Vol 57, No 3 (2017)

The paper examines so-called wave boundary layers arising in a bounded stratified fluid for large times. Each the layer is a narrow domain in the vicinity of the fluid surface and/or bottom characterized by sharp, growing with increasing time, vertical gradients of the buoyancy and horizontal velocity. The layers arise as a result of free linear wave evolution of the initial fields if the initial buoyancy at the boundaries depends on the horizontal coordinates. An asymptotic solution for the boundary layer for large times is presented, and it is shown that this solution describes exact fields fairly well even for moderate times.

Long-term hourly data from 12 tide gauge stations were used to examine the character of tidal oscillations in the Caspian Sea. Diurnal and semidiurnal tidal peaks are well-defined in sea level spectra in the Middle and South Caspian basins. High-resolution spectral analysis revealed that the diurnal sea level oscillations in the Middle Caspian Basin have a gravitational origin, while those in the South Caspian Basin are mainly caused by radiational effects: the amplitude of diurnal radiational harmonic S₁ is much higher than those of gravitational harmonics О₁, P₁, and K₁. In the North Caspian Basin, there are no gravitational tides and only weak radiational tides are observed. A semidiurnal type of tide is predominant in the Middle and South Caspian basins. Harmonic analysis of the tides for individual annual series with subsequent vector averaging over the entire observational period was applied to estimate the mean amplitudes and phases of major tidal constituents. The amplitude of the M₂ harmonic reaches 5.4 cm in the South Caspian Basin (at Aladga). A maximum tidal range of 21 cm was found at the Aladga station in the southeastern part of the Caspian Sea, whereas the tidal range in the western part of the South Caspian Basin varies from 5 to 10 cm.

Using data from laboratory, field, and numerical experiments, we investigated regularities in changes in the relative limit height of breaking waves (the breaking index) from peculiarities of nonlinear wave transformations and type of wave breaking. It is shown that the value of the breaking index depends on the relative part of the wave energy in the frequency range of the second nonlinear harmonic. If this part is more than 35%, then the breaking index can be taken as a constant equal to 0.6. These waves are spilling breaking waves, asymmetric on the horizontal axis, and are almost symmetric on the vertical axis. If this part of the energy is less than 35%, then the breaking index increases with increasing energy in the frequency range of the second harmonic. These waves are plunging breaking waves, asymmetric on the vertical axis, and are almost symmetric on the horizontal axis. It is revealed that the breaking index depends on the asymmetry of waves on the vertical axis, determined by the phase shift between the first and second nonlinear harmonic (biphase). It is shown that the relation between the amplitudes of the second and first nonlinear harmonics for an Ursell number less than 1 corresponds to Stokes’ second-order wave theory. The empirical dependences of the breaking index on the parameters of nonlinear transformation of waves are proposed.

Based on satellite and in situ data, we quantitatively estimated the penetration of PAR (photosynthetically available solar radiation in the visible spectral range of 400–700 nm) into waters of the central Barents Sea in the summer seasons of 2014–2016. The effects of cloudiness and coccolithophore blooms on the incidence and penetration of PAR was examined. These blooms occur in the Barents Sea almost every year. We estimated the impact of visible solar radiation on the SST against a background of incoming warmer waters of the Norwegian Current and established that the PAR impact is the most pronounced under clear sky conditions in July and August.

The results of integrated optical measurements of Black Sea water samples using a spectrophotometer, laser spectrometer, and fluorometer with pulse-modulated excitation light are discussed. A linear correlation between the intensities of chlorophyll absorption at 673 nm and chlorophyll fluorescence (680–750 nm) is observed. Phycoerythrin-containing organisms are recorded in phytoplankton in layers below 20 m. The data of 1-week monitoring of phytoplankton abundance and functional activity in Golubaya Bay with a Mega-25 flow fluorometer are described.

A study of core sections from the northern tropical Pacific (Northeast Basin) revealed that pelagic sediments are represented by Quaternary oxidized carbonate-free clayey‒radiolarian to radiolarian‒clayey mud, miopelagic clays locally enriched in radiolarians, and pre-Pleistocene miopelagic to eupelagic clays, zeolite‒clayey varieties, and zeolitites. The study also included an analysis of the grain-size and chemical composition, abundance, mass, and distribution of manganese micronodules (MN) contained in these sediments. The data made it possible to calculate relative sedimentation rates (Vr) in sections. In calculations, the estimate of the absolute (V) sedimentation rates for the upper (homogenous) core layer (approximately 5 ka old) was taken to represent a basic value. The values of relative sedimentation rates give grounds to assume that the mass of micronodules (Pmn), average mass of a single micronodule (Pmn/Nmn), mass of the sediment fraction (Pfr > 005 mm), and abundance of micronodules in sediments (Pmn/Psed) reflect the trends of variations in pelagic sedimentation rates.

Planktonic foraminiferal and ice-rafted debris count data, as well as the mean size of mineral particles of the 10–63 μm fraction (sortable silt, \(\overline {SS} \)) were used as a proxy for surface and near-bottom paleocurrent intensity variations. The data obtained support our hypothesis about turbiditic origin of the lower (80–370 cm) section of the studied AMK-4515 core. Stratigraphic subdivision of the upper section (0–80 cm) makes it possible to allocate two marine isotope stages (MIS) covering the last 27 ka. The main intervals of the North Atlantic Polar Front (PF) migrations were recorded: south of the modern PF position during early MIS 2 (24–27 ka) with PF presence in the study area during MIS 2 (20–24 ka); south of the study area during the last glacial maximum (18–20 ka). Influence of the near-bottom currents within the investigated interval led to beginning of the channel-related drift formation on the northern slope of the southern channel of the Charlie-Gibbs Fracture Zone. There is a weak relationship between intensity of near-bottom contour currents and long-term climatic cyclicity. However, intervals corresponding to Heinrich events coincide with decrease in bottom currents activity.

A possible approach to, and preliminary results of, amplitude calibration of acoustic signals backscattered from an ADCP moored at the bottom of the near-shelf zone of the Black Sea is considered. The aim of this work is to obtain vertical profiles of acoustic scattering signal levels, showing the real characteristics of the volume content of suspended sediments in sea water in units of conventional acoustic turbidity for a given signal frequency. In this case, the assumption about the intervals of maximum acoustic transparency and vertical homogeneity of the marine environment in long-term series of ADCP measurements is used. According to this hypothesis, the intervals of the least values of acoustic backscattered signals are detected, an empirical transfer function of the ADCP reception path is constructed, and it is calibrated. Normalized sets of acoustic backscattered signals relative to a signal from a level of conventionally clear water are obtained. New features in the behavior of vertical profiles of an acoustic echo-signal are revealed due to the calibration. The results of this work will be used in subsequent analysis of the vertical and time variations in suspended sediment content in the near-shelf zone of the Black Sea.