ICE EVENTS ON THE SIBERIAN RIVERS: FORMATION AND VARIABILITY

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 2002 International Association of Hydraulic Engineering and Research ICE EVENTS ON THE SIBERIAN RIVERS: FORMATION AND VARIABILITY Valery Vuglinsky 1 ABSTRACT Most Asian territory of Russia is occupied by the basins of three largest Siberian rivers Ob, Yenisey and Lena discharging to the Arctic Ocean; the total drainage area of these river basins exceeds 7 mln sq. km. A great climate variability in combination with variable relief caused great variations not only in the dates of beginning and duration of different phases of ice events on the rivers of the study region but in the ice cover thickness, too. A general trend is an earlier appearance of ice events and later ice cover break-up in the direction from west to east. The duration of the complete ice coverage on the rivers of the Ob basin varies within days on average; in the Yenisei river basin it varies from 160 to 220 days; in the Lena river basin from 180 to 230 days. In southern parts of the study basins differences in their ice regimes are insignificant. Principal regular features of regimes of different ice events in the Siberian rivers, their space-time variability, conditions of formation and break-up are discussed in the paper INTRODUCTION Siberia occupies the major part of the Asian territory of Russia; it is extending from the Urals in the west out to mountain ranges of the Pacific water divide in the east, and from the Arctic Ocean in the north to hilly steppes in the south at the boundary with Kazakhstan. The area of Siberia is about 10 million sq. km. There are four physiographic zones in this territory, i.e. tundra, taiga, forest steppe and steppe. Most Siberian rivers belong to the Arctic Ocean drainage area and are concentrated within the basins of large rivers, i.e. the Yenisei, Lena and Ob. These rivers rise in the southern mountain taiga at NL. Middle parts of these river basins are in the plain taiga zone. The lowest reaches flow across the tundra zone. Climate in most of the territory is severe and continental. Mean air temperatures in January vary from 16 C in the south of the West Siberia (the Ob river basin) to 43 C in east Yakutia where one of the cold poles in the Northern Hemisphere is found. A great extension of the territory, variable topography, a complicated heat and moisture exchange causing great differences in hydrometeorological conditions in different parts of the territory, are the basic reasons explaining great differences in the ice regimes of the rivers which are formed here. A complete (up to the bottom) freezing of not only small rivers but mid-size rivers (with 1 State Hydrological Institute, 2-nd Line, 23, St. Petersburg, Russia

2 drainage areas of several thousands of square kilometers) in winter time, different types of cryogenic events formation in river channels (aufeis, aufeis shields, etc.), intensive increase of river ice coverage during severe winters when maximum ice thickness attains 2.5 m to 3.0 m are distinctive features of ice regimes in the upper reaches of the Yenisei and Lena rivers. Basic specific features of the ice regimes in the Siberian rivers are discussed in the paper; long-term variability of ice characteristics is analysed with the account of the climate factors dynamics. BASIC DATA AND INFORMATION Data of standard observations of ice events and ice cover thickness made in hydrological network have been used for the analysis of ice regimes in the Siberian rivers. Dates of the start of ice events formation in autumn and dates of the end of ice events in spring and duration between these dates have been used as major characteristics of ice events. The first day of the permanent shore ice formation, the first day of frazil ice motion, ice drift or complete ice cover formation is used as the date of the start of ice events in spring, depending on the fact what event was observed before. The last day of observations when all ice events were over is the date of the end of ice events in spring. The analyses of these ice events characteristics have been made for the streams in the Ob river basin from 1978 (the year of the beginning of these data publication in Russian hydrological year-books) up to 1990 (1996); for streams in the Yenisei river basin mainly for the period of ; and for the streams in the Lena river basin for Data of ice cover thickness at the end of each month of the cold season, as well as maximum annual ice thickness and dates of its measurements have been used for the analyses of ice cover thickness. For streams in the Ob river basin these data have been mainly considered for ; for the streams in the Yenisei river basin since the early 1950s up to 1989; for the streams in the Lena river basin data for have been mainly used. The present paper gives information on the ice regime characteristics in rivers at 22 stations altogether (7 stations on the streams in the Ob river basin,6 stations on the streams of the Yenisei river basin, and 9 stations on the streams of the Lena river basin). These stations have been selected in such a way that it would be possible to show the most typical landscape zones in the basins of three largest rivers in Siberia and to characterise the ice regimes in the streams not only different in the drainage areas (from several thousands to hundreds of thousands of sq. km), but changes in ice regimes downstream the channels of the Ob, Yenisei and Lena rivers from heads to mouths. Fig. 1 is a map with stations where ice regimes have been observed; these observation data are used in the present paper. Table 1 contains information about drainage areas and periods of ice events and ice cover thickness at the selected observation stations. DATES OF THE START OF ICE EVENTS AND THEIR DURATION The earliest ice events in small and mid-size streams in the Ob, Yenisei and Lena river basins within NL are observed during the first 10-day period in October.

Table 1: List of stations, data from which are used in the paper Station Drainage Period of observations No River - station area, sq.km Ice cover Ice events (in Fig.

3 Table 1: List of stations, data from which are used in the paper Station Drainage Period of observations No River - station area, sq.km Ice cover Ice events (in Fig. 1) thickness Ob river basin Ob at Salekhard Ob at Oktiabrskoye Ob at Kamen-na-Obi KetatMaximkinYar Kargat at Gavrilovsky Aley at Lokot UrsulatOnguday Yenisei at Potapovo YeniseiatSelivanikha Yenisei at Vorogovo Yenisei at Kysyl N.Tunguska at Podvoloshino Angara at Kodinskaya Shivera Lena at Kiusiur Lena at Sangary Lena at Pokrovskoye Lena at Macha Markha at Malykay Aldan at Krest-Khaldzhai Aldan at Ugino Timpton at Ust Baralas Biriuk at Kilier Yenisei river basin Lena river basin If autumn is warm, the early ice events in the upper reaches of the Ob and Irtysh rivers may be observed later, during the first 10-day period in November. This wide time range (up to 40 days) in the dates of the first ice events is characteristic of the above area in the Ob river basin only under the influence of often intrusions of warm air fluxes

4 in autumn from Kazakhstan. Small and mid-size streams in the Yenisei and Lena river basins within the above latitudes are characterised by a narrower range of the dates of the first ice events formation and varies from 30 days (in the upper reaches of the Nizhniaya Tunguska river and in the Angara river) to 20 days in the Aldan river basin which is explained by a more severe climate in these territories in the piedmonts of the Sayans and Stanovoi Range. The dates of the end of ice events in the upper and middle reaches of the Ob and Irtysh tributaries (the Ursul, Aley and Kargat rivers) as well as the dates of the start of ice events vary within a wide range, i.e. from the second 10-day period in March to the first 10-day period in June. A narrower range (20 or 30 days) between the earliest and the latest dates of ice events are characteristic of the Angara and Nizhniya Tunguska rivers and of the streams in these basins as well as of the streams discharging to the Lena and Aldan rivers in their middle reaches. Here, dates of the end of ice events vary within the first to third 10-day periods in May. In the lower reaches of the Aldan river and in the streams of the Viliuy river basin (the Markha-Malykay river) this period occurs during the second 10-day period in May to the first 10-day period in June. When considering the character of changes in the dates of ice events start and end at the stations along the Ob, Yenisei and Lena rivers, a general specific feature should be noted, i.e. the earliest dates of ice events are observed during the first 10-day period in October at each station except Kamen-na-Obi where the earliest ice events are observed during the second and third 10-day periods in October. As to the dates of the end of ice events, another specific feature should be noted, i.e. these dates are postponed to a later time all down the rivers. In the Ob river this period from the second 10-day period in April to the beginning of May at the Kamen-na-Obi is postponed to the middle of May up to the second 10-day period in June at Salekhard. In the Yenisei river the period of the end of ice events from the first to the second 10-day periods in May in the upper reaches (at Kyzyl) is postponed to the early days in June (at Potapovo); in the lower reaches of the Lena river from the second and third 10-day periods in May (at Macha) to the second 10-day period in July (at Kiusiur). Fig. 2 shows a diagram of changes in mean dates of the beginning and ending of ice events (during the study period) at the stations downstream the main channels of the Ob, Yenisei and Lena rivers. The analysis of long-term changes in the dates of ice events beginning and ending at each selected hydrological station has resulted in a discovery of trends oriented in different directions. In small streams within the upper Ob river basin (Ursul and Aley streams) during a negative trend has been discovered relative to the dates of the earliest ice events (they are observed earlier as usual). In the Kargat river discharging to the Lake Chany and flowing northward, at 50 NL in the Ob and Irtysh interfluve this trend has not been discovered. In the Kem river (a left-hand tributary of the Ob river) flowing within NL a distinct positive trend has been discovered relative to the change in the dates of ice events beginning. Positive trends of the study characteristics have been also discovered at the stations on the Nizhniaya Tunguska river and in the Angara river within the Yenisei basin as well as for all stations in the tributaries of the Lena river discussed at the present paper. The analysis of the dates of ice events beginning at the stations along the main channels of the Ob, Yenisei and Lena rivers has resulted in a significant negative trend only at Kamen-na-Obi (upper Ob

reaches). Slightly positive trends or no trends have been discovered at the other stations. Fig. 3 demonstrates graphs of long-term variations in the dates of ice events beginning at some stations.

5 reaches). Slightly positive trends or no trends have been discovered at the other stations. Fig. 3 demonstrates graphs of long-term variations in the dates of ice events beginning at some stations. According to the above long-term dynamics of the changes in the dates of ice events beginning and ending the appropriate changes have been discovered in the duration of ice events. In the upper Ob reaches this duration increased by days during In the rivers of the Yenisei and Lena basins negative trends of this characteristics have been discovered; the duration of ice events became days shorter for the study period. Fig. 4 demonstrates graphs of changes in the duration of ice events at different stations downstream the main channel of the Lena river.

6 ICE COVER THICKNESS AND ITS VARIATION Analysis of data on the ice cover thickness on the study rivers at the end of each month with ice events on maximum ice cover thicknesses and appropriate dates shows that within NL in the small and mid-size streams within the basins of the three main rivers the maximum ice cover thickness during different years varies from 50 cm to 100 cm, the maximum ice cover thickness mainly occurs in March and April. Small streams in the upper Lena basin usually freeze-up to the bottom in November-December at the ice cover thickness of cm with a subsequent ice cover thickness increment on its surface. Water appears in the channels of such streams in May or even in June. This event is observed in the study rivers every year, e.g. in the Biriuk river at Kilier (drainage area equals 2660 sq. km). The river here is completely frozen to the bottom in December; runoff in it is observed in May only. In the streams of the Lena river basin northward the 60 C maximum ice cover thickness varies within cm; it may be 200 cm and thicker during some years, e.g. ice cover thickness exceeding 2 m was observed in the Aldan river at Krest-Khadzhai on 30 April 1965 (217 cm) and on 30 April 1971 (208 cm). The main channels of the Ob, Yenisei and Lena rivers are characterised by a gradual increase of maximum ice cover thickness from upper reaches to the lower reaches. The minimum range of difference in the ice cover thickness ( cm) is between the upper station (Kamen-na-Obi) and lower station (Salekhard) in the Ob river. Eastward, this range tends to increase and equals cm between the upper and lower stations on the Yenisei river; it attains cm for the appropriate stations on the Lena river. In the lower Ob reaches ice cover thickness is maximum in April May; in the lower Yenisei reaches it is maximum in May; in the lower Lena reaches the ice cover thickness is maximum in March April. The analysis of long-term changes in the ice cover thickness at the study stations for a long period (Table 1) shows that a tendency towards decreased maximum ice cover thickness is characteristic of the majority of stations since the middle of the 1950s to the end of the 1980s. On average, this decrease equals cm; it turned to be greater, however, at 4 stations. A great decrease of maximum ice cover thickness was observed in the Kargat river at Gavrilovsky in the Ob basin; in the Yenisei river at Kyzyl, in the Aldan river at Krest-Khardzhai and in the Lena river at Kiusiur. Besides, a slightly positive trend towards maximum annual ice cover thickness increase has been discovered at two stations in the lower Ob basin (in the Ket river at Maximkin Yar and in the Aley river at Lokot) where a trend toward earlier ice events formation has been observed during recent years. Fig. 5 demonstrates graphs of changes in the maximum ice cover thickness at the above six stations. It also

7 shows graphs (for the same periods) of changes in mean annual temperature at meteorological stations nearest to the study hydrological stations (data from Zhigansk station 500 km southward have been used for Kiusiur station at the Lena river).

8 A positive trend towards mean air temperature rise is seen in all the graphs, which proves a general climate warming in the study territories which may be a reason for a thinner maximum ice cover thickness at four stations in Fig. 5. A thicker ice cover at two other stations cannot be explained in this case. Therefore, it is necessary to make a more detailed analysis of changes in the temperature regime on the basis of mean seasonal and mean monthly data to get more reliable conclusions about positive and negative anomalies in the change of maximum ice cover thickness. REFERENCES Magnuson, J.J., Robertson, D.M., Benson, B.J., Wynne, R.H., Livengstone, D.M., Tadashi, Arai, Assel, R.A., Barry, R.G., Card, V., Kuusisto, E., Granin, N.G., Prowse, T.D., Stewart, K.M. and Vuglinsky, V.S. Historical trends in lake and river ice cover in the northern hemisphere. Science 289: (2000). Vuglinsky, V.S. Extremely early and late dates of lake freezing and ice break-up in Russia. In Proceedings of the 27 th Congress of the International Association of Theoretical and Applied Limnology 27-5, Stuttgart (2001) Vuglinsky, V.S. and Markov, M.L. Specific features of winter runoff formation in the Southern Lena River Basin. In Proceedings of the Fifth International Study Conference on GEWEX in Asia and GAME 3. Inernational GAME Project Office, Nagoya, Japan (2001) Vuglinsky, V.S. Peculiarities of ice events in Russian Arctic rivers. Hydrological Processes 16: (2001).

Regional and seasonal inhomogeneity of climatic variability in the Far-Eastern Seas

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