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Observed Atlantification of the Barents Sea Causes the Polar Front to Limit AUGUST 2018 B A R T O N E T A L . 1849 Observed Atlantification of the Barents Sea Causes the Polar Front to Limit the Expansion of Winter Sea Ice BENJAMIN I. BARTON Laboratoire d’Océanographie Physique et Spatiale, UMR 6523, CNRS-Ifremer-UBO-IRD, Brest, France YUENG-DJERN LENN School of Ocean Sciences, Bangor University, Bangor, United Kingdom CAMILLE LIQUE Laboratoire d’Océanographie Physique et Spatiale, UMR 6523, CNRS-Ifremer-UBO-IRD, Brest, France (Manuscript received 8 January 2018, in final form 21 June 2018) ABSTRACT Barents Sea Water (BSW) is formed from Atlantic Water that is cooled through atmospheric heat loss and freshened through seasonal sea ice melt. In the eastern Barents Sea, the BSW and fresher, colder Arctic Water meet at the surface along the Polar Front (PF). Despite its importance in setting the northern limit of BSW ventilation, the PF has been poorly documented, mostly eluding detection by observational surveys that avoid seasonal sea ice. In this study, satellite sea surface temperature (SST) observations are used in addition to a temperature and salinity climatology to examine the location and structure of the PF and characterize its variability over the period 1985–2016. It is shown that the PF is independent of the position of the sea ice edge and is a shelf slope current constrained by potential vorticity. The main driver of interannual variability in SST is the variability of the Atlantic Water temperature, which has significantly increased since 2005. The SST gradient associated with the PF has also increased after 2005, preventing sea ice from extending south of the front during winter in recent years. The disappearance of fresh, seasonal sea ice melt south of the PF has led to a significant increase in BSW salinity and density. As BSW forms the majority of Arctic Intermediate Water, changes to BSW properties may have far-reaching impacts for Arctic Ocean circulation and climate. 1. Introduction extent in May and Barents Sea sea surface temperature (SST) for the same month, as well as with SST in the The Arctic has been predicted to be free of sea ice in preceding December. The variability of the Barents Sea summer by the middle of the twenty-first century (Wang ice edge location has also been associated with atmo- and Overland 2012; Snape 2013; Notz and Stroeve 2016). spheric circulation (Sorteberg and Kvingedal 2006)and This follows an Arctic-wide decline in sea ice extent ice exported from the Arctic to the Barents Sea due to over recent decades (Screen and Simmonds 2010). The local winds (Koenigk et al. 2009; Kwok 2009). On longer Barents Sea alone has seen a 50% reduction in annual time scales, the reduction in annual and winter sea ice area sea ice area between 1998 and 2008 (Årthun et al. 2012), in the Barents Sea is thought to be driven by an increase in associated with a strong sea ice decline in all seasons the heat transport into the Barents Sea due to the com- including winter (Onarheim and Årthun 2017). Seasonal bined increase in advection and temperature of Atlantic sea ice extent variations are very predictable in the Water (AW; Årthun et al. 2012; Onarheim et al. 2015). Barents Sea compared with other parts of the Arctic AW temperature and salinity in the Barents Sea are also (Sigmond et al. 2016). For instance, Day et al. (2014) varying on multidecadal time scales (Levitus et al. 2009; found significant correlations between Arctic sea ice Smedsrud et al. 2013), making it challenging to distin- guish between long-term trends and natural variability. Corresponding author: Benjamin I. Barton, benjamin.barton@ Along with Fram Strait, the Barents Sea Opening univ-brest.fr (BSO) is a gateway for the warm and salty AW (defined DOI: 10.1175/JPO-D-18-0003.1 Ó 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 1850 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 48 TABLE 1. Definitions of the water masses present in the Barents Sea used in this study, along with definitions used in previous studies. Note that Barents Sea Water can be referred to as Modified Atlantic Water in literature. 2 Water mass Source Temperature (8C) Salinity (PSU) Density (r 2 1000 kg m 3) Atlantic Water (AW) Present study T . 3.0 S . 35.0 Oziel et al. (2016) T . 3.0 S . 34.8 Loeng (1991) T . 3.0 S . 35.0 Arctic Water (ArW) Present study T , 0.0 S , 34.7 Oziel et al. (2016) T , 0.0 S , 34.7 Loeng (1991) T , 0.0 34.3 , S , 34.8 Coastal Water (CW) Present study T . 2.0 S , 34.7 Oziel et al. (2016) T . 3.0 S , 34.4 Loeng (1991) T . 2.0 S , 34.7 Barents Sea Water (BSW) Present study T , 2.0 S . 34.7 s . 27.85 Schauer et al. (2002) s . 27.85 Oziel et al. (2016) T , 2.0 S . 34.8 s . 27.8 Loeng (1991) 21.5 , T , 2.0 34.7 , S , 35.0 in Table 1) entering the Arctic Ocean and its marginal an SST signature. This PF should be distinguished from seas (Fig. 1). The branch of AW entering through the another nearby SST front (hereafter thermal-surface BSO transits the Barents Sea, where it is modified en front) that is also expected to be present in the surface route, forming Barents Sea Water (BSW; Table 1; layer of the northern Barents Sea following the sea ice Schauer et al. 2002; Harris et al. 1998). This trans- edge, due to the transition from freezing-point water to formation into BSW is driven mainly by surface in- ice-free water. In the range of the temperatures and teractions with the atmosphere resulting in winter salinities of BSW and ArW, salinity and temperature convection and entrainment of freshwater. Heat is lost tend to contribute equally to the determination of den- from the ocean through turbulent heat flux and long- sity (Parsons et al. 1996; Våge et al. 2014). Thus, both wave radiation (Long and Perrie 2017), while freshwater surface temperature and surface salinity contribute to input mostly comes from seasonal sea ice import and the PF’s surface density gradient, suggesting that the rivers (Ellingsen et al. 2009). Thus, the length and lo- variability of the PF position can be influenced by other cation of the pathway along which AW flows determines processes than just the position of the sea ice edge. to what extent its properties are modified by surface BSW exits the Barents Sea, entering the Arctic Ocean fluxes, sea ice, and rivers before it enters the Arctic mainly through St. Anna Trough (Rudels et al. 2000; basin. The Barents Sea bathymetry is known to strongly Smedsrud et al. 2013). In the Arctic Ocean, BSW is influence the path of AW inflow (Fig. 1; Loeng 1991). entrained into Arctic Intermediate Water (AIW), ac- Part of the AW inflow crosses Murmansk Rise, south of counting for 50%–80% of the volume of AIW (Schauer Central Bank, into the Central Basin (Skagseth 2008; et al. 1997; Maslowski et al. 2004). AIW is ultimately Ingvaldsen 2005). The Central Basin acts as a reservoir exported to the North Atlantic through Fram Strait and for AW until it loses enough buoyancy to propagate in turn contributes to the deeper branch of the Atlantic northward below Arctic Water (ArW) as BSW. As a re- meridional overturning circulation (AMOC; Aagaard sult, the water column is stratified in the northern Barents et al. 1985; Fahrbach et al. 2001). There is some debate in Sea, with the upper 100 m occupied by relatively fresh and the literature about the extent to which BSW properties cold ArW (Table 1) and the lower layer occupied by BSW at the exit of the Barents Sea are preserved into the Arctic (Harris et al. 1998; Lind and Ingvaldsen 2012). Ocean and beyond. Observations have revealed that In situ observations in the western Barents Sea have some mixing of BSW occurs on continental slopes and revealed that the surface expression of the front sepa- within the Arctic Ocean (Shapiro 2003; Rudels et al. rating AW from ArW follows isobaths in the range of 2015), but model results of Lique et al. (2010) show 150–275 m (Gawarkiewicz and Plueddemann 1995; modifications to BSW properties within the Arctic Harris et al. 1998; Våge et al. 2014; Fer and Drinkwater Oceanaresmallcomparedtothemodificationwithinthe 2014). In the eastern Barents Sea, the northern front Barents Sea. In either case, the properties with which [referred to as the Polar Front (PF) hereafter] is defined BSW exits the Barents Sea are important as they pre- as the location where BSW meets ArW, but its geo- condition it for the target depth at which it may settle and graphic position is poorly defined (Oziel et al. 2016). The mix with ambient water masses within the Arctic basin. PF is a water mass boundary and therefore should have Anomalies in BSW density have been traced to Denmark AUGUST 2018 B A R T O N E T A L . 1851 FIG. 1. Bathymetry of the Barents Sea. The different lines and boxes indicate the area used for EOF analysis of SST (green box), the region used for Hovmöller analysis (blue dashed box), the cross-front transect (light-blue line), the area selected for calculating the contribution of sea ice to AW/BSW (dark-blue box), the area selected for 100–300-m BSW properties from EN4 data and 0–100-m ArW properties from EN4 data (cyan dashed box), the area selected for 0–100-m surface BSW properties from EN4 data south of the PF (yellow dashed line), the Kola section (orange line), and the Fugløya–Bear Island section (red line).
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