On low-pass digital filters in oceanography

-Two types of filters are widely used to remove semidirunal and diurnal tidal signals and other high frequency noises in oceanography. The first type of filters uses moving average with weights in time domain, and can be easily operated. Some data will be lost at each end of the time series, especially for the low low-pass filters. The second type of filters uses the discrete Fourier transform filter (DFTF) which operates in the frequency domain, and there are no data loss at the ends for the forward transform. However, owing to the Gibbs phenomenon and the discrete sampling (Nyquist effect) , ringing appears in the inverse transformed data, which is especially serious at each end. Thus some data at the ends are also discarded. The present study tries to find out what causes the ringing and then to seek for methods to overcome the ringing. We have found that there are two kinds of ringings, one is the Gibbs phenomenon, as defined before. The other is the "Nyquist"ringing due to sampling Nyquist critical     

Radioelement distribution in river,beach, and offshore areas and their significance to Chavara placer deposit,Southern Kerala Coast of India

The radioelement and heavy mineral distribution in river, beach and innershelf areas of the southern Kerala coast is related to placer mineral concentration on the beaches at Chavara. Southern Kerala rivers—Neyyar, Karamana and Vamanapuram—transport higher amounts of radioactive elements than the larger Kallada River due to higher radioactive minerals in the hinterland rocks. Coastal configurations and the seasonal longshore current pattern seems to control along-shore distribution of minerals. The proposed model for placer concentration suggests that the energy difference and seasonal current direction along this coast is important.     

AN ECOSYSTEM MODEL ANALYSIS OF THE EFFECT OF MINE TAILINGS ON THE EUPHOTIC ZONE OF A PELAGIC ECOSYSTEM

A computer analysis was performed on experimental results obtained when mine tailings were added to seawater. The Mixed Upper-Layer Ecotrophie Simulation (MULES) model was tested by changing the extinction coefficient and the abundance of heterotrophic zooflagellates. Increasing the extinction coefficient resulted in a delay of phytoplankton growth, an increase in zooplankton standing stock and better growth of autotrophic flagellates compared with diatoms. Zooflagellates in the ecosystem influence the growth of zooplankton; secondary production by zooplankton was markedly depressed at low levels of zooflagellates. These results are believed to be of general significance for the diagnosis of suspended sediment effects on planktonic ecosystems.     

海带的一个自然突变型

在多细胞藻的遗传学研究中,被发现的突变型很少[1].绿藻中有一些突变型,其中较多的是某种石莼(Ulva mutabilis)[2].三浦昭雄等在紫菜中发现突变型.在褐藻中未见有突变型的报道.这里所报道的是海带(Laminaria japonica)的一个突变型.     

Numerical Simulation of Residual Circulation due to Bottom Rouglmess Variability Under Tidal Flows in A Semi-Enclosed Bay

Nowadays there are some chronic serious environmental problems, such as eutrophication, blue tide and so on, in a complicated coastal zone or a semi-enclosed bay, because the water exchanges between an inner bay and an outer sea is weak compared with the supply of contaminant. Under this situation, a method to improve the water quality by 3-dimensional small unsymmetrical structures has been proposed by Komatsu et al. In this paper, several numerical simulations of the tidal current and concentration for various arrangements of bottom roughness in a semi-enclosed model bay are carfled out with a depth-averaged 2-D numerical model. The model is solved by the hybrid finite analytic method with nonstaggered grid. And the SIMPLES algorithm with Rhie and Chow＇ s momentum interpolation technique is used for the simulation. The effect of Komatsu＇ s method for water purification is examined by numerical simulation. The result of numerical experiment indicates that it is possible to generate a new tidal residual current and to activate a tidal exchange by bottom roughness arrangement only.     

Nonlinear response of a moored buoy

The nonlinear dynamic analysis of a multipoint slack moored buoy is performed under the action of first and second order wave forces. The nonlinearity of the system is caused by the geometric nonlinearity of the mooring lines. The resulting nonlinear equation of motion is solved by an incremental time marching scheme. The nonlinear responses of the system are analysed to investigate different kinds of dynamic instability phenomena that may arise due to the nonlinearity of the system. As an illustrative example, a hollow cylindrical buoy anchored to the sea bed by means of six slack mooring lines is considered. The responses of the system are obtained and analysed for three regular waves namely, 5 m/5 s, 12 m/10 s and 18 m/15 s. The results of the study show that different kinds of instability phenomena like nT subharmonic oscillations, symmetry breaking bifurcation and aperiodic responses may occur in slack mooring systems. Further, a second order wave force may considerably influence the dynamic stability of such systems.     

A nitrate and silicate budget in the equatorial Pacific Ocean: a coupled physical–biological model study

A coupled physical–biological model was developed to simulate the low-silicate, high-nitrate, and low-chlorophyll (LSHNLC) conditions in the equatorial Pacific Ocean and used to compute a detailed budget in the Wyrtki box (5°N–5°S, 180–90°W) for the major sources and cycling of nitrogen and silicon in the equatorial Pacific. With the incorporation of biogenic silicon dissolution, NH₄ regeneration from organic nitrogen and nitrification of ammonia in the model, we show that silicon recycling in the upper ocean is less efficient than nitrogen. As the major source of nutrients to the equatorial Pacific, the Equatorial Undercurrent provides slightly less Si(OH)₄ than NO₃ to the upwelling zone, which is defined as 2.5°N–2.5°S. As a result, the equatorial upwelling supplies less Si(OH)₄ than NO₃ into the euphotic zone in the Wyrtki box, having a Si/N supply ratio of about 0.85 (2.5 vs. 2.96 mmolm⁻² day⁻¹). More Si(OH)₄ than NO₃ is taken up with a Si/N ratio of 1.17 (2.72 vs. 2.33 mmolm⁻² day⁻¹) within the euphotic zone. The difference between upwelling supply and biological uptake is balanced by nutrient regeneration and horizontal advection. Excluding regeneration, the net silicate and nitrate uptakes are nearly equal (1.76 vs. 1.84 mmolm⁻² day⁻¹). However, biogenic silica export production is slightly higher than organic nitrogen (1.74 vs. 1.59 mmolm⁻² day⁻¹) following a 1.1 Si/N ratio. In the central equatorial Pacific, low silicate concentrations limit diatom growth; therefore non-diatom new production accounts for most of the new production. Higher silicate supply in the east maintains elevated diatom growth rates and new production associated with diatoms dominate upwelling zone. In contrast, the new production associated with small phytoplankton is nearly constant or decreases eastward along the equator. The total new production has a higher rate in the east than in the west, following the pattern of surface silicate. This suggests that silicate regulates the diatom production, total new production, and thereby carbon cycle in this area. The modeled mean primary production is 48.4 mmolCm⁻² day⁻¹, representing the lower end of direct field measurements, while new production is 15.0 mmolCm⁻² day⁻¹, which compares well with previous estimates.     

Modeling bed-load transport of coarse sediments in the Great Bay Estuary, New Hampshire

Current, sea level and bed-load transport are investigated in the Lower Piscataqua River section of the Great Bay Estuary, New Hampshire, USA—a well-mixed and geometrically complex system with low freshwater input, having main channel tidal currents ranging between 0.5 and 2 m s⁻¹. Current and sea level forced by the M₂M₄M₆ tides at the estuarine mouth are simulated by a vertically averaged, non-linear, time-stepping finite element model. The hydrodynamic model uses a fixed boundary computational domain and accounts for flooding–drying of tidal flats by making use of a groundwater component. Inertia terms are neglected in comparison with pressure gradient and bottom friction terms, which is consistent with the observed principal dynamic balance for this section of the system. The accuracy of hydrodynamic predictions in the study area is demonstrated by comparison with four tidal elevation stations and two cross-section averaged current measurements. Simulated current is then used to model bed-load transport in the vicinity of a rapidly growing shoal located in the main channel of the lower system. Consisting of coarse sand and gravel, the shoal must be dredged every five to eight years. Two approaches are taken—an Eulerian parametric method in which nodal bed-load flux vectors are averaged over the tidal cycle and a Lagrangian particle tracking approach in which a finite number of sediment particles are released and tracked. Both methods yield pathways and accumulations in agreement with the observed shoal formation and the long-term rate of sediment accumulation in the shoal area.     

The bottom currents in the area of abyssal hills in the north-east tropical Pacific Ocean

The results of simultaneous measurements of the bottom (6.25 and 35 m above the bottom) currents, deep currents, and surface currents made at three points in the north-east tropical Pacific Ocean are given. The bottom intensification of the current velocity is revealed in a layer of 35–25 m above the bottom. The estimation of the thickness of the bottom boundary layer (BBL) indicates that the velocity intensification is observed over the boundary layer upper border. A 10-day long benthic storm with a maximum measured velocity of 13 cm/s was revealed 6 m above the bottom. As was found, the origin of the benthic storm is associated with the penetration of an anticyclonic eddy down to the bottom.Translated by Mikhail M. Trufanov.