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Global deep-water circulation plays a crucial role in regulating long-term carbon storage in both the ocean and atmosphere. During the last glacial period, it is reported that this circulation slowed down, causing glacial intermediate water to descend to depths of 2,000 m in the Pacific. This process is thought to be a key mechanism in restricting global deep-water circulation and reducing atmospheric carbon dioxide concentrations during glacial periods. Conversely, the emergence of a potential deep-water formation zone in the northwestern Pacific during deglaciation adds complexity to these circulation patterns. Addressing the scarcity of sedimentary records in the subtropical western Pacific for paleoceanographic reconstruction, sediment core YK15-01 PC13 (23.5°N, 124.24°E; southeast of Ishigaki Island at a depth of 2,520 m) used in this study, collected in 2015. By analyzing coexisting planktonic and benthic foraminifera, we have reconstructed radiocarbon ventilation ages in this region since the last glacial maximum. Our findings, including ventilation age records compiled from the Atlantic and Pacific Oceans, reveal opposing deep-water circulation patterns from the last glacial to the interglacial period, including during the Heinrich Event 1 (HE1) and Younger Dryas (YD). This supports the theory of a seesaw-like oscillation in ocean circulation. Furthermore, the improved ventilation observed during HE1 and YD suggests a contribution from deep water formed in the high-latitude Pacific, influenced by cooler climate conditions. This glacial North Pacific deep water, likely confined to the northwestern Pacific, has left its mark in the subtropical western Pacific records. This research enhances our understanding of deep-water circulation interactions between the Atlantic and Pacific and contributes new insights into the role of northern Pacific deep water in influencing atmospheric carbon dioxide variations during the last deglaciation.

The overturning pathways for the surface-ventilated North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) and the diffusively formed Indian Deep Water (IDW) and Pacific Deep Water (PDW) are intertwined. The global overturning circulation (GOC) includes both large wind-driven upwelling in the Southern Ocean and important internal diapycnal transformation in the deep Indian and Pacific Oceans. All three northern-source Deep Waters (NADW, IDW, PDW) move southward and upwell in the Southern Ocean. AABW is produced from the denser, salty NADW and a portion of the lighter, low oxygen IDW/PDW that upwells above and north of NADW. The remaining upwelled IDW/PDW stays near the surface, moving into the subtropical thermoclines, and ultimately sources about one-third of the NADW. Another third of the NADW comes from AABW upwelling in the Atlantic. The remaining third comes from AABW upwelling to the thermocline in the Indian-Pacific. Atlantic cooling associated with NADW formation (0.3 PW north of 32°S; 1 PW = 1015W) and Southern Ocean cooling associated with AABW formation (0.4 PW south of 32°S) are balanced mostly by 0.6 PW of deep diffusive heating in the Indian and Pacific Oceans; only 0.1 PW is gained at the surface in the Southern Ocean. Thus, while an adiabatic model of NADW global overturning driven by winds in the Southern Ocean, with buoyancy added only at the surface in the Southern Ocean, is a useful dynamical idealization, the associated heat changes require full participation of the diffusive Indian and Pacific Oceans, with a basinaveraged diffusivity on the order of the Munk value of 10⁻⁴ m² s⁻¹.

While a rapid sea ice retreat in the Arctic has become ubiquitous, the potential weakening of the Atlantic meridional overturning circulation (AMOC) in response to global warming is still under debate. As deep mixing occurs in the open ocean close to the sea ice edge, the strength and vertical extent of the AMOC is likely to respond to ongoing and future sea ice retreat. Here, we investigate the link between changes in Arctic sea ice cover and AMOC strength in a long simulation with the EC-Earth–Parallel Ice Sheet Model (PISM) climate model under the emission scenario RCP8.5. The extended duration of the experiment (years 1850–2300) captures the disappearance of summer sea ice in 2060 and the removal of winter sea ice in 2165. By introducing a new metric, the Arctic meridional overturning circulation (ArMOC), we document changes beyond the Greenland–Scotland ridge and into the central Arctic. We find an ArMOC strengthening as the areas of deep mixing move north, following the retreating winter sea ice edge into the Nansen Basin. At the same time, mixing in the Labrador and Greenland Seas reduces and the AMOC weakens. As the winter sea ice edge retreats farther into the regions with high surface freshwater content in the central Arctic Basin, the mixing becomes shallower and the ArMOC weakens. Our results suggest that the location of deep-water formation plays a decisive role in the structure and strength of the ArMOC; however, the intermittent strengthening of the ArMOC and convection north of the Greenland–Scotland ridge cannot compensate for the progressive weakening of the AMOC.

A significant portion (∼2.1 Sv, 1 Sv = 106 m3 s−1) of deep water penetrates into the Philippine Sea through the Yap‐Mariana Junction, the sole passage of the Philippine Sea below 4,000 m, and is then upwelled into shallower layers, closing regional overturning circulation. Yet, the structure and variability of this diapycnal upwelling remain poorly understood. Here, we report on a fine‐resolution hydrographic observation conducted at the most significant topographic feature in the Philippine Sea, the Kyushu‐Palau Ridge (KPR). Enhanced mixing up to O(10−2) m2 s−1 near the KPR is manifested, indicating the presence of substantial upwelling herein. Besides, the ridge‐related topography contributes more deep‐water mass transformation than abyssal basins in the Philippine Sea. This study highlights the significant role of rough bathymetry features in generating diapycnal upwelling in the North Pacific. Plain Language Summary In the North Pacific, a significant portion of deep water from the Antarctic enters the Philippine Sea below 4,000 m, where it is upwelled into shallower layers, closing regional overturning circulation. However, where the deep water gains buoyancy in the deep Philippine Sea required to upwell into shallower layers remains largely unknown, rendering our understanding of the western Pacific overturning circulation incomplete. To fill this gap, we conducted observations at two transects over the Kyushu‐Palau Ridge (KPR), the most significant topographic feature in the Philippine Sea. Based on this observational program, we reveal the mixing structure down to the bottom across the KPR and estimate the deep water transformation in the deep Philippine Sea. This study illuminates the role of ridge‐related topography in driving diapycnal upwelling in the deep ocean and improves our understanding of the deep circulation in the North Pacific Ocean. Key Points A three‐dimensional distribution of diapycnal mixing over the Kyushu‐Palau Ridge is presented The spatially variable of mixing across the Kyushu‐Palau Ridge results in varying vertical velocities The Kyushu‐Palau Ridge contributes more deep‐water mass transformation than abyssal basins in the Philippine Sea

Ocean circulation critically affects the global climate and atmospheric carbon dioxide through redistribution of heat and carbon in the Earth system. Despite intensive research, the nature of past ocean circulation changes remains elusive. Here we present deep-water carbonate ion concentration reconstructions for widely distributed locations in the Atlantic Ocean, where low carbonate ion concentrations indicate carbon-rich waters. These data show a low-carbonate-ion water mass that extended northward up to about 20° S in the South Atlantic at 3–4 km depth during the Last Glacial Maximum. In combination with radiocarbon ages, neodymium isotopes and carbon isotopes, we conclude that this low-carbonate-ion signal reflects a widespread expansion of carbon-rich Pacific deep waters into the South Atlantic, revealing a glacial deep Atlantic circulation scheme different than commonly considered. Comparison of high-resolution carbonate ion records from different water depths in the South Atlantic indicates that this Pacific deep-water expansion developed from approximately 38,000 to 28,000 years ago. We infer that its associated carbon sequestration may have contributed critically to the contemporaneous decline in atmospheric carbon dioxide, thereby helping to initiate the glacial maximum.Carbon-rich Pacific deep water extended into the South Atlantic some 38,000 to 28,000 years ago, potentially contributing to a reduction in atmospheric carbon dioxide and the onset of the Last Glacial Maximum, according to deep-water carbonate chemistry reconstructions.

Changes in deep-ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide concentrations. It has been recently proposed that such changes are associated with variations in Antarctic sea ice through two possible mechanisms: an increased latitudinal extent of Antarctic sea ice and an increased rate of Antarctic sea ice formation. Both mechanisms lead to an upward shift of the Atlantic meridional overturning circulation (AMOC) above depths where diapycnal mixing is strong (above 2000 m), thus decoupling the AMOC from the abyssal overturning circulation. Here, these two hypotheses are tested using a series of idealized two-basin ocean simulations. To investigate independently the effect of an increased latitudinal ice extent from the effect of an increased ice formation rate, sea ice is parameterized as a latitude strip over which the buoyancy flux is negative. The results suggest that both mechanisms can effectively decouple the two cells of the meridional overturning circulation (MOC), and that their effects are additive. To illustrate the role of Antarctic sea ice in decoupling the AMOC and the abyssal overturning cell, the age of deep-water masses is estimated. An increase in both the sea ice extent and its formation rate yields a dramatic “aging” of deep-water masses if the sea ice is thick and acts as a lid, suppressing air–sea fluxes. The key role of vertical mixing is highlighted by comparing results using different profiles of vertical diffusivity. The implications of an increase in water mass ages for storing carbon in the deep ocean are discussed.

A quantitative theoretical model of the meridional overturning circulation and associated deep stratification in an interhemispheric, single-basin ocean with a circumpolar channel is presented. The theory includes the effects of wind, eddies, and diapycnal mixing and predicts the deep stratification and overturning streamfunction in terms of the surface forcing and other parameters of the problem. It relies on a matching among three regions: the circumpolar channel at high southern latitudes, a region of isopycnal outcrop at high northern latitudes, and the ocean basin between. The theory describes both the middepth and abyssal cells of a circulation representing North Atlantic Deep Water and Antarctic Bottom Water. It suggests that, although the strength of the middepth overturning cell is primarily set by the wind stress in the circumpolar channel, middepth stratification results from a balance between the wind-driven upwelling in the channel and deep-water formation at high northern latitudes. Diapycnal mixing in the ocean interior can lead to warming and upwelling of deep waters. However, for parameters most representative of the present ocean mixing seems to play a minor role for the middepth cell. In contrast, the abyssal cell is intrinsically diabatic and controlled by a balance between the deep mixing-driven upwelling and the residual of the wind-driven and eddy-induced circulations in the Southern Ocean. The theory makes explicit predictions about how the stratification and overturning circulation vary with the wind strength, diapycnal diffusivity, and mesoscale eddy effects. The predictions compare well with numerical results from a coarse-resolution general circulation model.

Much of the existing theory for the ocean’s overturning circulation considers steady-state equilibrium solutions. However, Earth’s climate is not in a steady state, and a better understanding of the ocean’s non-equilibrium response to changes in the surface climate is urgently needed. Here, the time-dependent response of the deep-ocean overturning circulation to atmospheric warming is examined using a hierarchy of idealized ocean models. The transient response to surface warming is characterized by a shoaling and weakening of the Atlantic meridional overturning circulation (AMOC)—consistent with results from coupled climate simulations. The initial shoaling and weakening of the AMOC occurs on decadal time scales and is attributed to a rapid warming of northern-sourced deep water. The equilibrium response to warming, in contrast, is associated with a deepening and strengthening of the AMOC. The eventual deepening of the AMOC is argued to be associated with abyssal density changes and driven by modified surface fluxes in the Southern Ocean, following a reduction of the Antarctic sea ice cover. Full equilibration of the AMOC requires a diffusive adjustment of the abyss and takes many millennia. The equilibration time scale is much longer than most coupled climate model simulations, highlighting the importance of considering integration time and initial conditions when interpreting the deep-ocean circulation in climate models. The results also show that past climates are unlikely to be an adequate analog for changes in the overturning circulation during the coming decades or centuries.

Climate during the last glacial period was marked by abrupt instability on millennial timescales that included large swings of temperature in and around Greenland (Daansgard–Oeschger events) and smaller, more gradual changes in Antarctica (AIM events). Less is known about the existence and nature of similar variability during older glacial periods, especially during the early Pleistocene when glacial cycles were dominantly occurring at 41 kyr intervals compared to the much longer and deeper glaciations of the more recent period. Here, we report a continuous millennially resolved record of stable isotopes of planktic and benthic foraminifera at IODP Site U1385 (the “Shackleton Site”) from the southwestern Iberian margin for the last 1.5 million years, which includes the Middle Pleistocene Transition (MPT). Our results demonstrate that millennial climate variability (MCV) was a persistent feature of glacial climate, both before and after the MPT. Prior to 1.2 Ma in the early Pleistocene, the amplitude of MCV was modulated by the 41 kyr obliquity cycle and increased when axial tilt dropped below 23.5∘ and benthic δ18O exceeded ∼3.8 ‰ (corrected to Uvigerina), indicating a threshold response to orbital forcing. Afterwards, MCV became focused mainly on the transitions into and out of glacial states (i.e. inceptions and terminations) and during times of intermediate ice volume. After 1.2 Ma, obliquity continued to play a role in modulating the amplitude of MCV, especially during times of glacial inceptions, which are always associated with declining obliquity. A non-linear role for obliquity is also indicated by the appearance of multiples (82, 123 kyr) and combination tones (28 kyr) of the 41 kyr cycle. Near the end of the MPT (∼0.65 Ma), obliquity modulation of MCV amplitude wanes as quasi-periodic 100 kyr and precession power increase, coinciding with the growth of oversized ice sheets on North America and the appearance of Heinrich layers in North Atlantic sediments. Whereas the planktic δ18O of Site U1385 shows a strong resemblance to Greenland temperature and atmospheric methane (i.e. Northern Hemisphere climate), millennial changes in benthic δ18O closely follow the temperature history of Antarctica for the past 800 kyr. The phasing of millennial planktic and benthic δ18O variation is similar to that observed for MIS 3 throughout much of the record, which has been suggested to mimic the signature of the bipolar seesaw – i.e. an interhemispheric asymmetry between the timing of cooling in Antarctica and warming in Greenland. The Iberian margin isotopic record suggests that bipolar asymmetry was a robust feature of interhemispheric glacial climate variations for at least the past 1.5 Ma despite changing glacial boundary conditions. A strong correlation exists between millennial increases in planktic δ18O (cooling) and decreases in benthic δ13C, indicating that millennial variations in North Atlantic surface temperature are mirrored by changes in deep-water circulation and remineralization of carbon in the abyssal ocean. We find strong evidence that climate variability on millennial and orbital scales is coupled across different timescales and interacts in both directions, which may be important for linking internal climate dynamics and external astronomical forcing.

Using a high-resolution regional ocean model, the impact of tidal mixing on water mass transformation and circulation in the South China Sea (SCS) is investigated through a set of numerical experiments with different configurations of tide-induced diapycnal diffusivity. The results show that including tidal mixing in both the Luzon Strait (LS) and SCS has significant impact on the LS transport and the intermediate–deep layer circulation in the SCS Basin. Analysis of the density field indicates that tidal mixing in both the LS and SCS are essential for sustaining a consistent density gradient and thus a persistent outward-directed baroclinic pressure gradient both between the western Pacific and LS and between the LS and SCS Basin, so as to maintain the strong deep-water transport through the LS. Further analysis of water mass properties suggests that tidal mixing in the deep SCS would strengthen the horizontal density gradient, intensify the basin-scale cyclonic circulation, induce more vigorous overturning, as well as generate the subbasin-scale eddies in the abyssal SCS. The results imply that tidal mixing in both the LS and SCS plays a key dynamic role in controlling water mass properties and deep circulation features in the SCS and thus need to be deliberately parameterized in ocean circulation models for this region.