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The Role of the Barents Sea in the Arctic Climate System

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The Role of the Barents Sea in the Arctic Climate System THE ROLE OF THE BARENTS SEA IN THE ARCTIC CLIMATE SYSTEM Lars H. Smedsrud,1,5 Igor Esau,2,5 Randi B. Ingvaldsen,3,5 Tor Eldevik,4,5 Peter M. Haugan,4,5 Camille Li,4,5 Vidar S. Lien,3,5 Are Olsen,3,5 Abdirahman M. Omar,1,5 Odd H. Otterå,1,5 Bjørg Risebrobakken,1,5 Anne B. Sandø,3,5 Vladimir A. Semenov,6,7 and Svetlana A. Sorokina2,5 Received 14 September 2012; revised 3 May 2013; accepted 30 June 2013; published 16 September 2013. [1] Present global warming is amplified in the Arctic and reduced sea ice formation in the Barents Sea during winter. accompanied by unprecedented sea ice decline. Located The missing Barents Sea winter ice makes up a large part along the main pathway of Atlantic Water entering the of observed winter Arctic sea ice loss, and in 2050, the Arctic, the Barents Sea is the site of coupled feedback Barents Sea is projected to be largely ice free throughout processes that are important for creating variability in the the year, with 4°C summer warming in the formerly entire Arctic air-ice-ocean system. As warm Atlantic Water ice-covered areas. The heating of the Barents atmosphere flows through the Barents Sea, it loses heat to the Arctic plays an important role both in “Arctic amplification” and atmosphere. Warm periods, like today, are associated with the Arctic heat budget. The heating also perturbs the high northward heat transport, reduced Arctic sea ice cover, large-scale circulation through expansion of the Siberian and high surface air temperatures. The cooling of the High northward, with a possible link to recent continental Atlantic inflow creates dense water sinking to great depths wintertime cooling. Large air-ice-ocean variability is evident in the Arctic Basins, and ~60% of the Arctic Ocean carbon in proxy records of past climate conditions, suggesting that uptake is removed from the carbon-saturated surface this the Barents Sea has had an important role in Northern way. Recently, anomalously large ocean heat transport has Hemisphere climate for, at least, the last 2500 years. Citation: Smedsrud, L. H., et al. (2013), The role of the Barents Sea in the Arctic climate system, Rev. Geophys., 51, 415–449, doi:10.1002/rog.20017. 1. INTRODUCTION AND FOCUS with a good historical record of conditions in the BS. For in- stance, old sailing logs [Vinje, 2001] reveal that the winter of [2] The Barents Sea (BS) is the northernmost Arctic shelf sea with partly ice-free ocean during winter in the present cli- 1866 was particularly severe, with the April sea ice edge mate. Because of this, the BS exhibits a number of special reaching its southernmost recorded position. In comparison, properties that make it an interesting region for studying in- recent years have seen the smallest ice extents with less than teractions between the atmosphere, sea ice, and ocean. 30% of the BS ice covered annually [Årthun et al., 2012b]. Second, the limited depth (< 300 m for the most part) and sur- [3] First, the relatively easy access due to open waters has fi allowed for good, year-round observations. The first rounding islands create a con ned oceanic basin that is reason- explorers visited the region more than 500 years ago [van ably simple to monitor. Despite limited observations in the Linschoten, 1601]. They discovered abundant wildlife and rich northern straits (Figure 1), the net heat transport (volume and fishing on the many shallow banks of the BS. These bounties temperature) over the last decade is well known [Smedsrud have drawn a steady stream of visitors over time, providing us et al., 2010]. This information has been used to evaluate regional and global models and to constrain the previously 1 wide ranges [Simonsen and Haugan, 1996] in heat budgets. Uni Climate, Uni Research, Bergen, Norway. fl 2Nansen Environmental and Remote Sensing Centre, Bergen, Norway. [4] The northward owing Atlantic Water (AW) that 3Institute of Marine Research, Bergen, Norway. keeps the BS partly ice free also keeps the Greenland Sea 4Geophysical Institute, University of Bergen, Bergen, Norway. ice free during winter. These regions provided the first obser- 5Also at Bjerknes Centre for Climate Research, Bergen, Norway. 6Helmholtz Centre for Ocean Research Kiel, GEOMAR, Kiel, Germany. vations of decadal-scale oscillations in the air-ice-ocean sys- 7Also at A. M. Obukhov Institute of Atmospheric Physics, Russian tem [Ikeda, 1990]. Around 1970, there was a long cold period Academy of Sciences, Moscow, Russia. with lower temperatures than average over the Arctic “ ” Corresponding author: L. H. Smedsrud, Uni Climate, Uni Research, (Figure 2). We term this period Cool 70s here, although Allegaten 55, Bergen 5007, Norway. (larsh@gfi.uib.no) it also incorporates some years at the end of the 1960s. The ©2013. American Geophysical Union. All Rights Reserved. Reviews of Geophysics, 51 / 2013 415 8755-1209/13/10.1002/rog.20017 Paper number 2012RG000418 SMEDSRUD ET AL.: THE ROLE OF THE BARENTS SEA Figure 1. The mean oceans transports to the Barents Sea and present-day mean value of surface flux of heat (TW =1012 W) and carbon (MtyrÀ1 =1012 gyrÀ1). Locations of station data used are shown as dia- mond, triangle, square, and circle; and the Kola section is shown as the dashed black line. Locations of the Barents Sea Opening (BSO) and the Barents Sea Exit (BSX) are also included. cooling was particularly pronounced in the Barents and atmosphere, the resulting decline of winter sea ice cover Greenland Seas, which were about 2°C colder than average [Screen and Simmonds, 2010] has been suggested to increase [Ikeda, 1990]. In the recorded air temperature from Novaya the probability of cold winters in Europe by triggering circula- Zemlya (Figure 2b), the cold anomaly is ~2°C, while the cold tion changes over a much larger region [Yang and anomaly in ocean temperature in the Kola section is ~0.5°C Christensen, 2012], though the mechanisms behind such a link (Figure 2a). The Cool 70s was also characterized by are debated [Honda et al., 2009; Overland and Wang, 2010; greater-than-average Arctic sea ice extent, primarily caused Inoue et al., 2012]. In these ways, the BS may influence larger by anomalies in the Barents and Greenland Seas [Ikeda, regions, both the Arctic to the north and Europe to the south, 1990]. In the same records, there is a warm period termed via atmospheric and oceanic pathways. the “Early Warming,” between the mid-1920s and the [6] A series of cause and effects in the coupled Barents air- 1940s (Figure 2). At its peak in the 1930s, the anomaly in ice-ocean climate appears well established from the pub- Northern Hemisphere (> 60°N) surface air temperature lished literature. Taken in sequence, these links suggest the (SAT) was of the same magnitude as that during the Cool possibility of positive feedback loops operating in the 70s. Increased ocean heat transport into the BS, driven by lo- Barents system. Consider an anomalously high transport of cal wind forcing, with an associated sea ice retreat, has been Atlantic heat in the BS Opening (BSO) (Figure 1). This suggested as the main drivers of the Early Warming may occur because of increased volume transport and/or tem- [Bengtsson et al., 2004]. perature of the water being transported (Figure 3a). This [5] How much influence can the small, shallow BS have on leads to a warmer BS, which requires more cooling before an Arctic scale? The BS area is about 10% of the Arctic Ocean, it reaches the freezing point, such that less winter sea ice is or about 1.4 million km2, and the mean depth is only 230 m. formed, and there is a larger temperature contrast between Despite this limited volume, the BS occupies a key position the cold air and the warm ocean (Figure 3b). As a conse- on the eastern side of the main gateway between the Arctic quence, there is increased net surface heat loss from the ocean and the other world oceans, and here, much of the heat trans- to the atmosphere (Figure 3c). This three-step series of events port occurs, both in the air and ocean. The BS is thus favorably was introduced more than 20 years ago by Ikeda [1990] and located to couple the oceanic heat brought northward by the Ådlandsvik and Loeng [1991] and forms the common axis of AW to the atmosphere. Being small and shallow may be an the two positive feedback loops that will be our focus here. advantage in some respects: The AW transported to the BS [7] Atmospheric circulation responses to the large surface by the Norwegian Atlantic Current is well exposed to the air heat loss in the BS have been proposed as one way to facilitate above and is therefore vertically mixed and cooled very effi- a positive feedback loop and thus maintain decadal oscillations ciently. For the ocean, the heat loss creates dense water that in the Arctic air-ice-ocean system [Ikeda 1990; Mysak and sinks below the upper mixed layer of the Arctic Ocean, bring- Venegas, 1998; Ikeda et al., 2001; Bengtsson et al., 2004]. ing along CO2 sequestered from the atmosphere. For the We call this loop the “wind feedback.” In this feedback loop, 416 SMEDSRUD ET AL.: THE ROLE OF THE BARENTS SEA 10.2 9.0 ) North AW [8] Complementary to the wind feedback, we hypothesize 4.4 a Modified North AW “ ” Kola 8.5 10.0 the existence of a new positive ocean feedback (Figure 3). ) 4.2 The existing, relevant ocean observations are reviewed in 9.8 8.0 section 2.2, and climate model simulations are presented in 4.0 temperature (°C 9.6 section 3.4.2 to support this new idea.
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