Globally, rivers deliver significant quantities of nitrogen (N) and phosphorus (P) to the coastal ocean each year. Currently, there are no viable estimates of how much of this N and P escapes biogeochemical processing on the shelf to be exported to the open ocean; most models of N and P cycling assume that either all or none of the riverine nutrients reach the open ocean. We address this problem by using a simple mechanistic model of how a low-salinity plume behaves outside an estuary mouth. The model results in a global map of riverine water residence times on the shelf, typically a few weeks at low latitudes and up to a year at higher latitudes, which agrees well with observations. We combine the map of plume residence times on the shelf with empirical relationships that link residence time to the proportions of dissolved inorganic N (DIN) and P (DIP) exported and use a database of riverine nutrient loads to estimate the global distribution of riverine DIN and DIP supplied to the open ocean. We estimate that 75% of DIN and 80% of DIP reaches the open ocean. Ignoring processing within estuaries yields annual totals of 17 Tg DIN and 1.2 Tg DIP reaching the open ocean. For DIN this supply is about 50% of that supplied via atmospheric deposition, with significant east-west contrasts across the main ocean basins. The main sources of uncertainty are exchange rates across the shelf break and the empirical relationships between nutrient processing and plume residence time.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016GB005483

What proportion of riverine nutrients reaches the open ocean

Mesoscale eddies in the Black Sea basin are investigated by automated eddy identification method (winding angle) using satellite altimetry data for 1992–2011. Seasonal and interannual eddy variability is strongly affected by the intensity of basin-scale circulation. Weakening of large-scale circulation in response to the decrease of the wind curl leads to the baroclinic instability of the Rim current and enhanced formation of mesoscale anticyclones. Increasing wind curl and circulation induce intensive formation of mesoscale cyclones. Intense developed large-scale circulation leads to the decrease in the number of both cyclonic and anticyclonic eddies. Eddies properties (orbital velocity, energy and lifetime) on the interannual time scales also depend on the intensity of the large-scale circulation. It was shown that the most powerful and long-lived anticyclones are generated in years with intense currents, right after the currents begin slowing down, which is related to the transfer of energy from large- to meso-spatial scales. Mechanisms of eddies formation and dissipation are discussed.

Seasonal and interannual variability of the Black Sea eddies and its dependence on characteristics of the large-scale circulation

Seasonal and interannual variability of the Black Sea surface temperature as revealed from satellite data (1982–2000)

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.

https://link.springer.com/content/pdf/10.1134/S0001437010050036.pdf

The upper desalinated layer in the Kara Sea

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.

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

This paper presents new results of the research and development project on the moored profiler Aqualog aimed at multidisciplinary studies and ecological monitoring of the marine environment. The data on the profiler’s operation are summarized based upon the field experiments in the northeastern Black Sea in 2011. An important scientific result obtained by using the profiler during the experiments was the discovery of the countercurrent below the Black Sea Rim Current in the layer between the 500 m and 900 m depth.

Autonomous System for Vertical Profiling of the Marine Environment at a Moored Station

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.

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

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.

A. G. Zatsepin

Papers

The upper desalinated layer in the Kara Sea

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

Autonomous System for Vertical Profiling of the Marine Environment at a Moored Station

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

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