Increased presence of Atlantic Water on the shelf south-west of Spitsbergen with implications for the Arctic fjord Hornsund
Introduction
Svalbard archipelago lies on the pathway of Atlantic Water (AW) heading towards the Arctic Ocean (Schauer and Fahrbach, 2004). This pathway embraces the archipelago from the west, in the Svalbard Branch of the West Spitsbergen Current (WSC), and from the south-east, taking the route via the Barents Sea (Fig. 1) (Loeng, 1991). This far-north reaching tongue of warm and saline AW makes the area subject of considerable zonal variability in ocean properties and to strong air-sea-ice interaction owing to the presence of different air and water masses and seasonal sea ice cover. Pan-Arctic amplified warming, however, has intensified the feedbacks arising from the complex interplay between the main climate components (Serreze and Barry, 2011), resulting in enhanced warming of the atmosphere and the ocean.
There is clear evidence for the transformation of the Barents Sea to a warmer state. The overall warming is attributed not only to the elevated temperature, salinity and transport of AW leading to the expansion of its domain (Årthun et al., 2012), but also to the smaller loss of heat to the atmosphere (Asbjørnsen et al., 2020). Observations show that the increased extent of AW has led to a dramatic shift in the water column to a warmer and more saline one and a significant reduction in Arctic Water (ArW) occupation (Lind et al., 2018). Progressive Atlantification is also a crucial factor reducing the Barents Sea’s efficiency as a cooling machine, so important for the climate and deep circulation in the Arctic Ocean (Skagseth et al., 2020, Shu et al., 2021). As a result of this shift to a warmer state, the Barents Sea is currently losing sea ice remarkably quickly, and is predicted to become ice-free in winter in the second half of the 21st century (Onarheim and Århtun, 2017). Atlantification of the east Greenland Sea and the Fram Strait (with respect to the properties and transport of AW) is not so explicit. Beszczyńska-Möller et al. (2012) found a significant trend of 0.06 °C y−1 for the mean AW temperature increase (AW warmer by 2 °C), but no distinctive changes in AW volume transport for the period 1997–2010. Walczowski et al. (2017), in turn, documented increasing AW salinity and pointed out that the temperature trend depended strongly on AW parametrization. After investigating the heat budgets of the main AW pathways, Asbjørnsen et al. (2020) suggested that the warming trend of AW carried with the WSC via the Fram Strait is only significant for the southern part of the region. Thus, Atlantification in the east Greenland Sea and the Fram Strait is much more complex and spatially variable. Nevertheless, some studies report an intensified inflow of AW through the Nordic Seas and into the Arctic Ocean in recent decades in response to declining Arctic sea ice (Stroeve et al., 2012) and reduced sea ice export via the Fram Strait (Smedsrud et al., 2017, Wang et al., 2020). Since AW has an impact on the local climate of Svalbard (Walczowski and Piechura, 2011), the observed Atlantification may further drive the loop of positive feedbacks, largely contributing to overall warming (Isaksen et al., 2016, Nordli et al., 2014, Wawrzyniak and Osuch, 2020) and sea ice decline (Onarheim et al., 2014, Muckenhuber et al., 2016).
Another manifestation of amplified warming is the increased frequency of extreme events, among which the most prominent in west Spitsbergen (the biggest island of the Svalbard archipelago, Fig. 1) are exceptionally warm winters or AW flooding events (Nordli et al., 2014, Nilsen et al., 2016). For the Arctic islands, including the Svalbard archipelago, a strong positive trend has been found in the occurrence and intensity of extremely warm, winter weather events over the last 50 years, and a further, up to threefold increase in the frequency of such events is predicted for the next 100 years (Vikhamar-Schuler et al., 2016). The West Spitsbergen Shelf (WSS) and fjords are experiencing a heightened frequency of AW flooding events during winter in response to changes in winter storm trajectories (Nilsen et al., 2016, Rogers et al., 2005). Skogseth et al. (2020) confirmed that this phenomenon has been common in Isfjorden in recent decades. AW flooding events during winter temporarily disrupt the typical Arctic conditions (Cottier et al., 2007) and their extent in fjords during winter influences the hydrographic conditions in the following summer (Tverberg et al., 2019). It has been shown that the last two decades have been dominated by Winter Open winters, which are characterized by the advection of AW into the fjords higher up in the water column, and by convection of the AW down to the bottom. Consequently, the following summer is marked by very warm surface waters (Tverberg et al., 2019). This stands in contrast with Winter Deep and Winter Intermediate winters, which prevailed in the 1990 s. In those years, whatever advection of AW during winter did take place was limited to the deepest part of the fjord’s water column. As a result, the following summer was characterized by relatively cold surface and intermediate water, and warming in the deeper layers.
The direct influence of AW on the west Spitsbergen marine environment can be partly mitigated by the Arctic Water (ArW), that enters the Barents Sea either between Spitsbergen and Franz Josef Land or between Franz Josef Land and Novaya Zemlya (Fig. 1). The inflow of ArW is typically concomitant with the import of sea ice, so it is initially cold and relatively fresh. ArW is also formed locally in the Barents Sea as a result of cooling and brine release during ice formation, the same way as the cold Arctic halocline (Rudels et al., 1996). The ArW propagates south-westwards along the eastern coast of the Svalbard archipelago, then turning north after having passed the southern tip of Spitsbergen. Here, this coastal current is named the Spitsbergen Polar Current (SPC; reintroduced by Nilsen et al. (2016) after Helland-Hansen and Nansen (1909)). While the properties and structure of the WSC are well documented (Cokelet et al., 2008; Walczowski, 2014), the SPC is poorly described and understood. Helland-Hansen and Nansen (1909) described ArW with a temperature below 0 °C and a salinity of 32–34 in summer (south-east of Edgeøya Island). This contrasts with the salinity given by Loeng (1991) (34.3 < S < 34.8), while Cottier et al., 2005, Tverberg et al., 2019 give higher ranges of temperature (-1.5 °C < θ < 1 °C). This considerable disparity results from the different locations of the studies and shows the substantial transformation of ArW which, when crossing the Barents Sea, is modified by cooling and sea ice formation during winter, whereas in summer its upper layer is much influenced by solar heating and freshwater input from the coast and sea ice melting. ArW can also be influenced by upward fluxes from the AW layer below (Lind et al., 2018) and lateral fluxes from the WSC west of Spitsbergen (Saloranta and Haugan, 2004). Thus, the degree to which the SPC is supplied with ArW is closely dependent on the thermohaline conditions in the Barents Sea on the one hand, on the other, since the SPC is a surface current, the supply will also be controlled by atmospheric conditions (Goszczko et al., 2018).
A constant interplay (mixing and exchange) between AW and ArW occurs along the Polar Front (PF) (Loeng, 1991; Walczowski, 2013), an area of enhanced physico-chemical gradients of water properties (grey line in Fig. 1). The PF in the Barents Sea is a complex feature with slightly different characteristics in the eastern and western Barents Sea (Barton et al., 2018). It is a persistent, shelf slope current constrained by potential vorticity (Barton et al., 2018). In the eastern Barents Sea it is weakly delineated, defined by mixing between ArW and Barents Sea Water (BSW). It is divided into a northern branch (dominated by salinity gradients) and a southern branch (dominated by temperature gradients) (Barton et al., 2018, Oziel et al., 2016). It has been shown that the Sea Surface Temperature (SST) gradient characterizing the PF increased significantly after 2005, thereby limiting the southward expansion of winter sea ice (Barton et al., 2018). In contrast, the PF west of Spitsbergen becomes narrower and its location is strongly bounded by the topography to the WSS edge (Oziel et al., 2016). This is where AW from the WSC mixes and exchanges with ArW carried by SPC. The subsurface (below 50 m) part of the West Spitsbergen PF is related to the eastern, largely density-compensated boundary of the WSC (Saloranta and Svendsen, 2001). The surface part of the West Spitsbergen PF is characterized by strong density gradients in the upper 50 m resulting from the presence of the fresh and cold SPC (Salorants and Svendsen, 2002). The strength of the PF seems to weaken in a northerly direction (Fig. A1 in Supplementary materials), probably because of gradual warming of shelf waters in contact with AW (Saloranta and Svendsen, 2001, Saloranta and Haugan, 2004, Tverberg et al., 2014). Thus, a typical gradation of water properties is observed when comparing Hornsund, located in the south (a typical Arctic fjord), with Kongsfjorden, which is located in the north (a typical Atlantic fjord) (Promińska et al., 2017).
The aims of this work are twofold. Firstly, bearing in mind the previous study of Promińska et al. (2018), which highlighted that the importance of ArW carried by the SPC, especially in the case of Hornsund, we attempt to characterize the hydrographic conditions over the shelf area, focusing in a holistic manner on AW, ArW and the Frontal Zone (FZ). Secondly, we address the following question: How has the extent of AW been varying over the last two decades and what changes has it brought the west Spitsbergen fjords? Bearing in mind the present-day conditions of increased warming and the greater frequency of extreme events, we hypothesize that greater amounts of water of AW origin (pure AW or modified AW) are occurring on the shelf and that the warming observed on the west Spitsbergen shelf and in fjords is associated with this rather than with the properties of AW in the WSC.
The details of the dataset used in this study and the methods applied are described in the next section. This is followed by the results and a discussion of the variability in water properties on the shelf, the extent of AW, changes in the PF’s position and how all these phenomena influence the Hornsund fjord. A brief conclusion is provided in the last section.
Section snippets
Overview of the hydrographic dataset
In this study we utilized the CTD dataset, relevant to the shelf area south-west of Spitsbergen (Fig. 2a), that was collected during summer AREX cruises on board R/V Oceania between 1999 and 2020 (Fig. 2b) (Walczowski et al., 2017). Over the years, different types of instruments were used to collect the CTD data (Table A1 in Supplementary materials).
The data from section V2 (measured from the southern tip of Spitsbergen to Bear Island (here called the Spitsbergen-Bear Island Gate, SBG, Fig. 2b)
Water mass distribution across the Spitsbergen-Bear Island Gate
The mean state of water properties between Spitsbergen and Bear Island is illustrated in Fig. 3. From the southern tip of Spitsbergen (station V38) to Bear Island (station V21) the SBG section crosses two troughs, which cut across the shallow shelf area. These are Storfjordrenna in the northern part of the section, with a depth of over 300 m, and the shallower Kveithola (∼250 m) in the southern part (Fig. 2b). The AW flow here is strongly bounded by the topography. It enters these areas, but
Discussion
Even though Atlantic Water supplies vast amounts of heat and salt to the Arctic Ocean (Schauer and Fahrbach, 2004), some of it is still mostly lost to the atmosphere and, more importantly, from the point of view of this study, to the surrounding West Spitsbergen Shelf and fjords (Saloranta and Haugan, 2004, Tverberg et al., 2014). Based on hydrographic observations (summers 1999–2020) along the shelf south-west of Spitsbergen, we documented a significant increase in the volume fraction of AW
Conclusions
This study has demonstrated a substantial increase in the presence of AW on the shelf south-west of Svalbard during the summers between 1999 and 2020, a process which has been very marked in the last ten years. Over the period studied, a positive trend of 8% y−1 in the presence of AW was observed, with a concomitant decrease in ArW, which is in line with the ongoing Atlantification of the both the Greenland and Barents Seas surrounding the Svalbard Archipelago. The increased AW index can also
CRediT authorship contribution statement
Agnieszka Strzelewicz: Conceptualization, Formal analysis, Investigation, Data curation, Writing – original draft. Anna Przyborska: Formal analysis, Writing – review & editing. Waldemar Walczowski: Investigation, Data curation, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the long-term monitoring program AREX as a contribution to the IO PAN statutory research areas (I.4). Part of the data were collected under Polish - Norwegian project ALKEKONGE (PNRF-234-AI-1/07). This study is an extension of analysis carried out during PhD study in the Center for Polar Studies KNOW (Leading National Research Centre 2014 - 2018). We thank Joanna Pardus for providing data for the Hornsund basemap preparation. We would like to thank the crew of RV
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