S.K.Monakhov O.I. Kholina
"Research Center of Southern Seas Ecology" Ltd.
Key words: Caspian Sea, current climate change, analysis and assessment, methods and results
Introduction
Currently the 2nd Report on the State of the Caspian Sea environment is being prepared under the umbrella of the Interim Secretariat of the Tehran Convention. One of the Report sections will be devoted to the current changes of the Caspian Sea climate change. In this respect, the publication of the relevant materials on the website of the Coordinating Committee on Hydrometeorology of the Caspian Sea (CASPCOM) is timely and can be used for the preparation of the 2nd Report. The objective of the report is to give an overview of the approach used in CASPCOM materials for the analysis and assessment of climate change, and to present some of the results obtained while implementing the approach.
Materials and methods
The materials for the analysis and assessment of the current climate change were based on annually updated CASPCOM data catalogues: catalogues of air and water temperature, surface runoff and sea level, most of which date back to 1961 (some of them even earlier).
CASPCOM data catalogues include the data provided by hydrometeorological services of all the five Caspian littoral states.
In the course of the analysis, the time series were smoothed by moving 30 year periods (1st derived series), then annual anomalies were calculated as a difference between the mean (monthly and/or annual) value of a parameter in the current year and for the previous 30 years (2nd derived series). The next step was to determine the annual increments as a difference between the following and the previous values (3rd derived series) and smoothing of this series by moving 30 year periods (4th derived series). To analyse the changes in the amplitude of annual anomalies and increments, them modulo series were used (5th and 6th derived series).
All the derived series were visualized and approximated by linear and non-linear (polynomial) trends. The statistical validity of the linear trends was assessed by Student's and Fisher's methodologies using different significance levels (0.1; 0.05; 0.01). To assess annual anomalies and increments, it is important to calculate quartiles, which allows subdividing them into weak, moderate and strong. Conducting comparative assessment of the growth rate of the climatic parameters can be implemented for n years, where n equals 0, 15, 20, 25, 30, 40, and 50 years
Results and discussion
According to CASPCOM data, the ongoing global warming has affected the Caspian Sea area, where the average air temperature for the past 30 years (1987 - 2006) has grown in comparison to the mean temperature for 1961-1990 from 9.9 to 10.7 оС in Astrakhan, from 12.2 to 12.5 оС in Makhachkala and from 12.7 to 13.5 оС in Derbent. In the first thirty-year intervals of the period under study, the average temperature fell slightly (by 0.1 - 0.2оС).
Long-term positive linear trend of air temperature (an example is set by the graph made by
observations data in Derbent)
vel is low (р=0.10).
CASPCOM materials contain data on annual air temperature anomalies on the Caspian Sea coast starting from 1991. According to CASPCOM data, air temperature anomaly was generally positive in all the years starting from mid 1990s. In some years (1997, 2003, 2011) the anomaly was close to zero (weakly positive or weakly negative), but in Derbent, for instance, it was positive during all the years starting from
value in these years was not so significant as in 2010.
Smoothed by moving 30-year periods, the rate of air temperature increment is increasing.
The linear trend which approximated this increase is statistically valid at p=0.01 According to the data of Derbent HMS, the average rate of air temperature increment for the past 30 years amounted to 0.07оС. An obvious fact is that when the averaging period increases (up to the past 50 years) or reduces (to the past 10 years), the mean rate of air temperature increment decreases, and it is negative for the 5 past years (2012-2016)
According to CASPCOM data, the Caspian Sea water temperature in 1961-2016 was rising as fast as the air temperature above the sea area (CASPCOM data catalogues include the data of coastal observations of the temperature of the sea water surface layer). In particular, in Makhachkala area in 1986-2015, it measured on average 12.9оС, which is by 0.4оС more than the average for the period of 1961-1990.
The positive linear trend is statistically valid at р=0.01 (by Student) and р=0.1 (by Fisher).
However the time series of water temperature anomalies is different from that of the air temperature. Stable positive anomalies of water temperature were observed only throughout 1997-2007; before and after this period positive anomalies alternated with negative ones.
Smoothed by moving 30-year periods, the rates of water temperature increment, despite the positive linear trend, are not so convincing as the rates of air temperature increase (which is confirmed by the statistical estimate). Only the 30-year periods which fell in 1968-2009 interval were characterized by positive water temperature increment rate. Before and after this period the increment rate varied from positive to negative and vice
ver
According to the data of Makhachkala HMS, the average rate of sea water temperature increment for the past 30 years amounted to 0.06оС. Similarly to air temperature, when the averaging period increases (up to the past 50 years) or reduces (to the past 10 years), the mean rate of water temperature increment falls, and it is negative for the past 5 and 10 years
recent years.
Throughout 1961-2015, the curve of the Volga flow smoothed by moving 30-year periods points to its continuous growth and the following decrease, though the linear trend
approximating the runoff changes in this time period is generally positive and statistically valid. The break point of the curve is the period 1978-2007, when the Volga runoff measured on average 260.3 km3/year. In contrast, in 1968-2015 interval, the Volga flow made on
average 248.8 km3/year.
The smoothed fluctuations of the Ural flow in 1961-2015 were synchronous to those of the Volga flow: it was increasing for some time, and then fell. The breaking point was as well in 1978-2007, when the Ural runoff measured on average 8.68 km3/year. In contrast, in 1968-2015 interval, the runoff of the river was on average 8.28 km3/year.
The flow of the Kura river in 1961-2010 was decreasing (negative linear trend, valid statistically at p=0.01). The sharp decline was observed in the first half of this period. In 1961-1990 the flow of this river made on average 14.7 km3, while in 1970-1999 it measured 13.2 km3. In the following 30-year periods, the flow stabilized at this average valu
The analysis of annual anomalies and increments of the river runoff and its average growth rates did not reveal any signs of its future growth. However, according to the information published in CASPCOM Bulletin, the Volga runoff in 2016 after an extended low-water period rose to 261 km3, which is by 5% higher, than the average for 1961-2015.
favour of the desired and probable increase of the surface runoff to the Caspian Sea The changes of the water flow to the Caspian Sea in 1961-2015 were accompanied by its level fluctuations. This fact is confirmed by the data of the Caspian Sea level catalogue created by CASPCOM.
1961-1977 the sea level fell by 52 cm, and its highest fall rate (9 cm/year) was recorded in 1971-1977. Then, the sea level was rising at a high rate (14 cm/year) for a long period of time (from 1978 to 1995). In 1996 the sea level started to fall: first at a slow rate (by 10 cm for 1996-2005), and then rapidly (by 11 cm/year in 2006-2015). In 2016 the sea level finally stabilized at the elevation of -28.0 m B.S.
Conclusion
Summarizing the facts stated above, we can say that in the last quarter of the 20th century the Caspian Sea was affected by the global climate warming, which lead to the increase of air temperature above its water area by 0.7-0.8оС, while the surface water temperature rose by 0.4-0.5 оС. First, the warming was accompanied with the rise of the Volga and the Ural runoff to the Caspian Sea (whereas the Kura flow reduced) and a rapid growth of the sea level. The flow to the sea decreased at the turn of the century, and the sea level started to fall slowly first, and rapidly since 2006. Simultaneously, the warming rate decreased, which resulted in normalization of the runoff in 2016-2017 and stabilization of the sea level.