V. V. Kremenetskiy

Submesoscale Eddies at the Caucasus Black Sea Shelf and the Mechanisms of Their Generation

The results of observations of submesoscale eddies (with a diameter of 2–8 km) on the narrow Black Sea shelf are presented. These observations were carried out in the Gelendzhik region in the autumn seasons of 2007–2008 using traditional and new methods of hydrophysical investigations. The mechanisms of generation of such eddies are discussed.

Circulation in the southwestern part of the Kara Sea in September 2007

During cruise 54 of the R/V Akademik Mstislav Keldysh to the southwestern Kara Sea (September 6 to October 7, 2007), a large amount of hydrophysical data with unique spatial resolution was obtained on the basis of measurements using different instruments. The analysis of the data gave us the possibility to study the dynamics and hydrological structure of the southwestern Kara Sea basin. The main elements of the general circulation are the following: the Yamal Current, the Eastern Novaya Zemlya Current, and the St. Anna Trough Current. All these currents are topographically controlled; they flow over the bottom slopes along the isobaths. The Yamal Current begins at the Kara Gates Strait and turns to the east as part of the cyclonic circulation. Then, it turns to the north and propagates along the Yamal coast over the 100-m isobath. The Eastern Novaya Zemlya Current (its core is located over the eastern slope of the Novaya Zemlya Trough) flows to the northeast. Near the northern edge of Novaya Zemlya, it encounters the St. Anna Trough Current, separates from the coast, and flows practically to the east merging with the continuation of the Yamal Current. A strong frontal zone is formed in the region where the two currents merge above the threshold that separates the St. Anna Trough from the Novaya Zemlya Trough and divides the warm and saline Arctic waters from the cooler and fresher waters of the southwestern part of the Kara Sea. This threshold, whose depth does not exceed 100–150 m, is a barrier that prevents the spreading of the Barents Sea and Arctic waters to the southwestern part of the Kara Sea basin through the St. Anna Trough.

Subsatellite polygon for studying hydrophysical processes in the Black Sea shelf-slope zone

The first data on the creation of the subsatellite polygon on the Black Sea shelf and continental slope in the Gelendzhik area (designed in order to permanently monitor the state of the aquatic environment and biota) and the plans for maintaining and developing this polygon are presented. The autonomous measuring systems of the polygon in the composition of bottom stations with acoustic Doppler current profilers (ADCP), Aqualog robotic profilers, and thermo-chains on moored buoy stations should make it possible to regularly obtain hydrophysical, hydrochemical, and bio-optical data with a high spatial-time resolution and transmit these data to the coastal center on a real-time basis. These field data should be used to study the characteristics and formation mechanisms of the marine environment and biota variability, as well as the water-exchange processes in the shelf-deep basin system, ocean-atmosphere coupling, and many other processes. These data are used to calibrate the satellite measurements and verify the water circulation numerical simulation. It is assumed to use these data in order to warn about the hazardous natural phenomena and control the marine environment state and its variation under the action of anthropogenic and natural factors, including climatic trends. It is planned to use the polygon subsatellite monitoring methods and equipment in other coastal areas, including other Black Sea sectors, in order to create a unified system for monitoring the Black Sea shelf-slope zone.

The upper desalinated layer in the Kara Sea

An area of about 40000 km2 of desalinated upper layer waters with a salinity of less than 25 psu was found during cruise 54 of the R/V Akademik Mstislav Keldysh in the southwestern part of the Kara Sea (September 2007). Close to the boundary of this region located near the eastern coast of Novaya Zemlya, the salinity was less than 16 psu. The thickness of the desalinated layer was about 10 m. The results of the chemical analysis revealed that the observed desalination of the sea water was produced, first of all, by the Yenisei River, while the contribution of the Ob River’s waters was secondary. However, the most desalinated region near the eastern coast of Novaya Zemlya was separated from the Ob-Yenisei estuary and corresponded to a quasi-isolated lens. It is likely that the formation of this lens, as well as the major part of the desalinated upper layer waters, occurred in June when the flood of the Yenisei was maximal, while the further drift of the desalinated waters to the west of the Ob-Yenisei estuary was forced by the prevailing northern wind. The additional desalination (by 2–3 psu and even more) of the upper layer waters near the eastern coast of Novaya Zemlya might be related to the melting of the Novozemelskiy ice massif. The regularities of the temporal evolution of the upper desalinated layer, as well as the influence of this layer on the hydrological structure and dynamics of the southwestern Kara Sea, are discussed.

Structure of the zooplankton communities in the region of the Ob River’s estuarine frontal zone

The studies were carried out on September 27–30, 2007, in the area of the Ob estuarine frontal zone and over the adjacent inner Kara Sea shelf. Based upon the latitudinal changes in the salinity, the 100 nautical mile wide estuarine frontal zone was marked out. The frontal zone was inhabited by a specific zooplankton community dominated by species that occurred outside the frontal zone in only minor amounts. The biomass of the mesozooplankton averaging 984 mg/m3 in the frontal zone exceeded by 1.5 and 6 times the corresponding values in the inner desalinated area of the estuary and the adjacent areas of the Kara Sea shelf. At the inner southern periphery of the frontal zone, at maximal latitudinal salinity gradients (>2 psu per mile), the maximal development of the mesoplankton with the mean biomass for the water column of 3.1 g/m3 (37 g/m2) and up to 5.8 g/m3 in the subpycnocline layer was observed. The latitudinal extension of the biomass in the maximum zone did not exceed 10 miles. More than 90% of the maximum was composed of herbivorous zooplankton with the strong domination of the copepod Limnocalanus macrurus. The daily consumption within the zooplankton maximum area was estimated at 820 mgC/m2 per day. This value exceeds by two orders of magnitude the local primary production. At that level of consumption, the available phytoplankton biomass was consumed by grazers in less than 8 hours (!). A zooplankton aggregation at the southern periphery of the estuarine front exists due to the advection of phytoplankton from the adjacent river zone. The aggregation forms a natural pelagic biofilter where new allochthonous organic matter delivered by the river flow is accumulated and high secondary production is formed on its basis. An anomalously high concentration of planktic predatory Parasagitta elegans with biomass of over 1 g/m3 (46% of the total zooplankton biomass) was associated with the outer northern periphery of the estuarine frontal zone.

On the nature of short-period oscillations of the main Black Sea pycnocline, submesoscale eddies, and response of the marine environment to the catastrophic shower of 2012

Field studies performed at the Shirshov Institute of Oceanology, Russian Academy of Sciences (SIO RAS), Black Sea hydrophysical polygon in 2012 are illustrated. The variations in the vertical distribution of the hydrophysical characteristics (water temperature, salinity, and density, as well as current velocity) in the upper 200-m layer of the Black Sea above the continental slope in the cold season, obtained using an Aqualog autonomous profiler on a moored buoy station, have been analyzed. It has been established that the position of the permanent pycno-halocline and the hydrosulphuric zone upper boundary intensively oscillate with a characteristic period of 5–10 days. These oscillations cause short-period variations in the thickness of the oxigenated layer by 20–40 m, which reaches one-third of the total thickness of the layer. Measurements performed with autonomous stations (bottom ADCP, thermochain) at the experimental subsatellite polygon in the Gelendzhik coastal zone, as well as meteorological, ship, and satellite data obtained during the catastrophic rains and flooding on July 6–7, 2012, and afterward, have been simultaneously analyzed. It has been established that a catastrophic flow of turbid fresh water into the sea caused the formation of a belt of freshened (by 1.0–2.7 psu) less dense water with a high suspension concentration on the shelf and the upper continental slope. This water formed a quasi-geostrophic northwestward along-shore current, the velocity of which reached 40–50 cm/s. Therefore, the freshened and turbid water mostly escaped from the Gelendzhik region northwestward for two days after the flood, and the remaining water became free of suspension owing to its settlement during approximately the same period. The fields of the current velocity and suspension concentration in a submesoscale cyclonic eddy, identified on the satellite image, were measured at the hydrophysical polygon. It has been established that a high (when compared to the background values) suspension concentration in the surface-water layer in an eddy is related to intense upwelling at the eddy center and the rising of suspension (apparently phytoplankton) from the thermocline layer, where the suspension concentration is maximal.

Propagation and transformation of waters of the surface desalinated layer in the Kara Sea

A new method for the calculating propagation of brackish waters from the Ob-Yenisei estuary in the Kara Sea is proposed. This method is based on satellite altimetry measurements and meteorological data. Surface currents in the upper layer are estimated as the sum of geostrophic and parameterized wind-driven transport. Geostrophic velocities are calculated using altimetry-derived sea-level anomalies and mean dynamic topography. The method has been used previously to calculate surface currents in the Black Sea [7, 12]. In this paper it has been successfully verified on the basis of comparisons with field observations of surface salinity and satellite images of sea-surface chlorophyll in the Kara Sea.

Formation of the coastal current in the Black Sea caused by spatially inhomogeneous wind forcing upon the upper quasi-homogeneous layer

A poorly known mechanism of formation of a warm coastal current by spatially nonstationary and nonuniform wind forcing in the northeastern part of the Black Sea is described. Owing to the blocking influence of the Caucasian Mountains, the northeasterly wind (nordost) west and east of Tuapse is characterized by a spatially nonuniform speed distribution over the Russian sector of the sea. In the first half of the year, during the nordost wind, the intense wind forcing upon the upper quasi-homogeneous layer (UQL) leads to its fast cooling due to the turbulent entrainment of cold waters from the shallow seasonal thermocline. Satellite data and measurements from ships obtained at the end of June-beginning of July 2006 showed that, during two days, the temperature of the UQL west of Tuapse dropped by 7–10°C, whereas, east of Tuapse where the wind was weak, it practically did not change. As a result, a narrow frontal zone between the warm (less dense) and cold (denser) upper layer waters oriented quasi-normally to the shore was formed. A hydrodynamic analysis showed that, in such situation, an intense jet of a near-shore density current is developed that transports warm waters to the northwest at a velocity of 40–60 cm/s. This estimate agrees well with satellite data and observations from ships. The satellite data allowed us to find that this jet reached the Kerch Strait region in a few days. During the time of its existence (approximately 2 weeks), the jet transports a large volume of water and can change the functioning regime of the coastal ecosystem. The mechanism of the near-shore current formation revealed may also function in other seas and oceanic regions.

Hydrophysical features of deep water troughs in the western Kara Sea

In the 59th cruise of the RV Akademik Mstislav Keldysh (September–October, 2011) a large amount of hydrophysical data was obtained with the help of various measuring systems. The data allowed us to study the dynamics and hydrological structure of deep-sea troughs of the western part of the Kara Sea at a high spatial resolution. In particular, the analysis of this material showed that a cyclonic jet exists in the central spur of the St. Anna Trough at depths of 150–300 m and penetrates down to the bottom. The jet is able to play a role of a dynamic barrier that prevents the penetration of water from the southwestern part of the Kara Sea to higher latitudes. In the eastern spur of the St. Anna Trough, the jet weakens, becomes less barotropic, and can no longer inhibit the northward spreading of the shelf waters of the Kara Sea. The weakest and least coherent currents were recorded in the Novaya Zemlya trough, in the southern part of which we traced waters of a Barents Sea origin.

Phytoplankton of the south-western part of the Kara Sea

The research was performed along a transect from the Yamal Peninsula coast towards the outer shelf of the southwestern part of the Kara Sea in September 2007. 130 phytoplankton species have been identified, among which 63 were found in the area for the first time. The total phytoplankton numbers varied within the range of 0.2 to 11.3 × 109 cells/m2, while biomass within the range of 43 to 1057 mgC/m2. A well pronounced cross-shelf zoning in the phytoplankton communities was ascertained. The inner shelf zone about 30 km wide with depths down to 30 meters was characterized by the predominance of diatoms (up to 80% of the total algae numbers and biomass). The second group by value was dinoflagellates. Seaward in the area of the depth increase from 30 to 140 m, the zone of the Yamal Current was located, which was 40 km wide and notable for its active water dynamics. The total cell numbers in the zone reached a maximum for the entire investigated area: up to 11.3 × 109 cells/m2. The leading group in the phytoplankton was autotrophic flagellates, whose share in the total numbers reached 56–82%. Further than 70 km from the shore, the outer shelf zone was found with the water column rigidly stratified. The highest for the whole area phytoplankton biomass was identified here (up to 1.06 gC/m9), 80% of which was concentrated above the halocline. Diatoms dominated in the phytoplankton numbers (up to 92%) and biomass (up to 90%), which was related to the mass development of two species: Chaetoceros diadema and Leptocylindrus danicus.