Climate changes in the Black Sea region
Air temperature and precipitation
In climatology, there are different approaches to classification of climates but the most
commonly used is the classification offered by V. Köppen, which is based on the seasonal course
of air temperature and precipitation, and takes into account the prevailing types of landscape and
vegetation. In the recent years, estimates of geographical distribution of the Black Sea climate
types, according to Köppen approach, have undergone significant changes. Originally, according
to Köppen's estimations (Köppen, 1936), which were regularly mentioned by other researchers,
the northern coast was associated with continental steppe climate, the western coast of the
Caucasus was related to temperate maritime climate, the coast of Turkey and the southern coast
of Crimea was considered to have the Mediterranean climate (Fig.1). In the most recent
evaluations of the global climate types, distribution during period of 1950–2000 on the 0.5×0.5°
grid (Kottek et al, 2006) and 0.1×0.1° (Peel et al, 2007), the most significant changes for the
Black Sea basin are related to distribution of the steppe and mediterranean types of climate
(Fig.1). The boundary of the steppe climate shifted far eastward to the Caspian Sea, so the
continental climate zone has been expanded. The steppe climate zone in the Eastern Crimea can
only be identified at high spatial resolution (Peel et al, 2007). Only the southwestern part of the
coast near Istanbul is referred to the Mediterranean climate, while the temperate climate
(subtropical or maritime) prevails over the rest of the Black Sea coast.
Figure 1 Distribution of climate types on the Black Sea coast, based on Köppen's classification:
Cfa – temperate climate with uniform humidity and hot summers (subtropical),
Cfb – temperate climate with uniform humidity and warm summers (maritime),
Csa – temperate climate with dry and hot summers (mediterranean),
Bsk – dry cold (steppe),
Dfa – continental climate with uniform humidity and hot summers,
Dfb – continental climate with uniform humidity and warm summers.
According to climatologies for the last WMO period of 1961–1990 the average annual
temperature rises evenly from the north to the south, from 10°C in Odessa area to values
exceeding 14°C in the southern coast from Istanbul to Batumi. The range of seasonal fluctuations
of air temperature, in its turn, decreases from 22 – 23°C on the northern coast to 15°C in the
south. The average air temperature for the whole coast is 12.8°C, which is 2°C lower than the
average sea surface temperature. The warming influence of the Black Sea is the most noticeable
from December till January, when temperature difference between air and water in the coastal
zone reaches 3 – 4° C. The cooling effect of the sea in summer is less noticeable, temperature
difference between air and water in the coastal zone is about –1°С. Those values of the
temperature difference between air and water are large enough and close to those in sub-Arctic
seas.
The minimum amount of atmospheric precipitation falls to the north-western coast (400–
450 mm). The area of precipitation exceeding the annual averages of 1000 mm covers almost
whole Anatolian and the Caucasian coast from Zonguldak to Tuapse, with values above 2000
mm – from Trabzon to Poti. The maximum annual average air temperature (14.6° C) and
precipitation (2750 mm) were observed on the coast of Georgia in Batumi. Annual variation of
precipitation along the Anatolian and the Caucasian coast, as well as the southern coast of
Crimea, with apparent predominance in the cold season of the year, belongs to the Mediterranean
type. Over the rest coastal regions, there is a weak dominance of rainfall during the warm period,
which is more typical for continental climate.
Based on estimates for the continental Europe made by (Blüthgen, 1966), Berg’s index
(frequency of continental air masses) in the Black Sea is more than 50%. In the northwestern part
of the sea, Gorchinsky’s index (relative amplitude of the air temperature seasonal variation in a
range from 0 to 100) exceeds the value of 30, which means that this region is more continental
than majority of the European seas. On the rest of the Black Sea coast, the values of the index are
less than 30, which correspond to the conditions of the Mediterranean Sea.
Long-term changes of air temperature (Ilyin, Repetin, 2006) are that secular linear trends of
annual values are positive in the most of coastal regions, which superimposed by oscillations of
70-80 year period. Winter and summer oscillations are in antiphase: warm winters correspond
cold summers and vice versa. Secular linear trends of winter warming prevail the summer ones.
There is increase of continentality in recent decades, manifested in increase of the seasonal cycle
magnitude.
In [Repetin et al, 2006] it was reported on general increase of precipitation in Black Sea
region and close negative correlation of decadal variations with NAO index.
Wind conditions
Research of wind conditions has traditionally received much attention. As for the Black
Sea studies, there are numerous publications and reference materials (Samoylenko et al, 1956;
Leonov, 1960; Rzheplinskiy et al, 1969; Sorkina et al, 1974; Altman, Matushevsky, 1987;
Simonov, Altman et al, 1991; Belokopytov et al, 1998; Repetin, Belokopytov, 2003–2009;
Naumova et al, 2010; Efimov, Anisimov, 2011).
Regular wind observations at the coastal stations show that the southeastern and the
southern coast of the sea are characterized by weak winds (average annual wind speed <3 m/s);
in the western and northwestern parts of the sea, as well as in the Kerch Strait area, stronger
winds are observed (average annual wind speed >4 m/s, at some stations >5 m/s).
Annual variations are well defined for the entire basin, wind speed increases in 1.2 – 1.5
times from the spring and summer to autumn and winter period. Fig. 2 shows the seasonal
variation of wind speed for the climatic period of 1961 – 1990 for meteorological stations located
along the Black Sea coast of the former USSR, and for the open sea area, by three sources. The
lowest wind speeds are typical for ERA-40 re-analysis (Uppala et al, 2005). Study of (Romanou
et al, 2010) also presents a comparison of seasonal variation of wind speed over the Black Sea
from various sources, where ERA-40 re-analysis is characterized by the lowest values, the ERAInterim data are close to NCEP data (Kalnay et al, 1996).
Evaluations of wind characteristics over the open sea at an early stage of research were
done on the basis of voluntary observing ships. Fig. 3 depicts distribution of mean annual wind
speed over the Black Sea where both ship observations and shore-based observations were used.
The work of (Samoylenko et al, 1956) with more than 50 thousands ship observations for the
period of 1880 – 1953 was the background. Later, in the work of (Sorkina et al, 1974) the data
was complemented with materials till mid- 1960s, and in the study by (Simonov, Altman et al,
1991) fields were adjusted with data of coastal stations until the 1980s.
Figure 2 Seasonal variation of wind speed (m/s). (а) Coastal stations for the period of 1961–
1990: 1–Odessa, 2–Chernomorskoye, 3– Yevpatoriya, 4–Chersonesus Lighthouse, 5–Yalta, 6–
Feodosiya, 7– Anapa, 8–Tuapse, 9–Batumi. (b) Open sea (total area average): re-analyses
ERA-40 and NCEP/NCAR and synoptic charts.
Figure 3 Annual average wind speed (m/s) according to (Sorkina et al, 1974) and
(Simonov, Altman et al, 1991).
To obtain reliable statistical estimates, a large amount of ship observations is required,
because of their low quality. The use of the coastal stations to characterize wind conditions in the
coastal zone is limited, due to local effects of the coastal orography. Therefore, wind
characteristics over the sea are now mainly estimated by means of computational methods:
global re-analyses of atmospheric fields, such as ERA-40, NCEP/NCAR, JRA, as well as
regional re-analyses with high spatial resolution (Efimov, Anisimov, 2011) or processing pressure
fields from synoptic charts (Belokopytov et al, 1998). The maps of average annual wind speed
presented on the Fig.2.1.7, for the period of 1961 – 1990 taken from ERA-40 (1.1°×1.1°) and
synoptic charts, are quite close to the fields shown in Fig. 3. The regional re-analysis (Efimov,
Anisimov, 2011), with resolution of 25 × 25 km for the period of 1958–1991, is most consistent
with the work (Simonov, Altman et al, 1991) by the spatial distribution and absolute values of
wind speed.
Much of the wind variability refer to synoptic and mesoscale variability (85% of the total
variance, according to (Belokopytov et al, 1998). It is caused by the synoptic pressure systems,
breeze circulation (Sorkina et al, 1974; Efimov, Barabanov, 2009, 2010), mountain winds,
mesoscale cyclones and anticyclones (Efimov et al, 2009).
Investigations of interannual and decadal variability of wind and wave characteristics
[Repetin et al, 2003, 2008, 2009; Goryachkin and Repetin, 2009; Voskresenskaya et al, 2006,
2009; Naumova et al, 2010] reveal a general tendency of the wind decrease over the last 50
years. As a sequence, weakening sea circulation and atmospheric surface fluxes follows, in
particular, evaporation [Lipchenko et al, 2006].
Variability of air-sea heat and water fluxes in the Black Sea on various time scales was
discussed in papers [Golubeva, 1987; Simonov, Altman et al, 1991; Schrum et al, 2001; Kara et
al, 2005; Romanou et al, 2010].
Long-term and decadal changes of various atmospheric variables in the Black Sea region
are closely linked with the large-scale atmospheric circulation. In [Polonsky et al, 2007] it is
shown that frequency of cyclones passing over the Black Sea decreased between 1960s and
1990s as a result of NAO intensification and shifting cyclones tracks to the north. Such a
conclusions were drawn in [Lionello, 2006; Voskresenskaya et al, 2003-2009].
Thermohaline structure
The thermohaline structure of the basin was studied in detail in the most well-known
generalizing works on the Black Sea oceanography such as (Knipovich, 1932; Leonov, 1960;
Philippov, 1968; Blatov et al, 1984; Simonov, Altman et al, 1991). A brief but exhaustive
description of the vertical structure of water was provided in the monograph (Blatov et al, 1984),
which outlined the following:
(i) crucial contribution of salinity in density stratification of water, except for the upper 50
m layer in the warm season of the year;
(ii) extremely large vertical contrasts in salinity, temperature and density;
(iii) concentration of these contrasts in relatively thin layers (seasonal and permanent
thermocline /halocline);
(iv) clear asymmetry of the vertical hydrographic structure, consisting of a thin upper
brackish layer and a thick saline lower quasi-homogeneous layers, which are separated with
halocline;
(v) existence of a cold intermediate layer in the upper part of the permanent halocline,
below which temperature tends to increase with depth.
In recent decades, despite the overall decline in field research activities in the oceans, new
methods of in-situ and remote measurements of the ocean have appeared. This allows to refine
and complement the previously established concept of the thermohaline structure of the Black
Sea, which has been done, particularly, in the works of (Özsoy, Ünlüata, 1997; Belokopytov,
2004; Tuzhilkin, 2008, 2008a; BSC, 2008; Ivanov, Belokopytov, 2011).
The average temperature over the entire volume of the sea is 8.96 °C. This is well above
the average temperature in the oceans, but below the mean temperature of the adjacent
Mediterranean Sea.
For the spatial distribution of surface temperature in the Black Sea, the most prominent
characteristic is its growth in the direction from northwest to southeast in all seasons (Fig.4).
This is due to the general atmospheric conditions in the region: the north-west of the Black Sea is
characterized by temperate climate, while the climate is more subtropical in the eastern part of
the sea. In winter, low water temperatures are not limited to the north-west, but also occur in the
central part of the sea, because of the intense cooling of the surface layer in the centers of
cyclonic gyres.
Figure 4 Spatial distribution of average monthly temperature (°C) at the sea surface.
The spatial distribution of temperature in the main pycnocline is completely determined
by a system of vertical movements, ascending in the center and descending to the periphery.
Throughout the year the temperature is characterized by high values in the central part of the sea,
and lower value on the continental slope. The spatial variance of the temperature at the depth 75
m, compared with values on the sea surface drops in 102 times, at the depth 300 m – in 104
times.
Interannual and decadal temperature oscillations in the upper layer of 0-30 m are clearly
distinguishable and only seasonal variations exceed it. Below 30m low-frequency fluctuations
prevail over seasonal variability. Long-term variations reach maximum in summer, the northern
part of the sea is the region being most undergone to this impact (σ = 1.4 – 2.0°С).
Surface temperature time-series on the basis of ship data (Fig.5) and coastal data (Fig.6)
show that during last 50 years a negative trend changes to a positive one in the end of 1970s –
beginning of 1980s for summer season and in the middle of 1990s for winter season. Annual
values turning point fits to the middle of 1980s. In comparison with other regions of World
Ocean, in particular midlatitude Atlantic and whole Northern Hemisphere (Smith and Reynolds,
2004; Rayner et al, 2006), trend turning point in the Black Sea took place decade later such as in
Mediterranean (Rixen et al, 2005).
There are several explanations for the negative temperature trend from 1950s to 1980s.
The most common view is that sea temperature is strongly correlated with advection of cold air
especially in winter (Belokopytov, 1998; Ivanov LI et al, 2000). The positive temperature trend
of last 20–30 years besides archival data is also confirmed by satellite observations [Ginzburg et
al, 2002; Kara et al, 2008). In general long-term temperature variations in the Black Sea
corresponds basic tendency in Northern Hemisphere. Differences in the phases of oscillations
for summer and winter seasons lead to corresponding differences in trend turning point comings.
26
August
Temperature, oС
25
24
23
9
22
7
Temperature, oС
8
21
6
February
5
1950
1960
1970
1980
1990
2000
Figure 5 Monthly sea surface temperature time-series calculated on ship data in the
western part of the Black Sea (43 – 45° N, 30 – 33° E) for February and August.
28
August
Temperature, oС
26
24
10
22
6
February
Temperature, oС
8
20
4
2
1900
1920
1940
1960
1980
2000
2020
Figure 6 Monthly sea surface temperature time-series at coastal station
Sevastopol in February and August.
The Cold Intermediate Layer (CIL), or a layer of minimum temperature between seasonal
and permanent pycnocline, is a sub-surface water mass which is the result of the winter
convective mixing in the centers of cyclonic gyres and in shelf areas. Conditions of CIL
formation in the Black Sea are similar to those in the seas, where winter convection is limited by
the shallow depth of halocline, such as in the Baltic Sea, the Sea of Okhotsk, the Gulf of St.
Lawrence and others. In those areas of the Black Sea, where winter convection does not occur
(southeastern part of the sea), the CIL has advective origin.
Being one of the most distinctive features of the thermohaline structure of the Black Sea,
the CIL has become a subject of many research papers. The greatest disputes among the authors
are devoted to difference in opinions about defining of the main areas of the winter water
renewal and about the role of various physical processes. Until the early 1950s, a "convection"
hypothesis (Spindler, Wrangell, 1899; Zubov, 1938) dominated: according to it, renewal of CIL
waters occurs almost every year throughout the basin due to the convective mixing during winter
season. Later, a hypothesis about advective origin of the CIL was proposed by (Kolesnikov,
1953), which was believed to form in the northwestern part of the sea and spread about the basin
by large-scale currents. Then, the central part of the sea was also attributed to areas with
predominance of convective and wind mixing (Georgiev, 1967, 1972). In the works of
(Ovchinnikov and Popov, 1984, 1987), the role of the central part of the sea was strengthened by
a hypothesis that the CIL is mainly formed in the main centers of cyclonic gyres, similarly to
deep convection in the Greenland Sea and the Gulf of Lyons, Mediterranean. Recently, most
researchers believe that the CIL is formed both in the centers of cyclonic gyres, and in the
northwestern part of the sea (mainly on the continental slope).
Quantitative estimation of the CIL volume, formed in various geographical areas by results
of volumetric TS analysis of climatic data, was obtained in (Belokopytov, 2004): in the western
and eastern cyclonic gyres and in the northwestern part of the sea, respectively, 60, 15 and 25%
of the CIL water is formed. In (Stanev et al, 2003), based on the results of mathematical
modeling, contribution of the continental slope of the northwestern part of the sea, of cyclonic
gyres of the central part of the sea, of the northwestern shelf and of the extreme eastern part of
the sea, were determined as 42, 28, 20 and 10%, respectively. Full renewal of the CIL waters
was estimated as 5.5 years. In (Polonsky and Popov, 2011) it is shown that from 16 to 26% of all
waters of the Black Sea CIL is formed on the shelf.
Many studies have been devoted to the interannual variability of the CIL, which is most
often associated with weather conditions – the cooling of the sea surface in winter (Filippov,
1968; Georgiev, 1972; Blatov, Kosarev, Tuzhilkin, 1980; Belokopytov, 1998; Ivanov LI et al,
2000; Krivosheya et al, 2002; Titov, 2003; Belokopytov, Shokurova, 2005; Belokopytov, 2010;
Polonsky and Popov, 2011).
Figure 7 Yearly temperature time-series in the Cold Intermediate Layer
core averaged for the May-November period. Bars represent ± 1 σ
range.
Interannual and decadal temperature variability of subsurface waters is stipulated by
processes on sea surface in winter. Interannual variations of CIL volume are about 20% of total
volume variance, year-to-year time-series of CIL core temperature (at the depth of temperature
minimum) is shown on Fig.7. CIL volume and its temperature are integral indices of ventilation
of upper layer in winter. There is an alternation of ventilation intensity in the Black Sea during
the last 50 years: in 1950s – high CIL renewal, in 1960–1970s weak renewal, in 1985–1995 high
renewal and after 1995 again weak winter mixing. Spatial distribution of sea surface winter
temperatures based on satellite observations of last 20 years shows that till 2002 CIL renewal
was most intensive within the eastern cyclonic gyre, and since 2003 in the western part of the
sea.
Salinity field in the Black Sea is formed by the balance between the fresh water budget and
the water exchange through the Bosporus. Excess of freshwater input with river runoff and
precipitation over evaporation leads to a relatively low salt content compared to most seas. The
salinity of the surface layer of the Black Sea (17.85 PSU) is two times less than the salinity of
the World Ocean surface waters. As noted by (Tuzhilkin, 2008a) the Black Sea is the world's
largest brackish basin.
Salinity field at the sea surface (Fig. 8) is largely determined by river runoff and
precipitation. Low salinity is typical for the northwestern shelf (rivers: Danube, Dnieper,
Dniester), southeastern part of the sea (rivers: Rioni, Chorokh, Inguri, Kodori, a regional
maximum precipitation), and some parts of the Anatolian coast: the central part (rivers: Kizil
Irmak, Eshil Irmak) and western one (rivers: Sakarya, Filyos (Yenice)).
Figure 8 Spatial distribution of mean monthly salinity (PSU) at the
sea surface.
Spatial distribution of salinity in the permanent halocline layer is closely related to the
intensity of the general circulation. The higher values of salinity in the central part of the sea and
lower values in the coastal zone are due to the general pattern of vertical upward movement in
the center and descending motion in the periphery. At the depth of 75 m salinity difference
between the center and the periphery of the sea is 1–1.5 PSU.
Long-term variability of salinity is the dominant part of temporal variance spectrum and in
contrast to temperature it prevails over seasonal variations. It is maximal for surface layer during
dilution period in May-July and during winter-spring intensification of general circulation for
permanent halocline. Areas with peaks of increased variability comprise the north-western shelf
close to Danube and Dnieper mouths (σ = 1.5-2 PSU) and coastal zone of the southeastern part
of the sea (σ = 0.5-1 PSU). In the central part of the sea interannual variability of salinity is
minimal (σ = 0.2 PSU).
The surface salinity time-series in the Black Sea, Fig. 9, shows rise from 1920s to 1950s
and gradual decrease over last 50 years. The tendency towards dilution of the upper layer is
repeatedly mentioned in literature (Belevich, Orlova, 1996; Polonsky, Lovenkova, 2004;
Shokurova et al, 2004).
Against the background of low-frequency changes decadal oscillations of salinity with
approximately 20 years period are significant. The modern negative trend ~0.04 PSU per decade
corresponds to negative trends of salinity in the North Atlantic and North Pacific (Dickson et al,
2002; Boyer et al, 2005), despite of evident regional differences of salt budget, and it is contrary
to positive trend in Mediterranean (Rixen et al, 2005). Comparison of interannual variations of
salinity with fresh budget, Fig.10 (Simonov, Altman et al, 1991), i.e. river plus precipitation
minus evaporation, reveals that general decline of salinity correspond to increase of fresh water
input.
19
S, PSU
18.5
18
17.5
1950
1960
1970
1980
1990
2000
2010
Figure 9 Monthly sea surface salinity time-series in the western part of the Black Sea(43
– 45° N, 30 – 33° E). On the lower plate the period of 1955–2010 is shown in expanded
scale with coastal Crimean stations data added (dashed line)
500
Volume, km3
400
300
200
100
0
1920
1930
1940
1950
1960
1970
1980
1990
2000
Figure 10 Annual fresh water budget of the Black Sea (Simonov, Altman et al, 1991)
In the main pycnocline (100–300 m) salinity decreased from 1920s to 1960s, then rose till
mid-1980s and decreased again (Fig.11), see also (Tsimplis et al, 2004; Polonsky, Lovenkova,
2004, 2006; Belokopytov, Shokurova, 2005). Reasons of the positive trend in 1960-1980s were
discussed in literature: sharpening stratification and reducing salt exchange due to surface
diluting (Simonov, Altman et al, 1991), salinity increase of Mediterranean waters penetrating into
the Black Sea via straits (Tsimplis et al, 2004). Most common view is that variations of
temperature and salinity in pycnocline are caused by vertical motions linked with intensity of
general circulation, which induce lifting/lowering deep saline layers.
Figure 11 Monthly salinity time-series at the depth 200 m in the
western part of the Black Sea(43 – 45° N, 30 – 33° E).
Sea level
H,сm
The sea level is the indicator of the global climatic changes effecting level of the World
Ocean (melting of continental glaciers and thermal expansion of water), variability of a regional
climate, its humid and a thermal regime.
Seasonal and interannual sea-level fluctuations in the Black Sea derived from coastal
location data and satellite altimetry data were analyzed in variety of works, in particular
(Simonov, Altman et al, 1991; Belokopytov and Goryachkin, 2000; Stanev and Peneva, 2002;
Tsimplis et al, 2003, 2004; Goryachkin and Ivanov, 2006; Kara et al, 2008).
The annual cycle of sea level in the Black Sea is well prominent, the average range of
seasonal fluctuations is about 10 cm, Fig. 12. The maximum level is observed in June, minimum
– in October and November. The main components of a sea level: fresh balance, steric and
barometric input have relative phase shifts in seasonal cycle and can compensate mutually each
other. So, for example, steric contribution is maximal in August while the fresh balance is
minimal. As a whole, the contribution of fresh balance twice is more than steric and barometric
components.
Figure 12 Average seasonal cycle of sea level in the Black Sea and its
components as monthly anomalies (from Belokopytov and Goryachkin, 2000).
500
Evpatoriya
H, cm
490
480
470
Sevastopol
460
1950
1960
1970
1980
1990
2000
2010
Figure 13 Yearly sea-level values at coastal locations Sevastopol, Yalta
and Evpatoriya in the Black Sea.
In the Black Sea, like in the World Ocean, general rise of sea level is observed, Fig. 13.
Mean rate of rising in 1960-1990 is about 1.3 mm yr-1 that was a little below World Ocean trend,
from the beginning of 1990s it is about 6 mm yr-1 that corresponds to trend estimations in other
ocean basins (Holgate and Woodworth, 2004). Relatively steady state of the sea level in 19601990s could be caused by steric effect connected with a negative trend of a water temperature
which existed in that period.
Sea circulation
Conception of the general circulation pattern in the Black Sea as a cyclonic motion with
two large gyres has already been developed in the late 19th, early 20th centuries, as shown in
works of F.F. Wrangell, I.B. Spindler, N. Andrusov, S. Zernov. The cyclonic wind rotation over
the sea and the river runoff were proposed as the main reasons for the circulation. Based on field
research of the 1920s–1930s, N.M. Knipovich offered a scheme of circulation which was
generally accepted to this day; he explained the dome-shaped distribution of hydrographic
properties by adaptation of the density field to cyclonic rotation (Knipovich, 1932, 1938). Since
then, the scheme of general circulation has not been revised, but only refined.
Very similar patterns of circulation can be found in (Neumann, 1942; Leonov, 1960;
Filippov, 1968; Bogatko et al, 1979; Blatov et al, 1984). The most recent general circulation
pattern based on oceanographic surveys of the 1980s–1990s and altimetry data was given in
(Oguz et al, 1993; Korotaev, Oguz et al, 2003), Fig.14.
All known schemes suggest the following main features of general circulation: the Main
Black Sea Current (or Rim Current), located over the continental slope, and two large-scale
cyclonic gyres in the eastern and western parts of the sea; quasi-stationary anticyclonic eddies in
the coastal zone, such as Batumi, Sevastopol, Caucasian, Sakarya, Sinop, etc.
In contrast to the suffisiently consistent views on general pattern of circulation in the Black
Sea, there is a wide variety of opinions about its seasonal variability. Only the annual maximum
of circulation intensity is usually admitted, which occurs in late winter and early spring period,
when the mean current speed increases in 1.5 times compared to the seasonal minimum. A
secondary maximum of circulation intensity at the end of the summer season is often picked out
as described in (Cheredilov, 1967; Blatov et al, 1984, 1989), the estimates of its magnitude can
vary from small values up to half the range of seasonal variation. In (Blatov et al, 1984), the peak
of summer maximum was estimated as almost equal to the winter-spring maximum. In many
descriptions of the annual variability of circulation, in contrast, the summer maximum is
completely absent, the circulation of the sea is considered to be weak during the summer season
(Filippov, 1968; Bogatko et al, 1979; Krivosheya et al, 1979–1981, Simonov, Altman et al, 1991;
Titov, 1993, 2003).
Figure 14 Circulation pattern of the surface layer of the Black Sea (Oguz et al, 1993;
Korotaev, Oguz et al, 2003).
Estimates of seasonal variations of current speed based on data, obtained at moorings in
the coastal zone, include the minimum in the beginning of summer (May–June) and the
maximum in December and January (Ovchinnikov et al, 1986), or February–March (Krivosheya
et al, 1980). In general, this corresponds to the seasonal run averaged over the sea. However,
there is an opinion that the existing currents measurement data, obtained at moorings can not
reliably identify annual variability (Tuzhilkin, 2008b).
According to the seasonal variation of surface currents on the basis of drifters data for the
period of 1999 – 2003 (Poulain et al, 2005), the maximum occurs in March, while the minium in
June (and December). The secondary velocity maximum is identified in August, but the authors
express doubts about his statistical significance, subject to the data paucity for this month.
According to estimates based on geostrophic calculations, hydrodynamic modelling
(Demyshev et al, 2005, 2007; Knysh et al, 2011), as well as altimeter data (Korotaev et al, 2003)
the mimimum of the Black Sea circulation intensity falls on autumn.
Qualitative differences in the estimates of annual variability of circulation intensity are
explained in (Polonsky, Shokurova, 2010) by change in seasonal cycle in the second half of the
20-th century, when the summer maimum dissapears. Significant year-to-year variability of the
seasonal cycle of circulation on basis of altimeter data over a 7-year period is also found in
(Korotaev et al, 2003). One of the reasons for the discrepancy of estimates of mean seasonal run,
taking in consideration that it mainly refers to the summer-autumn season, is intensification of
mesoscale variability in this period, while general circulation weakens. Spatial and temporal
irregularity by increasing the mesoscale "noise" may lead to unreliable averaged values.
Seasonal variability of circulation in the Black Sea is characterized not only by change in
the velocity of general flow of the basin, but also with fluctuations in the intensity separately for
the main cyclonic gyres and quasi-stationary anticyclonic eddies.
One of the views of the spatial structure of the seasonal cycle of currents is the
maintenance of the general circulation pattern throughout the year with little change in the
position and size of its individual components (Bogatko et al, 1979; Blatov et al, 1984, 1989,
Simonov, Altman et al, 1991; Eremeev, Kochergin, 1991).
Figure 15 Mean monthly dynamic topography of the Black Sea 0/300 db,
calculated by climatic density fields. Interval of isolines is 2 dyn.cm.
Another view is that during the seasonal cycle, a qualitative change in the circulation
pattern of the sea takes place (Oguz, Malanotte-Rizzoli, 1996; Trukhchev, Ibrayev, 1997; Stanev
and Beckers, 1999; Stanev and Staneva, 2000; Staneva et al, 2001; Belokopytov, 2003, 2004;
Korotaev et al, 2001, 2003; Tuzhilkin 2008b; Knysh et al, 2011; Demyshev et al, 2005, 2007;
Polonsky, Shokurova, 2010). Circulation may look like a single cyclonic movement centered in
the western or eastern part of the sea, or it may consists of well-defined cyclonic gyres.
Seasonal cycle of the geostrophic circulation calculated from climatic density fields for the
period 1950 – 2000 years can be represented by the following scheme (Belokopytov, 2004):
January - March: one cyclonic gyre with the center in the eastern part of the sea, western
gyre is developed weakly;
April - May: one cyclonic gyre with the center in the western part of the sea, the eastern
gyre is developed weakly;
June - July: two gyres, the western one is more intensive;
August - September: two gyres, the eastern one is more intensive;
October - December: two gyres of equal intensity.
Fig. 15 shows the monthly mean values of dynamic topography field, which served as the
basis of this scheme of circulation. Similar estimations of seasonal variations of circulation was
also obtained by other authors (Polonsky, Shokurova, 2010; Knysh, et al, 2011; Demyshev et al,
2007). As a consequence of alternate strengthening and weakening of cyclonic gyres in the east
and the west, seasonal cycles of circulation in these parts of the sea differ each other.
Long-term variability of the sea circulation is much less studied. Analysis of thermohaline
changes (Polonsky and Lovenkova, 2006) have concluded winter intensification of the western
cyclonic gyre in the end of period 1960-1970s and weakening of the eastern cyclonic gyre from
the middle 1960s till the end of 1980s. According to (Polonsky and Shokurova, 2009) by results
of geostrophic calculations, there is intensification of upper layer circulation in winter, kinetic
energy has increased from 1950 to 1995 almost twice, and the maximum increase refers to the
last decade. Significant increase of circulation in upper layer takes place in the western part of
the sea, while in the south-western part it weakens. In the lower part of pycnocline (200-300 m),
in contrary, kinetic energy diminishes down to 30% all over the sea. A similar conclusion is
made in (Knysh et al, 2011) by results of modeling Black Sea reanalysis (1971-1993).
Key variables to climate monitoring
The main climate changes in the Black Sea oceanography during last decades are as
follows: in the upper layer a stable freshening persists, negative temperature trends have been
changed by positive ones in late 1980s. In the layer of permanent pycnocline temperature and
salinity are increasing. Wind velocity and evaporation diminish, fresh budget and sea level rise,
sea circulation intensifies.
To estimate reliably long-term climatic variations and study physical causes of climate
changes it’s necessary to carry out regular observations. The following variables can be
representative to describe the Black Sea as whole:
 Sea level, precipitation, river runoff as indicators and components of fresh water
budget;
 Temperature and salinity of upper layer as a measure of the sea response on
external forcing;
 Temperature of Cold Intermediate Layer as an indicator of winter ventilation;
 Temperature and salinity of permanent pycnocline as an index of the sea density
stratification;
 pH as an index of the sea acidification by Carbone Dioxide;
 Dissolved oxygen as an important biological variable and an indicator of sea
ventilation.
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