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Vertical exchange of heat and carbon in the ocean regulates Earth's climate. Convection, a driver of near-surface exchange, occurs when dense water overlies light water. In a 1957 study, N. Fofonoff pointed out that when lighter overlying cold-fresh water mixes with denser underlying warm-salty water, the mixture can become denser than the underlying water due to a nonlinear process known as cabbeling. He suggested that such profiles, despite being gravitationally stable, could be classed as being unstable to cabbeling. Fofonoff hypothesized that, by mixing away such profiles, cabbeling may be shaping the thermohaline structure of polar oceans. We investigate this hypothesis here. In a one-dimensional model, we find that convective mixing occurs in temperature inverted profiles that are unstable to cabbeling even when they are initially gravitationally stable. In data from an observationally constrained global circulation model, we find profiles with a temperature inversion larger than −0.5°C are unstable to cabbeling less than 0.02% of the time, and in high-quality in situ observations, they are unstable less than 12% of the time. We find that due to cabbeling, larger temperature inversions, which should weaken stratification, make profiles more stable. Our results suggest that cabbeling limits the stability behavior of temperature inverted profiles and influences the thermohaline structure in parts of the ocean where cold-fresh water overlays warm-salty water. [ABSTRACT FROM AUTHOR]

The formation of dense water in the Aegean Sea is important as it affects the deep circulation and the hydrography of the Eastern Mediterranean Sea. In this study, the variability of dense water formation is investigated in relation to forcing mechanisms from 1947 to 2023 in the subbasins of the Aegean Sea, utilising in situ observations from various sources, which have been analysed in combination with satellite altimetry and reanalyses products. The analysis reveals that the Aegean Sea has been in a state of increased dense water formation since 2017 due to the combination of increased surface buoyancy loss and reduced Black Sea water inflow. Extremely high salinity has been recorded in the intermediate layers of the Aegean Sea since 2019. The anticyclonic circulation of the North Ionian gyre during 2017 and 2018 probably also contributed to the rapid transport of highly saline waters in the intermediate and, through dense water formation, the deep layers of the Aegean Sea in 2019. Until 2022, the dense waters formed during the peak of the Eastern Mediterranean Transient still occupied the bottom layers of some deep subbasins of the North and South Aegean; however, the 29.4 kg m−3 isopycnal in the North Aegean and the 29.3 kg m−3 isopycnal in the Southeastern Aegean have gradually deepened by 800 m, permitting the waters forming in the last ten years in the Aegean Sea to settle at ever greater depths. Temperature controls the density variability of the Cretan intermediate water up to the decadal time scale. Increased data availability since 2010 was sufficient to clarify that intrusions of dense water from the North–Central Aegean Sea contributed to the erosion of the Eastern Mediterranean transitional waters in the South Aegean Sea after 2017, as well as to raising the intermediate water masses of the South Aegean to shallower depths. The erosion of the transitional Mediterranean waters in the South Aegean Sea between 1947 and 1955 and 1973 and 1980 coincided with increased dense water formation in the North–Central Aegean Sea. During the peak of the Eastern Mediterranean Transient, the North Ionian circulation, the Black Sea water inflow, the Atlantic Multidecadal Oscillation, and the surface buoyancy fluxes favoured dense water formation in the Aegean Sea.

Decadal thermohaline anomalies carried northward by the North Atlantic Current are an important source of predictability in the North Atlantic region. Here, we investigate whether these thermohaline anomalies influence surface-forced water mass transformation (SFWMT) in the eastern subpolar gyre using the reanalyses EN4.2.2 for the ocean and the ERA5 for the atmosphere. In addition, we follow the propagation of thermohaline anomalies along two paths: in the subpolar North Atlantic and the Norwegian Sea. We use observation-based datasets (HadISST, EN4.2.2, and Ishii) between 1947 and 2021 and apply complex empirical orthogonal functions. Our results show that when a warm anomaly enters the eastern subpolar gyre, more SFWMT occurs in light-density classes (27.0–27.2 kg m−3). In contrast, when a cold anomaly enters the eastern subpolar gyre, more SFWMT occurs in denser classes (27.4–27.5 kg m−3). Following the thermohaline anomalies in both paths, we find alternating warm–salty and cold–fresh subsurface anomalies, repeating throughout the 74-yr-long record with four warm–salty and cold–fresh periods after the 1950s. The cold–fresh anomaly periods happen simultaneously with the Great Salinity Anomaly events. Moreover, the propagation of thermohaline anomalies is faster in the subpolar North Atlantic (SPNA) than in the Norwegian Sea, especially for temperature anomalies. These findings might have implications for our understanding of the decadal variability of the lower limb of the Atlantic meridional overturning circulation and predictability in the North Atlantic region. Significance Statement: Anomalously warm–salty or cold–fresh water, carried by the North Atlantic Current toward the Arctic, is a source of climate predictability. In this study, we investigate 1) how these ocean anomalies influence the transformation of water masses in the eastern subpolar gyre and 2) their subsequent propagation poleward and northwestward. The key findings reveal that anomalously warm waters entering the eastern subpolar gyre lead to increased transformation in lighter water masses, while cold anomalies affect denser water masses. These anomalies propagate more than 2 times faster toward the Greenland coast (northwestward) than toward the Arctic (poleward). Our findings contribute to enhancing the understanding of decadal predictability in the northern North Atlantic, including its influence on the Atlantic meridional overturning circulation. [ABSTRACT FROM AUTHOR]

The formation of dense water in the Aegean Sea is important as it affects the deep circulation and the hydrography of the Eastern Mediterranean Sea. In this study, the variability of dense water formation is investigated in relation to forcing mechanisms from 1947 to 2023 in the subbasins of the Aegean Sea, utilising in situ observations from various sources, which have been analysed in combination with satellite altimetry and reanalyses products. The analysis reveals that the Aegean Sea has been in a state of increased dense water formation since 2017 due to the combination of increased surface buoyancy loss and reduced Black Sea water inflow. Extremely high salinity has been recorded in the intermediate layers of the Aegean Sea since 2019. The anticyclonic circulation of the North Ionian gyre during 2017 and 2018 probably also contributed to the rapid transport of highly saline waters in the intermediate and, through dense water formation, the deep layers of the Aegean Sea in 2019. Until 2022, the dense waters formed during the peak of the Eastern Mediterranean Transient still occupied the bottom layers of some deep subbasins of the North and South Aegean; however, the 29.4   k g   m − 3 isopycnal in the North Aegean and the 29.3   k g   m − 3 isopycnal in the Southeastern Aegean have gradually deepened by 800 m, permitting the waters forming in the last ten years in the Aegean Sea to settle at ever greater depths. Temperature controls the density variability of the Cretan intermediate water up to the decadal time scale. Increased data availability since 2010 was sufficient to clarify that intrusions of dense water from the North–Central Aegean Sea contributed to the erosion of the Eastern Mediterranean transitional waters in the South Aegean Sea after 2017, as well as to raising the intermediate water masses of the South Aegean to shallower depths. The erosion of the transitional Mediterranean waters in the South Aegean Sea between 1947 and 1955 and 1973 and 1980 coincided with increased dense water formation in the North–Central Aegean Sea. During the peak of the Eastern Mediterranean Transient, the North Ionian circulation, the Black Sea water inflow, the Atlantic Multidecadal Oscillation, and the surface buoyancy fluxes favoured dense water formation in the Aegean Sea. [ABSTRACT FROM AUTHOR]

A large part of the variability in the Atlantic meridional overturning circulation (AMOC) and thus uncertainty in its estimates on interannual time scales comes from atmospheric synoptic eddies and mesoscale processes. In this study, a suite of experiments with a 1/12° regional configuration of the MITgcm is performed where low-pass filtering is applied to surface wind forcing to investigate the impact of subsynoptic (<2 days) and synoptic (2–10 days) atmospheric processes on the ocean circulation. Changes in the wind magnitude and hence the wind energy input in the region have a significant effect on the strength of the overturning; once this is accounted for, the magnitude of the overturning in all sensitivity experiments is very similar to that of the control run. Synoptic and subsynoptic variability in atmospheric winds reduce the surface heat loss in the Labrador Sea, resulting in anomalous advection of warm and salty waters into the Irminger Sea and lower upper-ocean densities in the eastern subpolar North Atlantic. Other effects of high-frequency variability in surface winds on the AMOC are associated with changes in Ekman convergence in the midlatitudes. Synoptic and subsynoptic winds also impact the strength of the boundary currents and density structure in the subpolar North Atlantic. In the Labrador Sea, the overturning strength is more sensitive to the changes in density structure, whereas in the eastern subpolar North Atlantic, the role of density is comparable to that of the strength of the East Greenland Current. Significance Statement: A key issue in understanding how well the Atlantic meridional overturning circulation is simulated in climate models is determining the impact of synoptic (2–10 days) and subsynoptic (shorter) wind variability on ocean circulation. We find that the greatest impact of wind changes on the strength of the overturning is through changes in energy input from winds to the ocean. Variations in winds have a more modest impact via changes in heat loss over the Labrador Sea, alongside changes in wind-driven surface currents. This study highlights the importance of accurately representing the density in the Labrador Sea, and both the strength and density structure of the East Greenland Current, for the correct representation of overturning circulation in climate models. [ABSTRACT FROM AUTHOR]

Using a CESM1 control simulation, we conduct a follow-up study to advance our earlier theoretical research on the multicentennial oscillation (MCO) of the Atlantic meridional overturning circulation (AMOC). The modeled AMOC MCO primarily arises from internal oceanic processes in the North Atlantic, potentially representing a North Atlantic Ocean–originated mode of AMOC multicentennial variability (MCV) in reality. Specifically, this AMOC MCO is mainly driven by salinity variation in the subpolar upper North Atlantic, which dominates local density variation. Salinity anomaly in the subpolar upper ocean is enhanced by the well-known positive salinity advection feedback that is realized through anomalous advection in the subtropical to subpolar upper ocean. Meanwhile, mean advection moves salinity anomaly in the subtropical intermediate ocean northward, weakening the subpolar upper salinity anomaly and leading to its phase change. The salinity anomalies have a clear three-dimensional life cycle around the North Atlantic. The mechanism and time scale of the modeled AMOC MCO are consistent with our earlier theoretical studies. In the theoretical model, artificially deactivating either the anomalous or mean advection in the AMOC upper branch prevents it from exhibiting AMOC MCO, underscoring the indispensability of both the anomalous and mean advections in this North Atlantic Ocean–originated AMOC MCO. In our coupled model simulation, the South Atlantic and Southern Oceans do not exhibit variabilities synchronous with the AMOC MCO; the Arctic Ocean's contribution to the subpolar upper salinity anomaly is much weaker than the North Atlantic. Hence, this North Atlantic Ocean–originated AMOC MCO is distinct from the previously proposed Southern Ocean–originated and Arctic Ocean–originated AMOC MCOs. [ABSTRACT FROM AUTHOR]

Van Vliet-Lanoë, B.; Andrieu, V.; Cliquet, D.; Authemayou, C.; Le Roy, P., and Renouf, J.C., 2024. Sea ice and the Middle to Recent Quaternary: Marine highstands in Western Europe. Journal of Coastal Research, 40(3), 445–473. Charlotte (North Carolina), ISSN 0749-0208. This study analyzes the raised beaches of less than 700 kilo annum (ka) above present sea level along the English Channel, which are associated with negative anomalies in δ18O and evidence for anchor ice. The dated coastal events were compared in terms of recent storminess, sea-ice extents, and permafrost with regional paleoclimates. In western France, late Eemian and early interstadial highstands (HSs) indicate an extended annual sea-ice duration during early transgressions, with orbitally forced cold winters and a still-cool Southern Ocean. During regressions, storminess developed with a still-warm intertropical ocean but Arctic cooling. The Northern Atlantic ice sheets starved during early stadials, with aridity because of the seasonal sea-ice extent. (1) A delayed sea-level drop resulted from some ice storage in permafrost and sea ice, which caused depleted δ18O seawater (δ18Osw) values and limited ice caps. (2) The large ice sheets along the Northern Atlantic spread progressively and were delayed until marine isotope stage 4 (MIS 4). Their development resulted from summer precipitation rise caused by the cyclic restoration of thermohaline circulation during Dansgaard–Oechger (DO) events alternating with multiyear sea-ice cover. This led to a more unstable North Atlantic Current, which was repeatedly destabilized. (3) Early glacial storage partly or fully vanished during early DO or interstadial HSs, with climate conditions close to those of the present day. [ABSTRACT FROM AUTHOR]

Recent news headlines suggest the Gulf Stream current could shut down in just a few years--or perhaps a few decades--bringing about a catastrophic change in global climate. Will the Gulf […]

The analysis conducted in this study examines the relationships between changes in cloud cover and the occurrence of cloud genera in Krakow, Poland, and the variations in the thermal state of the North Atlantic (NA) from 1951 to 2020. The existence of areas where annual sea surface temperature (SST) changes exhibit statistically significant correlations with the annual frequency of specific cloud genera was observed. These relationships vary in space: An increase in SST over the NA, particularly in the western and central regions of the subtropical NA, leads to a decrease in the frequency of stratiform clouds at all levels (Cs, As, Ns and St) and an increase in the frequency of convective clouds (Cu, Sc). An attempt to explain this phenomenon demonstrates that there is a correlation between the frequency of specific cloud genera and the variability of meridional SST gradients, as well as changes in the intensity of the thermohaline circulation in the NA (NA THC), which control the variability of mid‐tropospheric circulation (500 hPa). During positive phases of NA THC (the "warm" state of the NA), zonal circulation prevails over Europe, leading to an increase in the height of h500, an increase in the frequency of anticyclonic weather and a decrease in the frequency of cyclonic weather, including a significant proportion of frontal weather systems. Consequently, there is a reduction in frequency of stratiform clouds and an increase in the occurrence of vertically developed convective clouds, thereby increasing the possibility of observing middle‐level (Ac) and high‐level (Ci, Cc) clouds. In negative phases of NA THC (the "cool" state of the NA), the situation is reversed, with meridional circulation dominating over Europe, h500 lowering, an increase in the frequency of cyclonic systems with fronts, and an increase in the frequency of stratiform clouds. This results in decreased sunshine duration and a reduction in the amount of solar energy reaching the Earth's surface. [ABSTRACT FROM AUTHOR]

Ocean circulation is a key channel and mechanism for the budget, allocation, balance, maintenance, and change of ocean material and energy. From the global ocean perspective, based on the current understanding of multi-scale processes and trend evolution of ocean circulation dynamics, this paper focuses on the overview of basin-scale three-dimensional linkage mechanism, interoceanic exchange and synergy, meridional transport changes and related marine climate and environmental ecological effects of ocean circulation under global climate change, and puts forward heuristic suggestions according to the current research status and deficiencies. The results show that under the regulation of global consistent warming pattern and process, due to the evolution and stress of driving forces, the changes of ocean circulation play an important role in regulating the inter-annual and inter-decadal climate and environment variations and can generate outstanding environmental ecological effects and serious disaster risks in subtropical mid to high latitude regions. It is suggested to increase the independent research and development of refined, special and new observation instruments, enhance the leadership and participation of mid to high latitude comprehensive marine investigation of multi-scale dynamic processes of deeper ocean circulation through international cooperation, highlight the integration and cross-disciplinary research efforts, and effectively improve the capability for forecasts, projection, assessment and prevention of the comprehensive risks of climate change affected by ocean circulation variability and the dynamic environmental and ecological disasters, with the aim of providing necessary hydrodynamic reference for marine energy development, climate change adaptation and disaster risk governance in the marine field.

During the Quaternary, the Eastern Mediterranean Sea (EMS) experienced cyclical events of stagnation driven by natural climate variability. The resulting deoxygenation left well-preserved evidence in the sedimentary record as organic carbon-rich deposits referred to as sapropels. Although drastic modifications in the degree of dense-water formation over the EMS shelves exerted first-order control on the deoxygenation, most of the focus has been traditionally placed on the deep EMS. To provide a shallow-water perspective, here we investigated the sapropel S5 in the Adriatic shelf (borehole PRAD1-2) deposited during MIS5e (129-116 ka). This archive is strategically located in a region where the Northern Adriatic Dense Water (NAdDW) interacts with the seabed before cascading across the continental slope. We used Zr/Rb and MgO/Al2O3 to assess bottom current energy and north-to-south sediment transport dynamics, both regulated by the changes in NAdDW production intensity. In addition, we used stable isotopes (δ13C and δ18O) of foraminifera, redox sensitive elements (U, Mo and Sb), foraminifera assemblages as well as alkenones to reconstruct the paleo-environmental conditions during the S5 formation.Our study provides an unprecedented reconstruction of the physical forcing controlling the deoxygenation during the S5 formation. Results reveal that the shutdown of the NAdDW occurred in a few centuries (0.67 ± 0.22 kyrs), when freshening of surface waters combined with warming of winter temperatures mutually hampered the dense water formation. A few centuries after the NAdDW shutdown, the Adriatic shelf experienced euxinic waters for about 2 kyrs followed by a progressive reoxygenation that lasted 4 kyrs. We explain this second phase as a general recovery driven by increased surface salinity over the EMS combined with winter cooling. This favoured surface water mixing without, however, producing dense water in the Northern Adriatic and thus collectively the interruption of the dense water production lasted for 6 kyrs since the onset of MIS5e. Overall, our finding highlights that the thermohaline forcing responded to climate change much quicker than inferred by earlier studies that suggested instead a millennial-scale prelude necessary to develop stagnation. In addition, our results provide solid evidence about the large-scale impact of the deoxygenation during S5 that is capable of invading the continental shelf. Comparison with the latest regional models illustrates how none of the future simulations covering different climate change scenarios reproduces an event over the EMS margins comparable with what described in this study.

In this paper we present the analytical derivation of a local water mass transformation (WMT) framework for an individual water column. We exactly formulate the mapping of the governing equations from geopotential coordinates to an arbitrary tracer space. Unique definitions for the local effective vertical diasurface fluxes are given. In tracer space we derive new relations between the local diatracer fluxes and the mixing per tracer class. The key relation between the effective vertical diatracer velocity and the mixing per tracer class directly formulates how the overturning circulation is linked to local tracer variance dissipation. Horizontal integration of the governing equations in tracer space and the relations between the diatracer quantities finally recovers the well-known integral WMT formulations. [ABSTRACT FROM AUTHOR]

Biomass burning aerosol (BBA) emissions in the Coupled Model Intercomparison Project phase 6 (CMIP6) historical forcing fields have enhanced temporal variability during the years 1997–2014 compared to earlier periods. Recent studies document that the corresponding inhomogeneous shortwave forcing over this period can cause changes in clouds, permafrost, and soil moisture, which contribute to a net terrestrial Northern Hemisphere warming relative to earlier periods. Here, we investigate the ocean response to the hemispherically asymmetric warming, using a 100-member ensemble of the Community Earth System Model version 2 Large Ensemble forced by two different BBA emissions (CMIP6 default and temporally smoothed over 1990–2020). Differences between the two subensemble means show that ocean temperature anomalies occur during periods of high BBA variability and subsequently persist over multiple decades. In the North Atlantic, surface warming is efficiently compensated for by decreased northward oceanic heat transport due to a slowdown of the Atlantic meridional overturning circulation. In the North Pacific, surface warming is compensated for by an anomalous cross-equatorial cell (CEC) that reduces northward oceanic heat transport. The heat that converges in the South Pacific through the anomalous CEC is shunted into the subsurface and contributes to formation of long-lasting ocean temperature anomalies. The anomalous CEC is maintained through latitude-dependent contributions from narrow western boundary currents and basinwide near-surface Ekman transport. These results indicate that interannual variability in forcing fields may significantly change the background climate state over long time scales, presenting a potential uncertainty in CMIP6-class climate projections forced without interannual variability. [ABSTRACT FROM AUTHOR]

Marine dissolved organic matter (DOM) is a major global carbon pool, consisting of thousands of compounds with distinct lifetimes. While marine DOM persists for millennia, its molecular and isotopic composition imply that it is dynamic on shorter timescales. To determine the extent to which DOM deviates from conservative water mass mixing, we conducted a two‐endmember mixing analysis on dissolved organic carbon (DOC) concentration and DOM molecular composition in the Atlantic and Pacific. Endmembers were the deep water masses near their formation sites. For DOM composition, we considered 6118 molecular formulae (MF) identified via Fourier‐transform ion cyclotron resonance mass spectrometry in solid‐phase extracts (SPE) of 837 samples. Bulk DOC and SPE‐DOC concentrations behaved conservatively in both basins and ≥70% of the MF (14–20 μM SPE‐DOC) mixed conservatively. However, a small fraction (10%–20%) of the MF (<3 μM SPE‐DOC) were added or removed during mixing. These MF were more reduced and oxidized, respectively, than the conservative fraction. There were also MF absent from the endmembers; these accounted for ≤1 μM of SPE‐DOC and positively correlated with DOM lability. Based on their distribution across the two basins, we conclude that the conserved MF are formed in the surface subtropical ocean and modified in overturning areas. In the deep ocean, however, these MF are solely controlled by mixing. This finding contrasts with the current paradigm of slow, continuous degradation of recalcitrant DOM in the deep ocean. Our analysis illustrates the importance of the surface ocean in controlling DOM cycling in the deep. Key Points: 70% of the molecular composition of dissolved organic matter (DOM) follows two‐endmember water mass mixing in the deep Atlantic and PacificThere are small fractions of DOM with distinct chemical compositions that are added and removed relative to mixingMolecular formulae that behave conservatively in the deep are formed in the surface ocean and modified in overturning zones [ABSTRACT FROM AUTHOR]

The North Atlantic gyre experiences both a significant temperature rise at high latitudes and a considerable weakening of the geostrophic component of the Gulf Stream, which is reflected by the 64-year fundamental gyral Rossby wave (GRW). This singular behavior compared to the South Atlantic and South Indian Ocean gyres highlights a feedback loop of Arctic ice sheet melting on mid-latitude Atlantic Ocean temperature. The warming of the northern oceanic gyre at high latitudes due to the retreat of Arctic ice sheet via the Labrador Current decreases the thermal gradient between the high and low latitudes of the north Atlantic gyre. This results in a weakening of the geostrophic forces at the basin scale and a reduction in the amplitude of the GRWs. Reducing the amplitude of the variation of the upward and downward movement of the pycnocline modifies air–sea interactions, weakening vertical mixing as well as the evaporation processes and the departure of latent heat when the pycnocline rises. The resulting thermal anomaly stretching along the Gulf Stream from where it leaves the American continent is partly transferred to the Arctic sea ice via the drift current and thermohaline circulation, which contributes to the retreat of the ice sheet, and the closing of the feedback loop. The 64-year-period GRW should disappear around 2050 if its damping continues linearly, favoring an increasingly rapid warming of the ocean at mid-latitudes. These interactions are less acute in the southern hemisphere due to the circumpolar current. [ABSTRACT FROM AUTHOR]

As global temperatures rise due to increasing concentrations of greenhouse gasses in the atmosphere, melting in Antarctica and Greenland is accelerating, initiating feedback systems that will result in record losses in sea and land ice by the end of this century if left unabated. The freshening of polar waters changes the production of deepwater masses such as the Antarctic Bottom Water and the North Atlantic Deep Water, which rely on salinity and temperature gradients to form. Thermohaline Circulation, dependent on these gradients, transports heat, nutrients, and carbon around the globe, but is disrupted as freshwater forcing from ice melt reduces the capacity for overturning of the Atlantic Meridional Overturning Circulation (ΑMOC). As the ΑMOC weakens, it changes the trajectory of the Gulf Stream, which produces a southerly shift to the Intertropical Convergence Zone (ITCZ), altering the temperature of the lower atmosphere and thus precipitation over most of the Northern Hemisphere. Multidecadal and multiannual atmospheric circulation patterns are also affected, increasing the intensity and prevalence of storms in some regions, and drought conditions in others. Such climatic changes can have implications for food security through variations in land and water availability for agriculture, and through changes to ocean circulation that alter upwelling and marine primary productivity. Many countries that would be affected are the least developed and are already, or most at risk of becoming, food insecure. [ABSTRACT FROM AUTHOR]

Paleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean.

Simulating and reproducing the past Atlantic meridional overturning circulation (AMOC) with comprehensive climate models are essential to understanding past climate changes as well as to testing the ability of the models in simulating different climates. At the Last Glacial Maximum (LGM), reconstructions show a shoaling of the AMOC compared to modern climate. However, almost all state-of-the-art climate models simulate a deeper LGM AMOC. Here, it is shown that this paleodata–model discrepancy is partly related to the climate model biases in modern sea surface temperatures (SST) over the Southern Ocean (70°–45°S). Analysis of model outputs from three phases of the Paleoclimate Model Intercomparison Project shows that models with warm Southern Ocean SST biases tend to simulate a deepening of the LGM AMOC, while the opposite is observed in models with cold SST biases. As a result, a positive correlation of 0.41 is found between SST biases and LGM AMOC depth anomalies. Using sensitivity experiments with a climate model, we show, as an example, that changes in parameters associated with the fraction of cloud thermodynamic phase in a climate model reduce the biases in the warm SST over the modern Southern Ocean. The less biased versions of the model then reproduce a colder Southern Ocean at the LGM, which increases formation of Antarctic Bottom Water and causes shoaling of the LGM AMOC, without affecting the LGM climate in other regions. The results highlight the importance of sea surface conditions and clouds over the Southern Ocean in simulating past and future global climates. Significance Statement: To test the ability of comprehensive climate models, simulations of the Last Glacial Maximum (LGM) have been conducted. However, most models simulated a deeper Atlantic meridional overturning circulation (AMOC) in the LGM, which contradicts paleodata suggesting a shallower AMOC. Here, using multimodel analysis and sensitivity experiments with a climate model, we show that this paleodata–model discrepancy is partly related to model biases in the modern Southern Ocean. Improvements in Southern Ocean surface temperatures and clouds reproduce a colder climate over the Southern Ocean at the LGM, which causes an intense shoaling of the AMOC due to increased formation of Antarctic Bottom Water. These results demonstrate the important effect of model biases over the Southern Ocean on simulating past climates. [ABSTRACT FROM AUTHOR]

The paper presents effects of changes in climatic elements in the Greater Poland region (Poland), their causes and consequences for shaping the water balance of this area, copying with the most severe water deficit in Poland. The study period covers 70 years (1951–2020). The research identified an abrupt and significant change in the climate of Greater Poland, which started between 1987 and 1989, concerning not only air temperature but also a wider spectrum of climatic elements. The change in the state of the climate, which covers the entire Atlantic-Eurasian circulation sector, results from a sudden change in the macro-circulation conditions in the middle troposphere (500 hPa). The reason for the change in the mid-tropospheric circulation is an equally abrupt and simultaneous change in the intensity of the ocean heat transport by the North Atlantic thermohaline circulation (NA THC). Climate change observed in Greater Poland is manifested in an increase in sunshine duration (SD) and air temperature, a decrease in relative humidity, a change in the cloud structure, and an increase in the degree of sky coverage. The main, physical reason for an increase in air temperature is a rapid and strong increase in SD in the warm half-years, which began after 1988, and a significant increase in the frequency of positive North Atlantic Oscillation (NAO) phases in winters. The ongoing climate change entails various effects, among which the most important is considered to be hydrological consequences. The water balance of Greater Poland is becoming increasingly unfavourable, mainly as a result of a rapid increase in field evaporation.

Human-induced carbon emissions are altering the modern climate, with severe repercussions on ecosystems. Among others, anthropogenic pressure is causing deoxygenation of the bottom water, with the widespread establishment of hypoxic zones in several Mediterranean areas. The geological archives allow investigating past deoxygenation dynamics (sapropel events) and their impact on marine ecosystems. Here, we compare the causes and the evolution of deoxygenation dynamics which occurred during two different time periods (Messinian and Holocene) in different paleoceanographic settings based on their micropaleontological content. The Messinian sapropel events are the result of increased export productivity during a relatively cold and arid context, triggering bottom anoxic conditions. The Holocene sapropel formed in response to weakening/stopping of the thermohaline circulation due to increasing temperature and freshwater input. Our results suggest that the deoxygenation dynamics in the Mediterranean in the near future will not follow the trend characteristic of the Holocene deep-sea sapropel because of the predicted drying trend. Differently, the paleoceanographic setting triggering the Messinian shallow-sea sapropels is comparable with the modern situation in different Mediterranean areas, where human-induced eutrophication is promoting deoxygenation. Based on these results, we suggest that the patchy deoxygenation trend in the Mediterranean Sea caused by climate warming may lead to a drastic change in the ecosystem services which would likely impact human activities. [ABSTRACT FROM AUTHOR]

Long‐term transition in the deep water of the Japan Sea was investigated to understand how oceans respond to climate change. A basin‐scale density decrease in the upper portion of the Japan Sea Proper Water (UJSPW) by 0.01 σθ was detected from the period 1964–1985 (Period I) to that of 2001–2019 (Period II) using Argo‐ and ship‐based data sets. The density decrease was mainly due to a temperature increase of 0.3°C. Based on the pycnostad density, the UJSPW was redefined as the water mass of 27.30–27.33 σθ, along with the core density of 27.30–27.31 σθ. The dissolved oxygen (DO) and thickness in the UJSPW showed a notable decrease between the periods, along with the isopycnal surfaces flattening. Warming down to 700 m and freshening in the upper 300 m were observed in the winter UJSPW formation area in the northwestern Japan Sea. An influence of the hemispheric‐scale regime shift occurred in 1988/1989 on the UJSPW warming was suggested, along with the long‐term trend in the winter air temperature. The freshening in the formation area was attributable to the increase in winter precipitation in the Amur River basin and the decrease in winter sea‐ice extents in the Okhotsk and northern Japan Seas. The intensified stratification in the UJSPW formation area brought a decrease in the amount of the UJSPW formation through the prevention of winter deep convection, which resulted in the thinner thickness and lower DO in the UJSPW in Period II, along with the temperature increase. Plain Language Summary: Deep‐water formation is an important component of the global climate system. As the Japan Sea is a miniature ocean with a unique deep water formation, the long‐term transition in the deep water of this sea was investigated to understand how oceans respond to climate change. The deep water in the Japan Sea has been warmed and lightened since the period 1964–1985, along with the decreases in its volume and the dissolved oxygen concentration, suggesting a weakening in the deep‐water formation. Accordingly, the deep‐water formation area in winter exhibited warming and freshening in the upper layer associated with increases in winter air temperature and precipitation. This situation is similar to the present North Atlantic, where warming and freshening are progressing in the deep‐water formation area. Key Points: Basin‐scale density decrease in the deep water of the Japan Sea was detected using Argo‐ and ship‐based data setsIntensified stratification in the upper layer due to warming and freshening was observed in the winter deep water formation areaWarming and excess precipitation in recent winter in the relevant areas were considered as causes of the transition in deep water formation [ABSTRACT FROM AUTHOR]

Seamounts and oceanic currents play crucial roles in shaping the genetic diversity of species by either acting as barriers or pathways for species dispersal. In the meiofaunal samples collected using ROV facilities at the slope of the Koko Guyot (2172 m) in the southernmost part of the Emperor Seamount Chain in the Northwestern Pacific, we found representatives of three kinorhynch genera, Echinoderes cf. lupherorum Sørensen et al. 2018, Campyloderes cf. vanhoeffeni Zelinka, 1913 and Sphenoderes sp. 1., previously known from other locations in the Pacific. Kinorhynchs were studied and illustrated using light (LM) and scanning electron (SEM) microscopy to prove the species identity and to compare morphological variations with representatives of these species collected from other, very distant localities in the Pacific. We illustrate the pan-oceanic distribution of the collected species with a system of abyssal currents in the Pacific. The probable distribution pattern corresponds with the northward pathway of Antarctic Bottom Water out of the Southern Ocean in the Pacific through the deep-sea area off New Zealand. Echinoderes lupherorum appears to be the first representative of the Echinoderidae with the pan-Pacific distribution. Discussions on the biogeography of Campyloderes cf. vanhoeffeni Zelinka, 1913 and Sphenoderes in the Pacific are also provided. [ABSTRACT FROM AUTHOR]

Boundary currents along the Sri Lankan eastern and southern coasts serve as a pathway for salt exchange between the Bay of Bengal and the Arabian Sea basins in the northern Indian Ocean, which are characterized by their contrasting salinities. Measurements from two pairs of pressure-sensing inverted echo sounders (PIES) deployed along the Sri Lankan eastern and southern coasts as well as satellite measurements are used to understand the variability of these boundary currents and the associated salt transport. The volume transport in the surface (0–200-m depth) layer exhibits a seasonal cycle associated with the monsoonal wind reversal and interannual variability associated with the Indian Ocean dipole (IOD). In this layer, the boundary currents transport low-salinity water out of the Bay of Bengal during the northeast monsoon and transport high-salinity water into the Bay of Bengal during the fall monsoon transition of some years (e.g., 2015 and 2018). The Bay of Bengal salt input increases during the 2016 negative IOD as the eastward flow of high-salinity water during the fall monsoon transition intensifies, whereas the effect of the 2015/16 El Niño on the Bay of Bengal salt input is still unclear. The time-mean eddy salt flux over the upper 200 m estimated for the April 2015–March 2019 period along the eastern coast accounts for 9% of the salt budget required to balance an estimated 0.13 Sv (1 Sv ≡ 106 m3 s−1) of annual freshwater input into the Bay of Bengal. The time-mean eddy salt flux over the upper 200 m estimated for the December 2015–November 2019 period along the southern coast accounts for 27% of that same salt budget. Significance Statement: In the northern Indian Ocean, the highly saline Arabian Sea undergoes extreme evaporation while the Bay of Bengal (BoB) receives excess freshwater input. The focus of this study is the role of the observed time-variable circulation around Sri Lanka that permits the exchange between these basins to maintain their salinity distributions. The circulation fluctuates seasonally following the monsoon wind reversal and interannually in response to large-scale climate modes. The BoB freshwater export around Sri Lanka occurs during the northeast monsoon, whereas saline water import occurs during the fall monsoon transition of some years. However, rapid changes in both water volume transport and salt exchange can occur. The circulation over 0–200-m depth transports ∼9%–27% of the BoB salt budget. [ABSTRACT FROM AUTHOR]

The diabatic transformations of the middepth meridional overturning circulation (MOC) as it exits and reenters the South Atlantic to close the AMOC are studied using a state estimate assimilating data into a dynamically consistent ocean model. Virtual Lagrangian parcels in the lower branch of the MOC are followed in their global tour as they return to the upper branch of the MOC. Three return pathways are identified. The first pathway enters the abyssal Indo-Pacific as Circumpolar Deep Water, directly from the northern Antarctic Circumpolar Current (ACC), and before sampling the Antarctic margin. The second pathway sinks to abyssal densities exclusively in the Southern Ocean, then upwells while circulating within the ACC and eventually enters the Indo-Pacific or Atlantic at mid- to upper depths. The third pathway never reaches densities in the abyssal range. Parcels sinking in the Antarctic Bottom Water range upwell to mid- to upper depths south of 55°S. Parcels in all three pathways experience additional diabatic transformations after upwelling in the Southern Ocean, with more diabatic changes north of about 30°S than elsewhere. Diabatic changes are predominantly in the mixed layer of the tropical and subpolar gyres, enabled by Ekman suction. A simple model of the wind-driven flow illustrates that parcels always reach the surface in the tropical and subpolar gyres, regardless of their initial condition, because of coupling among gyres, the Ekman transport, and its return. [ABSTRACT FROM AUTHOR]

Collapse of the Atlantic Meridional Overturning Circulation (AMOC) is often invoked as an explanation of major past climate changes and as a major risk for future climate. Many of these arguments appear, from an observer’s point of view, as far-more definitive than is warranted. In the hypothetical event of a future collapse, the implications may be much less severe than those from many other elements of global change already underway. The Gulf Stream system, and its required return flow of mass, implies that changed circulations will nonetheless continue to carry significant amounts of heat, carbon etc., poleward even without any AMOC.

The North Atlantic gyre experiences both a significant temperature rise at high latitudes and a considerable weakening of the geostrophic component of the Gulf Stream, which is reflected by the 64-year fundamental gyral Rossby wave (GRW). This singular behavior compared to the South Atlantic and South Indian Ocean gyres highlights a feedback loop of Arctic ice sheet melting on mid-latitude Atlantic Ocean temperature. The warming of the northern oceanic gyre at high latitudes due to the retreat of Arctic ice sheet via the Labrador Current decreases the thermal gradient between the high and low latitudes of the north Atlantic gyre. This results in a weakening of the geostrophic forces at the basin scale and a reduction in the amplitude of the GRWs. Reducing the amplitude of the variation of the upward and downward movement of the pycnocline modifies air–sea interactions, weakening vertical mixing as well as the evaporation processes and the departure of latent heat when the pycnocline rises. The resulting thermal anomaly stretching along the Gulf Stream from where it leaves the American continent is partly transferred to the Arctic sea ice via the drift current and thermohaline circulation, which contributes to the retreat of the ice sheet, and the closing of the feedback loop. The 64-year-period GRW should disappear around 2050 if its damping continues linearly, favoring an increasingly rapid warming of the ocean at mid-latitudes. These interactions are less acute in the southern hemisphere due to the circumpolar current.