Winter intrusions of Atlantic Water in Kongsfjorden: oceanic preconditioning and atmospheric triggering

Kongsfjorden is an Arctic fjord in Svalbard, which is largely influenced by the West Spitsbergen Current (WSC), transporting warm and salty Atlantic Water (AW) into the Arctic. The geostrophic control typically prevents AW from entering the fjord in winter, whereas energetic wind events develop the Spitsbergen Trough Current (STC), ultimately flooding fjords facing the West Spitsbergen Shelf with AW. However, an exhaustive understanding of the interplay between these two opposite mechanisms and a clear knowledge of conditions leading to AW winter intrusions are still lacking. In this study, observational and reanalysis data show that wind reversal events trigger AW intrusions, while the ocean density is a key preconditioning factor limiting the occurrence of AW intrusions to specific winters only. Wind reversals are strong southerly wind events linked to the setup of a high pressure anomaly over the Barents Sea, followed by a circulation reversal with northerly winds. Winters with AW intrusions feature fresher and less dense fjord waters compared to AW, resulting in the breakdown of the geostrophic control mechanism at the fjord mouth, which opens the fjord to waters advected from the WSC by wind reversals. The low salinity signal is consistent with a large freshwater production through summer Arctic sea-ice melting in the Barents Sea. Another mechanism is observed only in winter 2014: southern winds blew continuously for two months and transported surface AW from the WSC to the fjord, eventually forcing AW to intrude near the surface, on top of denser local waters.