Variability and impacts of Atlantic Water entering the Barents Sea from the north

Abstract

Branches of the submerged Atlantic Water (AW) slope-current in the Nansen Basin enter the Barents Sea from the north between Svalbard and Franz Josef Land. Using hydrographic observations from annual surveys during 1970–2009, the mean state, variability and trend of the AW in the northern Barents Sea were documented, and the dominant driving forces were identified. The AW temperature has a strong positive trend over the last 40 years that accelerated in the late 1990s. The most important driving factor is the upstream temperature in the West Spitsbergen Current, which influences the entire region occupied by AW. This driving factor has pronounced multiannual variability and has a significant increasing trend, although it cannot account for the accelerated increase since the late 1990s. The secondary forcing is associated with the wind stress curl/Ekman pumping on the shelf-break towards the Arctic Ocean, causing cross-shelf exchange between the Barents Sea and the Arctic Ocean. This process increases the penetration of AW onto the shelf and is mostly confined to the northern shelf. The signal is dominated by multidecadal variability with a notable shift in the late 1990s/early 2000s, thereby amplifying the AW temperature increase compared with the upstream conditions. Additionally, coastal upwelling along northern Svalbard and the winter-mean surface air temperature were found to impact the AW temperature variability, although they were of less importance than the wind stress curl. Variability in the sea ice cover does not appear to influence the subsurface AW temperature.

Variability in the AW temperature is transferred to the Arctic Water (ArW), and the vertical extent of the ArW varies considerably. Before the early 2000s, the ArW temperature was stable and low; afterwards, both the variability and the temperature increased. Our results indicate that the ArW in the northern Barents Sea is mainly heated from below.

Introduction

During the last decade, pronounced changes in the Arctic climate have been reported. These changes include an accelerating sea ice decline (Comiso et al., 2008), strong positive surface air temperature anomalies, and a dipole pattern of the near surface atmospheric pressure, which favours stronger meridional winds compared with previous decades (Overland and Wang, 2005, Overland et al., 2008, Zhang et al., 2008).

The Barents Sea also experienced large climate changes in the last decade. The winter mean surface air temperature (SAT) in the northern Barents Sea has increased by 2.5 °C since the millennial shift, and the summer sea ice concentration has decreased by 14% (Zhang et al., 2008). Both changes were linked to the near-surface atmospheric dipole pressure pattern (Zhang et al., 2008). Additionally, there has been observed increasing oceanic heat flux to the Barents Sea from the Norwegian Sea (Skagseth et al., 2008). With the rapid climatic changes observed in the Arctic as a whole and the more regional warming in the Barents Sea, significant changes are expected in the ocean climate in the northern Barents Sea. Processes in the Barents Sea may, due to the water mass exchange across the northern and eastern boundaries of the Barents Sea, influence the properties of the halocline, the Atlantic Water (AW) and the intermediate water in the Arctic Ocean (e.g., Rudels et al., 1994, Rudels et al., 2004, Steele et al., 1995, Schauer et al., 1997, Aagard and Woodgate, 2001). Changing physical conditions in the Barents Sea will also have an impact on the Barents Sea ecosystem. In the 2000s, many species have been observed further north in the Barents Sea than previously (ICES, 2011).

The Barents Sea is bounded to the north by the Arctic Ocean. Following the definition used in Jakobsson et al. (2004), we use the Nansen Basin continental slope as the northern boundary of the Barents Sea (Fig. 1). This definition differs from that given by the International Hydrographic Organisation (1953) in “Limits of Oceans and Seas”, which uses a line between Nordaustlandet and Franz Josef Land. We find the definition of Jakobsson et al. (2004) more appropriate as it includes the entire shallow shelf in the Barents Sea.

The Barents Sea borders the warmer Norwegian Sea in the south, and warm AW enters the Barents Sea from the Norwegian Sea in the southwest. The AW in the south is separated from the Arctic Water (ArW) in the north by the oceanic Polar Front, a dominant hydrographic feature of the near-surface waters of the Barents Sea (Fig. 1; Loeng, 1991; Pfirman et al., 1994). The Polar Front is located in the southern portion of our study area. A detailed description of our study area is given in Section 3.

The northern Barents Sea is a seasonally ice-covered marginal ice zone (Kvingedal, 2005). There is a highly fluctuating sea ice transport to/from the Arctic Ocean (Kwok et al., 2005) due to cyclone activity (Sorteberg and Kvingedal, 2006), causing strong interannual variability in the extent and concentration of the sea ice. The region also has a complex topographic structure, with deep trenches cutting between shallow banks (Fig. 1).

Information on the water masses present in the northern Barents Sea can be found in, e.g., Rudels (1986), Loeng (1991), Pfirman et al. (1994), Steele et al. (1995) and Løyning (2001). Following the definitions in Loeng (1991), the water masses in the northern Barents Sea are surface water/melt water, ArW, (modified) AW and bottom water/Cold Dense Water

AW enters the northern Barents Sea from the north through the Northern Barents Sea Opening (NBSO, Fig. 1) (Mosby, 1938, Hanzlick and Aagard, 1980, Pfirman et al., 1994, Steele et al., 1995, Matishov et al., 2009). The AW is advected south-westward below the ArW and has been observed year-round in the Olga Basin (Abrahamsen et al., 2006). The AW entering the northern Barents Sea through the NBSO is a branch of the AW slope-current, the submerged AW boundary current coming from Fram Strait and propagating eastward along the Nansen Basin continental shelf slope (Mosby, 1938, Pfirman et al., 1994, Steele et al., 1995). The variability in the temperature and extent of the AW entering through the NBSO is largely unknown.

Smaller fractions of AW also enter the northern Barents Sea as a submerged flow from the south across the ∼200 m deep saddle point between the Hopen Trench and the Olga Basin (Novitskiy, 1961, Loeng, 1991, Pfirman et al., 1994, Aksenov et al., 2010). This AW is cold (∼0 to 0.5 °C) due to the strong atmospheric heat loss in the southern Barents Sea, but it maintains a high salinity. Thus, these waters are denser and flow below the AW entering through the NBSO, which is both freshened and cooled from mixing with ArW (Pfirman et al., 1994).

ArW usually occupies the layer between 20 and 100 m in the northern Barents Sea (Pfirman et al., 1994, Løyning, 2001) and has a temperature minimum ranging from −1.8 to −1 °C at 50–75 m (Loeng, 1991). Presently, there is no consistent understanding of the formation and advection of ArW in the Barents Sea. According to Mosby (1938), ArW develops locally due to sea ice formation and heat loss to the atmosphere in winter and is modified by mixing in summer, while Novitskiy (1961) and Tantsiura (1973) claimed that it is advected into the region from the northern Kara Sea and partly from the Arctic Ocean. Steele et al. (1995), however, argued that all the water masses in the northern Barents Sea were generated from AW by varying degrees of heat loss to the atmosphere and ice melting and that at least part of this water mass (the Cold Halocline Water) is advected from the Barents Sea toward the Arctic Ocean. ArW is subject to strong water mass modifications and vertical mixing in the regions where subsurface AW enters from the north (Pfirman et al., 1994, Sundfjord et al., 2007). Downward and upward heat fluxes of 15–20 W m⁻² between the ArW and the surrounding water masses have been estimated (Sundfjord et al., 2007). Thus, the variability in the AW and ArW might influence each other.

Cold Dense Water is formed through heat loss to the atmosphere and brine release during winter (e.g., Midttun, 1985). This water flows along the bottom and exits the Barents Sea through the Franz-Victoria Trough (e.g., Årthun et al., 2011, Platov, 2011). Characteristics of this outflow have been addressed by, e.g., Rudels (1986), Steele et al. (1995), Schauer et al. (1997) and Rudels and Friedrich (2000).

The hydrography in the northern Barents Sea has a complex vertical structure due to the multitude of active processes, such as the AW (e.g., Mosby, 1938, Pfirman et al., 1994) and ArW inflows (Novitskiy, 1961, Tantsiura, 1973); the melting, formation and import/export of sea ice (e.g., Steele et al., 1995, Kvingedal, 2005, Kwok et al., 2005); winter convection and mixing of the components to produce Cold Dense Water with a variety of densities (e.g., Midttun, 1985, Steele et al., 1995, Rudels et al., 2004, Årthun et al., 2011); the lateral exchanges with extremely small Rossby radius (∼1 km) due to the strong stratification and high latitude (e.g., Gill, 1982); and the interaction of topographically steered flows with cascading plumes (e.g., Schauer et al., 1997, Ivanov et al., 2004).

Although the water mass exchanges in the southern boundary of our study region are limited due to the Polar Front, the exchanges across the NBSO are not well known. Direct current measurements in and/or near the NBSO have only been performed twice: one east of Kvitøya (Aagaard et al., 1983) and the other in the AW slope-current north of Kvitøya (Ivanov et al., 2009). Several numerical model studies (Maslowski et al., 2004, Johansen, 2008, Aksenov et al., 2010) have provided volume and heat transports across the gateway, but the results differ considerably, ranging from −0.4 to 0.4 Sv and from 0.5 to 6 TW into the Barents Sea.

An overview of the ocean currents and circulation in the Barents Sea from model studies can be found (e.g., Maslowski et al., 2004, Aksenov et al., 2010, Postlethwaite et al., 2011). All the authors show the AW slope-current in the Nansen Basin, and Maslowski et al. (2004) and Aksenov et al. (2010) also show its branching off bringing AW into the NBSO. According to Aksenov et al. (2010), the AW makes excursions into the trenches of the NBSO, where it interacts with the water masses there before recirculating in the Franz Victoria Trough. This pattern is consistent with a recent model study by Platov (2011), although his main focus was on dense water flow from the Barents Sea toward the Arctic Ocean. However, all the published studies focus on the entire Barents Sea (or even larger regions) or on specific processes. Thus, inferring details on the ocean currents of the northern Barents Sea from these studies is difficult. Consequently, the general circulation of the northern Barents Sea is not well known.

At the current stage, basic knowledge of the distribution (both horizontal and vertical) and the temporal variability of the water masses in the northern Barents Sea is lacking. Details on vertical structure, vertical heat fluxes and other parameters in the region can be found in recent publications (e.g., Sundfjord et al., 2007, Sundfjord et al., 2008, Matishov et al., 2009). However, all of these studies are based upon and focus on short time scales or small spatial scales. Here, we utilise vertical temperature and salinity data covering most of the sea ice-free northern Barents Sea annually over the period 1970–2009. We establish a picture of the general oceanographic conditions (“the mean state”), we investigate variability on the multiannual and decadal time scales, and we investigate the oceanographic processes and external factors important for the observed variability. The objectives of this paper are (1) to give a basic description of the AW inflow to the Barents Sea from the north and its impact on the ArW, and (2) to identify the factors driving the multiannual variability of the AW temperature. We investigate heat advected with the AW slope-current and its upstream surface heat loss, ice cover and wind stress curl/Ekman pumping as possible driving factors.

The paper is organised as follows: the data and methods are described in Section 2. In Section 3, we use descriptive methods to give a background description of the mean state, variability and processes important for the temperature variability in the northern Barents Sea. A statistical analysis of the observational data are presented and discussed in Section 4, and the findings are summarised and the implications are discussed in Section 5.