Winter Intrusions of Atlantic Water in Kongsfjorden: Oceanic Preconditioning and Atmospheric Triggering

Kongsfjorden, an Arctic fjord in Svalbard, is largely influenced by the West Spitsbergen Current (WSC), transporting warm and salty Atlantic Water (AW) into the Arctic region. Despite the geostrophic control preventing AW from entering the fjord in winter, AW intrusions occasionally occur during energetic local wind events in this season. However, recent intrusions remain poorly characterized, and the underlying mechanism(s) and large-scale precursors are only partly understood. This study uses in-situ oceanographic and atmospheric measurements, alongside reanalysis data covering 2011-2020, to describe recent wintertime AW intrusions in Kongsfjorden. By discerning common traits in the observed events, the main triggering factors and controls of the phenomenon are described. Our results indicate that AW intrusions are typically triggered by wind reversals over the shelf, consisting of the sudden transition from a strong southerly to a northerly circulation linked to the setup and damping of a high-pressure anomaly over the Barents Sea. Ocean density is a critical preconditioning factor influencing the nature of the intrusion: when fjord waters exhibit a lower density compared to WSC waters, wind reversals induce AW intrusions by upwelling; in contrast, when fjord waters present higher or similar densities compared to WSC waters, reversals force AW inflows near the surface or at intermediate depths, respectively. Another mechanism was observed only in winter 2014: southerly winds prevailed for 2 months, transporting surface AW from the WSC into the fjord, promoting its intrusion near the surface, on top of denser local waters.Kongsfjorden, an Arctic fjord in Svalbard, is largely influenced by the WSC, which carries warm and salty Atlantic Water (AW) into the Arctic region. Occasionally, the fjord experiences sudden wintertime warming events due to intrusions of AW, whose underlying mechanism(s) and large-scale precursors remain only partly described. By examining events observed in the 2011-2020 decade, we found that changes in wind direction typically trigger such intrusions, precisely when energetic winds from the south are followed by winds from the north. These episodes lead to abrupt warming events, but AW intrusions occur only when the density of the fjord is lower than that of the WSC. When fjord waters have a similar or larger density than WSC waters, wind events generate AW inflows at intermediate or near-surface depths, respectively. Hence, the atmosphere triggers the AW intrusion, but its dynamics are determined by oceanic conditions, acting as a critical preconditioning factor. Additionally, a second intrusion mechanism was observed: prolonged winds from the south force AW from the WSC to intrude in the near-surface layers of the fjord, on top of denser waters.Sudden winter intrusions of Atlantic Water in Kongsfjorden are typically triggered by abrupt reversals of local meridional winds The dynamics of the intrusions are determined by the density difference between fjord and Atlantic waters The peculiar intrusion of winter 2014 was triggered by long-lasting southerly winds, transporting warm surface waters toward the fjord