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1.
With observational data from three Acoustic Doppler Current Profiler (ADCP) moorings, we detected strong near-inertial oscillations (NIO) in the continental shelf region of the northern South China Sea in July 2008. The amplitude of the near-inertial current velocity is much greater than that of diurnal and semi-diurnal tides. The power of the NIOs is strongest in the intermediate layer, relatively weak in the surface layer, and insignificant in the near-bottom layer. The spectral analysis indicates that the NIOs have a peak frequency of 0.0307 cph, which is 2% lower than the local inertial frequency, i.e., a red-shift. The near-inertial wave has an upward vertical phase velocity, which involves a downward group velocity and energy flux. The estimated vertical phase velocity is about 43 m day−1, corresponding to a vertical wave length of about 58 m. The horizontal scale of the NIOs is at least hundreds of kilometers. This NIO event lasted for about 15 days after a typhoon’s passage. Given the northeastward background flow with significant horizontal shear, both Doppler shift and shear flow modulation mechanisms may be responsible for the red-shift of the observed NIOs. For the shear flow mechanism, the observed negative background vorticity and the corresponding effective Coriolis frequency reduce the lower limit of admissible frequency band for the NIOs, causing the red-shift. Meanwhile, the mooring area with the broadened frequency band acts as a wave-guide. The trapping and amplification effects lead to the relatively long sustaining period of the observed NIOs.  相似文献   

2.
This study demonstrates reduced electron transfer system (ETS) activity of mixed copepods collected from 5,000 to 7,000 m depths [3.21 ± 1.25 μl O2 (mg protein)−1 h−1 at 10°C] as compared with mixed copepods from 0 to 200 m depths [5.93 ± 1.66 μl O2 (mg protein)−1 h−1 at 10°C] of the western subarctic Pacific. At the in situ temperature of 1.5°C, the 5,000–7,000 m ETS data, in terms of wet mass (WM)-specific respiration rates (R), is equivalent to [0.052 ± 0.021 μl O2 (mg WM)−1 h−1] which is similar to or greater than those reported for selected copepods or mixed mesozooplankton from <5,000 m depth by previous workers.  相似文献   

3.
The seasonal abundance of the dominant dinoflagellate, Ceratium fusus, was investigated from January 2000 to December 2003 in a coastal region of Sagami Bay, Japan. The growth of this species was also examined under laboratory conditions. In Sagami Bay, C. fusus increased significantly from April to September, and decreased from November to February, though it was found at all times through out the observation period. C. fusus increased markedly in September 2001 and August 2003 after heavy rainfalls that produced pycnoclines. Rapid growth was observed over a salinity range of 24 to 30, with the highest specific rate of 0.59 d−1 measured under the following conditions: salinity 27, temperature 24°C, photon irradiance 600 μmol m−2s−1. The growth rate of C. fusus increased with increasing irradiance from 58 to 216 μmol m−2s−1, plateauing between 216 and 796 μmol m−2s−1 under all temperature and salinity treatments (except at a temperature of 12°C). Both field and laboratory experiments indicated that C. fusus has the ability to grow under wide ranges of water temperatures (14–28°C), salinities (20–34), and photon irradiance (50–800 μmol m−2s−1); it is also able to grow at low nutrient concentrations. This physiological flexibility ensures that populations persist when bloom conditions come to an end.  相似文献   

4.
Concentrations of particulate organic nitrogen (PN), dissolved inorganic nitrogen (DIN), and their nitrogen isotope ratios (δ 15N) in the Kiso-Sansen Rivers were determined from monthly observations over the course of a year to assess variations in the form and sources of riverine nitrogen discharged into Ise Bay. The δ 15N values of NO3 − observed in the Kiso-Sansen Rivers showed a logarithmic decreasing trend from 8 to 0‰, which varied with the river discharge, indicating mixing between point sources with high δ 15N and non-point sources with low δ 15N. The influence of isotope fractionation of in situ biogeochemical processes (mainly DIN assimilation by phytoplankton) on δ 15N of NO3 − was negligible, because sufficient concentrations of NH4 + for phytoplankton demand would inhibit the assimilation of NO3 −. A simple relationship between river discharge and δ 15N of NO3 − showed that the fraction of total NO3 − flux arising from point sources increased from 4.0–6.3% (1.1–1.8 tN day−1) during higher discharge (>600 m3 s−1) to 30.2–48.3% (2.6–4.1 tN day−1) during lower discharge (<300 m3 s−1). Riverine NO3 − discharge from the Kiso-Sansen Rivers can explain 75% of the variations in surface NO3 − at the head of Ise Bay over the year.  相似文献   

5.
A finite-difference quasigeostrophic (QG) model of an open ocean region has been employed to produce a dynamically constrained synthesis of acoustic tomography and satellite altimetry data with in situ observations. The assimilation algorithm is based upon the 4D variational data interpolation scheme controlled by the model's initial and boundary conditions. The data sets analyzed include direct and differential travel times measured at the array of five acoustic transceivers deployed by JAMSTEC in the region of the Kuroshio Extension in 1997, Topex/Poseidon altimetry, CTD soundings, and ADCP velocity profiles. The region monitored is located within the area 27.5°–36.5°N, 143°–155°. The results of assimilation show that mesoscale variability can be effectively reconstructed by five transceivers measuring direct and reciprocal travel times supported by relatively sparse in situ measurements. The misfits between model and data lie within the observational error bars for all the data types used in assimilation. We have compared the results of assimilation with the statistical inversion of travel time data and analyzed energy balances of the optimized model solution. Energy exchange between the depth-averaged and shear components of the observed currents reveals a weak decay of the barotropic mode at the rate of 0.2 ± 0.7⋅10−5 cm2/s3 due to topographic interaction. Mean currents in the region are unstable with an estimate of the available potential energy flux from the mean current to the eddies of 4.7 ± 2.3⋅10−5 cm2/s3. Kinetic energy transition has the same sign and is estimated as 2.8 ± 2.5⋅10−5 cm2/s3. Potential enstrophy is transferred to the mesoscale at a rate of 5.5 ± 2.7⋅10−18 s−3. These figures provide observational evidence of the properties of free geostrophic turbulence which were predicted by theory and observed in numerical experiments. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

6.
Spatial distribution of the carbon and nitrogen content and their isotopic enrichment in suspended matter and sediments were measured in the Godavari estuary to identify the sources and transformation mechanism of organic matter. Significant variability in isotopic distribution was found over the entire length of the Godavari estuary, suggesting multiple sources of organic matter. The mean isotopic ratios (δ13Csed −25.1 ± 0.9, δ13Csus −24.9 ± 1, δ15Nsed 8.0 ± 2 and δ15Nsus 6.5 ± 0.9‰) and elemental concentrations (Csed 0.45 ± 0.2%, Csus 0.9 ± 0.7%, Nsed 0.07 ± 0.05% and Nsus 0.16 ± 0.1%) support a predominantly terrigenous source. Significant enrichment in the isotopic ratios of δ13C from the upper to lower estuary in both suspended (−27.5 and −24.3‰, respectively) and sedimentary (−26.2 and −24.9‰, respectively) phases indicates a decrease in the influence of terrigeneous material toward the mouth of the estuary. A significant positive relationship exists between the δ13C of suspended and sediment, which indicates that these two organic carbon pools are likely coupled in the form of a significant exchange between the two phases. A positive relationship exists between chlorophyll a and suspended organic matter, which may mean that a significant source of organic carbon is the in situ produced phytoplankton. But, applying a simple mixing model to our isotopes, data yielded about 46% as the contribution of the terrestrial source to suspended matter, which may support the excessive heterotrophic activity in the Godavari estuary reported earlier.  相似文献   

7.
Variability of Northeastward Current Southeast of Northern Ryukyu Islands   总被引：8，自引：4，他引：8  
To better understand the mechanism underlying the variation of the Kuroshio south of central Japan, we have examined the variability of current structure in its upstream region, southeast of Amami-Ohshima Island in the northern Ryukyu Islands. By combined use of ship-mounted Acoustic Doppler Current Profiler (ADCP) and the TOPEX/POSEIDON satellite altimeter data on Path 214, the sea surface absolute geostrophic currents were estimated every ten days from January 1998 to July 2002. The 4.5-year mean surface current was found to flow northeastward north of 26.8°N with a maximum speed of 14 cm s−1 over the shelf slope at 3000 m depth. The moored current-meter observations at three or four mooring stations from Dec. 1998 to Oct. 2002 suggested the existence of a northeastward undercurrent with a maximum core velocity of 23 cm s−1 at 600 m depth over the shelf slope at 1600 m depth. The mean volume transport in the top 1500 m between 27.9°N and 26.7°N is estimated to be 16 × 106 m3s−1 northeastward, including the subsurface core current related component of 4 × 106 m3s−1. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

8.
A profiling float equipped with a fluorimeter, a dissolved oxygen (DO) sensor, and temperature and salinity sensors was deployed in the subtropical mode water (STMW) formation region of the North Pacific. It acquired quasi-Lagrangian, 5-day-interval time-series records from March to July 2006. The time-series distribution of chlorophyll showed a sustained and sizable subsurface maximum at 50–100 m, just above the upper boundary of the STMW, throughout early summer (May–July). The DO concentration in this lower euphotic zone (50–100 m) was almost constant and supersaturated in the same period, becoming more supersaturated with time. On the other hand, the DO concentration at 100–150 m near the upper boundary of the STMW decreased much more slowly compared with the main layer of STMW below 150 m, even though oxygen consumption by organisms was expected to be larger in the former depth range. The small temporal variations of DO in the lower euphotic zone and near the upper boundary of the STMW were reasonably explained by downward oxygen transport because of large diapycnal diffusion near the top of the STMW. Assuming that the oxygen consumption rate at 100–150 m was the same as that in the main layer of STMW and compensated by the downward oxygen flux, the diapycnal diffusivity was estimated to be 1.7 × 10−4 m2 s−1. Nitrate transport into the euphotic zone by the same large diffusion was estimated to be 0.8 mmol N m−2 day−1. All of the transported nitrate could have been used for photosynthesis by the phytoplankton; net community production was estimated to be 5.3 mmol C m−2 day−1.  相似文献   

9.
马龙  郑彦鹏 《海洋学报》2020,42(1):144-153
本文基于中国南极考察第30航次、第32航次所获得的实测重力资料,结合NGDC资料,开展12个航次重力场数据的平差融合工作,全部386个交点平差后标准差减小为±1.53×10−5 m/s2,与卫星重力差值平均值为1.49×10−5 m/s2,均方差为±3.81×10−5 m/s2,并在此基础上采用频率域界面反演法计算莫霍面深度。研究发现,与沉积盆地对应重力异常低值相悖,在罗斯海北部盆地、维多利亚地盆地、中央海槽、东部盆地4个主要盆地腹地却表现为重力异常高值,跨度达100 km以上。莫霍面深度分布整体呈南深北浅之势,范围为10~28 km。伴随着罗斯海西部盆地的多次拉张及岩浆活动,该区域的地壳厚度和莫霍面深度高值和低值相间分布,并表现出越来越大的差异性。综合剖面结果表明,罗斯海重力异常值的长波长变化与莫霍面的起伏呈正相关关系,但是反演的莫霍面深度与区域重力场特征并非完全对应,所以岩浆底侵和地壳侵入仍不足以导致罗斯海盆地的重力异常或盆地几何形状。  相似文献   

10.
Bottom simulating reflectors (BSRs), known as the base of gas hydrate stability zone, have been recognized and mapped using good quality three-dimensional (3D) pre-stack migration seismic data in Shenhu Area of northern South China Sea. Additionally, seismic attribute technique has been applied to better constrain on the distribution of gas hydrate. The results demonstrate that gas hydrate is characterized by “blank” zone (low amplitude) in instantaneous amplitude attribute. The thickness of gas hydrate stability zone inferred from BSR ranges from 125 to 355 m with an average of 240 m at sea water depth from 950 to 1,600 m in this new gas hydrate province. The volume of gas in-place bound in hydrate is estimated from 1.7 × 109 to 4.8 × 109 m3, with the most likely value of around 3.3 × 109 m3, using Monte Carlo simulation. Furthermore, geothermal gradient and heat flow are derived from the depths of BSRs using a conductive heat transfer model. The geothermal gradient varies from 35 to 95°C km−1 with an average of 54°C km−1. Corresponding heat flow values range from 43 to 105 mW m−2 with an average of 64 mW m−2. By comparison with geological characteristics, we suggest that the distribution of gas hydrate and heat flow are largely associated with gas chimneys and faults, which are extensively distributed in Shenhu Area, providing easy pathways for fluids migrating into the gas hydrate stability zone for the formation of gas hydrate. This study can place useful constraints for modeling gas hydrate stability zone from measured heat flow data and understanding the mechanism of gas hydrate formation in Shenhu Area.  相似文献   

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