DOI: 10.25923/8q4y-bg95

I. V. Polyakov¹, R. B. Ingvaldsen^2,3, and B. A. Bluhm³

¹International Arctic Research Center and College of Natural Science and Mathematics, University of Alaska Fairbanks, Fairbanks, AK, USA
²Institute of Marine Research, Bergen, Norway
³Department of Arctic and Marine Biology, UiT The Arctic University of Norway, Tromsø, Norway

Headlines

• Atlantification—an influx of anomalous water properties and biota from lower latitudes—now reaches the central Arctic Ocean, with its fingerprint detected in the western Amerasian Basin and advancing toward Alaska.
• Atlantification weakens Arctic Ocean stratification, enhancing heat transfer to the surface and undermining sea ice resilience.
• Rapid atlantification drives Arctic change, altering ocean properties, reshaping ecosystems, and threatening climate stability.

Introduction

Oceanic currents play an important role in the Arctic region by bringing oceanic heat to the Arctic Ocean from the lower latitude regions (Fig. 1). In the early 1890s, Nansen and his crew aboard the Fram made the first measurements in the Eurasian Basin, observing that warm, salty water from the Atlantic Ocean flows northward into the Arctic Ocean at 150-800 meters depth. They also observed near-freezing and relatively fresh water in the upper ~50 m and a layer with large vertical salinity and density gradients (called the Arctic halocline) overlying the Atlantic Water (AW) at ~50-150 m depth.

Later expeditions showed that changes in the AW layer are often connected to ocean cooling or warming events (Fig. 2) related to the highly variable nature of AW inflows (Fig. 3e-g). System-wide changes in Arctic Ocean basins driven by anomalous AW inflows are referred to as atlantification (e.g., Polyakov et al. 2023; Ingvaldsen et al. 2021). Atlantification has a long historical perspective: a series of studies, including paleoceanographic reconstructions, demonstrate that AW inflows and sea ice conditions have varied over decades to millennia, highlighting the Arctic Ocean’s sensitivity to long-term climate variability (e.g., Fig. 2). In 1990, for instance, a warm pulse entered the Eurasian Basin and moved along the deep oceanic pathways, resulting in a strong, up to 1°C, AW warming in the Arctic. Another warm AW pulse was detected in Fram Strait and later in the eastern Eurasian Basin during 1999-2004. This latter pulse peaked in 2007-08, when the average AW temperature was ~0.2°C higher than in the 1990s. After 2007, AW temperature stabilized, showing no significant trend through 2017 (Fig. 2).

Arctic halocline changes

Despite >1°C AW warming in the 1990s-2000s, the strong halocline largely insulated the surface and sea ice from the AW heat. The warming did, however, affect the halocline stratification, with stratification weakening in the Eurasian Basin while strengthened in the Amerasian Basin. These trends continued toward present, and overall stronger stratification in the Amerasian Basin since 2007 is consistent with the region’s ongoing freshening and deepening of the surface fresh layer due to the intensification of the Arctic high pressure atmospheric system. Boosted regional stratification lowers oceanic heat fluxes, slowing sea-ice losses in the Amerasian Basin (Polyakov et al. 2023).

In contrast, by the mid-2010s, the Eurasian Basin halocline had lost its fundamental role as an effective barrier to AW heat, having experienced a ~30% decline in stability over the preceding three decades (Polyakov et al. 2025). Increased upward AW heat transport associated with atlantification has significantly lowered the regional rates of winter sea-ice formation (e.g., Polyakov et al. 2025). The primary contribution of atlantification to high-latitude climate change has been the decline of regional sea ice over the past decade (Fig. 3a,b). Furthermore, sea ice thickness declines caused by atlantification persist across the central Arctic Ocean and are prevalent along the Transpolar Drift (Belter et al. 2021; see essay Sea Ice). Atlantification also impacts the strength of upper ocean currents (Fig. 3c,d) and the efficiency of ocean mixing, thereby affecting carbon and nutrient exchanges and thus primary productivity (see essay Primary Productivity).

Role of Atlantic Water influxes from the Barents Sea

Changes brought on by atlantification are closely related to the advection of AW over two Arctic gateways: Fram Strait and the Barents Sea (Fig. 1). Alternating decadal phases of atmospheric circulation modulate the relative strength of these two branches (Polyakov et al. 2023). In 2007-21, this circulation pattern weakened the northward inflows, enhanced southward sea-ice export through Fram Strait, and increased the AW inflow from the Barents Sea (Fig. 3e-g). Moreover, since the mid-2000s, the along-track heat losses of the Fram Strait and Barents Sea throughflow has changed (e.g., Moore et al. 2022), suggesting a stronger warming of the AW entering the Arctic from the Barents Sea as compared to Fram Strait.

In addition, the Barents Sea and Nordic regions are the key contributors to the biological component of atlantification, evident in northward changes of boreal species, driving shifts in biological community compositions and food webs (Calvet et al. 2024; Husson et al. 2024; Fig. 4).

Ocean-heat/sea ice-albedo feedback

Atlantification of the Arctic Ocean can be viewed as occurring in two phases (Fig. 5). Phase 1 involves warming of the ocean interior and shoaling of the (AW)/nitricline—a layer where nutrient concentrations increase with depth—while the halocline remains relatively strong, preventing deep mixing. In Phase 2, the halocline weakens sufficiently to allow deep mixing to occur. The shoaling of AW from approximately 150 m in the early 2000s to up to 70 m in recent years in the eastern Eurasian Basin has coincided with the seasonal disappearance of the halocline, causing fast sea ice loss (Polyakov et al. 2025) and probably an increase in primary production. A positive ocean-heat/sea ice-albedo feedback mechanism is triggered, amplifying atlantification. In this feedback, weaker stratification and enhanced winter ventilation—when cold, dense surface waters formed through sea ice growth and brine rejection sink and mix with underlying layers—of the upper (>100 m) ocean increase the release of AW heat to the sea surface, contributing to sea ice loss (Polyakov et al. 2023). Thinner ice melts more quickly in the following summer, exposing the darker ocean surface sooner and intensifying the sea ice-albedo feedback. Reduced sea ice extent during the succeeding freezing season encourages more powerful currents, which in turn enhances AW ventilation (Polyakov et al. 2020). Stronger coupling between the atmosphere, ice, and ocean in the Eurasian Basin over the past decade has played a critical role in driving this feedback (Polyakov et al. 2020), which has been central to the sustained decline in sea ice cover in the region in recent decades (Fig. 3a,b).

Eastward progression and ramifications of atlantification

CMIP-6 model projections indicate that atlantification is unlikely to extend far into the Amerasian Basin during the current century (Muilwijk et al. 2023). However, there is observational evidence that atlantification is already extending beyond the Eurasian Basin. The transition of the western Amerasian Basin to conditions resembling those observed in the eastern Eurasian Basin 5-7 years earlier is a clear fingerprint of the eastward progression of atlantification (Fig. 5a, Polyakov et al. 2025). The powerful ocean-heat/sea ice-albedo feedback mechanism is the primary cause of these changes (phase 2 of atlantification in Fig. 5a).

In contrast, no evidence of deep ventilation of AW heat was found in the 2021-24 mooring records from the eastern East Siberian Sea. Shoaling of the AW and halocline, however, indicates that this region is experiencing a preconditioning phase (phase 1 of atlantification in Fig. 5a) similar to that found in the Eurasian Basin in the 2000s. This ongoing transition not only mirrors earlier changes but also sets the stage for broader ecosystem impacts. For example, the shoaling of AW and diminishing stability of the halocline resulted in a 70 m vertical elevation of the boundary between the Pacific and Atlantic halocline domains from 2015 to 2023 (Fig. 5b,c). This process lifted nutrient-rich waters into the zone where there is enough light for photosynthesis to occur, potentially enhancing nutrient use and influencing the occurrence of summer surface blooms, as seen in observational records by the relocation of the chlorophyll-a (Chl-a, a proxy for phytoplankton biomass and primary production) maximum nearer to the surface since 2015, and reflected in model projections of primary production (Fig. 5b,c) (Bluhm et al. 2020, see essay Primary Productivity). Thus, even in this early preconditioning stage of atlantification, physical changes had an immediate impact on the state of the local ecosystem. Notably, newly recovered mooring records from winter and spring (February-May) 2025 provide the first evidence of deep ventilation in the eastern East Siberian Sea.

Shifting species distributions and migrations are expected to impact the entire food web. At lower trophic levels, the growing potential and observed occurrence of harmful algal blooms pose concerns for both humans and marine ecosystems (Karlson et al. 2021). At higher levels, expanding commercial (boreal) stocks may demand new monitoring and management strategies, complicated by logistical and geopolitical challenges (Nascimento et al. 2025). In areas where boreal and Arctic species already overlap, competitive and trophic interactions suggest an increasing role for omnivorous predatory fish. Greater mixing of halocline waters could further facilitate species exchange between the Pacific and Atlantic, as has occurred in the past (e.g., Reid et al. 2007).

Concluding note

Climatic and ecological changes linked to Arctic atlantification are complex and far-reaching, already having major impacts on primary production and species distribution in the Pacific Arctic (Ershova et al. 2021). Altered nutrient fluxes and increased high-latitude productivity (Nishino et al. 2023) have led to shifts in species composition and food web structure (Ingvaldsen et al. 2021; Husson et al. 2024). The continued weakening of nutrient, temperature, and light limitations in upstream areas is expected to further accelerate atlantification (Noh et al. 2024), with uncertain but potentially severe consequences for the Arctic system.

Methods and data

A detailed description of the data and methods used in this synthesis can be found in Polyakov et al. (2023, 2025). Conductivity, temperature, density (CTD) data are available online via Pangaea Data Publisher (e.g., https://doi.org/10.1594/PANGAEA.691332), at https://uaf-iarc.org/nabos/data/ (NABOS [Nansen and Amundsen Basins Observational System] project) and https://www2.whoi.edu/site/itp/data/ (ITP [Ice Tethered Profilers] program).

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November 26, 2025