Mechanisms of warm-water intrusions onto the West Spitsbergen Shelf during winter

Abstract. The West Spitsbergen Current, flowing northward along the continental slope in the eastern Fram Strait, represents a key pathway for warm Atlantic Water entering the Arctic Ocean. However, along the west coast of Svalbard, parts of this Atlantic Water frequently diverge from its core, intruding eastward onto the West Spitsbergen Shelf and further toward the adjacent fjords. Here, the associated excess heat has a substantial impact on the regional hydrography, as well as on the regional marine biosphere and cryosphere. This study uses a high-resolution, fully dynamical regional ocean model to investigate the mechanisms driving such warm-water intrusions during winter. Our results show that warming events on the West Spitsbergen Shelf are associated with a variety of cross-shelf exchange processes, including surface Ekman transport, upwelling of Atlantic Water from deeper slope regions across the shelf break, and topographical steering of Atlantic Water onto the shelf along shallower isobaths. The eastward displacement of the West Spitsbergen Current core itself is most frequently involved in triggering shelf warming events. Regardless of the specific mechanism, the intrusion depth on the shelf is governed by the relative density difference between the intruding Atlantic Water and the ambient shelf water. As winter progresses, increased shelf density due to cooling, brine rejection, and previous Atlantic Water intrusions enhances the likelihood of near-surface intrusions, in contrast to intrusions typically penetrating the shelf at depth in early winter. These findings highlight the complexity and seasonality of the cross-shelf intrusions of Atlantic Water from the West Spitsbergen Current onto the West Spitsbergen Shelf. They also emphasize the need for high-resolution modeling and complementary observations to fully capture the dynamics of these intrusions and their broader implications for Atlantic Water heat transport and ecosystem variability in the Svalbard region.