Caroline A. Katsman

The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in the climate system, especially in the global meridional heat transport. Historical reconstructions indicate that the AMOC has weakened by about 15% since the mid‐20th century. Paleoclimate records, ocean theory, as well as a hierarchy of climate models suggest that the AMOC is a tipping element, sensitive to changes in buoyancy fluxes at the air‐sea interface, and could transition into a substantially weaker or fully collapsed state. Such a transition would have significant climate impacts on decadal to centennial timescales, potentially exceeding societal adaptability. Assessing the probability of such a transition, particularly before 2100, requires evaluating whether a collapsed AMOC state is possible under current forcing conditions. While conceptual and intermediate‐complexity models have long identified collapsed states, comprehensive global climate models have only recently done so. Based on integrating model diagnostics with observations and current AMOC theory this review article critically evaluates the current arguments for and against the evidence of a multi‐stable AMOC regime. We conclude that the evidence base in favor of such a regime has broadened over the last years and that the present‐day AMOC is in such a regime. This article is categorized under: Paleoclimates and Current Trends > Modern Climate Change Paleoclimates and Current Trends > Earth System Behavior Climate Models and Modeling > Knowledge Generation with Models

River plumes and estuaries are inherently coupled, yet their dynamics are often considered separately. Wind conditions, such as wind speed and direction, can influence both. Using a three-dimensional hydrodynamic model of the Rhine-Meuse Delta, this study investigates whether and how wind-induced modifications to the plume influence estuarine dynamics. Three three-day periods with similar river discharge and wind speed but different wind directions—upwelling, downwelling, and onshore (here, equivalent to up-estuary)—are examined. Results reveal that wind direction modifies the plume’s stratification, extent, and tidal plume fronts, fundamentally altering the estuarine boundary conditions. Upwelling winds stretch and stratify the plume, and associated onshore-directed bottom currents enhance the exchange flow. Downwelling winds thicken the plume and induce offshore-directed bottom currents that weaken the exchange flow. Strong downwelling winds can also indirectly suppress and even completely inhibit cross-shore straining on ebb by prolonging the alongshore flood. In contrast, onshore winds modify the cross-shore velocity structure throughout the tidal cycle, suppressing cross-shore straining directly. Even with similar offshore water levels and wind speeds, different wind directions result in markedly different estuarine responses: upwelling and downwelling winds are associated with reduced salt intrusion, whereas onshore winds lead to increased salt intrusion. These observed differences are attributed to wind-induced modifications of the plume structure and dynamics, underscoring the need to view river plumes and estuaries as a coupled system.

The Atlantic Meridional Overturning Circulation (AMOC) transports vast amounts of heat to high latitudes, and is largely responsible for Western Europe’s relatively mild climate. Climate models project the AMOC will weaken substantially over the 21st century, which impacts weather, climate, sea level and the oceanic carbon cycle. In many studies, the AMOC state is described in a condensed two-dimensional view or even by means of a single metric, which leaves many aspects of its complex 3D-structure underexposed. By revealing the sharp contrast in overturning strength between the western and eastern subpolar gyre (SPG), the recent OSNAP observations emphasized the importance of considering the AMOC in 3D.In this study, we explore this further by analyzing the characteristics of the overturning in density space in the North Atlantic SPG on a regional scale, and over time periods ranging from seasons to decades. For this, we use model data from the high-resolution GLORYS12 reanalysis, spanning the period 1993-2020. Following the approach applied in OSNAP, the overturning is assessed from alongstream changes in boundary current transport in specific density classes. This analysis is performed for the entire SPG, for its major basins (Iceland Basin, Irminger Sea, and Labrador Sea) and for smaller segments along the boundary currents, thus providing detailed insights in variations of the overturning varies along the entire SPG boundary.The mean overturning from GLORYS12 for 1993-2020 is 23.8 Sv, distributed as 41%, 29%, and 30% for the Iceland Basin, Irminger Sea, and Labrador Sea respectively, and peaking at increasingly higher densities in alongstream direction. Within each basin, a pronounced seasonal cycle can be identified, with the maximum overturning occurring in March and the minimum in September. Over the entire reanalysis period, the overturning strength in both the Iceland Basin and Irminger Sea exhibits a weak decreasing trend, whereas the Labrador Sea displays a weak increasing trend. The subdivision in shorter segments reveals large spatial differences in overturning, both with regard to its overall strength and its distribution over density classes. However, these outcomes are less robust than the analyses on the scale of the major basins, as the flow is highly variable and numerical uncertainties associated with offline overturning calculations become more prominent.Further research is needed to properly interpret these regional variations, and thereby improve our understanding of the AMOC dynamics and its sensitivity to changing oceanic and atmospheric forcing conditions. Linking them to local processes known to govern the overturning (i.e., formation of dense waters in the interior of marginal seas and their export, formation of dense waters within the boundary current system itself and the exchange of waters via overflows) seems a viable route.

Sea level variations in the coastal zone can differ significantly from those in the open ocean and can be highly spatiotemporally coherent in the alongshore direction. Yet, where and how coastal sea levels exhibit variations that emerge as persistent and recurrent patterns along the world's coastlines remain poorly understood. Here, we use a Bayesian mixture model to identify large‐scale patterns of coherent modes of monthly coastal sea level variations from coastal altimetry and tide gauge data. We determine nine clusters of coherent coastal sea level variability that explain a majority of the monthly variance measured by tide gauges (1993–2020). The analysis of along track altimetry data enables us to detect several additional clusters in ungauged regions, such as the Indian Ocean or around the South Atlantic basin, which have so far been poorly described. Although some clusters (e.g., at the eastern boundary of the Pacific, the western tropical Pacific, and the marginal and semi‐enclosed seas) are highly correlated with climate modes, other clusters share very little variability with the considered climate modes at the monthly timescale. Knowledge of these coherent regions thus motivates and enables further investigations on the impacts of local and remote forcing on coastal sea level variability, and the extent to which coastal sea level variability is decoupled from the adjacent deep ocean.

Starting in 2012, the eastern subpolar North Atlantic experienced the strongest surface freshening in the past 120 years. It is yet unknown whether this salinity anomaly propagated downward into the water column and affected the properties of the boundary currents of the subpolar gyre, which could slow down the overturning. Here, we investigate the imprint of this salinity anomaly on the warm and saline Irminger Current (IC) in the decade thereafter. Using daily mooring data from the IC covering the period 2014–2022 combined with hydrographic sections across the adjacent basins from 1990, the evolving signal of the salinity anomaly over the water column and its imprint on the transport variability is studied. We find that due to the salinity anomaly, the northward freshwater transport of the IC increased by 10 mSv in summer 2016 compared to summer 2015. In 2018, the salinity anomaly covered the water column down to 1,500 m depth. Hydrographic sections across the basin showed that this recent freshening signal spread across the Irminger Sea. Overall, the freshwater transport of the IC increased by a factor of three between 2014–2015 and 2021–2022. The associated density decrease over the upper 1,500 m of the water column resulted in an increase in the northward transport of waters lighter than σ₀ = 27.55 kg m⁻³ from 1.7 to 4.2 Sv. This change in northward IC transport by density class may impact the characteristics of the overturning in the Northeastern Atlantic, its strength and the density at which it peaks.

The Irminger Current (IC) brings relatively warm and saline waters northward in the North Atlantic subpolar gyre, contributing to the upper limb of the Atlantic Meridional Overturning Circulation. The IC is a two‐core current with surface‐intensified velocities. The eastern core, closest to the Reykjanes Ridge, is warmer and more saline than the western core. To investigate the source waters of the two IC cores, using a 1/10° ocean model, we track Lagrangian particles released in the IC at OSNAP East (∼59.5°N) in the upper 1,000 m backward in time for one model year. Over a 1‐year time scale, nearly all particles are sourced from nearby regions of the Irminger Sea and Iceland Basin. Those seeded in the western IC core mostly originate from the Irminger Sea (83%), while those in its eastern core mostly originate from the Iceland Basin (69%). Iceland Basin water feeding the IC predominantly crosses the Reykjanes Ridge near 57°N and 59°N. Generally, particles from the Irminger Sea are colder and fresher than particles from the Iceland Basin. The fraction of waters from the Iceland Basin and the Irminger Sea varies from month to month. So, to explain monthly variations of the two IC cores at the OSNAP East line, changes in hydrographic properties in both basins as well as their contributions must be considered. Based on this model study, we interpret the Irminger Sea circulation as a basin‐wide recirculation with an increasing contribution of Iceland Basin waters toward the ridge which is subject to monthly variations.

Variability in ocean currents, temperature and salinity drive dynamic sea level (DSL) variability on the Northwest European Shelf (NWES). It is dominated by mass variations, with steric signals relatively small. A mechanistic explanation of how ocean dynamics relates to the mass component of NWES sea level variability is required. We use regional ocean model experiments to isolate sources of variability and then investigate the effect on monthly to‐interannual DSL variability together with the simulated momentum budgets along the shelfbreak. Regional (local) wind and non‐regional (remote) forcing are important on the NWES. For the local wind forcing, the net mass flux onto the shelf, which drives a shelf‐mean mode of DSL variability, results from a combination of surface Ekman, bottom Ekman and geostrophic flows and explains 73% of the variance in transport across the shelf‐edge. The geostrophic flow is closely related to wind stress with a flow about half that of surface Ekman transport but in the opposite direction. For the remotely forced mass‐flux across the shelf‐edge, the geostrophic component explains 62% of the variance and bottom friction plays an important indirect role. The remotely forced variability, while relatively spatially uniform, is more important for explaining DSL variance over the western NWES. This mode of variability is sensitive to signals propagating northward via a thin strip of the southern boundary near the Portuguese coast, consistent with coastal trapped wave signals. It also appears to drive steric height in the Bay of Biscay, which is related to DSL on the shelf.

The North Atlantic subpolar gyre experienced strong freshening in recent years starting around 2012. Here, we investigate the imprint of this freshwater anomaly on the water column hydrography and transport variability of the Irminger Current (IC). The IC transports warm and saline waters northward along the western flank of the Reykjanes Ridge as part of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). To investigate if the salinity anomaly spread and propagated downward, we used high-resolution mooring data from the IC covering the period 2014 – 2022 combined with hydrographic sections from the Irminger Sea and Iceland Basin. We found that the IC experienced a strong freshening starting in summer 2016. By 2018, this salinity anomaly covers the whole water column down to 1500 m depth and freshened the IC until 2022. In 2022, the IC was at its freshest state observed since the early 1990’s. Hydrographic sections across the adjacent basins showed that the recent freshening spread across the Irminger Sea and was also comparable to its fresh state in the early 1990’s. The salinity anomaly increased the freshwater transport of the IC by a factor of three from 2014-2015 to 2021-2022 and caused a decrease in density over much of the water column. This resulted in an increase in the transport of waters lighter than 27.55 kg m-3, potentially strengthening the upper limb of the AMOC.

A document by Nora Fried. Click on the document to view its contents.