ICES Journal of Marine Science

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1 ICES Journal of Marine Science ICES Journal of Marine Science (2012), 69(5), doi: /icesjms/fss075 Atlantic water temperature and climate in the Barents Sea, Vladimir D. Boitsov, Alexey L. Karsakov, and Alexander G. Trofimov* Knipovich Polar Research Institute of Marine Fisheries and Oceanography (PINRO), 6, Knipovich Street, Murmansk , Russia *Corresponding author: tel: ; fax: ; Boitsov, V. D., Karsakov, A. L., and Trofimov, A. G Atlantic water temperature and climate in the Barents Sea, ICES Journal of Marine Science, 69: Received 25 May 2011; accepted 26 March 2012 Year-to-year variability in the temperature of Atlantic water (AW), which has a strong influence on the marine climate and ecosystem of the Barents Sea, was analysed using data from the Kola Section. With a positive trend in mean annual temperature during the late 20th century, only positive anomalies were registered during the past decade. In nine of those years, the temperature was warmer than the long-term mean by C, and in 2006, the historical maximum for the 110-year period of observations along the section was recorded. High air and water temperature coincided with reduced sea-ice cover, especially between October and April, when there is seasonal enlargement of the ice-covered area. An integral climate index (CI) of the Barents Sea based on the variability in temperature of AW, air temperature, and ice cover is presented. A prediction of future Barents Sea climate to 2020 is given by extrapolating the sixth degree polynomial approximating the CI. Keywords: Barents Sea, climate, water temperature. Introduction The Barents Sea is a shelf sea of the Arctic Ocean situated between northern Europe and the archipelagos of Spitsbergen, Franz Josef Land, and Novaya Zemlya (Figure 1). Being a transitory area between the North Atlantic and Arctic Basins, it plays a key role in water exchange between them. Advection of Atlantic water (AW) is one of the main sources of heat and salt in the Eurasian part of the Arctic Ocean (Nikiforov and Shpaikher, 1980; Alekseev et al., 1997; Schauer et al., 2002; Gerdes et al., 2003). AW masses arrive there mainly through the Barents Sea and the eastern Greenland Sea past the Spitsbergen archipelago. They are carried by the eastern branch of the Norwegian Atlantic Current, which then splits into two branches before entering the Barents Sea. The first branch enters the Barents Sea, crosses its southern and eastern parts, then flows into the Kara Sea through the strait between Franz Jozef Land and Novaya Zemlya. This modified AW then flows from the Kara Sea into the deep Arctic Ocean. The second branch proceeds north as the Spitsbergen Current, continuing as the West Spitsbergen Current, which flows along the continental shelf break through the Fram Strait west of Spitsbergen. The transformed AW rounds the northern tip of the archipelago and continues east along the continental slope, joining with modified AW flowing from the Kara Sea (Figure 1). In ice-covered areas, this water submerges under the cold surface layer and penetrates far into the eastern Arctic Ocean (Treshnikov and Baranov, 1976; Rudels et al., 1994). Interaction of Atlantic and Arctic waters increases the vertical mixing of water masses and warming of their upper layers, hampering ice-formation in winter and enhancing ice-melting in summer in the Arctic Ocean areas occupied by AW (Polyakov et al., 2010). A combination of these processes has an impact on climate and climate variability in high-latitude areas (Polyakov et al., 2004; Alekseev et al., 2007), which means that variations in volume, temperature, and salinity of AW can affect oceanographic conditions in both the Barents Sea and the Arctic Ocean. Studies of variability of these factors in the Barents Sea are therefore important for investigating climate variability in large areas of the Arctic Ocean. Material and methods Knipovich Polar Research Institute of Marine Fisheries and Oceanography (PINRO) has over the years accumulated a large array of oceanographic data on the Barents Sea and adjacent waters. Fluctuations in temperature and salinity of AW passing through the Barents Sea are studied at standard oceanographic sections. The longest dataseries are available for the Kola Section running along the E meridian from to N (Figure 1). Observations along the Kola Section started in May 1900 and are more frequent (6 15 times a year) than at other # 2012 International Council for the Exploration of the Sea. Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oup.com

2 834 V. D. Boitsov et al. Figure 1. Main flows of AW in the Barents Sea (arrows; after Rudels et al., 1994, and Ozhigin et al., 2000) and the Kola Section (black dots). 1, eastern branch of the Norwegian Atlantic Current; 2, Spitsbergen Current; 3, West Spitsbergen Current; 4, Murmansk Current; 5, Bear Island; 6, Vardø Island; 7, Cape Kanin Nos. standard sections. The Kola Section has been so far occupied more than 1100 times, and the PINRO database contains data on oceanographic stations. It is one of the world s longest oceanic time-series. Long-term variations in the Barents Sea climate and features of hydrographic conditions from 2000 to 2009 were analysed using the following data: annual mean air temperature at Vardø (70822 N31806 E) and Kanin Nos (68839 N43818 E) coastal stations, ; annual mean air temperature at Bear Island coastal station (74830 N19800 E), ; annual mean temperature in the m layer of the Kola Section, ; monthly mean temperature in the m layer of the Kola Section, ; May July averaged ice cover (the percentage of area covered by ice) of the Barents Sea, (provided by the Murmansk Department for Hydrometeorology and Environmental Monitoring); and monthly mean ice-covered area calculated by data from satellite observations, The water temperature observations were carried out with both bottles and CTDs, but the temperature data stored in the PINRO database and used here have the same accuracy of 0.018C for the whole period. The spatial distribution of temperature in the Barents Sea was analysed using data from the joint Norwegian Russian ecosystem survey in as well as data from hydrographic observations in the strait between Franz Josef Land and Novaya Zemlya in September of 2007 and Results and discussion Over the past two decades, water and air masses in the Barents Sea have been warmer than normal (Boitsov, 2006; Karsakov, 2009). Mean annual AW temperature in the m layer of the Murmansk Current in the Kola Section in the first decade of the 21st century was 4.78S (Figure 2a), the highest for the entire period of instrumental observations in the Barents Sea since During , the mean annual temperature exhibited positive anomalies of C relative to the mean, being close to the long-term mean in 2003 only, with an anomaly of 0.38S. There were anomalously high positive anomalies in 2000 and , with a historical maximum in 2006 (positive anomaly of 1.28S relative to the mean) for the entire 110-year period of observations along the Kola Section (Figure 2b). In some months in , positive anomalies were the highest since In 2006, anomalies were extremely high from May to October (Figure 2c). Positive anomalies in were generally 0.18C higher in the cold season than during summer. The first decade of the 21st century was characterized not only by warmer water temperature, but also by warmer air temperature (Figure 3), with the highest annual mean values in 2006/2007. In particular, air temperature off the Bear Island in those 2 years reached S, vs. the long-term ( ) mean of 1.48S. Similar to AW temperature in the Kola Section, positive air temperature anomalies in winter were higher than in summer. From 2000 to 2009, air temperature anomalies off Bear Island in November February were 2.4 times higher than in June August, and those off the west coast of Spitsbergen along 788N were 3.6 times higher in winter than in summer. Ice cover of the Barents Sea (the area covered with ice in relation to the entire marine area) is strongly correlated with air and water temperature (Boitsov, 2006), because ice conditions depend mainly on heat exchange between the sea and the atmosphere (Doronin and Kheysin, 1975; Bengtsson et al., 2004; Johannessen et al., 2007). The formation and melting of ice in the Barents Sea is seasonally and interannually variable (Malinin and Gordeeva, 2003; Boitsov, 2007). Long-term observations show that from May to July during the ice-melting season, there are two periods with extensive ice cover and two periods with little cover (Figure 4). The periods were identified based on the cumulative curve of ice cover anomalies, which shows the years of the transition from one period to the other. From 2000 to 2009, higher air and water temperatures in the Barents Sea resulted in little ice cover. Mean ice extent in the period was 10% less than in the previous decade, and 19% less than in the cold decade The least mean annual ice cover, 22 23% below the long-term mean, was observed in the Barents Sea in 2006/2007. The mean annual ice-covered area calculated using data from satellite observations in 2007 was km 2 smaller than the long-term mean. Data for show that in February April, when the extent of drifting ice is greatest, it was 13% lower than the normal. In August September of the same period, when ice extent is least, it was 7% less than the normal. This suggests that winter processes, principally heat exchange between the atmosphere and the sea causing ice formation, had a greater impact on the decrease in annual ice extent than summer processes during ice-melting. The formation and melting of ice depended on air temperature, which had higher positive anomalies in winter than in summer. From 2000 to 2009, the air temperature anomalies off Bear Island were 2.78C from December to February and 1.18C from June to August (relative to the long-term means), whereas those in the northern Kara Sea were 2.68C in winter and 0.38C in summer. Air temperature, water temperature and ice cover serve as indicators of marine climate. Their combination provides more

3 Atlantic water temperature and climate in the Barents Sea 835 Figure 2. Temperature in the m layer in the Murmansk Current of the Barents Sea: (a) mean decadal temperature, (the long-term average is for the years ); (b) mean annual temperature anomalies, (grey columns indicate anomalously high positive anomalies); (c) seasonal variations in mean monthly temperature anomalies, (absolute maxima for the period since 1951 are shown as circles). accurate estimates of long-term climate variability than each used separately, because combining these parameters smoothes shortterm climate variability and therefore allows better estimation of long-term climate variability (Boitsov, 2006). As a result, a new climate index (CI) for the Barents Sea was introduced and calculated for the period It was derived as a sum of mean annual anomalies of air temperature, water temperature, and ice-free area normalized by their standard deviations: CI = Ta M Ta + Tw M Tw + L M L, s Ta s Tw s L where Ta, Tw, and L are the mean annual air temperature (at Vardø and Kanin Nos coastal stations), water temperature (0 200 m layer of the Murmansk Current in the Kola Section), and ice-free area in the Barents Sea (May July), respectively, M Ta, M Tw, and M L are the long-term mean air temperature, water temperature, and ice-free area, respectively, and s Ta, s Tw, and s L are the standard deviations of air temperature, water temperature, and ice-free area, respectively. The individual contributions of air temperature, water temperature, and ice-free area to the CI were 20, 62, and 18%, respectively. Analysis of interannual variations in the CI and its sixth degree polynomial approximation demonstrated that, since 1900, long

4 836 V. D. Boitsov et al. Figure 3. Mean decadal anomalies of air temperature off Bear Island, cold periods twice alternated with long warm periods (Figure 5a). Vertical arrows in Figure 5b show years of the transition from cold to warm periods and vice versa. Currently observed steady warming of air and water masses in the Barents Sea began in the late 1980s. The CI was negative only from 1997 to 1999, because air and water temperature in that period were lower than the average. In 2006, mean annual water temperature in the Murmansk Current was the highest and ice cover in May July the lowest for the entire history of observations, which resulted in the highest CI. From 2000 to 2009, the CI for the Barents Sea was positive, with the largest inclination of cumulative curve compared with other decades (Figure 5b). However, since 2006, this index has been decreasing; in 2009, it was only half its historical maximum (2006). Extrapolation of the sixth degree polynomial approximating the CI Figure 4. Anomalies of ice cover in the Barents Sea, May July , and the corresponding non-linear trend (dotted line). Horizontal lines show the mean ice cover in specific periods. Figure 5. (a) CI for the Barents Sea (solid line), its sixth degree polynomial approximation (dotted line), and the forecast up to (b) Cumulative curve of the Barents Sea CI, where 1929, 1961, and 1988 are years of the transition from cold to warm periods and vice versa.

5 Atlantic water temperature and climate in the Barents Sea 837 Figure 6. Bottom temperature (8C) in the Barents Sea, August September has shown that the index is expected to decrease further soon (Figure 5a), and transition through the zero point will probably take place after 2025 (Boitsov, 2008). The predicted decreasing CI should coincide with decreasing air temperature, water temperature, and ice-free area, because the index is formed from these parameters and is related directly to them. Thanks to low ice extent in the Barents Sea in August September , oceanographic observations during the joint Russian Norwegian ecosystem survey covered almost the entire northern part of the sea (Figure 6), providing an opportunity to study the interannual variability of the area occupied by bottom waters of different temperature. Figure 7 shows the area

6 838 V. D. Boitsov et al. Figure 7. Areas (%) of the Barents Sea (68 778N E) occupied by bottom water of different temperature, August September occupied by bottom water of different temperature in the Barents Sea (68 778N E) in August September As mentioned above, the water temperature in the Kola Section in summer 2006 was extremely high. The value demonstrates that in August September 2006, 51% of the study area was occupied by bottom water of temperature.28s. During the subsequent 3 years ( ), that area was reduced by 5 6%, and in , just 32 39% of the area was occupied by water.28s (Figure 7). The area occupied by bottom water with temperature,08s was smallest in August September of 2007 (5%) and 2008 (4%). In August September 2006, it was three times larger than in the two subsequent years, despite the historical maximum of water temperature in the Kola Section and the largest area occupied by bottom water with temperature.28s in summer This was because rather cold water remained in the eastern Barents Sea in August September 2006 (Figure 6). However, in the first half of 2007 and 2008, the temperature in the m layer of the Kola Section remained anomalously warm ( S warmer than the normal), and in Figure 8. Temperature (8S) in the section between Franz Josef Land and Novaya Zemlya in September of 1991, 1992, 2007, and 2008.

7 Atlantic water temperature and climate in the Barents Sea 839 August September of 2007 and 2008, the temperature of the bottom water was,08s only in several small areas of the Barents Sea. In other years ( and 2009), the area occupied by bottom-water masses with,08s temperature was 3 8 times larger than in 2007/2008. The greatest extent of cold bottom water was observed in 2003 (Figure 7). Lower temperature in the m layer of the Kola Section in winter was correlated with larger areas occupied by bottom water with,08s temperature in August September (r ¼ 20.77, n ¼ 11, p ¼ 0.05) and smaller areas occupied by bottom water with temperature.28s (r ¼ 0.74, n ¼ 11, p ¼ 0.05), and vice versa. Oceanographic conditions in August September of 2007 and 2008 differed from those in other years of that decade, because positive bottom temperature anomalies were observed in almost 90% of the study area, exceeding the historical (since 1951) maximum in 35% of the area. Record high anomalies were mainly in the eastern Barents Sea. Bottom temperature in the entire Barents Sea was S above the normal. Positive anomalies in the areas occupied by the main AW flows reached 28S. In recent years, Arctic regions have also experienced stronger heat advection by atmospheric and ocean currents from the North Atlantic (Zhang et al., 1998; Alekseev et al., 2007; Rozhkova et al., 2008). Field observations and simulations indicate that the major outflow from the Barents Sea is in the strait between Novaya Zemlya and Franz Josef Land (Loeng et al., 1997; Rozhkova et al., 2008). In 2007/2008, PINRO and the Institute of Marine Research (Bergen, Norway) conducted oceanographic observations in the strait under the BIAC (Bipolar Atlantic Thermohaline Circulation) project of the International Polar Year 2007/2008. The investigations revealed a complex thermohaline structure in water of Atlantic and Arctic origin (Trofimov et al., 2010). Comparison of the results obtained in 2007/2008 with data from investigations performed in the same area in 1991/1992 (Loeng et al., 1993) showed that in September 2007/2008, the temperature of the surface waters in the strait between Novaya Zemlya and Franz Josef Land was.08s, i.e. considerably warmer than in 1991/1992 (Figure 8). In September 2007/2008, the temperature of water flowing from the Barents Sea above the southern slope of the St Anna Trough was also.08s, whereas in the less warm years of 1991/1992 it was,08s. Conclusions The decade was the warmest of the record starting in Throughout the year, air and water temperature in the Barents Sea was higher than the average. Temperature peaked in 2006/2007, and during the subsequent years ( ), the proposed Barents Sea CI almost halved. 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