Isabela Le Bras

The subpolar North Atlantic is a hotspot of deep ocean ventilation. In other words, it is a region where mixed layer waters are oxygenated and then move into the deep interior North Atlantic where they are sheltered from the atmosphere. In this study, we use two years of densely-spaced moored oxygen observations in the Irminger Sea's western boundary current from 2020 to 2022, as well as Biogeochemical Argo floats, to investigate the processes governing ventilation in this region. We find that oxygen transport variability is dominated by upper Irminger Sea Intermediate Water (upper ISIW), a dense subpolar mode water, rather than denser waters formed by deep convection. While some of these waters are within the boundary current's winter mixed layer and are oxygenated by direct air-sea exchange, the denser portion appears to be oxygenated by along-isopycnal exchange with mixed layer waters just offshore of the boundary current. Spice and oxygen variability are independently suggestive of this lateral isopycnal exchange, highlighting the added value of direct oxygen measurements. While the classical view is that springtime re-stratification is what allows mixed layer oxygen to enter the stratified interior, our results suggest that eddy subduction processes in the wintertime could also play an important role in shaping the ventilation of the deep North Atlantic.

The wind‐driven Beaufort Gyre is stabilized through frictional damping of momentum at topographic boundaries and eddy fluxes. Mesoscale eddies, located predominantly within the Beaufort Gyre's halocline stratification, contain most of its kinetic energy. The vertical distribution of kinetic energy is intrinsically linked to the Beaufort Gyre's stratification, which has changed significantly in recent decades in response to wind forcing. Notably, a prominent deepening of the lower halocline has been observed. We explore how these stratification changes affect dynamic mode structures and energy pathways (i.e., the exchange of energy between barotropic and baroclinic modes) in the Beaufort Gyre over 2003 to 2024. Triplet interaction coefficients, which are derived from the quasi‐geostrophic potential vorticity conservation equations, are used to quantify the efficiency of energy transfer between modes. We find that interactions within the second baroclinic mode (i.e., second mode velocity advecting second mode potential vorticity) are becoming increasingly efficient at keeping energy there, whereas interactions within the first baroclinic mode are becoming slightly less efficient. We couple our analysis of the interaction coefficients with an examination of mooring velocity data to determine how the modal distribution of kinetic energy has evolved over the past two decades. We find increased levels of kinetic energy concentrated in the first two baroclinic modes from 2019 to 2024, suggesting that the water column may be evolving to support a more active halocline eddy field as a balance on wind‐energy input. Our findings indicate that halocline eddies will likely play a greater role in stabilizing the Beaufort Gyre in the future.

Since 2004 the Atlantic meridional overturning circulation (AMOC) has been observed at 26°N by the RAPID project, and in 2014 it was joined by the Overturning in the Sub-Polar North Atlantic Programme (OSNAP) at around 58°N. These two array-based programmes have provided many insights into the AMOC and its variability over a range of timescales, however there is little coherence between the two time series, not helped by OSNAP's relatively short duration.The MEZCAL (Methods for Extending the Horizontal Coverage of the AMOC Latitudinally and retrospectively) project seeks to leverage indirect observations and existing reconstruction methods to create an AMOC framework covering the North Atlantic between these two existing observing systems. The primary reconstruction method was developed at 41°N, a latitude between the subpolar and subtropical gyres, and combines data from Argo floats and satellite altimetry to estimate the AMOC from 2002 onwards.Here we examine how the assumptions of this method hold as we move northwards and southwards from 41°N into the more active gyre regions. We evaluate the resulting AMOC estimates against three 0.25° ocean reanalysis products, the 1° ECCO v4r4 state estimate, and other observation-based AMOC time series, and compare how well each captures ocean processes over varying timescales. We also show at which latitude it becomes important to define the AMOC in density-space rather than depth-space, and how the reconstructions are affected by features like the Gulf Stream extension.The increased spatial coverage these reconstructions provide show AMOC changes and variability at a greater range of latitudes, give insight into the connectivity and coherence between latitudes, and provide a comparison basis for numerical models. The increased temporal coverage increases the likelihood of identifying climate-driven low frequency changes to the Atlantic meridional overturning circulation.

Denmark Strait Overflow Water (DSOW) is formed in the Nordic Seas and is a key water mass transported by the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). DSOW facilitates the transmission of climate signals into the deep ocean, both directly from the Nordic Seas and indirectly from the Irminger Sea via entrainment. Here, we use new hydrographic and oxygen observations from BGC-Argo profiling floats and moored instruments along with a simple streamtube model to investigate along-stream changes and seasonality in DSOW properties. We suggest that the along-stream changes in DSOW oxygen are consistent with mixing with entrained ambient waters. Novel year-round moored oxygen measurements offshore of Cape Farewell, Greenland reveal seasonality in DSOW oxygen properties that is similar to the seasonality in DSOW salinity. Model results suggest that this observed seasonality reflects seasonality in the source waters, rather than seasonality in the properties of the entrained waters or the strength of entrainment. We suggest that future changes in dense water properties in the Nordic Seas could substantially influence overflow properties. This finding may have consequences for future projections of AMOC strength and the ventilation of the deep North Atlantic.

The Atlantic Meridional Overturning Circulation (AMOC) redistributes heat, salt, oxygen, carbon, and nutrients in the Atlantic Ocean, providing warmth and moisture to the climate of northern Europe (Rhines and Häkkinen, 2003), displacing the Intertropical Convergence Zone northward of the equator (Ben-Yami et al., 2024), sequestering anthropogenic carbon in the deep ocean (Brown et al., 2021), and seeding the subpolar North Atlantic with sufficient nutrients to support the annual spring phytoplankton bloom (Williams et al., 2026). While models and proxies of the AMOC have been instrumental in understanding its past and future variability, large inter-model differences necessitate direct observations to provide a baseline from which to compare the models, as well as an unambiguous time series of recent AMOC variability. However, measuring a system as large and complex as the AMOC presents many challenges: spanning thousands of kilometers horizontally and covering full ocean depths, the AMOC is composed of currents that vary on daily to millennial time scales. Recirculating flows that can be an order of magnitude stronger than the AMOC itself further complicate observations.

The deep ocean is a critical yet under-observed component of the Earth’s climate system, acting as a long-term reservoir for heat, carbon, and nutrients. Over recent decades, observations have revealed significant changes in the deep ocean, including warming, freshening, deoxygenation, and acidification. However, regional patterns of change vary substantially, and the mechanisms connecting these changes to surface climate forcing remain poorly understood. To address these gaps, the joint US CLIVAR – OCB – DOOS workshop, titled “Pathways Connecting Climate Changes to the Deep Ocean: Tracing Physical, Biogeochemical, and Ecological Signals from Surface to Deep Sea,” was held in April 2024. The workshop brought together observationalists and modelers from physical, biogeochemical, and ecological disciplines to improve our understanding of the pathways connecting surface climate changes to the deep ocean, as well as linkages across disciplines. The primary objectives were to assess the state of the deep ocean, identify knowledge and observational gaps, and develop recommendations for improved detection and attribution of changes in the deep ocean system.

The Beaufort Gyre circulation system is a central part of the Arctic climate undergoing significant change in all environmental parameters. This paper presents two decades of intensive observations in the Beaufort Gyre and then introduces the publications that comprise this special collection on the changing Beaufort Gyre. Observations spanning 2003–2024 are analyzed to characterize the Beaufort Gyre's evolving freshwater and heat content, which are related to wind forcing modulated by the presence of sea ice. Following a period of sustained freshwater and heat accumulation in the first half of the record, freshwater and heat content have been declining in recent years. Trends indicate that prevailing winds are changing such that they are becoming less effective at spinning up the gyre and accumulating freshwater. This forcing is dominating over a trend to a weakening sea‐ice pack that is less effective at spinning down the gyre. Details on the changing Beaufort Gyre are examined in the publications in this special collection, including studies on its sea ice, upper‐ocean properties, freshwater sources and content, the fundamental dynamical balances of the Beaufort Gyre, water‐mass circulation, and biogeochemistry. These studies bring new understanding of the Beaufort Gyre that is essential for accurately characterizing the Arctic climate system and informing future projections.

The freshwater content of the Arctic Ocean has increased dramatically in the last two decades, particularly in the Beaufort Gyre. However, quantifying the sources of this change is an observational challenge and has historically been limited by methodological differences across studies. Here we derive observation-based freshwater budgets from volume and mass budgets for the Arctic Ocean and the Beaufort Gyre from 2003 to 2020. Our budgets include all sources and sinks (river runoff, precipitation minus evaporation, land ice melt, sea ice export, sea ice melt, and ocean fluxes) as well as volume and mass storage terms measured by satellite. We find that Arctic freshwater changes are dominated by changes in the Beaufort Gyre, and we reconcile this with previous studies that argue for freshwater compensation between the Beaufort Gyre and the rest of the Arctic. We use inverse methods to close the volume and mass budgets within observational uncertainty and link the observed Arctic freshwater changes to the sources and sinks. Our budget analysis demonstrates that small changes to the ocean fluxes (smaller than we can measure) can account for all freshwater storage changes in the Arctic, highlighting the need for more careful accounting and detailed ocean observations in this rapidly changing environment.

Formed by deep convection in the Nordic Seas, cold, oxygen-rich Denmark Strait Overflow Water (DSOW) is the densest water mass in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). As DSOW spreads southward from the Denmark Strait into the Irminger Sea, the overflow water entrains ambient water masses. This entrainment is critical for the transfer of climate signals and biogeochemical tracers from the surface to the deep ocean. Although the impact of entrainment on DSOW properties in the immediate vicinity of the Denmark Strait (< 100 km downstream of the sill) has been studied extensively, the entrainment dynamics contributing to DSOW transformation further downstream in the Irminger Sea and its influence on variability in DSOW properties remain unclear. Here, we use observations from BGC-Argo floats and moorings along with an idealized numerical model of entrainment mixing to investigate the dynamics contributing to the observed DSOW transformation in the western Irminger Sea. We find that DSOW experiences continual warming and deoxygenation as it spreads along the Deep Western Boundary Current in the Irminger Sea. Furthermore, novel dissolved oxygen measurements from moored instruments offshore of Cape Farewell, Greenland reveal seasonality in the oxygen content of DSOW for the first time. Numerical model simulations of supercritical entrainment dynamics predict DSOW properties offshore Cape Farewell that are cooler and more oxygenated than in the observations. This disagreement suggests that sub-critical entrainment influences DSOW transformation in the western Irminger Sea, which we test by accounting for this process in the model. Model sensitivity experiments further suggest that the observed seasonal variation in the oxygen content of DSOW likely originates from the source overflow water upstream of the Denmark Strait. Overall, this work is the first to explore the entrainment dynamics of DSOW in the western Irminger Sea and its influence on the oxygen content of the overflow water, with implications for the response of the AMOC structure and dissolved gas uptake to changes in the climate system.