Determination of the seasonal course and trends of the dissolved oxygen concentration and temperature in the upper layer of a deep water part of the Black Sea according to modern data

by

A.B. Polonsky, A.A. Valle

 Institute of Natural and Technical Systems, RF, Sevastopol, Lenin St., 28

 E-mail: apolonsky5@mail.ru, ogorodovaa.a@mail.ru

DOI: 10.33075/2220-5861-2020-2-134-143

UDC 551.465 (262.5)

Abstract:

   In this work, based on archival data for the period from 1923 to 2020, the intra-annual variability of dissolved oxygen and temperature in the 0–150 m layer of a deep-water part of ​​the Black Sea is analyzed. A comparative analysis of the results obtained with earlier data was carried out to highlight long-term trends, which showed that the water temperature in the 0 – 10 m layer from 1923 to 1932 from September to December was significantly higher, and in March lower than at present. The oxygen concentration for the period from 1955 to 2020 in the layer of 20 – 100 m is significantly lower than previously observed.

   It has been confirmed that in the upper mixed layer the concentration of dissolved oxygen reaches a maximum (about 330 μM) in March-April and a minimum (about 240 μM) in August. The amplitude of the seasonal variation as a whole decreases with depth and at the horizon 150 m it falls almost to the noise level. The seasonal variation of oxygen dissolved in the surface waters of the Black Sea, consistent with intra-annual temperature changes, has not fundamentally changed over an almost century-long period of instrumental observations. At the same time, its quantitative amplitude-phase characteristics significantly change under the influence of climatic variations. The intermediate layer (between 50 and 150 m) on the scales of several decades is characterized by a significant decrease in the concentration of dissolved oxygen and an increase in temperature, which have continued in recent years. Most likely, a sharp decrease in the oxygen concentration in the intermediate layer is related to an exacerbation of stratification, leading to a weakening of the vertical mixing of water [3]. In recent decades, this trend has been also supported by the increase in upward motions in the subsurface layer, related to the increase in wind vorticity after 1980 [17].

Keywords: dissolved oxygen in the upper 150-meter layer, temperature, decrease in oxygen concentration in the intermediate layer of the Black Sea waters.

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