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Kuroshio is an important western boundary current system in the North Pacific subtropical gyre. Mesoscale eddies play an important role in Kuroshio path variations, which significantly affect fisheries, marine navigation, and climate in regions along the Kuroshio path. However, the direct physical impacts of the Kuroshio variabilities on mixing dynamics within the water columns off the southern coast of Japan remain unclear due to the lack of observational records. The radiocarbon (Δ14C) in dissolved inorganic carbon (DIC) of seawater has been used as a reliable tracer of water circulation and mixing processes. Here, we present the high‐resolution dissolved inorganic radiocarbon (DIC ∆14C) measurements for water samples collected during the Kuroshio large‐meander (LM) period in March 2022 to observe the mixing dynamics within the Kuroshio area and elucidate the controls of mixing processes. In the present study, horizontal variations (12–175‰) in DIC Δ14C values were observed between 400 and 1,000 m and were attributed mainly to changes in the depth of isopycnal surfaces associated with mesoscale eddies and the position of the Kuroshio axis. Furthermore, fluctuations in DIC Δ14C values (10–25‰) were observed on the same isopycnal surfaces (σθ=${\sigma }_{\theta }=$25.5–27.0), suggesting the occurrence of diapycnal mixing. By comparing this newly obtained data with values previously reported at similar locations, variations in penetration depths of high bomb 14C signals from past observations are found to be mainly caused by changes in the depth of isopycnal surfaces associated with mesoscale eddies and Kuroshio path variations. The Kuroshio is a warm western boundary current that transports a huge amount of heat and chemical compounds from the western tropical Pacific Ocean to the mid‐latitude region of the western North Pacific (NP) Ocean. As the Kuroshio flows along the southern coast of Japan, it can take either a large‐meander (LM) path or a non‐large‐meander (NLM) path. The path variations and mesoscale eddies of the Kuroshio, accompanied by turbulent mixing, could impact local climate and marine ecosystems. The radiocarbon (∆14C) in dissolved inorganic carbon (DIC) in the ocean has been used as a reliable tracer of water mass sources and ages. In this study, we conducted ∆14C measurements for water samples collected during the Kuroshio LM period in March 2022. Large variations in DIC ∆14C suggest that physical processes such as mesoscale eddies influence DIC ∆14C values through isopycnal depth variations. Additionally, DIC Δ14C was demonstrated to be useful as a sensitive tracer to obtain direct evidence of the enhanced turbulent diapycnal mixing by analyzing DIC Δ14C anomalies on the same potential density surfaces. Distinctive horizontal variations (12–175‰) in Δ14C values were observed during the Kuroshio LM periodAnalysis of DIC Δ14C anomalies on isopycnal surfaces revealed the presence of enhanced turbulent diapycnal mixingThe penetration depths (200–700 m) of the high bomb 14C signals vary mainly due to mesoscale eddies and Kuroshio path variations Distinctive horizontal variations (12–175‰) in Δ14C values were observed during the Kuroshio LM period Analysis of DIC Δ14C anomalies on isopycnal surfaces revealed the presence of enhanced turbulent diapycnal mixing The penetration depths (200–700 m) of the high bomb 14C signals vary mainly due to mesoscale eddies and Kuroshio path variations

The spreading of dense shelf water (DSW) from Antarctic coastal margins to lower latitudes plays a vital role in the ocean thermohaline circulation and the global climate system. Through enhanced localized sea ice production in Antarctic coastal polynyas, cold and saline DSW is formed over the continental shelf regions as a precursor to Antarctic Bottom Water (AABW). However, the detailed fate of coastal DSW over the Southern Ocean is still unclear. Here we conduct extensive passive tracer experiments using a circumpolar ocean‐sea ice‐ice shelf model to investigate pathways of the regional polynya‐based DSW from the Antarctic margins to the deep Southern Ocean basins. In the numerical experiments, the Antarctic coastal margin is divided into nine regions, and a passive tracer is released from each region at the same rate as the local sea ice production. The modeled spatial distribution of the total concentration of the nine tracers is consistent with the observed AABW distribution and clearly demonstrates nine routes of the DSW over the Southern Ocean along its bottom topography. Furthermore, the model shows that while ∼50% of the total tracer is distributed northward from the continental shelf to the deep ocean, ∼7% is transported poleward beneath ice shelf cavities. The comprehensive tracer experiments allow us to estimate the contribution of local DSW to the total concentration along each of the pathways. Passive tracer experiments detail the export of DSW formed in Antarctic coastal polynyas to the offshore basins of the Southern OceanNine key pathways of DSW over the Southern Ocean are describedThe contributions of regional DSW formation and distributions are estimated

The subduction process of Pacific Antarctic Intermediate Water (PAAIW) in the Pacific is investigated using output from an eddy‐resolving ocean model. Focus is on contribution of eddies to the subduction process. To separate the subduction rate into contributions by eddies and mean flows, the temporal residual mean (TRM) velocity is used. In the mean subduction rate, lateral induction caused by the strong eastward flow of the Antarctic Circumpolar Current (ACC) is dominant. The largest rate is located in the Drake Passage. The estimated eddy‐induced subduction rate is comparable with the mean subduction rate, and it tends to cancel the vertical mean component in many regions. In the west of the Drake Passage, however, the eddy‐induced subduction is larger than the vertical mean component, and this eddy‐induced subduction was not detected in previous studies using the thickness diffusion parameterization and an eddy‐permitting model. Results of idealized sensitivity studies to model resolution suggest that the subduction rate would be larger using a model with higher vertical resolution. Therefore, the vertical resolution should be paid more attention in model studies investigating eddy‐induced subduction, and not just the horizontal resolution. The eddy subduction rate is estimated using the temporal residual mean velocityThe eddy subduction rate of Pacific Antarctic Intermediate Water is largeThe vertical resolution is important in estimating the eddy‐induced subduction

The heat influx of the Atlantic water and its interannual variability through the Fram Strait toward the Arctic Ocean are examined by using a realistically configured ice‐ocean general circulation model. The modeled routes of the Atlantic water and high eddy activity around the Fram Strait are consistent with many observations. Two‐thirds of the heat transported by the Atlantic water passing through the Fram Strait (78°N) is lost by the westward transport and the sea surface cooling, and the other one‐third is injected to the Arctic Ocean. The contribution of oceanic eddy to the westward heat transport is 5% of that of mean current. The variability of sea level pressure anomaly centered at the Nordic Seas explains the interannual variability of the heat passing through the Fram Strait, transported westward, and cooled at the sea surface in the north of the Fram Strait. The interannual variabilities of these heat fluxes have significant correlations with the NAO. The interannual variability of heat transported by the Atlantic water and entering the Arctic Ocean is caused by the variability of the Siberian high. The Atlantic water inflow and eddy at the Fram Strait are simulated by high‐resolution ocean modelThe contribution of oceanic eddy to the heat flux is examinedInterannual variability of heat flux is explained by the variability of wind

Dense Shelf Water (DSW) formation in the northwestern continental shelf of the Sea of Okhotsk is the beginning of the lower limb of the overturning circulation that ventilates the intermediate layer of the North Pacific Ocean. The upper limb consisting of surface currents in the Okhotsk Sea and the subarctic gyre has not been clarified. Using a high‐resolution North Pacific Ocean model with a curvilinear grid as fine as 3 km × 3 km in the Sea of Okhotsk, we succeeded in representing the three‐dimensional structure of the overturning circulation including the narrow boundary currents and flows through straits that constitute the upper limb, as well as the lower limb consisting of DSW formation and ventilation. In particular, pathways and time scales from the Bering Sea to the intermediate layer via the ventilation in the Sea of Okhotsk were examined in detail using tracer experiments. Further, we found that the overturning circulation that connects the surface and intermediate layer is sensitive to wind stress. In the case of strong winds, the coastal current under polynyas where DSW forms is intensified, and consequently diapycnal transport from the surface layer to the intermediate layer increases. Strong winds also induce a positive sea surface salinity anomaly in the subarctic region, causing a significant decrease in the density stratification and increase in the DSW salinity (i.e., density). These processes act together to produce intense overturning circulation and deep ventilation, which may subduct even to the bottom of the Sea of Okhotsk if the wind is strong. We represented the intermediate overturning in the Okhotsk‐North PacificSensitivity study revealed the overturning circulation is sensitive to windAdvection of saline water from subtropical to subarctic by wind is discussed

Barents–Kara (BK) sea ice conditions are considered a potential source of seasonal predictability in the midlatitudes, but confirmation or refutation of this possibility remains elusive. We focused on the Warm Arctic–Cold Eurasia (WACE) pattern and evaluated whether BK sea ice is related to winter conditions over Eurasia in multi‐model seasonal predictions. In the models analyzed, the WACE index is strongly depended on the BK temperature and did not rigorously reflect a sea ice–Eurasia causal link. The autumn BK sea ice anomaly was not a precursor of winter atmospheric conditions over Eurasia. Rather, the winter atmospheric circulation likely drives both winter BK sea ice and Eurasian temperature. However, the predicted winter sea ice–Eurasia links are likely weaker than the observed. This contrast is speciously related to the modest sea ice variance, but the actual influence of BK sea ice variation on Eurasian temperature anomalies is still questionable. Arctic sea ice coverage has decreased rapidly during recent decades, but whether this sea ice loss remotely influences continental wintertime temperatures is actively debated. We investigated the relationship between the Barents–Kara sea ice extent and winter Eurasian temperatures in seasonal predictions of model ensembles. In the model results, autumn sea ice conditions were not a precursor of winter temperatures in Eurasia; no measurable remote influence via the atmospheric teleconnection called the Warm Arctic–Cold Eurasia pattern was found. However, current seasonal prediction models likely underrepresent the observed relation between sea ice and atmospheric conditions in remote regions. The weak year‐to‐year fluctuations of sea ice extent by the models may partly relate to this inconsistency, but the degree of it is probably small. Thus, the influence by the Barents‐Kara sea ice variation on the Eurasian climate and its benefit to seasonal prediction are unclear presently. The contribution of autumn Barents‐Kara sea ice conditions to winter Eurasian temperatures is uncertain in seasonal prediction modelsThe Warm Arctic‐Cold Eurasia index is not suitable for assessing the causal link between Barents‐Kara sea ice and Eurasian temperatureAn observed link between sea ice and Eurasian temperature is stronger than the precited and close to the model's uncertainty range limit The contribution of autumn Barents‐Kara sea ice conditions to winter Eurasian temperatures is uncertain in seasonal prediction models The Warm Arctic‐Cold Eurasia index is not suitable for assessing the causal link between Barents‐Kara sea ice and Eurasian temperature An observed link between sea ice and Eurasian temperature is stronger than the precited and close to the model's uncertainty range limit

Abstract: This paper reviews recent progress and problems in modeling the thermohaline circulation of the world ocean by use of z-coordinate ocean general circulation models. Discussions focus on four issues: sea surface forcing, mixing in the deep ocean interior, eddy-induced tracer transport, and bottom boundary layer processes. Although some widely used techniques and parameterizations deal with these issues, some aspects are still overlooked and more sophistication is certainly required.

Deep Pacific circulation is investigated by using a World Ocean model with depth-dependent vertical diffusivity. Vertical diffusivity estimated from observations, 0.1 × 10−4m2s−1for the upper layer and 3.0 × 10−4m2s−1for the bottom layer, is adopted. Comparison is made between cases with different vertical diffusivity at middepths. With larger vertical diffusivity at middepths, the deep Pacific circulation becomes stronger. This is due to enhanced heat exchange between the thermocline water and the deep water through more intense diffusion at middepths. The water below the thermocline is warmed and that at the thermocline is cooled for the whole basin. The warmed deep water leads to larger heat loss through the sea surface, causing the enhanced deep-water formation in the deep-water formation region. On the other hand, the cooled thermocline water leads to larger heat gain through the sea surface where the thermocline water outcrops, counterbalancing the larger heat loss in the deep-water formation region. The deep water brought up to the middepths does not further upwell to the sea surface due to the small upper-layer vertical diffusivity, but it flows back to the deep-water formation region, slowly upwelling within the middepths. In this way, the enhanced meridional overturning forms in the deep Pacific. The layered deep Pacific meridional circulation is realistically reproduced when vertical diffusivity is larger at middepths. This circulation yields tracer distributions that compare well with observations. Such a strong deep Pacific circulation does not occur when vertical diffusivity is taken larger at middepths but is held constant below the middepths. For realistic reproduction of the deep Pacific circulation, vertical diffusivity needs to keep increasing with depth beginning at the lower thermocline depths.

The authors compare two classical advection schemes, the centered difference and weighted upcurrent, for coarse-resolution OGCMs, using an idealized ocean basin and a realistic World Ocean topography. For the idealized basin, three experiments are run, one with 12 vertical levels and the centered difference scheme, one with 12 levels and the weighted upcurrent scheme, and the other with 800 levels and the centered scheme. The last experiment perfectly satisfies the grid Péclet number stability criterion and is regarded as the “true solution.” Comparison of the coarse vertical resolution experiments with the true solution indicates 1) that with the centered scheme, when strong vertical motion crosses a strong stratification, false density values are created in the coarse resolution model and this leads to false convective adjustment, which transports those false density values downward; and 2) that because of computational diffusion, the weighted upcurrent scheme leads to a less dense deep water with a stronger stratification than those of the true solution. These characteristics also apply even to the World Ocean model with relatively small grid Péclet numbers (moderately high vertical resolution and relatively large vertical diffusivity): the centered scheme leads to artificial convective adjustment near the surface in the equatorial Pacific, creating an artificial circulation, and the weighted upcurrent scheme leads to a warmer deep water and more diffuse thermocline. Deep equatorial “stacked jets” are found in all idealized-basin experiments, in particular, in the super-high vertical resolution case. Horizontal diffusion is found to dominate the density balance at the bottom jet in the super-high-resolution model, as previously found in an OGCM with a moderately high vertical resolution. This is consistent with the hypothesis that the jets exist owing to diapycnal mixing.

Vertical diffusivity at the thermocline depths is now believed to be as small as 1 × 10−5m2s−1. In order to accomplish a reliable simulation of the World Ocean for the vertical diffusivity of 1 × 10−5m2s−1, two advective tracer transport schemes, the Uniformly Third-Order Polynominal Interpolation Algorithm (UTOPIA) of Leonard et al and the Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) of Smolarkiewicz, are incorporated into an ocean general circulation model. Intercomparison is made among simulations using UTOPIA, MPDATA, and the centered differencing scheme. When UTOPIA or MPDATA is adopted, features at the thermocline depths are realistically simulated. Increase in computational cost is moderate. Circulations associated with Antarctic Bottom Water (AABW) in the Atlantic and Circumpolar Deep Water (CDW) in the Pacific are not reproduced at all for such small vertical diffusivity, although the circulation associated with North Atlantic Deep Water (NADW) has reasonable intensity. Another experiment with UTOPIA for the vertical diffusivity of 5 × 10−5m2s−1shows that the circulation associated with NADW is relatively insensitive to vertical diffusivity, compared with the circulation associated with AABW and CDW.

During the early 20th century, the Arctic experienced a period of remarkable warming, often called the early 20th century warming (ETCW). However, the degree of the response to external climate forcing on the Arctic surface air temperature is not well understood. Climate simulations using a state‐of‐the‐art climate model of Meteorological Research Institute (MRI‐ESM2.0) and multimodel analyses were conducted to better understand ETCW. The MRI‐ESM2.0 historical simulations successfully reproduced the observed ETCW and the corresponding decreases in sea ice extent. Detection and attribution experiments using MRI‐ESM2.0 suggest that internal climate variability and external natural forcings by solar and volcanic activities had major influences on the model‐simulated ETCW, rather than external anthropogenic forcings. Multimodel analyses indicate that the Arctic warming trend during 1911–1940 induced by natural forcings is comparable to the unforced multidecadal internal variability, suggesting major contributions of the internal dynamics and natural forcings to ETCW. Instrumental records show that there was significant warming in the Arctic during the first half of the 20th century, known as the early 20th century warming (ETCW). Climate simulations using a state‐of‐the‐art climate model of Meteorological Research Institute (MRI‐ESM2.0) suggest that external natural forcings, i.e., long‐lasting quiet volcanic activity and increased solar activity, influenced the model‐simulated ETCW and associated sea ice decrease. Multimodel analyses indicate the robustness of the MRI‐ESM2.0 results and suggest that the climatic response of Arctic surface air temperature to external natural forcings in the ETCW period is comparable to the unforced multidecadal internal variability of the models, suggesting a major contribution of unforced multidecadal internal variability and natural forcings to ETCW. Climate model MRI‐ESM2.0 reproduces the Arctic surface air temperature rising and associated sea ice loss observed in the early 20th centuryMRI‐ESM2.0 results indicate that the natural forcings and the internal variability are main contributors to Arctic warming during 1920–1940Multimodel analyses suggest the Arctic warming response to natural forcings in 1911–1940 is comparable to the unforced internal variability Climate model MRI‐ESM2.0 reproduces the Arctic surface air temperature rising and associated sea ice loss observed in the early 20th century MRI‐ESM2.0 results indicate that the natural forcings and the internal variability are main contributors to Arctic warming during 1920–1940 Multimodel analyses suggest the Arctic warming response to natural forcings in 1911–1940 is comparable to the unforced internal variability

The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N—a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high‐resolution models. Furthermore, we will describe how high‐resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC. Observations and high‐resolution models have changed view on the AMOC pathwaysHigh‐resolution models suggest the presence of previously unknown high‐frequency AMOC variabilityHigh‐resolution models allow to estimate the intrinsic/chaotic component of the AMOC

Using an ice‐ocean coupled model with fine horizontal resolution around East Antarctica, sea ice production and dense shelf water (DSW) formation in coastal polynyas are investigated. The model reproduces well the locations of coastal polynyas and the high sea ice production there. DSW is formed over the continental shelves under a number of coastal polynyas. A threshold density, beyond which net production of DSW takes place, is largely different among coastal polynyas. The densest and most vigorous DSW formation occurs in the Cape Darnley and Mertz‐Ninnis Glacier polynyas followed by somewhat less but still significant DSW formation in the Prydz‐Barrier and Vincennes polynyas. Assuming mixing of the DSW outflowing across the shelf break with typical Modified Circumpolar Deep Water over the continental slope, the maximum possible formation rate of Antarctic Bottom Water (AABW) is estimated to be 7.58 Sv around East Antarctica between 60°E and 150°E, with the Cape Darnley and Mertz‐Ninnis Glacier polynyas exhibiting the most active formation rates of 2.13 and 1.97 Sv, respectively. From a series of numerical experiments, it is found that the treatment of coastline and grounded icebergs has a large impact on both sea ice production and formation of DSW and AABW.

Descent of Ice Shelf Water (ISW) overflowed from the Filchner Depression in the southern Weddell Sea and consequent Weddell Sea Bottom Water (WSBW) production is investigated through high resolution numerical experiments. The model reproduces the part of ISW which interacts with the submarine ridge near 36°W. The baroclinic instability wave developed near the exit of the Filchner Depression leads to intermittent outflow of ISW, and the intermittent passage of ISW‐origin dense water induces 6‐day period oscillation of velocity at the east side of the ridge which coincides well with mooring data. The existence of the ridge induces two different mechanisms for cross‐isobath transport of the ISW‐origin dense water: a strong downslope current at the east side of the ridge and offshore transport by eddies, which are generated when a gravity current makes a detour around the tip of the ridge, over a wide region to the west of the ridge. In a sensitivity experiment where thermobaricity is removed from the equation of state, there is almost no transport of the ISW‐origin water below 3000 m depth. WSBW production rate as a result of descent of the ISW‐origin dense water in the model domain is about ∼0.15 Sv.