S. V. Sheberstov

Bio-Optical Characteristics of the Aegean Sea Retrieved from Satellite Ocean Color Data

The specific goal of this work is to demonstrate the possibilities of new ocean color satellite sensors such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) for studying processes in the Eastern Mediterranean, with the Aegean Sea as an example. The Aegean Sea appears as an appropriate basin for study of contrasting ecosystems, because it is subjected to intrusion of water masses of different origin with pronounced contrast in bioproductivity: the eutrophic water from the Black Sea, and the oligotrophic water from the Levantine Basin.

Spatiotemporal variability of chlorophyll a in the Altantic and Indian sectors of the Southern Ocean during February–April of 2000 according to satellite and expeditionary data

In February and April 2000, in the Southern Ocean between Africa and Antarctica, the concentration of chlorophyll (Cchs) in the surface layer was not high (0.1–0.3 mg/m3). The zones of increased Cchs values (0.6–2.0 and over 2.0 mg/m3) were located between the Southern Subtropical and Subantarctic fronts, near the Polar front, and to the south of the Antarctic Divergence. From February to April, in the open areas of the Southern Ocean, a general Cchs decrease was observed. A similar trend was also noted in the near-shore seas of the East Antarctic. At the shelf of the Sea of Cosmonauts, within the upper mixed layer, the content of chlorophyll amounted to 0.34–0.37 mg/m3. In the area of the continental slope, we registered the formation of a subsurface chlorophyll maximum (0.52–0.56 mg/m3) at a 20-m depth, which deepened to 70 m when passing to the deep-water area. The positive correlation with a high coefficient (r = 0.939) between the field and satellite data (Cchs and Csat, respectively) allows one to conclude about the applicability of the SeaWiFS algorithm for the estimation of the chlorophyll content within the surface layer in the Antarctic areas studied. In the course of the ice formation in Prydz Bay, during five days, the content of chlorophyll in different phases of the new ice increased by a factor of 2.9, whereas the values of this parameter in the surrounding waters remained quite invariable. The Csat values were 6.3 times as high as those obtained experimentally for the sludge ice. Because of the large areas occupied by floating ice, the sole usage of satellite data may cause great errors in the productivity estimation of the East Antarctic seas.

Estimation of sea surface solar radiation at 400-700 nm using satellite ocean color data, and its validation by ship data

We present results of validating the algorithms used to estimate sea surface solar radiation at 400-700 nm, photosynthetically available radiation (PAR), from satellite ocean color data and an appropriate validation procedure when data are collected using a moving ship. The validation was performed using field measurements of PAR during a transit cruise from the Baltic to the White Sea during the summer of 2014. The PAR was measured at 10-minute intervals using a deck radiometer throughout the daylight hours. The satellite estimate of daily surface PAR with an acceptable error, on the scale of 1 day-102 km, is shown.

Correlation of the Distribution of Normalized Spectral Radiances as an Indicator of the Features of Water Exchange in the Black Sea

A set of 128 images of the Black Sea obtained using the SeaWiFS color scanner data in 1998–2004 was processed to estimate the new capabilities of indication of the vertical inhomogeneity of the upgoing normalized radiance. The basin of the sea was divided into 430 frames with a size of 30 × 30 km. Correlation coefficients R between all the spectral radiances were calculated for all the frames of all the images. The correlation between the radiance distributions at 490 and 670 nm was most sensitive to the conditions of the radiance formation. This allowed us to find the situations in which the variations in the shortwave and long wave radiances in the frame appear opposite and are characterized by estimates R < −0.7. It was shown that such distributions are indicators of the water exchange between the coastal and open sea areas. They are caused by a special optical stratification during the events of such exchange. A conclusion is made about the prospective of applying the estimates of the correlation of spectral radiances for diagnosis of the water exchange based on the current and archive materials of ocean color scanners.

Suspended matter in the surface layer of the southeastern Baltic from satellite data

The spatial and temporal variability of the concentration of suspended matter in the surface layer of the southeastern Baltic is analyzed using data from the satellite scanner MODIS-Aqua. The concentration of suspended matter was calculated by a regional algorithm developed on the basis of field data in the given region. The mean monthly distributions of the concentration of suspended matter from March to September 2003–2014 were calculated. The role of contributing factors is analyzed by using the data of the satellite scanner Landsat-8, with a spatial resolution of 30 m. Along with the whole area of studies, the seasonal and interannual variability of the concentration of suspended matter in different regions is considered. No meaningful trend for the average concentration of suspended matter over the considered period is shown.

Assessment of underwater irradiance and absorption of solar radiation at water column from satellite data

The volume absorption of solar radiation in water body determines important processes in the upper ocean such as primary bioproduction and heat balance. Assessment of penetration of solar radiation into water body can be performed with satellite data and the previous attempts in this direction were promising. This paper presents a package of the algorithms to compute the components of balance of photosynthetically available radiation (PAR) at sea level (incident, reflected from the rough sea surface, and water-leaving PAR) and to calculate the volume absorption of PAR in the upper layer from satellite ocean color data. Data measured by the SeaWiFS ocean color sensor and the ancillary data needed (such as ozone amount and wind speed) are used. Computations of the underwater irradiance are performed for the 0-25 m upper layer. The errors are estimated by direct comparison between the values of underwater irradiance and volume absorption derived by the algorithms developed and by the exact method. Monthly means of the components of PAR balance as well as the potential daily heating have been estimated in different regions.

Bio-optical characteristics of the seas surrounding Russia from satellite ocean color data

The mean monthly distributions of chlorophyll a concentration, the particle backscattering and yellow substance absorption coefficients in the Barents, Black, Caspian, and Japan Seas have been computed from data of the SeaWiFS satellite ocean color scanner from 1998 to 2005; the color maps constructed show spatial and temporal variability of photosynthetic phytoplankton, particulate matter and colored organic matter. The modified algorithms developed on the basis of the ship-measured data were used for computations; these regional algorithms provide reasonable agreement between in situ and satellite-based values of bio-optical characteristics in the seas under study. The monthly, seasonal and annual mean values for selected regions were calculated. A brief analysis of the obtained results is given. Some interesting phenomena in the above seas are observed; among them the coccolithophorid blooms in the Black Sea and in the Middle Barents revealed from satellite ocean color data and confirmed then by the field studies. In the Caspian Sea the satellite data displayed sharp increase of chlorophyll concentration and of the particle backscattering attributed to a consequence of invasion of Ctenophore Mnemiopsis.

New approach to atmospheric correction of satellite ocean color data

Requirements on accuracy of the atmospheric correction of satellite ocean color data are rather high, and so far attempts to solve this problem as needed have not been quite successful, especially at high latitudes and in coastal waters. New approach to the atmospheric correction problem consists in simultaneous retrieval of the unknown aerosol and ocean reflectances ρα(λi) and ρw(λi) from the top-of-the-atmosphere reflectance ρt(λi) with use of all spectral channels of a satellite sensor in infrared and visible ranges. The approach is based on parameterization of the spectral behavior of ρα(λi) and ρw(λi) with help of several basic functions. In this way the problem reduces to determination of several weight coefficients at these basic functions. This allows us to cut drastically the size and number of required lookup tables. The inversion problem is solved by a least square method with inequality constraints. It is shown that the new algorithm results in better agreement with in situ measured ρw(λi), compared with the standard SeaWiFS algorithm, in Case 2 waters.

Assessment of optical characteristics of atmosphere and ocean by data from satellite ocean color sensors

At present there are half a score of satellite ocean color sensors being in operation or prepared for launch; they offer strong possibilities of providing data on spatial and temporal variability of various characteristics of atmosphere and ocean, primarily of the optical ones. As of now, data from the satellite scanner SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are in most common use; they have been available since September 1997. The possibilities and problems of retrieval of optical characteristics of atmospheric aerosol, cloudiness, and sea water from SeaWiFS data are considered. The algorithms developed in the P.P. Shirshov Institute of Oceanology Russian Academy of Sciences are presented as well as some results of their validation by data of field measurements. The examples of spatial distributions and seasonal changeability of aerosol optical thickness and seawater optical characteristics derived from SeaWiFS data are given.

Estimation of Annual Kara Sea Primary Production

Primary production of phytoplankton and ice and under-ice flora of the Kara Sea and regions thereof has been assessed using region-specific models and MODIS-Aqua satellite data for 2002–2015. Average annual primary production of phytoplankton calculated for the growing season (April–October) amounted to 165 mg С m–2 day–1. Annual primary production of phytoplankton was 35 g C/m2. Annual primary production of phytoplankton in the entire Kara Sea was 13 × 1012 g C. Annual primary production of ice and underice flora calculated using an integrated biophysical model was 1.7 × 1012 g C, or 12% of total primary production of the Kara Sea; the ice cover dynamics and published data were taken into account for the calculations. The results have been compared to earlier primary production estimates for the Kara Sea. The extent of the increase in sea productivity during warming of the Arctic and the decrease in ice cover area are discussed.