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The Barents Sea is a primary gateway for Atlantic Water entering the central Arctic Ocean and ubiquitous water-mass transformation on the Barents shelf is key for mitigating increases in heat transport to the central Arctic through the St. Anna Trough. Using a mesoscale-permitting reanalysis spanning 40 years, we derive the first long-term estimate of heat transport through the St. Anna trough, finding that it has increased by 0.11 TW yr⁻¹ since 1980. However, this is only half of the 0.23 TW yr⁻¹ trend in the heat transport into the Barents Sea through the Barents Sea Opening. Decomposing the heat transports reveals that these trends are entirely due to warming temperatures at the sections with no discernible trend in the volume transports. We find that a northward migration of the largest heat fluxes from the ocean to the atmosphere have partially mitigated the increased temperature of Atlantic Water entering through the Barents Sea Opening, resulting in the less dramatic temperature trend in the waters exiting through the St. Anna Trough. Finally, we present evidence for a revised version of the “ocean feedback” hypothesis, which modulates volume transport into and out of the Barents Sea on sub-decadal timescales. The revised “ocean feedback” hypothesis relies on denser waters in the northeastern Barents Sea associated with increases in salinity and sufficient cooling from inflowing Atlantic Water. We find that these denser, more saline waters create lower sea surface height relative to the permanently elevated Kara Sea. The resulting gradient enhances downslope flow into the St. Anna Trough, driving increased inflow of saline Atlantic Water through the Barents Sea Opening and sustaining the feedback loop.

The Arctic surface mixed layer is an important moderator of the transfer of heat, salt, and momentum between the atmosphere and the warmer, more saline water that enter the Arctic Ocean from the North Pacific and North Atlantic oceans. However, prior observational studies of trends in mixed layer depth produce seemingly conflicting results due to limited temporal and spatial coverage. Using reanalyses which are consistent with the climatological mixed layer seasonal cycle, changes in the pan-Arctic mixed layer depth are assessed from 1980 to 2024. From the 1980s to the most recent decade, there was a shoaling of the winter (Oct–Mar) mixed layer and a slight deepening of the summer (Apr–Sep) mixed layer. Furthermore, time series of winter mixed layer depth anomalies show for the first time that the seemingly conflicting trends of prior studies can be reconciled as a result of significant decadal variability, particularly in the Canadian and Amerasian Basins. The decadal variability is found to be consistent with changes in the thickness of sea ice centered over the Canadian and Amerasian Basins. This work underscores how decadal variability in the sparsely observed Arctic can complicate the interpretation of long-term trends.

The CNES-CLS22 Mean Dynamic Topography (MDT; https://doi.org/10.24400/527896/A01-2023.003, Jousset, 2023) represents an incremental update to previous CNES-CLS solutions, combining altimetry, satellite gravity, and in situ observations (drifters, hydrography profiles, and HF radar data). The main improvement lies in the Arctic, where enhanced Mean Sea Surface (MSS) coverage eliminate artifacts present in CNES-CLS18 and enable a more physically consistent representation of circulation, including the Norwegian Atlantic Front Current along the Mohn Ridge. Globally, CNES-CLS22 remains close to CNES-CLS18, with modest improvements in validation against independent datasets: RMS differences in geostrophic velocities decrease by only ∼ 0.2 %–0.5 % at the global scale and the average variance reduction at the global scale compared to heights derived from profiles is ∼ 0.5 %. Though regional gains are significant in the Arctic and Nordic Seas. HF radar integration in the Mid-Atlantic Bight demonstrates progress but highlights persistent challenges in shelf regions dominated by ageostrophic processes. At very small scales (< 40 km), noise from in situ data may introduce unrealistic kinetic energy, underscoring the need for improved filtering. Overall, CNES-CLS22 consolidates previous advances and provides better representation of key circulation features, but further progress will require enhanced coastal observations and refined processing methods, particularly for high-latitude and shelf areas.

A novel method is developed and applied to identify high-dimensional weather and climate extremes located on the envelope of the data set within its state space. The method is based on formulating and integrating dynamical systems whose attractive set, that is, stable fixed points, is constituted of extreme states residing on the convex hull, namely archetypes. It is shown that these states tend to organize into clusters leading to the concept of ’hull clustering’. The method is applied to tropical monthly sea surface temperature (SST) anomalies over the period 1950–2014 and to the 850-K northern hemispheric winter potential vorticity over the period 1979–2021. The SST archetype clusters yield broadly five clusters reflecting El-Nino Southern Oscillation (ENSO) flavors, representing conventional and Modoki El-Niño and La-Niña phases. The application to the stratospheric polar vortex yields broadly three to five clusters representing different states of the winter polar vortex including splitting and displacement over parts of the northern hemisphere. Implication and usefulness of the developed method in long-range forecasting are also discussed.

The Oleander project, a program to monitor upper ocean currents between Bermuda and New Jersey, started in fall 1992, at about the same time modern satellite altimetry began. This study has two purposes. First, it revisits earlier work that compared Oleander estimates of sea level with altimetry. They agree well with respect to mean surface transport, but the Oleander velocity data exhibit significant temporal variability principally due to a varying Ekman layer. Second, we compare Oleander and altimetry-derived transport estimates with a set of oceanographic products (OSCAR, GLORYS12, GREPV2, ARMOR3D) as well as with transport estimates from hydrography. All agree with respect to surface transport reflecting the dominant influence of altimetry. Upper ocean (0–1,000 m) transports agree poorly except for acoustic Doppler current profiler estimates, and dynamic height. The analysis products give completely different results with respect to total (surface to bottom) transport.

Atlantic Water (AW) inflow plays a central role in Arctic Ocean warming and Atlantification, yet its transport into the Amerasian Basin remains poorly constrained. Using four ocean–sea ice reanalyses, including the new SODA4, we assess the structure, long-term trends, and variability of AW transport. While three of the reanalyses are consistent with prior transport estimates upstream of the Lomonosov Ridge, only SODA4 realistically captures the observed transports into the Amerasian Basin across the Lomonosov Ridge. One of the reanalyses, GLORYS12, significantly underestimates the heat content of Atlantic Water upstream of the Lomonosov Ridge as a result of anomalously high heat loss in the Barents Sea and excess cooling in the Barents Sea. We derive the first 40-year time series of heat transport into the Amerasian Basin. The ensemble mean of this time series shows that AW heat transport has increased by 0.36 TW/year (1984–2016) yielding a 5 ZJ increase in Amerasian Basin heat content. Interannual and interdecadal modes strongly modulate AW heat transport across the Lomonosov Ridge over the 40-year period. The leading mode of variability is associated with the phases of the Arctic Dipole anomaly, an atmospheric climate pattern which modulates the strength of the Siberian and Beaufort high-pressure systems. These results suggest that accurately resolving Atlantic Water structure and transport across the Lomonosov Ridge and in the Barents Sea is essential for accurately characterizing ocean-driven Arctic warming as far east as the Amerasian Basin and Beaufort Gyre.

The influence of subpolar North Atlantic hydrographic conditions on the North Sea is well recognized, yet the precise pathways taken by Atlantic Water to reach its gateway remain uncertain. Using satellite-derived velocity fields, we map the open-ocean routes leading to the North Sea. Our Lagrangian analysis shows that the Rockall Trough serves as the primary route in a time-mean sense, with its dominance becoming particularly evident under anomalously cold subpolar conditions. During anomalously warm periods, however, Atlantic Water is preferentially routed through the Iceland Basin. Empirical orthogonal function analysis of the Lagrangian trajectories reveals a dipole mode of variability between the Rockall Trough and the Iceland Basin, with its first principal component explaining 74% (R = 0.86) of the variance in multi-year ocean heat content variability. These trajectories further demonstrate that this variability is closely linked to the north-south shifts of the North Atlantic Current. Such spatial shifts are likely driven by variations in northward ocean heat transport at the intergyre boundary, with the strength of the subpolar overturning circulation in preceding years potentially playing a critical role. This connection suggests that the conditions in the North Sea as well as the pathways Atlantic Water is advected along to reach it could be predictable several years in advance.

The inflow of warm waters into the Nordic Seas, crucial for sustaining the climate-regulating Atlantic overturning circulation, can be reconstructed from hydrography using a north-south dynamic height gradient across the Greenland-Scotland Ridge. Variations in this influx are herein linked to northward-propagating thermohaline anomalies, initially observed at the intergyre boundary and likely driven by changes in ocean heat transport. As these anomalies reach the eastern subpolar North Atlantic, they modulate the cross-ridge dynamic height difference, thereby influencing both the Atlantic inflow and the Nordic Seas overflows on multi-year to decadal scales. Thus, these thermohaline anomalies play a dynamically active role in modulating the watermass exchanges across the ridge and downstream along the Atlantic Water path, rather than being a simple passive train of signals. This explains why these thermohaline signals are a key source of climate predictability and provides fresh insights into the functioning of the Nordic Seas overturning circulation from observations.

As the number of in situ measurements of ocean currents in the central Arctic Ocean remains very limited, much of our understanding of the Arctic Ocean circulation is based on idealized wind-driven models. In this paper, we make use of the latest available hydrography and Mean Dynamic Topography to study the Arctic Ocean time-mean circulation. Key concepts such as to what degree the flow is steered by bathymetry, equivalent barotropic, and consistent in direction along isobaths are evaluated. Comparing along- and cross-isobath velocities, we find that while the former generally has a larger magnitude, they are locally comparable in many regions. In these regions, the estimated cross-isobath velocities imply vertical velocities that can be orders of magnitude larger than typical surface Ekman pumping velocities. Even so, we find that the surface and bottom flow is generally well-aligned along closed ambient potential vorticity contours spanning local basins as well as the entire Arctic Ocean. In some regions of the central Arctic Ocean, where the hydrographic coverage is relatively sparse, bathymetric alignment is stronger in the surface. Despite this, our results suggest that the circulation is essentially equivalent barotropic and even barotropic in some regions. At some locations where water enters or exits the deep Arctic Basin, along-isobath flow reversals are observed at the surface as well as the bottom. Finally, the direction and magnitude of the bottom flow in regions of anti-cyclonic surface circulation are found to be sensitive to the choice of data set.