We observed the formation of an internal bore interacting with the vertically sheared flow generated during the previous phase of the internal tide, which resulted in strong turbulent mixing. The rate of turbulent kinetic energy dissipation reached on the order of 10−5 W kg−1 during the event. Numerical simulations reproduced the observed interaction of internal bores with the sheared flow and verified the hypothesized breaking and mixing mechanism. The numerical results indicated that the Iribarren number, or the ratio of the topographic slope to the internal wave slope, plays a major role in the mixing intensity and types of internal bores. It was found that waves with low Iribarren numbers lead to bores that interact with vertically sheared flows induced by the previous phase of the internal tide and are more likely to produce strong wave breaking and mixing.
A quantitative echo sounder (QES) was used to measure the bottom surface backscattering strength (SS) extensively off Southern Jawa Island, Indonesia. The average SS by the bottom echo integration method were plotted as a map with the depth contour to give a synoptic view of the topography. On the other hand, the results from the ring surface scattering (RSS) model gave us more detailed SS values in ping base. A simultaneous display of the fish volume backscattering strength and the bottom SS was useful to observe bottom fish habitats remotely. The bottom material estimated by the measured SS showed that the fish schools were abundant in the sand bottom in this area.
Fin whales (Balaenoptera physalus) undergo seasonal migration in the Arctic Sea. Because their migration and distribution is likely affected by changes in global climate, we aimed to examine the migration timing of fin whales, and the relationship with prey availability within the oceanographic environment of the Arctic Sea, using passive and active acoustic monitoring methods. Automatic Underwater Sound Monitoring Systems were deployed in the southern Chukchi Sea from July 2012 to 2014 to determine the acoustic presence of fin whales. Furthermore, water temperature and salinity were recorded by a fixed data logger. An Acoustic Zooplankton Fish Profiler was additionally deployed to estimate prey abundance through backscattering strength. Sea ice concentrations were obtained by remote sensing data. Fin whale calls were automatically detected using a custom-made software, and the per cent of half-hours containing calls were counted. Fin whale calls were detected from 4 August to 20 October 2012 (78 d) and 25 July to 1 November 2013 (100 d). The extended period of acoustic presence of fin whales during 2013 when compared with 2012 is likely related to a longer ice-free period during 2013. Furthermore, generalized linear model analyses showed that half-hour periods containing calls increased with a rise in water temperature and zooplankton abundance during the initial call presence period, while they decreased with a decrease in water temperature and salinity during the end of the call presence period. Our results suggest that the rise in water temperature and zooplankton abundance affect the timing of migration of fin whales in a way that is consistent with the expansion of their suitable habitats and the extension of their presence in the Arctic Sea.
We carried out a 24-h station experiment at Lake Biwa (Japan) to measure mixing events and concurrent biological signals using a free-fall microstructure profiler (TurboMAP-L), conventional hydrographic measurement device (F-probe), and the Tracker acoustic profiling system (TAPS). A clearly defined three-layer physical system was observed. Two layers were actively mixed: the surface-mixed layer and the subsurface-mixed layer. Both winds and night-time convection create the surface-mixed layer, and vertical shear due to a counterclockwise gyre maintains turbulence in the subsurface mixing layer. A strongly stratified layer between these two mixing layers is almost turbulence free, so no material flux is expected. A local oxygen maximum layer, a local oxygen minimum layer, and layers of increased chlorophyll and zooplankton abundance are all located in this strongly stratified layer. The data show the intricate influence of physical processes on the structure of biological systems and their combined influence on biogeochemical and trophic transfers in aquatic systems.
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