypothesis is then tested in high-resolution numerical experiments using a non-hydrostatic ocean model, which is run for different background stratifications observed during flood and ebb tide. |