681 Journal of Coastal Research, Special Issue 57, 2011 Sea ice shaping the shores K. Orviku†, J. Jaagus‡ ‡ and H. Tõnisson† † Institute of Ecology, Tallinn University, Tallinn, 10120, Estonia, Hannes.Tonisson@tlu.ee ‡ ‡ Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia, jaak.jaagus@ut.ee ABSTRACT Orviku, K.; Jaagus, J. and Tõnisson, H., 2011. Sea ice shaping the shores. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), – . Szczecin, Poland, ISSN 0749-0208 The Baltic Sea region is characterized by very variable winter weather conditions. In severe winters, the Baltic Sea near the Estonian coast is totally covered by pack ice, whose thickness may achieve 20-30 cm. Some storms carry drifting ice far from its original location. A combination of high sea level and prolonged strong winds from a single direction forms preconditions for catastrophic ice movement to the coast. Large accumulations of erratic boulders characterize both ancient and contemporary shore formations in the northern Baltics. The genesis of such accumulations has been attributed mainly to drifting ice from near-shore water to the shore. Evidence of transported boulders are visible in spring at low sea-level conditions after the ice has melted. Large boulders sometimes accumulate on beach ridges at elevations of a couple of meters above the sea level and tens of meters inland from the shoreline. Our first studies on the structure and formation conditions of the hummocks of drifted ice on shore originate from February 1957, when 15 m-high blocks of ice were deposited on a sandy beach in Pärnu Bay. The latest studies on the impact of drifting ice on the structure and development of the Estonian seashores are from March to July 1997 on the northern coast of Saaremaa Island. The main goal of the current study is to identify the role of sea ice in shaping the accumulative shores of Estonia and its dependence on climatic conditions. ADITIONAL INDEX WORDS: Baltic Sea, Drifting ice, Sediment transport, Hummocks, Winter storm. INTRODUCTION Accumulations of erratic boulders characterize both ancient and contemporary shore formations on the Estonian coast (Figure 1a, 1b). The genesis of these accumulations has been attributed mainly to drifting ice from near-shore water to the shore. A combination of high sea level and prolonged strong winds from a single direction form the preconditions for catastrophic ice movement to the coast (Figure 1c, 1d). These formations often consist of straight rows of boulders parallel to the shoreline. The boulders sometimes accumulate on beach ridges, which are composed of limestone shingle at elevations of a couple of meters above the sea level and tens of meters inland from the shoreline (Figure 2a). Sea ice drifting onshore and the formation of ice hummocks is usually accompanied by transport and accumulation of sea bottom sediment to the shore. These ice lobes are capable of moving boulders over 1 m in diameter and deposit ridges of cobble and boulders. Evidence of boulder transport is visible in spring (Figure 2b, 2c) in low sea-level conditions after the ice has melted as tracks of sediment transport up to 100 m long on the near-shore sea bottom. The maximum depth of the sea bottom is from where boulders can be transported by drifting ice remains uncertain, but transport funnels along the Estonian coast have been discovered at 3–4 m depths. Similar traces have been found at much deeper depths in the Gulf of Bothnia. Sea ice forms in the Baltic Sea near the Estonian coast almost every winter and is characterized by large temporal and spatial variability. The first sea ice forms generally in December (Jevrejeva, 2000; Jaagus, 2006a; Sooäär and Jaagus, 2007), although ice usually form in November in shallow bays and at the beginning of January in the open coast of the Baltic Proper. The formation of fast ice usually follows the formation of the first ice by one month. Break-up of the fast ice typically occurs in the first half of April, followed by the final melting of sea ice occurs one week later. The earliest sea ice was observed at Kunda (Figure 2d), on the southern coast of the Gulf of Finland, on 15 October 1976, and the latest on 19 May 1955 (Pärnu) and in 1956 (Kihnu). The mean number of days with sea ice along the Estonian coast varies spatially, ranging from 50 days in Pakri on the open sea coast to 142 days in Pärnu (Sooäär and Jaagus, 2007). The longest duration of sea ice was observed in Pärnu (180 days) in 1954– 1955. Thick sea ice allows use in winter of official roads over the ice connecting islands of the West-Estonian Archipelago with the mainland. No sea ice forms in mild winters on the coast of the Baltic Proper in western Estonia. The increase in average air temperature in Estonia during the second half of the 20th century in winter and spring exceeds the increase in annual air temperature (1–1.5°C) (Jaagus, 2006b). This increase has induced significant changes in the sea ice regime. During 1950-51–2004-05 the first formation of sea occurs later in western Estonia (Sooäär and Jaagus, 2007) while the final disappearance of sea ice occurs 20–30 days earlier. Thus, the sea ice season has shortened by 1–2 months in western Estonia. Warming in winter is caused by changes in atmospheric circulation. Intensification of westerly winds (Jaagus, 2006b) and cyclonic activity in the Baltic Sea Region (Sepp et al., 2005) has lead to an increased storminess (Orviku et al., 2003; Jaagus et al., 2008), a rise in mean sea level (Suursaar et al., 2006), and greater frequency and magnitude of storm surges and coastal damage (Orviku et al., 2003; Kont et al., 2007; Tõnisson et al., 2008) in winter. The Baltic Sea ice conditions are closely related to the North Atlantic and Arctic oscillations (Jevrejeva, 2002). Journal of Coastal Research SI 64 681 - 685 ICS2011 (Proceedings) Poland ISSN 0749-0208