Measurements of acoustic volume backscattering in the Indian and Southern Oceans

1981 ◽  
Vol 32 (6) ◽  
pp. 855 ◽  
Author(s):  
M Hall
Keyword(s):  
Indian Ocean ◽  
Acoustic Scattering ◽  
Average Depth ◽  
Night Time ◽  
Average Mass ◽  
Average Abundance ◽  
The Indian Ocean ◽  
Scattering Strength ◽  
Great Australian Bight ◽  
Good Agreement

Volume backscattering strengths have been measured at several positions in the Indian Ocean and the Southern Ocean (across the Great Australian Bight). The positions in the Indian Ocean varied from the vicinity of the Equator to a station off the coast of Western Australia near Fremantle. The backscattering strengths have been analysed at frequencies in third-octave steps from 2.5 to 20 kHz. The average daytime scattering strengths at the Equator and in the Bight are similar and range from around - 87 dB re m-1 at 2.5 kHz to between -75 and -70 dB re m-1 at 20 kHz. At night, the average scattering strengths in the Bight increase from about -75 dB re m-1 at 2.5 kHz to about -70 dB re m-1 at 20 kHz, whereas at the Equator the results increase from about -82 dB re m-1 at 2.5 kHz to -64 dB re m-1 at 20 kHz. Deep scattering layers (DSL) were observed both in the Bight and at the Equator. The DSL in the Bight had a resonance frequency of 4 kHz and the average depth of the bottom of the layer was 950 m. From the acoustic scattering strength. it is inferred that the average population density of the fish in the layer is 10-3 m-3, and that the average mass of the fishes is around 40 g. The DSL at the Equator had a flat frequency response at frequencies above 10 kHz (there was no peak in the spectrum) and the average depth of the layer was about 500 m. The average abundance of the scatterers in the layer is inferred to be of the order of 5 × 10-3 m-3. The backscattering strengths measured in the Great Australian Bight have been compared with predictions based on concurrent net hauls that were conducted to depths of 50 and 100 m. Good agreement occurs only at the higher frequencies and at night-time when most of the organisms are near the surface.

scholarly journals Atmospheric CO and CH<sub>4</sub> time series and seasonal variations on Reunion Island from ground-based in situ and FTIR (NDACC and TCCON) measurements

2018 ◽  
Vol 18 (19) ◽  
pp. 13881-13901 ◽  
Author(s):  
Minqiang Zhou ◽  
Bavo Langerock ◽  
Corinne Vigouroux ◽  
Mahesh Kumar Sha ◽  
Michel Ramonet ◽  
...  
Keyword(s):  
Time Series ◽  
Indian Ocean ◽  
Mole Fraction ◽  
Seasonal Cycles ◽  
Reunion Island ◽  
Time Period ◽  
Réunion Island ◽  
The Indian Ocean ◽  
Good Agreement

Abstract. Atmospheric carbon monoxide (CO) and methane (CH4) mole fractions are measured by ground-based in situ cavity ring-down spectroscopy (CRDS) analyzers and Fourier transform infrared (FTIR) spectrometers at two sites (St Denis and Maïdo) on Reunion Island (21∘ S, 55∘ E) in the Indian Ocean. Currently, the FTIR Bruker IFS 125HR at St Denis records the direct solar spectra in the near-infrared range, contributing to the Total Carbon Column Observing Network (TCCON). The FTIR Bruker IFS 125HR at Maïdo records the direct solar spectra in the mid-infrared (MIR) range, contributing to the Network for the Detection of Atmospheric Composition Change (NDACC). In order to understand the atmospheric CO and CH4 variability on Reunion Island, the time series and seasonal cycles of CO and CH4 from in situ and FTIR (NDACC and TCCON) measurements are analyzed. Meanwhile, the difference between the in situ and FTIR measurements are discussed. The CO seasonal cycles observed from the in situ measurements at Maïdo and FTIR retrievals at both St Denis and Maïdo are in good agreement with a peak in September–November, primarily driven by the emissions from biomass burning in Africa and South America. The dry-air column averaged mole fraction of CO (XCO) derived from the FTIR MIR spectra (NDACC) is about 15.7 ppb larger than the CO mole fraction near the surface at Maïdo, because the air in the lower troposphere mainly comes from the Indian Ocean while the air in the middle and upper troposphere mainly comes from Africa and South America. The trend for CO on Reunion Island is unclear during the 2011–2017 period, and more data need to be collected to get a robust result. A very good agreement is observed in the tropospheric and stratospheric CH4 seasonal cycles between FTIR (NDACC and TCCON) measurements, and in situ and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite measurements, respectively. In the troposphere, the CH4 mole fraction is high in August–September and low in December–January, which is due to the OH seasonal variation. In the stratosphere, the CH4 mole fraction has its maximum in March–April and its minimum in August–October, which is dominated by vertical transport. In addition, the different CH4 mole fractions between the in situ, NDACC and TCCON CH4 measurements in the troposphere are discussed, and all measurements are in good agreement with the GEOS-Chem model simulation. The trend of XCH4 is 7.6±0.4 ppb yr−1 from the TCCON measurements over the 2011 to 2017 time period, which is consistent with the CH4 trend of 7.4±0.5 ppb yr−1 from the in situ measurements for the same time period at St Denis.

A regional study of Bay of Bengal processes using radiation boundary condition in Modular Ocean Model

2020 ◽  
Author(s):  
Siddhesh Tirodkar ◽  
Manasa Ranjan Behera ◽  
Sridhar Balasubramanian
Keyword(s):  
Boundary Condition ◽  
Indian Ocean ◽  
Solar Radiation ◽  
Bay Of Bengal ◽  
Heat Budget ◽  
Regional Scale ◽  
Ocean Model ◽  
The Indian Ocean ◽  
Modular Ocean Model ◽  
Good Agreement

&lt;p&gt;The ocean exchanges heat and mass with the atmosphere in form of shortwave and longwave radiations, precipitation, and evaporation. The regional scale ocean processes governed by this exchange play a vital role in modulating the local dynamics of the Indian Ocean. For instance, the meso-scale eddies and waves control the ocean vertical temperature structure, mixed layer depth, and the thermocline. The Indian Ocean Observing System (IndOOS) recommends the need of proper understanding of heat budget in the Indian Ocean to resolve the mesoscale and submesoscale processes, which trigger large scale ocean circulation, cyclonic eddies, plumes etc. In a regional domain, the stability of ocean also depends on the local parameters, namely, wind pattern, precipitation, runoff and exchange of heat and mass fluxes near the domain boundary. The main objective of this study is to understand the effect of atmospheric wind and solar radiation on the ocean surface and sub-surface characteristics using Modular Ocean Model (MOM5).&lt;/p&gt;&lt;p&gt;A regional domain in the Bay of Bengal (BoB) is selected, which has unique features, such as, large amount of freshwater flux, seasonal wind reversal and high amount of solar radiation due the geographic location. The dynamics in BoB is important for understanding the Indian summer and winter monsoon seasons and associated weather patterns. A regional ocean modelling approach is adopted using MOM5 with horizontal grid resolution (0.25&lt;sup&gt;0&lt;/sup&gt;) while maintaining the vertical grid-size as 1m near the surface region which increases with depth. For the regional domain, radiation open boundary condition (OBC) is implemented on three lateral boundaries of domain, based on the technique proposed by Orlanski (1976). The OBC at the lateral boundaries help in smooth exchange of current and tracers. K-profile parameterization (KPP) vertical mixing scheme is used that accounts for effects of shear, wave breaking, and double diffusion. The model is started from a state of rest and simulated for a period of 10 years using 6-hourly solar radiation (Japanese 25-year reanalysis (JRA-25)) and daily averaged wind stress (SODA reanalysis) dataset. After five years of model spin-up, the last five years of simulated output is considered to ensure consistency of model results. Heat budget calculation shows good agreement with WHOI OA Air-Sea Fluxes (OAFlux). Smooth exchange of mass and fluxes is observed near boundary, which confirms successful implementation of OBC. Implementation of KPP scheme enhances mixing in the upper ocean layers with more realistic thermocline formation and turbulent kinetic energy (TKE). The model is able to mimic the seasonal variability in the ocean currents enforced due to winds. The Sea Surface Temperature (SST) is in good agreement with SODA reanalysis data.&lt;/p&gt;&lt;p&gt;A plume like mesoscale feature in the SST plot is captured in the present study (that is also observed in microwave SST), but found to be missing in earlier BoB study with sponge boundary conditions. Finer scale resolution (0.125&lt;sup&gt;0&lt;/sup&gt;) study is in progress, which is expected to show secondary mesoscale structures and their evolution. The results from this study would help in better understanding of the influence regional-scale processes on local ocean dynamics.&lt;/p&gt;

scholarly journals Comparisons of day-time and night-time hydroacoustic surveys in temperate lakes

2020 ◽  
Vol 33 ◽  
pp. 9
Author(s):  
Michaël Girard ◽  
Chloé Goulon ◽  
Anne Tessier ◽  
Pascal Vonlanthen ◽  
Jean Guillard
Keyword(s):  
Acoustic Scattering ◽  
Target Strength ◽  
Fish Stock ◽  
Size Estimation ◽  
Temperate Lakes ◽  
Night Time ◽  
Fish Size ◽  
Schooling Behaviour ◽  
Scattering Strength ◽  
Lower Depth

In recent years, due to an increased need for non-intrusive sampling techniques, hydroacoustics has attracted attention in fishery science and management. Efforts to promote standardisation are increasing the accuracy, efficiency, and comparability of this method. The European Water Framework Directive and the Standard Operating Procedures for Fisheries Hydroacoustic Surveys in North American Great Lakes has recommended that surveys be conducted at night. At night, fish usually disperse in the water column, thus allowing for single echo detection and subsequent accurate fish size estimation, while day-time schooling behaviour hampers the estimation of fish size. However, sampling during the day would often be safer and cheaper. This study analyses how fisheries hydroacoustic results differ between day-time and night-time surveys, using data from 14 natural temperate lakes of various size. Data collected during the day and night at two depth layers linked to thermal stratification were compared in terms of acoustic scattering strength, target strength, and biomass estimates. The results showed a significant correlation between day-time and night-time estimates, though biomass in the upper layer was biased for day-time surveys, mainly due to incorrect fish size estimates resulting from rare single echo detections and schooling behaviour. Biomass estimates for the lower depth layer did not significantly differ between the two diel periods. Thus, this study confirms that hydroacoustic sampling in temperate lakes should be performed at night for accurate fish stock biomass estimates.

scholarly journals DINAMIKA SPASIAL IKAN MESOPELAGIS (ceratoscopelus warmingii LÜTKEN, 1892) DI SAMUDERA HINDIA

2017 ◽  
Vol 22 (4) ◽  
pp. 263
Author(s):  
Andria Ansri Utama ◽  
Wudianto Wudianto
Keyword(s):  
Indian Ocean ◽  
Marine Biodiversity ◽  
Mesopelagic Fish ◽  
Average Depth ◽  
Spatial Dynamic ◽  
High Concentration ◽  
Significant Information ◽  
Vertical Distributions ◽  
The Indian Ocean ◽  
High Seas

Kajian mengenai ikan mesopelagis di perairan Samudera Hindia masih sangat terbatas, sehingga informasi terkait kelimpahan jenis ikan mesopelagis di Samudera Hindia sangat penting. Survei trawl lapisan pertengahan dilakukan pada tanggal 26 Juni-16 Juli 2015 di perairan laut lepas (high seas) Samudera Hindia untuk memperoleh data dan informasi tersebut dengan menggunakan kapal penelitian R.V. Dr. Fridtjof Nansen. Hasil penelitian menunjukkan distribusi kedalaman vertikal di malam hari jenis yang dominan C.warmingii sesuai dengan kedalaman operasi trawl yaitu 86,9 ± 38,6 m. Namun pada siang hari tidak ditemukan spesies C. warmingii saat operasi trawl pada kedalaman rata-rata 444,3 ± 45,96 m. Diperkirakan ketika siang hari distribusi C. warmingii terkonsentrasi pada lapisan perairan lebih dalam sehingga tidak terjangkau oleh jaring trawl tersebut. Distribusi spasial secara horizontal pada malam hari menunjukkan pola konsentrasi tertinggi berada pada area gyre yang diindikasikan dengan pola geostrophic circulation. Sementara, prosentase C. warmingii yang merupakan hasil tangkapan seluruh stasiun trawl selama penelitian terdiri dari 2,58% fase larva, 27,21% juvenile, dan 60,21% dalam keadaan dewasa.Studies on mesopelagic fishes in the Indian Ocean are relatively limited, therefore data and information regarding spatial dynamic of the most abundant mesopelagic fish species Ceratoscopelus warmingii in the Indian Ocean would contribute a significant information on deepsea marine biodiversity. Pelagic-trawl stations were used to collect and identify the spatial dynamic of C. warmingii in the main scattering layers of the water column. Survey were carried out by RV Dr. Fridtjof Nansen in the high seas of Indian Ocean. Observation were made during day and night on 26th June to 16th July 2015 as part of the second International Indian Ocean Expedition (IIOE 2). The result shows that vertical distributions of C. warmingii were concentrated at the average depth 86.9 ± 38.6 m during nighttime. There were no individuals found during the daytime at the average depth 444.3 ± 45.96 m, diurnal migrations of C. warmingii  to more than the depth of trawl operation might explain the absent of this species. Horizontal spatial distribution of trawl catches (number per hour) during nighttime show high concentration of C. warmingii close to gyre indicated by geostrophic circulation. Moreover, the total catches of C. warmingii across the Indian Ocean are dominated by 60.21% adult, 27.21% juvenile, and 2.58% larvae, respectively.   

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