This paper presents a research synthesis of one of the essential features of the global climate system — Arctic amplification: a higher rate of change of surface air temperature in the Arctic region compared to that of the Northern Hemisphere or global average. Arctic amplification is a regional manifestation of the more common phenomenon — Polar amplification. However, Antarctic amplification is significantly weaker than the Arctic one. The major mechanisms defining the Arctic amplification are various climate feedbacks, operating differently at different latitudes, and a poleward heat transport induced by the atmospheric and oceanic circulation. The state-of-the-art scientific results have mostly demonstrated a relative role of different climate feedbacks in forming the Arctic amplification. From the more important to the less important ones, these are the lapse rate, Planck and surface albedo feedbacks. However, several other possible mechanisms are remained poorly studied. In particular, the contribution from the time varying meridional heat transport is quite unclear. Moreover, meridional advection of heat by the atmosphere and ocean can play a significant role in the observed variations of the intensity of the Arctic amplification at different time scales.

The lenses of the Mediterranean water (LMW), also known as Meddies, well distinguishable in the Atlantic Ocean water. The field observations devoted to study Meddies are presented in numerous publications and provide information on their origination, distribution, spatial scales and temporal dynamic activities. The salt-fingers and double-diffusion at the Meddies top and bottom borders may be considered as only mechanisms causing the Meddies disappearance as the thermal — saline mesoscale anomalies in the surrounding Atlantic Ocean waters. Taking into account the Medies’ realistic scales it is shown that in temporal scales of about and less than a year the Meddies water mass may be considered as invariable. Then the Meddies conservation mass is valid in such temporal scales.
The Meddies (LMW) temporal variability analysis is carried out using the theoretical approach of intrusive lens in stratified fluid extended taking into account the Coriolis force. The Meddies (LMW) are considered with the center of gravity sitting at the level of equal density. The temporal variability occurs due to the resulting effect of the set of forces: the surplus pressure originated due to the density difference between the water inside the LMW and the ambient density field; the forces originated due to both the lens’ rotation (centrifugal) and the Earth rotation (Coriolis); the forces caused by the internal wave radiation and the viscosity action. The temporal variability of LMW constitutes of two principal stages:
1) the initial, inviscid stage (“young lens”), when balance of forces is formed by the forces of inertia, the surplus pressure, the centrifugal force, the Coriolis force and the force of wave resistance due to the internal wave radiation; the Coriolis force is a key factor supporting and keeping compact the anti-cyclonic LMW, the Meddies; it prevents the LMWs from breakup, limits their geometrical dimensions, affects the lens angular velocity variations. The cyclonic LMWs are destabilized by the Coriolis force, getting small thickness and under certain conditions may disappear as a density anomaly already at this stage;
2) the viscid stage (“old lens”) is characterized by a slow decreasing of the LMW thickness up to its limit value, that is determined by the LMW initial thickness, the initial LMW stratification as well as ambient stratification; during the final period of the viscous stage the exchange of heat and salinity with ambient water-mass may, on a characteristic temporal scales about a year, significantly influence on the degenerations of the LMW as a density anomaly; at this stage the anti-cyclonic LMW, the Meddy, continues to have the anti-cyclonic rotation.

Based on ECMWF ORAS4 and ORAS5 reanalysis data volumetric flow rate and also heat and salt fluxes and their balances have been calculated on the lateral boundaries of the Lofoten Basin area in the Norwegian Sea. These results have been compared with results based on CMEMS reanalysis GLORYS12V1 data. We revealed, that the most physically justified in the volumetric flow rate is the ORAS5 reanalysis with a 6.0 % discrepancy. Heat balance estimation based on the GLORYS12V1 reanalysis data is more realistic because it shows an increased heat influx that participates in the ocean-atmosphere heat exchange. We conclude that the influx from the southern boundary dominates while the outflux through the eastern boundary is prevalent in the study area, which is mainly determined by the region’s waters dynamics. All of three reanalysis shows an excess of water and salt inflow over outflow for the Lofoten Basin. That indicates the balances for individual basins are not closed and approaches to using these data are limited.
All reanalysis data demonstrate the irregular fluctuations in the temporal variability of the fluxes through the boundaries with the different variance rate. Using the ORAS5 reanalysis data from 1993–2016, we estimate significant linear trends of the studied fluxes through all boundaries, except the northern one. It can be a real proof of increasing transport through the Lofoten Basin. The linear trends of the balance calculated using the ORAS5 data of the same period show the accumulation of water in the region as well as the cooling and accumulation of salt. The linear trends of the balance calculated using the GLORYS12V1 reanalysis demonstrate positive values indicating the warming of the basin.

The Lofoten Basin in the Norwegian Sea is a real reservoir of the Atlantic waters. The shape of the Basin in the form of a bowl and a great depth with its monotonous increase to the center results in the Atlantic water gradually deepen and fill the Basin. The deepening of the Atlantic waters in the Lofoten Basin determines not only the structure of its waters but also the features of the ocean-atmosphere interaction. Flowing through the transit regions, the Atlantic waters lose heat to the atmosphere, mix with the surrounding water masses and undergo a transformation, which causes the formation of deep ocean waters. At the same time, the heat input with the Atlantic waters significantly exceeds its loss to the atmosphere in the Lofoten Basin.
We study isopycnal advection and diapycnal mixing in the Lofoten Basin. We use the GLORYS12V1 oceanic reanalysis data and analyze four isosteric δ-surfaces. We also calculate the depth of their location. We establish that δ-surfaces have the slope eastward with maximal deepening where the quasi-permanent Lofoten Vortex is located. We analyze the temperature distribution on the isosteric δ-surfaces as well as the interannual and seasonal variability of their location depth.
The maximal depth on the isosteric surfaces is observed in 2010, which is known as the year of the largest mixed layer depths in the Lofoten Basin according to the ARGO buoys. We demonstrate the same correspondence to in 2000, 2010, 2013.
The maximal depth on the isosteric surfaces is observed is reached in summer. The maximal areas with the greatest depths also are observed in summer in contrast to a minimum in winter. This means a certain inertia of changes in the thermohaline characteristics of Atlantic Waters as well as a shift of 1–2 seasons of the influence of deep convection on isosteric surfaces.
It is shown that isopycnic advection in the Lofoten Basin makes a significant contribution to its importance as the main thermal reservoir of the Nordic Seas.

Algorithm of assessment of photosynthetically available radiation near the bottom of the Black Sea shelf was developed and validated. Two-weekly, monthly, and long-term average maps of photosynthetically available radiation were obtained for the period from September 1997 to 2018, based on regional satellite ocean color products and regional spectral bio-optical properties of optically active substances, which profiles were obtained with statistical analysis of long-term measurements from profiling Bio-Argo floats. Estimates of the photosynthetically available radiation near the bottom of the Black Sea shelf are presented on two-weekly maps with a spatial resolution of 2.5 km by 2.5 km of longitude and latitude for the period from January 1998 to December 2018 on the website at http://blackseacolor.com/browser3.html. Interannual variability of photosynthetically available radiation near the shelf bottom was analyzed for six small designated areas: Zernov’s Phyllophora field, Karkinitsky Bay, in the center of the shelf slope zone, near Odessa, the mouth of the Danube River and Western Crimea. The analyses for summer months revealed an increase of photosynthetically available radiation near the shelf bottom in some areas during the period from 1998 to 2018. However, it was not observed for the winter months. The accuracy of the photosynthetically available radiation assessment is ~50%.

The results of hydrological, hydrooptical and hydrobiological studies performed in the photic zone of the northern part of the Black Sea in October 2016 are presented. The main features of the bio-optical structure formation and its relationship with the hydrological regime were revealed. It is shown that in the zones of the Sevastopol and Feodosiya anticyclones, together with the deepening of the upper mixed layer, the corresponding deepening of the maximum values layer of the total suspended matter concentration is observed. In the southern part of the survey, closer to the center of the Rim Current gyre of the Black Sea, where the depth of the lower boundary of the upper mixed layer is minimal, the rise of the maximum layer of the total suspended matter concentration is noticed. In October 2016 there was noted anomalously high transparency of waters with a small range of spatial variability. It is shown that during this period, there were markedly lower correlations between the vertical distributions of the total suspended matter concentration and the temperature, salinity and density in comparison with the summer period of 2016.
In the water samples, microalgae of nine classes typical for this season were identified. It was found that the values of the metabolic active biomass of microplankton in the sea open part corresponded to the values typical for oligo-mesotrophic waters, while at the Crimean coast it was close to eutrophic ones. The stage of microplankton production/destruction succession is developing, with a forecast for biomass growth in the future.

An experimental study of the distribution paths in the Neva River aquatic area one of the most polluted tributaries the Izhora River was carried out. Identification of tributary water in the Neva River was made based on the measurements of total dissolved solids values (TDS), as well as additional integral characteristic of water quality — redox potential (Eh). Based on the analysis of the results of the field experiment, it is shown that the waters of the tributaries (possibly polluted), after entering the Neva River, are pressed by the main stream to the shore and spread along it in the direction of the current. The effect of transverse turbulent diffusion, which contributes to the mixing of water masses, is insignificant. For the mathematical description of the mass transfer process in the Neva River, a conjugate mathematical model IL_MTRiver of the water flow, sediment transport and dissolved impurities in the open channel, developed at the на Institute of Limnology of the Russian Academy of Sciences, is used. The model implements an interconnected calculation of the variables of the state of the water flow and solid flow. A unified model allows to track the movement of water and suspended matter along the river channel and estimate the particles deposition rate. The interaction in the model of the moving stream and the river channel is represented by physically justified parameters — the coefficient of internal friction of the soil and the adhesion parameter of the soil particles in case of displacement. It is shown that the model adequately estimates the coastal distribution of impurities in the absence of mixing over the main flow cross section. In the future, the IL_MTRiver model can be used to solve the problems of assessing the possible pollution of Neva water in the area of urban water intakes and to predict the consequences of emergencies associated with the discharge of contaminated effluents at nearby enterprises, including the Krasny Bor toxic waste landfill.

The paper proposes a methodology for determining the speed and direction of the sea surface current from measurements of Doppler radar panoramas with an X-band Doppler radar. Numerical simulation of the Doppler velocity of the Bragg waves in the field of wind waves and currents were carried out. The range of distance was selected for measuring the velocity of the surface current at which the effect of shading of the sea surface sections by wave crests can be ignored. Long field experiments were conducted, during which the proposed method was tested. The velocity and direction of the surface current were calculated as the vectorial sum of the velocity of the water column and 3% of the wind speed, while at the same time Doppler radar panoramas of the sea surface were measured. It was shown that, for upwind/upwave radar sensing, the average Doppler velocity of the scattering microwave waves of sea surface elements are significantly higher than the predictions of the two-scale scattering model. To restore the velocity of the surface current the registration was carried out empirically. For downwind/downwaves radar sensing, good agreement with the simulation results was observed. A correlation analysis of the surface current, calculated through hydro meteorological parameters and Doppler radar panoramas, showed a maximum correlation coefficient for a velocity value is about 0.88 with a root mean square error of 8 cm/s, and for a direction is about 0.98 with a root mean square error of 14 degrees. It is noted that film slicks on the sea surface lead to a significant decrease in the average Doppler velocity, which may serve as an additional criterion for the remote detection of oil spills.