Interannual variability of the Caspian Sea hydrometeorological parameters
(the 1990
th
– 2017)
Ginzburg, A.I. & A.G. Kostianoy
Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, 117997, Russia
Keywords: Caspian Sea level, river discharge, sea surface temperature, atmospheric
precipitation, air temperature
Introduction
The Caspian Sea is the largest enclosed water basin of the world. The changes in its natural
state are determined to a large extent by climatic factors and characterized by considerable
interannual variability. This long-term variability of the Caspian Sea regime, as well as need of
control of an ecological condition of the sea necessitates comprehensive monitoring of
changes in its main parameters (sea level, sea surface temperature (SST), ice cover) as well as
the determining factors (river discharge, air temperature, atmospheric precipitation, etc.).
With a sharp decrease of the number of ship-borne measurements and the
number of
hydrometeorological stations on the Caspian Sea coast since the beginning of the 1990th, the
known data systems using satellite measurements and modeling became a reliable source of
regular information on various parameters of the Caspian Sea. In the present study, long-term
dynamics of different hydrometeorological parameters of the Caspian Sea based on such data
bases and available instrumental measurements is considered, namely, changes in sea level
(1993-2017), Volga river discharge (1992-2016), SST (2003-2017), surface air temperature
(2003-2017), and atmospheric precipitation (2003-2017). Trends of the listed parameters for
the indicated time intervals are compared with those obtained earlier (Kostianoy et al., 2014)
for the period from the beginning of the 1980s to 2009-2012.
Materials and methods
- Data on the variability of the level of the Caspian Sea and Kara-Bogaz-Gol (KBG) Bay were
obtained from the DAHITI (Database for Hydrological Time Series of Inland Waters) (
Schwatke
et al., 2015).
- Data on annual discharge (km
3
) in the delta apex of the Volga river mouth are
provided by
L.P. Ostroumova (N.N. Zubov State Oceanographic Institute).
- The Caspian Sea monthly SST time series are produced with the Giovanni online data system
using measurements with Modis-Aqua radiometer. These data are analyzed for the three
regions of the Caspian Sea (Northern, Middle and Southern) and the KBG Bay separately.
- Time series of basin-averaged monthly surface air temperature and total atmospheric
precipitation (mm/month) are taken also from the Giovanni online data system using MERRA-2
Model.
Results
The variability of the Caspian Sea level (CSL) according to the altimetry data of the DANITI
system was traced from September 28, 1992 to March 24, 2018. The performed comparison of
these data with those obtained earlier using the software of the Geophysical Center of the
Russian Academy of Sciences for the period 1993-2012 (Kostianoy et al., 2014) showed that,
with the same character of the interannual
variability, CSL values in the DAHITI system were
overestimated by approximately 0.4 m. Average rate of the Caspian Sea level decrease during
the 25-year period 1993-2017 has appeared to be about -5 cm/year. The highest rate of fall of
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the level of about 16 cm/s was observed in the period from 2010 to 2015. In 2016 and 2017,
CSL practically was not changed. The character of the interannual
variability of the KBG Bay
level since 1996 (the completion of the refilling of the bay after the destroying the dam
separating it from the sea) was the same as in the Caspian Sea.
The level of the KBG Bay was
about 0.5 m lower than in the Caspian Sea.
Annual discharge of the Volga River, which accounts for 80% of the river inflow to the
Caspian Sea, during the period 1992-2016 corresponding to the period of altimetry
observations of CSL changed within 68 km
3
ranging from a maximum of 346 km
3
in 1994 to a
minimum of 178 km
3
in 1996. The negative trend of the discharge in 24-year time interval was
about -2.46 km
3
/year.
The time series of the monthly mean SST have shown that warming
of the Caspian Sea and
the KBG Bay, on the average, occurred in 2003-2017. Positive SST trends during this 15-year
period in the Northern, Middle, Southern Caspian Sea and the KBG Bay were about 0.05, 0.07,
0.09 and 0.11 °C/year, respectively.
The trend of surface air temperature during the period 2003-2017 was found to be +0.04
°C/year. The lowest winter air temperatures were observed in the winters 2007-2008 and
2011-2012, the highest winter temperature was related to the winter 2015-2016. The highest
summer temperature was observed in 2010, the lowest one was in 2009.
The trend of atmospheric precipitation during the same period appeared to be about -0.22
mm/month/year. Thus, precipitation over the Caspian Sea from 2003 to 2017 has dropped by
about 10%.
Discussion
Over the 25-year period from January 1993 to December 2017, the average level of the
Caspian Sea dropped at a rate of approximately 5 cm/year on the background of alternating
periods of CSL rise and drop. This rate exceeds the value of -2.78 cm/year
obtained for the
period 1993-2012 (Kostianoy et al., 2014). In 2013-2017, CSL changed within 1.3 m from a
maximum of about -26.6 m BS in 1995 to a minimum of about -27.9 m in 2015-2017. The
character of the annual variability of CSL did not correspond to the interannual variability of
the Volga discharge, except for a period of sharp rise in the level and an increase in river
discharge in 1993-1994. The sharp drop in the level of the Caspian Sea in 2010-2015 occurred
with an alternating increase and a decrease in the Volga discharge in the range from 234 to
184 km
3
.
During the period from 1992 to 2016, Volga discharge decreased by an average of 59 km
3
,
which for a sea area of 371,000 km
2
corresponded to a mean drop in the sea level of about 16
cm. The maximum discharge of 346 km
3
was observed in 1994, the minimum 178 km
3
corresponded to 1996.
The trend of the amount of precipitation from January 2003 to December 2017 (-0.22
mm/month/year) turned out to be an order of magnitude greater than that determined earlier
for the period 1979-2010 (-0.01 mm/month/year) (Kostianoy et al., 2014). The average
amount
of precipitation in 2003-2017 decreased by 38 mm/year.
Thus, both components of the incoming part of the water balance of the Caspian Sea during
the period of CSL drop from 1994 to 2015 decreased. However, the proportion of precipitation
in the water balance is much lower than that of the Volga River. Therefore, significant
decrease of precipitation in the last 15 years (and, in particular, in the period of 2010-2015),
with a relatively small negative trend of the Volga's flow in this period, does not explain the
sharp drop in CSL in 2010-2015. Apparently, the main role here was played by increasing
surface evaporation which is the main component of the expenditure part
of the water
balance. This assumption is supported by an increase in SST trend in the Middle and Southern
Caspian Sea from 0.06 and 0.05 °C/year, respectively, in 1982-2009 up to 0.07 and 0.09 °C/year
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