Abstract
Isfjorden, a broad Arctic fjord in western Spitsbergen, has shown significant changes in hydrography and inflow of Atlantic Water (AW) the last decades that only recently have been observed in the Arctic Ocean north of Svalbard. Variability and trends in this fjord’s climate and circulation are therefore analysed from observational and reanalysis data during 1987 to 2017. Isfjorden experienced a shift in summer ocean structure in 2006, from AW generally in the bottom layer to AW (with increasing thickness) higher up in the water column. This shift, and a concomitant shift to less fast ice in Isfjorden are linked to positive trends in the mean sea surface temperature (SST) and volume weighted mean temperature (VT) in winter (SSTw/VTw: 0.7 ± 0.1/0.9 ± 0.3 °C 10 yr−1) and summer (SSTS/VTS: 0.7 ± 0.1/0.6 ± 0.1 °C 10 yr−1). Hence, the local mean air temperature shows similar trends in winter (1.9 ± 0.4 °C 10 yr−1) and summer (0.7 ± 0.1 °C 10 yr−1). Positive trends in volume weighted mean salinity in winter (0.21 ± 0.06 10 yr−1) and summer (0.07 ± 0.05 10 yr−1) suggest increased AW advection as a main reason for Isfjorden’s climate change. Local mean air temperature correlates significantly with sea ice cover, SST, and VT, revealing the fjord’s impact on the local terrestrial climate.In line with the shift in summer ocean structure, Isfjorden has changed from an Arctic type fjord dominated by Winter Deep and Winter Intermediate thermal and haline convection, to a fjord dominated by deep thermal convection of Atlantic type water (Winter Open). AW indexes for the mouth and Isfjorden proper show that AW influence has been common in winter over the last decade. Alternating occurrence of Arctic and Atlantic type water at the mouth mirrors the geostrophic control imposed by the Spitsbergen Polar Current (carrying Arctic Water) relative to the strength of the Spitsbergen Trough Current (carrying AW). During high AW impact events, Atlantic type water propagates into the fjord according to the cyclonic circulation along isobaths corresponding to the winter convection. Tides play a minor role in the variance in the currents, but are important in the side fjords where exchange with the warmer Isfjorden proper occurs in winter. This study demonstrates that Isfjorden and its ocean climate can be used as an indicator for climate change in the Arctic Ocean. The used methods may constitute a set of helpful tools for future studies also outside the Svalbard Archipelago.
Highlights
The decline in Arctic sea ice is one of the most striking manifestations of climate change (Onarheim et al, 2018; Serreze and Barry, 2011; Stroeve and Notz, 2018), and in the recent decades, Svalbard fjords have experienced a substantial reduction in winter sea ice extent (Muckenhuber et al, 2016)
In order to make a robust analysis of interannual variability and trends in the hydrography and circulation in the Isfjorden system, all non-published hydrographic profiles obtained by UNIS and collaborating partners were collected and combined with hydrographic profiles available in public databases
In this review of the Isfjorden hydrography, and in order to make a robust analysis of variability and trends during the time period 1987 to 2017, all non-published hydrographic profiles obtained by UNIS and collaborating partners have been combined with hydrographic profiles available in public databases
Summary
The decline in Arctic sea ice is one of the most striking manifestations of climate change (Onarheim et al, 2018; Serreze and Barry, 2011; Stroeve and Notz, 2018), and in the recent decades, Svalbard fjords have experienced a substantial reduction in winter sea ice extent (Muckenhuber et al, 2016). Onarheim et al (2014) have shown that the sea ice area north of Svalbard has been decreasing for all months since 1979 with the largest ice reduction occurring during the winter months at a rate of 10% per decade This is related to inflow of warmer AW higher up in the water column when entering the Arctic Ocean north of Svalbard (Onarheim et al, 2018; Polyakov et al, 2017; Walczowski et al, 2017), and cannot be linked to any increase in AW volume flux in the WSC (Beszczynska-Moller et al, 2012). More detailed information about the data and analysis in this study is provided in Supplementary data, where interpolated winter and summer distributions of hydrography and water masses along the Isfjorden Transect are displayed for every year with enough data to produce interpolated sections
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