A scientific cruise in the northern Arabian Sea captures turbulent characteristics during winter 2018 | POGO

A scientific cruise in the northern Arabian Sea captures turbulent characteristics during winter 2018

 

In the Arabian Sea, the semi-enclosed basin in the northern Indian Ocean, sea surface temperature and near surface circulation is primarily modulated by seasonally reversing monsoon winds. The large evaporation over precipitation, particularly in the northern region of the Arabian Sea, and nearby marginal seas, the Persian Gulf and Red Sea, leads to the formation of unique water masses that occupy the upper 1000 m in the Arabian Sea. These water masses lead to rich structure of thermohaline layers interleaving warm-saline and cold-fresh water in the thermocline and formation of double diffusive layering. The Arabian Sea is also known for high biological productivity and thick Oxygen Minimum Zone.

 

Small scale turbulent mixing across isopycanal has significant role in physical and biological processes in the ocean and in the redistribution of heat, salt and nutrients. In the near surface layer the turbulent mixing is primarily generated by wind, current shear and buoyancy flux and in the interior ocean the major source of energy is provided by the internal tides, near-inertial motions and double diffusive convection. Several observation campaigns were carried out in the Bay of Bengal, the eastern counterpart of Arabian Sea in the north Indian Ocean, to measure the turbulence in the upper ocean. But, systematic measurements of fine-scale fast evolving turbulent mixing processes were non-existent in the Arabian Sea till the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad carried out an expedition in the northern Arabian Sea on-board research vessel Sagar Nidhi during the winter 2018. One of the major achievements during this cruise was the nine day time series (27 Jan-4 Feb, 2018) measurements of the microstructure of temperature, conductivity and shear using Vertical Microstructure Profiler (VMP-250) at every 3 hour interval in the upper 300 m of northern Arabian Sea (19°N, 67°E). The preliminary analysis shows that, the rate of dissipation of turbulent kinetic energy (ε; 10-5-10-7 W kg–1) trapped in thin surface layer (15 m) during morning hours due to stratification associated with net surface heat flux into the ocean and during night, the high ε layer penerates into the deeper depth (60 m) due to net surface heat loss from the ocean. A region of relatively high value of ε (~10-8) is observed in the depth range between 190 to 230 m throughout the observation period, which is approximately one order magnitude higher than the region above. The presence of double diffusion due to the interleaving water mass of different temperature and salinity must be the plausible explanation for this.

 

This article was provided by Jofia Joseph, research student, N. Sureshkumar, B. Murali, V. P. Thangaprakash, S. Shivaprasad, M. S. Girishkumar, Pattabhi Rama Rao, S. S. C. Shenoi, , Scientist at INCOIS.

 

Temporal evolution of vertical profiles of the turbulent kinetic energy dissipation rate (log ε; W Kg-1) estimated from VMP-250 at 18.481°N, and 67.445°E in the Arabian Sea.

Temporal evolution of vertical profiles of the turbulent kinetic energy dissipation rate (log ε; W Kg-1) estimated from VMP-250 at 18.481°N, and 67.445°E in the Arabian Sea.

 

INCOIS research team with VMP-250: Photo credit INCOIS

INCOIS research team with VMP-250: Photo credit INCOIS

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