Structure and Dynamics of Plumes Generated by Small Rivers

The total share of small rivers in the influxes of fluvial water and suspended matter to the world ocean is estimated at between 25 and 40%. On a regional scale, this contribution can be even more significant for many coastal regions. In this chapter, we show that dynamics of small river plumes is significantly different from that of plumes generated by large rivers. Spatial structure of small plumes is generally characterized by sharper horizontal and vertical gradients. As a result, small plumes exhibit more energetic temporal variability in response to external forcing. In this chapter, we address several dynamical features typical for small plumes. We describe and discuss the response of small plumes to wind forcing and river discharge variability, the interaction between neighboring small plumes, and the generation of high-frequency internal waves in coastal ocean by small rivers. We also substantiate the Lagrangian approach to numerical modeling of small river plumes

Continuity preserving modified maximum cross-correlation technique

The maximum cross-correlation (MCC) method reconstructs the surface advective velocity fields from the displacements of spatial patterns in pairs of sequential satellite (normally infrared) images. However, the performance of the conventional MCC method is not always satisfactory. One of the main reasons for this is the fact that the method can correctly estimate only the velocity component parallel to the gradient of the property depicted in the images, while any small displacement perpendicular to the gradient (i.e., directed along the isolines) essentially maps the spatial pattern onto itself and therefore can not be detected using the conventional MCC technique. In the present work we propose a modification of the MCC method that allows circumventing this basic deficiency and improving the performance of the MCC technique. In this approach, the "cross-isoline'' components of the velocity field are obtained as in the conventional MCC scheme; however, the "along-isoline'' components derived from the MCC are disregarded as unreliable. Instead, the "true'' along-isoline components are then reconstructed from the given cross-isoline velocity field based on the continuity requirement and on the condition of no normal flow at solid boundaries. This inverse problem is solved by constructing the two-dimensional stream function in the curvilinear coordinate frame associated with the image isolines. The method is illustrated using AVHRR images from the southwestern Atlantic Ocean and the Black Sea. The results are compared with some direct drifter and current meter measurements and geostrophic estimates

The internal seiche field in the changing South Aral Sea (2006-2013)

Internal standing waves (seiches) in the South Aral Sea are studied for the first time. The study, based on numerical simulations and field data, focuses on two different campaigns: the first in autumn 2006, when the stratification was weak and there was a mild prevailing northeasterly wind, and the second in autumn 2013, when the stratification was strong and there was a mild easterly wind. Between these two campaigns, the sea surface level decreased by 3.2m. The periods of the fundamental modes were identified as 36 and 14h, respectively. In both years, either second or third vertical modes were found. In general, the vertical modes in 2013 were higher because of the broad and strong pycnocline. For both years, it was found that the deep quasi-homogeneous mixed upper layer could sustain internal waves under mild wind conditions. The observed first and second vertical modes in 2006 are the first and second horizontal modes and the second and third vertical modes in 2013 are the second and third horizontal modes. The results suggest that, due to sea level variations, the neck connecting the Chernyshev Bay to the main body of the lake can become a critical location for the development of a nodal line for all principal oscillation modes. Rotation effects on waves were not analyzed in this studyThe present work was carried out within the framework of the CLIMSEAS exchange project funded by the European Commission (ref. FP7-IRSES-2009-247512). Field work was supported by Russian Science Foundation grant 14-50-0009

Assessment of Santa Catarina shelf currents through the analysis of indirect measurements

Preliminary analyses on the use of drift measurements of a buoy in the study of shelf currents are presented. The equipment was moored at Santa Catarina Shelf in Southern Brazil. The drift was measured by means of a GPS which provided buoy's geographical coordinates on an hourly basis. The data was decomposed in meridional and zonal position components and along with wind and sea level data, covered a period of 203 days from late summer (March 21 ) to late spring (October 11 ) of 2002. Statistical comparison between mean sea level (msl) and the meridional buoy positioning displayed very high correlation (R =0.83) with a 3 h delay in relation to msl. The best correlation between meridional wind and buoy's drift was attained at 10 h time lag (R =0.63). Sea level analysis characterizes the tide as mixed, mainly semidiurnal. Diurnal and Semi-Diurnal bands accounted for 63.6% of energy spectra in comparison to 36.4% on the Meteorological-forcing band. Low frequency signal in Meteorological band were clearly dominant in the buoy's meridional series, accounting for 90% of energy spectra. The buoy mooring system seems to work as a very useful indicator of the sense of geostrophic currents over the shelf, which seems to respond to both regional as well remote wind forcing

Geodynamical processes in the channel connecting the two lobes of the Large Aral Sea

Reasons for the existence of the channel connectin the two lobes of the present Large Aral Sea are discussed. In situ measurements in 2005 show that differences between the measured depths and those contained in the available digital bathymetry of the lake are considerably different at the channel (7.5±0.9 m, at nine measurement stations along it) and at the northern part of the eastern lobe (1.6±0.3 m, at six stations from the western to the eastern shore). Differences in the misfits observed in the two zones are discussed and thought to be a consequence of the variation of the transversal area of the channel as it enters the eastern lobe, which would affect the flow velocity and thus the strength of the erosion process at the bottom. Field data together with satellite images have been used to modify an original digital bathymetry of the lake and have been implemented into a 3-D hydrodynamical model. A numerical simulation shows that a wind of 12 m/s blowing from the east (112°) generates velocities of up to 45 cm/s in the channel, allowing denser water from the eastern lobe (salinity: 132 g/kg) to flow about 38 km towards the fresher western lobe (salinity: 98 g/kg) in one day. The effect of the inflow on the vertical structure of the western lobe is also illustrated. Although nowadays the channel will be soon a thing of the past, in a more general context, the Aral Sea is presented as an example showing that geomorphologic and geophysical processes, along with hydrological and atmospheric processes, must be taken into account for short-term predictionsWe thank D. Soloviov and S. Stanichniy (Marine Hydrophysical Institute, Ukraine), who kindly provided the satellite images used in this study. We are grateful to Professor Micklin and one anonymous reviewer for the interesting points they have raised in their review and for the opportunity to incorporate them in the text. This paper has been done within the frame of the NATO Collaborative Linkage Grant ESP-NR-NRCLG-982592. The work was also supported by the Russian Foundation for Basic Research and, partially, by the Spanish Ministry of Education and Science, grant FIS2008-03608This paper has been done within the frame of the NATO Collaborative Linkage Grant ESP-NR-NRCLG-982592. The work was also supported by the Russian Foundation for Basic Research and, partially, by the Spanish Ministry of Education and Science, grant FIS2008-0360

The world’s largest heliothermal lake newly formed in the Aral Sea basin

The Aral Sea desiccation is one of the worst aquatic ecological disasters of the last century, important for understanding the worldwide trends to degradation of arid lakes under water use and climate change. Formerly the fourth largest lake worldwide, the Aral Sea has lost ∼90% of its water since the early 1960s due to irrigation in its drainage basin. Our survey on the seasonal thermal and mixing regime in Chernyshev—a semi-isolated hypersaline part of the Aral Sea—revealed a newly formed two-layered structure with strong gradients of salinity and water transparency at mid-depths. As a result, the Chernyshev effectively accumulates solar energy, creating a temperature maximum at the water depth of ∼5 m with temperatures up to 37 °C. Herewith, this part of the Aral Sea has evolved to an unprecedently large (∼80 km ^2 ) heliothermal lake akin to artificial solar ponds used for ‘green energy’ production. The newly formed heliothermal lake, with transparent and freshened layer on top of the hypersaline and nutrient-rich deep water, acts as a solar energy trap and facilitates intense biogeochemical processes. The latter reveal themselves in practically 100% opacity of the deep layer to the solar light, permanent deep anoxia, and growing methane concentrations. The recent emergence of the Chernyshev as a heliothermal lake provides an opportunity for tracing the biogeochemical and ecological response of aquatic ecosystems to suddenly changed environmental conditions

On thermohaline structure and circulation of the Western Large Aral Sea from 2009 to 2011: Observations and modeling

The shrinkage of the Aral Sea in the second half of the past century has significantly affected the hydrophysical regime of the lake. The objective of this paper is to report on a hydrological structure and circulation of the today's Aral Sea based on both direct field observations and modeling results. We focus on the results of three field surveys to the Aral Sea which took place in the period from 2009 to 2011. In addition, series of numerical experiments using Princeton Ocean Model adapted to the Aral Sea was undertaken to investigate the contributions from bathymetry and water stratification in the formation of the basin scale circulation. The hydrological structure of the Aral's western basin in autumn season exhibited a three-layered pattern with two local salinity maxima, separated by a fresher intermediate layer. According to direct observations, water circulation in the surface layer has anti-cyclonic character, while circulation in the bottom layer has cyclonic sign under the predominant northerly winds. The simulation experiments demonstrated clearly that the main cause of the anti-cyclonic circulation in the surface layer of the lake is the "asymmetric" bathymetry with broad shallow area along the eastern coast and relatively steep and deep western slope. However, strong stratification is a necessary condition for the formation of the cyclonic circulation gyre in the bottom laye

Dissolved Methane in Coastal Waters of the Northeastern Black Sea

As the largest methane reservoir in the world, the Black Sea is characterized by significant variability in its dissolved methane distribution patterns. Aerobic waters in the coastal regions are influenced by various factors governing the biogeochemical processes in the water column; however, their impact on the distribution pattern of dissolved methane is not always well studied, especially in the shallow areas of the northeastern Black Sea, which have hitherto not been thoroughly covered by observations. Here, we consider the measurements of methane content in several regions of the northeastern Black Sea carried out from 2013–2017 with large and detailed spatial coverage in order to identify the key factors determining the dissolved methane pattern in each region of the study area. The CH4 pattern in the region of Sochi is dominated by the impact of river flow. The increased methane saturation in surface waters of the southeastern Crimean shelf (up to 40 nM) is caused by the influence of the Azov Sea outflow. The waters of the Feodosia Bay and to the south of it are mainly characterized by an increase in methane content towards the bottom (up to 100 nM), which is associated with water–sediment interactions and gas emission from the bottom