\documentclass[twoside]{article}
\usepackage[silent,pdf,hyper]{elxshort}
\usepackage{color}
\usepackage{hyperref}
\journalid{ES}{19}{3}{19}
\citnumber{}{1}
\paperid{2019ES000657}
\papertype{RAR}
\cpright{}{2019}
\lefthead{stont and bukanova}
\righthead{general features of air temperature} % over coastal waters of the south-eastern Baltic Sea for 2004-2017
\received{23}{October}{2018}
\accepted{30}{November}{2018}
\published{1}{May}{2019}
\definecolor{brown}{RGB}{157,23,4}

\authaddr{T. V. Bukanova,  Shirshov Institute of Oceanology RAS, 36, Nahimovskiy Pr., % ospekt,
117997 Moscow,  Russia.
%\qquad  \ \qquad  \ \qquad  \ \qquad  \qquad
(tatiana.bukanova@gmail.com)}


\begin{document}

\title{General features of air temperature over coastal waters of the south-eastern Baltic Sea for 2004--2017}

\author{1}{Stont}{Z. I.}{}{1}{}{}
\author{2}{Bukanova}{T. V.}{}{1}{}{}

\affil{1}{Shirshov Institute of Oceanology RAS}{Moscow}{}{Russia}

\keywd{Variability}
\keywd{air temperature}
\keywd{fluctuations}
\keywd{Baltic Sea}
\keywd{circulation}
\keywd{correlation}


\abstract{The paper is devoted to the analysis of air temperature variability over the
coastal waters of the south-eastern part of the Baltic Sea. The study uses the data
from the automatic hydrometeorological station located 20 km from the coast on the
offshore ice-resistant fixed platform. The annual mean air temperature is $8.8 \pm 0.6$\deg C,
which is 1.4\deg C higher than in the middle of the last century. The annual minimum of
air temperature ($- 0.3$\deg C) has shifted towards February, therefore the climate of the
south-eastern Baltic Sea has become more marine. The rise of monthly mean air
temperature in 2004--2017 has revealed at a rate of $+ 0.005$\deg C/month, i.e.
$+ 0.8$\deg C/period (according to the hourly observations of air temperature the
increase accounts for 0.95\deg C for the same period). The greatest contribution to the
variability of annual mean values of air temperature is provided by variations in the
cold season (February and March). A close relationship between the North Atlantic
Oscillation index and the air temperature is shown (correlation coefficient $r = 0.7$),
which explains the dependence of detected air temperature variability from the
circulation conditions. The lower values of the North Atlantic Oscillation index
stipulate the intensification of anticyclonic activity and noticeable decrease of air
temperature in the study area. The predominance of a positive phase of the North
Atlantic Oscillation index causes the strengthening of the Icelandic Low and a
west-east transfer what consequently results in the increase of air temperature. Western
form of circulation by Wangenheim-Girs is prevailing in the study area. However, the
enhancement of the eastern form of circulation by Wangenheim-Girs slows down the
increase of air temperature.}


\section{Introduction}
\enlargethispage{-.65pc}

\begin{sloppypar}
     Recently, the variations of hydrometeorological conditions have become
evident in the Baltic region, as well as in the other regions of Europe. In most studies
these variations are associated with anthropogenic impact
[\itc{IPCC,}  \reflink{IPCC08}{2008};
\itc{Stocker et al.,}  \reflink{Stoc13}{2013}].
The instrumental data indicate the increase of subsurface air temperature in temperate
latitudes and cyclic variations in temperature over several decades
[\itc{Alekseev et al.,} \reflink{Alek10}{2010};
\itc{Kotlyakov,} \reflink{Kotl10}{2010};
\itc{Panin,} \reflink{Pani10}{2010}].
Following
\itc{Alekseev et al.} [\reflink{Alek10}{2010}]
and
\itc{Frolov et al.} [\reflink{Frol10}{2010}],
the observed warming is caused by natural factors which vary over a time
scale of tens to hundreds of years, and is not affected by anthropogenic influence.
Despite the regional differences, the common patterns of long-term variability of
mean air temperatures are noticeable in the coastal areas of the Baltic Sea
[\itc{Chubarenko and Chubarenko,} \reflink{Chub02}{2002}].          %%% ???
The paper
[\itc{Avotniece et al.,} \reflink{Avot10}{2010}]
 showed that
the duration of vegetation and frost-free period increased because of the high summer
temperature and the decrease of extreme values of temperature in winter. The
increased frequency of the exceptionally hot summer months and even entire summer
seasons was revealed in Europe in the last decade of the XX century and in the first
decade of the XXI century
[\itc{Twardosz and Kossowska-Cezak,} \reflink{Twar13}{2013}].
 The variability
of the minimum and maximum temperature and the daytime temperature range in
Poland were analyzed using the data from 9 stations with different data periods (the
longest dataset is 98 years)
[\itc{Wibig and G{\l}owicki,} \reflink{Wibi02}{2002}].
The increase of the
minimum temperature was accompanied by a slight rise of the maximum temperature
and a decrease of the daytime temperature range. It has been established that these
changes correlate well with cloudiness while strong temperature fluctuations are
associated with the intensity of the North Atlantic Oscillation (NAO) especially in
winter and spring. In the Baltic countries average air temperature anomalies in 1991--2007
accounted for 0.8--0.9\deg C relative to the climatic period of 1961--1990 and the
increase of precipitation was also observed (1--6\%)
especially in winter
[\itc{Kriauciuniene et al.,} \reflink{Kria12}{2012}].
 The linear trend of air temperature for the entire Baltic
coast of Poland was equal to 0.3\deg C/decade for the period 1966--2009
[\itc{Tylkowski,} \reflink{Tylk13}{2013}].
But the regional differences were notable. Thus, the maximum increase of air
temperature (0.32\deg C/decade) occurred on the Szczecin Coast, located in the western
part of the Baltic Sea Polish coast, while in the eastern part, on the Gdansk Coast, the
trend was weaker (0.28\deg C/decade)
[\itc{Michalska,} \reflink{Mich11}{2011}].
The seasonal linear trends of
air temperature in the area of Szczecin were obtained for 1950--2009: 0.0296\deg C/year
in spring and 0.032\deg C/year in winter (1.8 and 1.9\deg C/60 years respectively)
[\itc{Kirschenstein,} \reflink{Kirs11}{2011}].
\end{sloppypar}


     The total solar radiation reaching the south-eastern Baltic Sea equals to 360.0--376.8~kJ/cm$^2$
     per year
     [\itc{Pomeranets,} \reflink{Pome64}{1964};
\itc{Samoylenko,} \reflink{Samo57}{1957};  % Climate and hydrological atlas of the Baltic Sea, 1957;          %%% ???
\itc{Terziev et al.,} \reflink{Terz92}{1992}].            % Hydrometeorology and hydrochemistry of the seas, 1992;   %%% ???].
The
distribution of cloudiness and atmospheric transparency significantly affects the solar
radiation supply. The south-eastern Baltic Sea receives more than 45\%
of the annual
sum of absorbed solar radiation in summer, a little less in spring (approximately 35\%),
 and 20\%
 in cloudy autumn-winter season. The radiation balance is negative
during 4--5 months (from October--November to February--March) in the study area.
The minimum of solar radiation is observed in December and the maximum values
are noted in June
[\itc{Dubravin and Stont,} \reflink{Dubr12}{2012};
\itc{Terziev et al.,} \reflink{Terz92}{1992}].  %   Hydrometeorology and hydrochemistry of the seas, 1992;      %%% ???

     There is an opinion
[\itc{Pinker et al.,} \reflink{Pink05}{2005};
\itc{Wild et al.,} \reflink{Wild05}{2005}],
that the amount of
solar radiation reaching the Earth's surface has recently increased in comparison with
1990. This is one of the reasons for the increase of air and sea surface temperatures.
The total solar radiation shows the positive trend since the mid-1980s for the Moscow
region
[\itc{Chubarova et al.,} \reflink{Chub14}{2014}].

     Following
     \itc{Smirnova et al.} [\reflink{Smir88}{1988}],
     the increase of air temperature in different
coastal areas of the White and the Baltic Sea has been observed since the beginning
or middle of 1980s, and in spite of the global warming, the solar activity secular
minimum was expected in 2016. According to the World Data Center for the
production, preservation and dissemination of the international sunspot number of
Royal Observatory of Belgium
% [World Data Center],
(World Data Center for the production, preservation and dissemination of the international sunspot number, Royal Observatory of Belgium, URL:  \href{http://sidc.be/silso/home}{http://sidc.be/silso/home}).
the deepest part of the
minimum should be in 2018--2019 and the increase of solar activity is predicted from
the middle of 2019.

     The goal of the study is to analyze the variability of air temperature in the
Russian sector of the south-eastern Baltic Sea.

\begin{figure*}[t]                     % Fig 1
\figurewidth{35pc}
\setimage{}{}{}{34pc}{2019es000657-f01}
\caption{Study area. The asterisk shows the location of offshore ice-resistant platform (D6) and
automatic hydrometeorological station (AHMS), red lines -- the boarders of the Russian
exclusive economic zone.}
\end{figure*}

\section{Materials and Methods}

     The research was carried out on the basis of a 14-year series of observations in
the open part of the sea, where the influence of land is excluded
 (\figref{1}).           % (Figure 1).
Hydrometeorological observations in this part of the south-eastern Baltic Sea are
almost absent in recent years. The study area is located outside the active shipping
routes, i.e. there are no simultaneous measurements on board transport, passenger,
and fishing vessels.\enlargethispage{-.95pc}

\begin{sloppypar}
     We used the data of the automatic hydrometeorological station mounted on the
offshore ice-resistant fixed platform OIFP (D6), located approximately 20 km from
the coast
(\figref{1}).           % (Figure 1).
The automatic hydrometeorological station (AHMS)
``MiniKrams-4'' is installed on the offshore platform (D6) at a height of 27~m above
the sea level and provides continuous measurements. The AHMS software is able to
encode meteorological parameters using the international code FM 12-IX SYNOP
and transmit the data via the Internet channel, which allows the operational
processing of observational data. We analyzed the hourly measurements of air
temperature, which represent a continuous dataset for 14 years. The air temperature
was not reduced to a standard 2~m level, since it was difficult to take into account the
factor of atmospheric stratification.
\end{sloppypar}

     The annual mean air temperature was calculated by averaging the hourly
measurements with a uniform pitch (1~hour). Afterwards spline interpolation was
used to smooth the dataset.

\begin{table*}[t]                                   % Table 1
\tablewidth{41pc}
\ttt\caption{Interannual Variations of Air Temperature From OIFP D6 for 2004--2017}
\vspace{5pt}
\fitopen{0.9}
\begin{tabular}
{@{}l@{\hspace{65pt}}
c@{\hspace{70pt}}
c@{\hspace{55pt}}
c@{\hspace{55pt}}
c@{}}
\hline
\\ [-7pt]
Year & Mean, \deg C & \multicolumn{3}{c}{Extreme values, \deg C }\\
     &        &   minimum&  maximum&  amplitude\\
        [7pt]  \hline   \\ [-7pt]
2004 &   8.4  &   $-10.6$  &   25.9  &  36.5   \\
2005 &   8.9  &   $-9.7 $  &  24.4   & 34.1 \\
2006 &  9.0   &   $-19.3 $ &   29.8  &  49.1 \\
2007 &  9.3   &   $-12.3$  &   26.4  &  38.7 \\
2008 & {\itshape 9.4}   &  $-10.3 $  &   22.9  &  33.2 \\
2009 &  8.7   &  $-10.9 $  &   26.7  &  37.6 \\
2010 &  {\itshape7.2}   &  $-15.3 $  &   31.1  &  46.4 \\
2011 &  8.9   &  $-13.1 $  &   28.0  &  41.1 \\
2012 &  8.2   &  $-16.4 $  &   29.4  &  45.8 \\
2013 &  8.7   &  $-10.9 $  &   30.7  &  41.6 \\
2014 &  9.2   &  $-12.3 $  &   26.8  &  39.1 \\
2015 &  {\itshape 9.4}   &  $ -7.2 $  &   28.5  &  35.7 \\
2016 &  9.2   &  $-13.2 $  &   29.6  &  42.8 \\
2017 &  8.8   &  $-10.1 $  &   25.2  &  35.2 \\
Mean $\pm \sigma$ & $8.8\pm 0.6$ & $-12.2 \pm 3.1$& $27.5 \pm 2.5$ & $39.8 \pm 4.9$\\
[4pt] \hline
\end{tabular}
\parbox{41pc}{\footnotesize\baselineskip=10pt
Note: Extreme values are marked in italics.}\vspace*{-.5pc}\fitclose
\end{table*}



     The interannual variations of air temperature were investigated using variance
and harmonic analyzes
[\itc{Bruks and Karuzers,} \reflink{Bruk63}{1963};
\itc{Kozubskaya and Konyaev,} \reflink{Kozu77}{1977};
\itc{Rozhkov,} \reflink{Rozh08}{2008}].          %%% ???      +
The long-term trends were calculated by linear approximation of the
time series using the least squares method. The statistical significance of the linear
dependence coefficients and the reliability of the obtained results were evaluated
using the Student's t-test. The possibility of sampling by seasons and months was
provided for investigation of the linear model parameters. The spline interpolation of
data was additionally performed to obtain series with even time step and identify
long-term variations. Seasonal statistics and the general trend (the whole dataset) are
non-additive.

\begin{sloppypar}
     The daily NAO index was used to distinguish the NAO phases. The daily NAO
indices are provided by the National Weather Service of National Oceanic and
Atmospheric Administration, USA
% [Daily NAO index]
(Daily NAO index since January 1950, National Weather Service of National Oceanic and Atmospheric Administration, USA, URL:
\href{http://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.nao.index.b500101.current.ascii}{http://ftp.cpc.ncep.noaa. gov/cwlinks/norm.daily.nao.index.b500101.current. ascii})         %%% ???
and calculated using the
rotated principal component analysis
[\itc{Barnston and Livezey,} \reflink{Barn87}{1987}].
\end{sloppypar}

\begin{figure*}[t]                     % Fig 2
\figurewidth{35pc}
\setimage{}{}{27pc}{}{2019es000657-f02}
\crosscaption{Interannual variations of air temperature from OIFP D6 for 2004--2017. The linear trend is
shown with a red line.}\vspace*{-1pc}
\end{figure*}

\enlargethispage{-.85pc}
\section{Results}\vspace*{-1.2pc}

     Following
     % Hydrometeorological conditions, 1983 \itc{Hydrometeoizdat} [\reflink{Hydr83}{1983}]
 [\itc{Sustavov,} \reflink{Sust83}{1983}]
     the annual mean air
temperature in the study area was 7.4\deg C at the end of the XX century, and the lowest
monthly temperature ($-2.0$\deg C) was observed in January. During summer the average
temperature was 16.8\deg C, and the highest values occurred in August (16.9\deg C). The
annual amplitude of monthly mean temperature was 18.9\deg C
[\itc{Sustavov,} \reflink{Sust83}{1983}]. %  [\itc{Hydrometeoizdat,} \reflink{Hydr83}{1983}]. %  Hydrometeorological conditions, 1983].

     The main features of air temperature interannual variability obtained from
OIFP D6 for the period 2004--2017 are presented in
\tabref{1}           % Table 1
and
\figref{2}.           % Figure 2.
The annual
mean air temperature is higher than in the middle of the last century
[\itc{Sustavov,} \reflink{Sust83}{1983}] % [\itc{Hydrometeoizdat,} \reflink{Hydr83}{1983}] % Hydrometeorological conditions, 1983]
and accounts for $8.8 \pm 0.6$\deg C, the annual
amplitude of monthly mean values equals to 19.7\deg C
(\tabref{2}).           % (Table. 2).
 The maximum
annual mean temperature is observed in 2008 ($9.4 \pm 5.8$\deg C) and in 2015
($9.4 \pm 5.6$\deg C). The variations of monthly mean values range from 2.4\deg C (January) to 18.0\deg C
(July) and from 1.9 (January) to 19.3\deg C (July) respectively. The minimum annual
mean temperature occurs in 2010 ($7.2 \pm 8.6$\deg C), and monthly mean temperature
values vary from $-5.4$ to $20.8$\deg C.

\def\bottomfraction{.7}
\def\textfraction{.2}

\begin{table}[b]                                   % Table 2
\tablewidth{41pc}
% \noindent\rule{41pc}{1pt}\\
\crosstablecaption{Statistical Characteristics of Monthly Mean Air Temperature Measured on OIFP D6 in
2004--2017 and Values of Linear Trend Increment per Month}
\vspace{5pt}
\fitopen{.9}
\begin{tabular}
{@{}l@{\hspace{15pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{6pt}}
c@{\hspace{5pt}}
c@{}}
\hline
\\ [-7pt]
Statistical     &                        \multicolumn{12}{c}{Months}\\
characteristics &           I  &  II  &  III & IV  &    V   & VI  &   VII  & VIII &  IX  &  X  &    XI  & XII\\
        [7pt]  \hline   \\ [-7pt]
Mean, \deg C            &  $-0.2$ & $-0.3$&  2.2 &    6.0 &   10.9 &14.5 & 18.2 &  18.6& 15.6 &  10.5&  6.4 &  3.5\\
Standart & & & & & & & & & & & &\\
deviation, $\pm \sigma$ & 2.76 & 2.44 & 2.02 &   1.16 &  1.14& 1.55 &1.54 &  0.65  & 0.95 &  1.42 & 1.03 & 2.76\\
Absolute               &&& & & & & & & & & &\\
maximum, \deg C        &  10.3 & 7.4 & 15.8 &   21.3 &  26.1 &29.8 &31.1 &  29.1  & 24.1  & 18.2  &  12.8 & 12.3\\
Absolute               && & & & & & & & & & & \\
minimum, \deg C        & $-19.3$ & $-16.4$& $-9.7$ &  $-1.1$  &  2.8 & 7.7 & 12.1 &  11.7  &  7.0 &  1.1 &  $- 9.2$ & $- 11.5$ \\
Trend                  & & & & & & & & & & & & \\
increment, \deg C      &  $-0.6$ &  2.3 &  2.2 &  0.1  &   1.8 & 0.5 & $-0.8$ &  0.04  & $-0.6$ &  $-2.4$  &0.7 &  $-0.1$ \\
Student's              && & & & & & & & & & &  \\
coefficient            & 5.844 &17.602 &28.815& 0.970* &13.648 &4.702 & 8.290 & 0.520* &7.534 &23.425& 6.984& 0.739* \\
Number of              & & & & & & & & & & & & \\
observations           &10083  & 9219 & 10221 &  9790 &  10189 &9720 & 10045& 10149  & 9765& 10152 &  9794 & 10057 \\
[4pt] \hline
\end{tabular}
\parbox{41pc}{\footnotesize\baselineskip=10pt
Note:    * statistically not significant ($p$-level $> 0.01)$.}
\end{table}

     The interannual distribution of the linear trend increment of annual mean air
temperature in 2004--2017 ranges from $-1.5$ to $1.7$\deg C (the mean value is $0.3 \pm 0.8$\deg C).
The interannual trend increment of the maximum monthly mean values varies
from $-0.3$ to $2.2$\deg C, while the increment of minimum monthly mean values
fluctuates from $-2.1$ to $2.3$\deg C that is approximately 2 times greater than for the
warmest months of the year. This result indicates that temperature variations during
the cold period of the year provide the greatest contribution to the variability of
annual mean values of air temperature. Thus in 2008 (the warmest year) only 10 days
with frosts down to $-10$\deg C were observed during winter. In winter period in 2010
(the coldest year) the total duration of freezing days (up to $-15$\deg C) was 60 days
[\itc{Stont and Demidov,} \reflink{Ston15}{2015}].


     In winter the monthly mean air temperature of two months (January and
February) declines to negative temperatures ($-0.2$\deg C and $-0.3$\deg C respectively). The
minimum of annual variation moves towards February ($-0.3$\deg C) in comparison with
the minimum that appeared in January ($-2.0$\deg C) in the middle of the last century
(\tabref{2}).           % (Table 2).
 This phenomenon is probably caused by the circulation processes.

\enlargethispage{-20\baselineskip}

     Meanwhile, the recurrence of the winds of the east points has increased in
February in the same years, when the negative temperatures were observed. The shift
of the monthly mean temperature minimum from January to February reveals that the
climate of the south-eastern Baltic Sea has become more marine. The absolute
minimum $-19.3$\deg C (in January) was observed in 2006, the maximum amplitude of
extreme values was 49.1\deg C this year
(\tabref{1}).           % (Table 1).
The air temperature is most
changeable in winter -- for December and January the standard deviation ($\pm \sigma$) is the
highest and accounts for 2.76\deg C
(\tabref{2}).           % (Table 2).
 This fact is provoked by the circulation
conditions: the frequent alternation of thaws during invasion of Atlantic cyclones and
cold air inflows under increasing influence of anticyclones.

\begin{figure}[t]                     % Fig 3
\figurewidth{20pc}
\setimage{}{}{20pc}{}{2019es000657-f03}
\caption{Interannual variations of air temperature and NAO indices. The red line shows the air
temperature distribution, the blue line -- the NAO index. The linear trend of air temperature is
shown with a solid line, the linear trend of the NAO index -- dotted line.}
\end{figure}


     The seasonal increase of air temperature occurs evenly from March to August --
from 2.2\deg C (March) to 18.6\deg C (August)
(\figref{2},           % (Figure 2,
\tabref{2}).           % Table 2).
Since 2004 the monthly
mean temperature of the warmest month (August) has increased by 1.7\deg C relative to
the second half of the XX century. The monthly mean temperature in August is the
least changeable ($\pm \sigma = 0.65$\deg C). The temperature of July is 0.4\deg C lower (18.2\deg C),
while in June the monthly mean temperature is 14.5\deg C. The air temperature during
the spring transitional period is estimated as 2.2\deg C in March, 6.0\deg C in April, and
10.9\deg C in May. The autumn period shows the following values of monthly mean
temperature: 15.6\deg C in September, 10.5\deg C in October, and 6.4\deg C in November. The
periods of temperature increase and decrease in the seasonal cycle are approximately
equal and amount to 6 months, i.e. the seasonal variation of air temperature in the
study area is determined by the annual wave.

\begin{sloppypar}
     The variation of air temperature in the study area is characterized by a linear
trend of $+0.02$\deg C/year. Thus, a warming rate accounts for 0.20\deg C/decade and 0.28\deg C/period
(\figref{2}).           % (Figure 2).
This result totally corresponds to the estimations of linear
trends, obtained for the eastern coast of Poland ($+0.28$\deg C/decade for 1966--2009) by
\itc{Michalska} [\reflink{Mich11}{2011}]
and for Kaliningrad ($+0.20$\deg C/decade for 1996--2010) by
\itc{Navrotskaya and Stont} [\reflink{Navr14}{2014}].
\end{sloppypar}

     A positive trend increment of the monthly mean air temperature is observed
from February to June. The maximum positive increments appear in February (2.3\deg C
per period) and March (2.2\deg C per period) with high statistical significance ($p<0.01$)
(\tabref{2}).           % (Table 2).
 The maximum negative increment occurs in October $-2.4$\deg C/period. In
August the increment is close to 0\deg C/period (statistically not significant $p>0.01$).
Following
\itc{Jurgel\.{e}nait\.{e} et al.} [\reflink{Jurg12}{2012}]
the rate of change in the air temperature in
Lithuania was estimated as 0.06\deg C/warm season for 1991--2010, which is in a good
agreement with our estimation.


     \section{Discussion}

     Western (W) circulation form by Wangenheim-Girs has prevailed in the
Atlantic-European region over recent decades
[\itc{Dmitriev et al.,} \reflink{Dmit18}{2018}].
The increase of
solar radiation reaching the Earth's surface has declared
[\itc{Pinker et al.,} \reflink{Pink05}{2005};
\itc{Wild et al.,} \reflink{Wild05}{2005}].
 These two factors induced the rise of monthly mean air temperature in
winter (up to $-0.3$\deg C against $-2$\deg C in the middle of the last century) and the shift of
annual minimum temperature towards February.

     The intensification of anticyclonic activity in the study area occurs during the
periods with lower values of the NAO index (in our study 2005--2012, excluding
2011)
(\figref{3},           % (Figure 3,
 \tabref{3}).           % Table 3).
Thus, in 2010 the negative NAO phase prevailed, as a
result winter frosts reached $-15$\deg C, while in summer temperature increased up to 31\deg C.
Overall 2010 appeared as the coldest year in the period of observation
(\tabref{1}).           % (Table 1).

\begin{table}%%[t]                                   % Table 3
\tablewidth{20pc}
\ttt\caption{Annual mean values of air temperature, western (W), eastern (E), meridional (C)
circulation forms by Wangenheim-Girs, and the NAO index}
\vspace{5pt}
\begin{tabular}
{@{}l@{\hspace{10pt}}
c@{\hspace{10pt}}
c@{\hspace{20pt}}
c@{\hspace{20pt}}
c@{\hspace{18pt}}
c@{}}
\hline
\\ [-7pt]
Year& Air        & W & E & C & NAO  \\
    &temperature &   &   &   & index \\
      [7pt]
     \hline   \\ [-7pt]
2004 & 8.4  &       144 &136 & 86 & 0.155\\
2005 & 8.9  &       134 &147 & 84 &$-0.091$\\
2006 & 9.0  &       123 &153 & 89 &$-0.085$\\
2007 & 9.3  &       138 &132 & 95 &  0.1\\
2008 & 9.4  &       179 &119 & 68 &$-0.172$\\
2009 & 8.7  &       126 &155 & 84 &$-0.119$\\
2010 & 7.2  &       108 &163 & 94 &$-0.582$\\
2011 & 8.9  &       143 &153 & 69 & 0.164\\
2012 & 8.2  &       143 &141 & 82 &$-0.202$\\
2013 & 8.7  &       122 &145 & 98 & 0.12\\
2014 & 9.2  &        94 &177 & 94 & 0.094\\
2015 & 9.4  &       145 &128 & 92 & 0.264\\
2016 & 9.2  &       103 &160 & 103& 0.02\\
2017 & 8.8  &           &    &    & 0.108\\
[4pt] \hline\vspace*{2pc}
\end{tabular}
%%%\end{table}
\vspace*{-1pc}
%%%\begin{figure}[b]                     % Fig 4
\figurewidth{20pc}
\setimage{}{}{20pc}{}{2019es000657-f04}%%\vspace*{-2pc}
\bottomcaption{Interannual variations of air temparature and circulation indices by Wangenheim-Girs
for 2004--2016. The purple line shows the air temparature, the red line shows the eastern circulation
form, the blue line shows the western circulation form, the green line -- meridional circulation form.
The dotted lines show the respective linear trends. The equation of air temparature linear trend is
$y= 0.02x + 8.672$ $(R^2 = 0.02)$; the linear trend of the eastern circulation form is
$y = 1.242x + 138.15$ $(R^2 = 0.09)$; the linear trend of the western circulation form
$ y = -2.253x + 146.69$ $(R^2 = 0.16)$; the
linear trend of the meridional circulation form is
$y = 1.011x + 80.462$ $(R^2 = 0.14).$}
%%\end{figure}
\end{table}


     The positive phase of the NAO index has dominated since 2013
(\figref{3}).           %      (Figure 3).
This stipulates the strengthening of the Icelandic Low, the reinforcement of the west-east transfer, and consequently the increase of air temperature. The correlation
coefficient $r$ calculated between the NAO index and air temperature for 2004--2017
equals to 0.7 ($p$-level $< 0.05$), what proves a quite close relationship (according to the
Cheddock scale).

     In 2004--2016 activation of the eastern (E) circulation form was observed
(\figref{4}).           % (Figure 4).
 A blocking anticyclone over Eastern Europe took place and Atlantic
cyclones, enveloping Scandinavia, penetrated far into the Arctic and carried the heat
[\itc{Dmitriev et al.,} \reflink{Dmit18}{2018}].
 This high-latitude zonality has led to retardation of air
temperature increase in the south-eastern part of the Baltic Sea.


     Comparison between interannual variability of the air temperature and the
Wangenheim-Girs circulation indices reveals that the positive air temperature trend
stems from the positive trends of the eastern and meridional circulation forms and the
negative trend of the western circulation form
(\figref{4}).           % (Figure 4).
The maximum annual
temperature (2008, 2015) corresponds to the high value of the western form index
and the minimum value of the eastern circulation form index. The minimum annual
air temperature (2010) appears when the maximum values of the eastern circulation
index occur (with dominating of anticyclones with eastern winds in winter)
 [\itc{Stont and Demidov,} \reflink{Ston15}{2015}].

     The analysis of monthly mean linear trend increments for air temperature and
circulation indices for 2004--2016 detected the following features
 (\tabref{4}):           % (Table 4):
 \begin{itemize}
 \item
 	the maximum trend increment for air temperature is observed in
February ($+2.3$\deg C);
 \item
 	there is an increase in the western (W) circulation form and the NAO
index with a decrease in the eastern (E) and meridional (C) circulation forms;
 \item
 	the minimum trend increment for air temperature is observed in October,
which is associated with a decrease in both the western (W) and the eastern (E)
circulation forms and a maximum enhancement of the meridional (C) circulation
form.
\end{itemize}

\begin{table}[t]                                   % Table 4
\tablewidth{20pc}
%\noindent\rule{20pc}{1pt}\\
\ttt\caption{Monthly Mean Values of Linear Trend Increment for Air Temperature, Western (W), Eastern
(E), Meridional (C) Circulation Forms by Wangenheim-Girs, and the NAO Index}
\vspace{5pt}
\begin{tabular}
{@{}l@{\hspace{3pt}}
c@{\hspace{3pt}}
c@{\hspace{8pt}}
c@{\hspace{8pt}}
c@{\hspace{7pt}}
r@{}}
\hline
\\ [-7pt]
Month & Air        & W & E & C & NAO  \\
      &temperature &   &   &   & index  \\
      [7pt]
     \hline   \\ [-7pt]
I & $-0.6$ & $-2.45$ &  1.3  &  1.3 & 0 \\
II & 2.3 &  7.61 &$-10.99$ &$-7.84$ &0.952 \\
III& 2.2 &  0.84 &  1.3  &$-2.14$ &0.602 \\
IV &$-0.3$ &  1.92 &$-10.99$ & 9.07 &0.322 \\
V  & 1.4 & $-1.46$ & 5.15  &$-3.68$ &$-0.238$ \\
VI & 0.3 & $-12.0$ & 8.06  & 3.92 & 0.182 \\
VII& $-0.1$& $-0.38$ &$-0.84$  & 1.22 &$-0.588$ \\
VIII& 0.1& $-12.6$ &3.99   & $-2.6$ &$-0.336$ \\
IX  &$-0.3$&  10.9 &7.46   & 3.46 &$-0.126$ \\
X   &$-2.1$& $-11.8$ &$-2.91$  &14.76 & 0.602 \\
XI  & 0.6& $-4.84$ &6.15   & $-1.3$ & 0.154 \\
XII &$-0.7$& 3.37  &$-1.37$  &$-1.99$ & 0.588 \\
[4pt] \hline
\end{tabular}
\end{table}


     \section{Conclusions}

      The changes in the air temperature characteristics for the Russian sector of the
south-eastern Baltic Sea in 2004--2017 (climatic half-period) was notable compared to
the middle of the last century -- the annual mean air temperature has increased (8.8
and 7.4\deg C respectively).

      The annual minimum of air temperature shifted towards February ($-0.3$\deg C),
which is associated with variations in the circulation conditions and indicates that the
climate of the South-Eastern Baltic Sea has become more marine.

      Variations of temperature in the cold period of year contribute greatly to the
variability of annual mean values of air temperature.

      The revealed variability of air temperature is closely associated with
circulation conditions: a direct noticeable relationship ($r = 0.7$, $p$-level $< 0.05$) is
established with the NAO index.

     Deceleration of air temperature rise is caused by strengthening of the eastern
circulation form along with a concurrent dominance of western circulation form.

\acknowledgements{The study was done with a support of the state assignment of IO RAS (Theme
No. 0149-2019-0013).}


\begin{references}
\vspace*{-\baselineskip}

\reference{Alek10}
\reftype{1}
\refauthf{Alekseev}{G. V.}{}
\refauth{Radionov}{V. F.}{}
\refauth{Aleksandrov}{E. I.}{}
\refauth{Ivanov}{N. E.}{}
\refauth{Kharlanenkova}{N. E.}{}
\refyear{2010}
\reftitle{Climate changes in the Arctic and the Northern Polar Region}
\refjour{Arctic and Antarctic Research}
\refvol{1}
\refnumb{84}
\refpages{67--80}
\endreference{}

\reference{Avot10}
\reftype{1}
\refauthf{Avotniece}{Z.}{}
\refauth{Rodinov}{V.}{}
\refauth{Lizuma}{L.}{}
\refauth{Briede}{A.}{}
\refauth{K\c{l}avi\c{n\v{s}}}{M.}{}
\refyear{2010}
\reftitle{Trends in the frequency of extreme climate events in Latvia}
\refjour{Baltica}
\refvol{23}
\refnumb{2}
\refpages{135--148}
\endreference{}

\reference{Barn87}
\reftype{2}
\refauthf{Barnston}{A. G.}{}
\refauth{Livezey}{R. E.}{}
\refyear{1987}
\reftitle{Classification, seasonality and persistence of low-frequency atmospheric circulation patterns}
\refjour{Mon. Wea. Rev.}
\refvol{115}
\refpages{1083--1126}
\refdoi{10.1175/1520-0493(1987)115<1083:CSAPOL>2.0.CO;2}
\endreference{}

\reference{Bruk63}
\reftype{3}
\refauthf{Bruks}{K.}{}
\refauth{Karuzers}{N.}{}
\refyear{1963}
\refbook{Application of Statistical Methods in Meteorology}
\refpubl{Gidrometeoizdat}
\reflocat{Leningrad}
\refpages{416}
\refdoi{10.1126/science.142.3590.416}
\endreference{}

\reference{Chub02}
\reftype{1}
\refauthf{Chubarenko}{I. P.}{}
\refauth{Chubarenko}{B. V.}{}
\refyear{2002}
\reftitle{General waterdynamics of the Vistula Lagoon}
\refjour{Environm. and Chemical Physics}
\refvol{24}
\refnumb{4}
\refpages{213--217}
\endreference{}

\reference{Chub14}
\reftype{2}
\refauthf{Chubarova}{ N. E.}{}
\refauth{Nezval'}{ E. I. }{}
\refauth{Belikov}{ I. B. }{}
\refauth{Gorbarenko}{E. V.}{}
\refauth{Eremina}{I. D.}{}
\refauth{Zhdanova}{E. Yu.}{}
\refauth{Korneva}{I. A.}{}
\refauth{Konstantinov}{P. I.}{}
\refyear{2014}
\reftitle{Climatic and environmental characteristics of Moscow megalopolis according to the data of the Moscow State University Meteorological Observatory over 60 years}
\refjour{Meteorology and Hydrology}
\refvol{9}
\refpages{49--64}
\refdoi{10.3103/S1068373914090052}
\endreference{}

\reference{Dmit18}
\reftype{3}
\refauthf{Dmitriev}{A. A.}{}
\refauth{Dubravin}{V. F.}{}
\refauth{Belyazo}{V. A.}{}
\refyear{2018}
\refbook{Atmospheric Processes in the Northern Hemisphere (1891--2018), Their Classification and Use}
\refpubl{SUPER Izdatelstvo}
\reflocat{St. Petersburg}
\endreference{}

\reference{Dubr12}
\reftype{1}
\refauthf{Dubravin}{V. F.}{}
\refauth{Stont}{Zh. I.}{}
\refyear{2012}
\reftitle{Evolution of Hydrometeorological Fields over the South-Eastern Baltic Sea}
\refjour{Russian Geographical Society Herald}
\refvol{144}
\refnumb{5}
\refpages{37--48}
\endreference{}

\reference{Frol10}
\reftype{1}
\refauthf{Frolov}{ I. E.}{}
\refauth{Gudkovich}{ Z. M. }{}
\refauth{Karklin}{ V. P. }{}
\refauth{Smolyanitsky}{V. M.}{}
\refyear{2010}
\reftitle{Climate change in the Arctic and Antarctic -- result of natural causes}
\refjour{Arctic and Antarctic Research}
\refvol{2}
\refnumb{85}
\refpages{52--61}
\endreference{}


\reference{IPCC08}                %%% ???
\reftype{3}
\refauthf{IPCC}{}{}
\refyear{2008}
\refbook{Fourth Assessment Report: Climate Change}
\refpubl{Cambridge University Press}
\reflocat{Cambridge, United Kingdom and New York, NY, USA}
\refpages{175}
\endreference{}

\reference{Jurg12}
\reftype{2}
\refauthf{Jurgel\.{e}nait\.{e}}{A.}{}
\refauth{Kriau\u{c}i\={u}nien\.{e}}{J.}{}
\refauth{\u{S}arauskien\.{e}}{D.}{}
\refyear{2012}
\reftitle{Spatial and temporal variation in the water temperature of Lithuanian rivers}
\refjour{Baltica}
\refvol{25}
\refnumb{1}
\refpages{65--76}
\refdoi{10.5200/baltica.2012.25.06}
\endreference{}

\reference{Kozu77}
\reftype{1}
\refauthf{Kozubskaya}{G. I.}{}
\refauth{Konyaev}{K. V.}{}
\refyear{1977}
\reftitle{The spectral analyses of the processes and areas}
\refjour{Izvestiya atmospheric and oceanic physics}
\refvol{13}
\refnumb{1}
\refpages{61--71}
\endreference{}

\reference{Kirs11}
\reftype{1}
\refauthf{Kirschenstein}{M.}{}
\refyear{2011}
\reftitle{The air temperature variations in Szczecin and its dependence on the North  Atlantic Oscillation (NAO), Baltic coastal zone}
\refjour{Journal of Ecology and Protection of the Coastline}
\refvol{15}
\refpages{5--23}
\endreference{}

\reference{Kotl10}
\reftype{1}
\refauthf{Kotlyakov}{V. M.}{}
\refyear{2010}
\reftitle{Cryosphere and climate}
\refjour{Ecology and life}
\refvol{11}
\refpages{51--60}
\endreference{}

\reference{Kria12}
\reftype{1}
\refauthf{Kriauciuniene}{J.}{}
\refgroup{et al.}\refyear{2012}
\reftitle{Variability in temperature, precipitation and river discharge in the Baltic States}
\refjour{Boreal Env.}
\refvol{17}
\refnumb{2}
\refpages{150--162}
\endreference{}

\reference{Mich11}
\reftype{1}
\refauthf{Michalska}{B.}{}
\refyear{2011}
\reftitle{Recent trends of air temperature in Poland}
\refjour{Pracei Studia Geograficzne}
\refvol{47}
\refpages{67--75}
\endreference{}

\reference{Navr14}
\reftype{1}
\refauthf{Navrotskaya}{S. E.}{}
\refauth{Stont}{Zh. I.}{}
\refyear{2014}
\reftitle{Regional features of hydrometeorological conditions variability over the coast of the south-eastern Baltic Sea (Kaliningrad region)}
\refjour{Russian Geographical Society Herald}
\refvol{146}
\refnumb{3}
\refpages{54--64}
\endreference{}

\enlargethispage{-22\baselineskip}

\reference{Pani10}
\reftype{1}
\refauthf{Panin}{G. N.}{}
\refyear{2010}
\reftitle{On climate change in the polar zones of the Earth}
\refjour{Doklady Earth Sciences}
\refvol{427}
\refnumb{3}
\refpages{397--402}
\endreference{}

\reference{Pink05}
\reftype{2}
\refauthf{Pinker}{R. T.}{}
\refauth{Zhan}{B.}{}
\refauth{Dutton}{E. G.}{}
\refyear{2005}
\reftitle{Can satellites observe trends in surface solar radiation?}
\refjour{Science}
\refvol{308}
\refpages{850--854}
\refdoi{10.1126/science.1103159}
\endreference{}

\reference{Pome64}
\reftype{1}
\refauthf{Pomeranets}{K. S.}{}
\refyear{1964}
\reftitle{Heat balance of the Baltic Sea}
\refjour{Proceedings of Zubov State Oceanographic Institute}
\refvol{82}
\refpages{87--109}
\endreference{}

\reference{Rozh08}          %%% ???
\reftype{4}
\refauthf{Rozhkov}{V. A.}{}
\refyear{2008}
\reftitle{Methods of statical hydrometeorology}
\refbook{Geographical and Geoecological Aspects of Nature and the Society Development, N. V. Kaledin (ed.)}
\refpubl{SPBU}
\reflocat{St. Petersburg}
\refpages{121--132}
\endreference{}

\reference{Samo57}                             %%% ???
\reftype{3}
\refauthf{Samoylenko}{V. S.}{(ed.)}
\refyear{1957}
\refbook{Climate and Hydrological Atlas of the Baltic Sea}
\refpubl{Gidrometeoizdat}
\reflocat{Moscow}
\refpages{106}
\endreference{}

\reference{Smir88}
\reftype{4}
\refauthf{Smirnova}{A. I.}{}
\refauth{Minin}{N. I.}{}
\refauth{Yakovleva}{N. P.}{}
\refyear{1988}
\reftitle{Probabilistic analysis of annual variations and interannual variability of Wolf numbers, atmospheric circulation forms, water exchange between the Baltic sea and the North sea and its components}
\refbook{Moderforming Factors, Hydrometeorological and Hydrochemical Processes in Seas of the USSR, A. I. Smirnova and  M. A. Borisovskiy, (eds.)}
\refpubl{Gidrometeoizdat}
\reflocat{Leningrad}
\refpages{5--15}
\endreference{}

\reference{Stoc13}          %%% ???
\reftype{3}
\refauthf{Stocker}{T. F.}{}
\refgroup{et al. (eds.)}
\refyear{2013}
\refbook{Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change}
\refpubl{Cambridge University Press}
\reflocat{Cambridge, United Kingdom and New York, NY, USA}
\refpages{1535}
\refdoi{10.1017/CBO9781107415324}
\endreference{}

\reference{Ston15}
\reftype{1}
\refauthf{Stont}{Zh. I.}{}
\refauth{Demidov}{A. N.}{}
\refyear{2015}
\reftitle{Current trends in air temperature variability over the water area of the South-eastern Baltic}
\refjour{Moscow University Bulletin. Series 5, Geography}
\refvol{2}
\refpages{50--58}
\endreference{}

\reference{Sust83}                %%% ???
\reftype{3}
\refauthf{Sustavov}{Y. V.}{(ed.)}
\refyear{1983}
\refbook{Hydrometeorological Conditions of the Shelf Zone of the Seas of the USSR (1983), Vol. 1. Baltic Sea. Issue 1. The Baltic Sea Without Lagoons}
\refpubl{Hydrometeoizdat}
\reflocat{Leningrad}
\refpages{175}
\endreference{}

\reference{Terz92}
\reftype{4}
\refauthf{Terziev}{F. S.}{}
\refauth{Rozhkov}{V. A.}{}
\refauth{Smirnova}{A. I.}{(eds.)}                      %%% ???
\refyear{1992}
\reftitle{Hydrometeorology and hydrochemistry of the seas of the USSR}
\refbook{Project ``The Seas of the USSR'', Vol. 3, The Baltic Sea, Issue 1, Hydrometeorological Conditions}
\refpubl{Hydrometeoizdat}% Gidrometeoizdat}
\reflocat{St. Petersburg} %%% ???
\refpages{450}
\endreference{}

\reference{Tylk13}
\reftype{1}
\refauthf{Tylkowski}{J.}{}
\refyear{2013}
\reftitle{Temporal and spatial variability of air temperature and precipitation at the Polish coastal zone of the southern Baltic Sea}
\refjour{Baltica}
\refvol{26}
\refnumb{1}
\refpages{83--94}
\endreference{}

\reference{Twar13}
\reftype{2}
\refauthf{Twardosz}{R.}{}
\refauth{Kossowska-Cezak}{}{}
\refyear{2013}
\reftitle{Exceptionally hot summers in central and eastern Europe (1951--2010)}
\refjour{Theoretical and Applied Climatology}
\refvol{112}
\refpages{617--628}
\refdoi{10.1007/s00704-012-0757-0}
\endreference{}

\reference{Wibi02}
\reftype{2}
\refauthf{Wibig}{J.}{}
\refauth{G{\l}owicki}{B.}{}
\refyear{2002}
\reftitle{Trends of minimum and maximum temperature in Poland}
\refjour{Climate Research}
\refvol{20}
\refpages{123--133}
\refdoi{10.3354/cr020123}
\endreference{}

\reference{Wild05}
\reftype{2}
\refauthf{Wild}{ M.}{}
\refauth{Gilgen}{ H. }{}
\refauth{Roesch}{ A. }{}
\refauth{Ohmura}{ A. }{}
\refauth{Long}{C. N.}{}
\refauth{Dutton}{E. G.}{}
\refauth{Forgan}{B.}{}
\refauth{Kallis}{A.}{}
\refauth{Russak}{V.}{}
\refauth{Tsvetkov}{A.}{}
\refyear{2005}
\reftitle{From dimming to brightening: decadal changes in solar radiation at Earth's surface}
\refjour{Science}
\refvol{6}
\refnumb{308 (5723)}
\refpages{847--850}
\refdoi{10.1126/science.1103215}
\endreference{}

\vspace*{.5pc}

\end{references}

\end{document}


