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The equatorial Kelvin waves, remotely excited by basin‐scale climate modes, and subsequent coastal trapped waves significantly influence the intraseasonal variations, their low‐frequency modulations, and the frequency of extreme sea level events along the western coast of India. This study demonstrates that the frequency of extreme events are linked to the phase of the Indian Ocean Dipole mode. The temporal changes in the occurrence frequency of extremes are simulated in an eddy‐resolving ocean model consistently with observations. However, a non‐eddying model significantly underestimate the occurrence frequency of extreme sea level events, suggesting the importance of coastal trapped wave propagations regulated by the horizontal scale with the Rossby radius of deformation. This result implies that many state‐of‐the‐art climate models with a one‐degree ocean horizontal resolution may underestimate future coastal sea level variability and the frequency of extreme events under global warming and potential modulations of major internal climate modes. Plain Language Summary: Sea level variations in the northern Indian Ocean are influenced by ocean waves near the coast, typically in a horizontal scale less than 100 km. It remains unclear whether there is a relationship between extreme events associated with coastal waves and climate variability. Also, if such a relationship exists, it is uncertain how well it is represented in climate simulations, which often have relatively coarse horizontal resolution. To highlight the role of relatively small scale coastal waves, this study compared sea level variations along the western coast of India using two ocean models with coarse and fine horizontal resolutions. We found that the high‐resolution model adequately simulates the generation and propagation of coastal waves, and thus successfully simulate sea level variations along western India modulated by large scale climate variability with a 20–150‐day time scale. This result suggests that many recent climate simulations may have underestimated the frequency of extreme sea level events in coastal regions. Key Points: The eddy‐resolving model represents the intrasesonal sea level variability along the coast of India explained by coastal trapped wavesThe occurrence frequency of extreme intraseasonal sea level anomalies is significantly underestimated in the non‐eddying modelChanges in the probability distributions of sea level associated with the Indian Ocean Dipole are simulated in the eddy‐resolving model [ABSTRACT FROM AUTHOR]

Landfast ice is near-motionless sea ice attached to the coast. Despite its potential for modifying sea ice and ocean properties, most state-of-the-art sea ice models poorly represent landfast ice. Here, we examine two crucial processes responsible for the formation and stabilization of landfast ice, namely sea ice tensile strength and seabed–ice keel interactions. We investigate the impact of these processes on the Arctic sea ice cover and halocline layer using the global coupled ocean–sea ice model NEMO-LIM3. We show that including seabed–ice keel stress improves the seasonality and spatial distribution of the landfast ice cover in the Laptev and East Siberian Seas. This improved landfast ice representation sets the position of flaw polynyas to new locations approximately above the continental shelf break. The impact of sea ice tensile strength on the stability of the Arctic halocline layer is far more effective. Incorporating this process in the model yields a thicker, more consolidated, and less mobile Arctic sea ice pack that further decouples the ocean and atmosphere. As a result, the available potential energy of the Arctic halocline is decreased (increased) by ∼ 30kJ/m 2 (∼ 30kJ/m 2 ) in the Amerasian (Eurasian) compared to the reference simulation excluding sea ice tensile strength and seabed–ice keel stress. Our findings highlight the need to better understand landfast ice physical processes conjointly with the subsequent influences on the ocean and sea ice states. [ABSTRACT FROM AUTHOR]

The ocean general circulation model (OGCM) is an essential tool for researching oceanography and atmospheric science. The LASG/IAP climate system ocean model version 3 (LICOM3) is a parallel version of the OGCM. Our goal is to implement and optimize a GPU version of LICOM3 based on compute unified device architecture (CUDA) called LICOM3-CUDA. Considering the characteristics of LICOM3 and CUDA, we design and implement some pivotal optimization methods, including redesigning the numerical algorithms of complicated functions, decoupling data dependency, avoiding memory write conflicts, and optimizing communication. In this paper, we selected two experiments, including 1 ∘ (small-scale) and 0.1 ∘ (large-scale) resolutions to evaluate the performance of LICOM3-CUDA. Under the experimental environment of two Intel Xeon Gold 6148 CPUs and four NVIDIA Quadro GV100s, the LICOM3-CUDA (1 ∘ ) achieves a simulation speed of 114.3 simulation-year-per-day (SYPD). Compare with the performance of LICOM3, the LICOM3-CUDA can run much faster with 6.5 times, and the compute-intensive module achieves over 70 × speedup. In addition, the energy consumption for the simulation year is reduced by 41.3%. As for high-resolution and large-scale simulation, the number of GPUs increased from 96 to 1536 as well as the LICOM3-CUDA (0.1 ∘ ) time consumption decreased from 3261 to 720 seconds with approximately 4.5 × of speedup. [ABSTRACT FROM AUTHOR]

In the ocean, zinc (Zn) is an important element for biological activity and biogeochemistry. The distribution of dissolved Zn in the global ocean is similar to that of silica (Si). Previous model-based experiments proposed the Southern Ocean hypothesis: high Zn/P uptake ratio by phytoplankton in the Southern Ocean leads to Zn-depleted surface water and this anomaly is transported into the interior ocean associated with mode water formation, resulting in a distribution similar to Si. However, recent observational data from the North Pacific showed that there is decoupling of Zn and Si: the correlation between Zn and Si breaks down in the North Pacific. This study investigates the process of the Zn cycle that causes the decoupling of Zn and Si in the North Pacific using a model. We conducted the model experiment with various Zn uptake speeds in the surface ocean, but it was not easy to reproduce Zn concentrations in the North Pacific, indicating that additional mechanisms are required to produce the decoupling of Zn and Si in the North Pacific. By considering additional Zn sources from the continental shelves of the Sea of Okhotsk and the Bering Sea, we found that high Zn concentration and the Zn–Si decoupling in the North Pacific were reproduced, consistent with observational data. Our result suggests that the Zn supply from the coastal regions in the North Pacific has an important role in causing the Zn–Si decoupling. [ABSTRACT FROM AUTHOR]

By analyzing the European Centre for Medium-Range Weather Forecasts reanalysis version 5 (ERA5) dataset, we found increased frequency of marine heatwaves (MHWs) in East Asian marginal seas (EAMS) during the boreal summer (June-July-August) in the recent past. To examine which processes are responsible for the upward trend of MHW occurrence, we performed three numerical simulations using Modular Ocean Model version 5 (MOM5) forced by ERA5 dataset. The first experiment used historical atmospheric variables to force the MOM5 for 1982 to 2020, which reasonably simulated the upward trend of MHWs as well as its dominant variability in terms of temporal and spatial structure in EAMS. The second (third) experiment is the same as in the first except that the atmosphere variables used to force the MOM5 consisted of thermodynamic (dynamic) variables only. The upward trend of MHW occurrence in EAMS is simulated in the first and the second experiment only. We argue that the atmosphere thermodynamic processes, in particular, the shortwave radiative forcing, play a key role in inducing the upward trend of MHW occurrence in EAMS during the boreal summer compared to the ocean dynamic processes.

State-of-the-art coupled general circulation models (CGCMs) are used to predict ocean heat uptake (OHU) and sea-level change under global warming. However, the projections of different models vary, resulting in high uncertainty. Much of the inter-model spread is driven by responses to surface heat perturbations. This study mainly focuses on the response of the ocean to a surface heat flux perturbation F, as prescribed by the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP). The results of ocean model were compared with those of a CGCM with the same ocean component. On the global scale, the changes in global mean temperature, ocean heat content (OHC), and steric sea level (SSL) simulated in the OGCM are generally consistent with CGCM simulations. Differences in changes in ocean temperature, OHC, and SSL between the two models primarily occur in the Arctic and Atlantic Oceans (AA) and the Southern Ocean (SO) basins. In addition to the differences in surface heat flux anomalies between the two models, differences in heat exchange between basins also play an important role in the inconsistencies in ocean climate changes in the AA and SO basins. These discrepancies are largely due to both the larger initial value and the greater weakening change of the Atlantic meridional overturning circulation (AMOC) in CGCM. The greater weakening of the AMOC in the CGCM is associated with the atmosphere—ocean feedback and the lack of a restoring salinity boundary condition. Furthermore, differences in surface salinity boundary conditions between the two models contribute to discrepancies in SSL changes. [ABSTRACT FROM AUTHOR]

Variations of Caspian Sea water budget in the epoch of the Last Glacial Maximum (LGM, ~21 thousand years ago) relative to the preindustrial conditions (~1850 CE) are considered using data of climatic modeling under PMIP4-CMIP6 projects. The variations of evaporation from the Caspian Sea surface and the volumes of river flow required to maintain the equilibrium state of the lake under various climate conditions and under various lake levels were also evaluated with the use of a regional configuration of the eddy-resolvingoceanic model INMIO-CICE. The boundary conditions in the oceanic model were derived from experiments with INMCM climate model reproducing LGM climate and the climate of preindustrial period. According to the data of PMIP4 simulation, both Volga runoff and evaporation from the Caspian Sea surface decreased in LGM epoch compared with the control experiment. The difference between the estimates by different models is rather high: from −5 to −50% for the Volga, and from −2 to −20% for the Caspian Sea. Such results are not enough to make unambiguous conclusions about whether the position of the Caspian in the LGM period was regressive or transgressive; however, deep regression of the Caspian is excluded. Even if the Caspian level is estimated with the use of the results of the model that has demonstrated the maximal decrease in the Volga runoff, the volume of river runoff from the territory of the Caspian basin will be enough to prevent the sea level dropping below –45 m above sea level. [ABSTRACT FROM AUTHOR]

In this study, we report our ocean general circulation model simulations of the global distribution of rare earth elements (REEs) in the ocean. As previously reported (Oka et al. in Glob Biogeochem Cycles 23:1–16, 2009), the vertical profiles of REEs in the North Pacific Ocean are strongly controlled by the reversible scavenging process, and the systematic differences between REEs can be reproduced in the model by selecting an appropriate model parameter which controls affinity to particles. We here demonstrate that the external REE input from the coastal regions also plays a role in controlling the vertical profiles of dissolved REE and their inter-basin differences. The role of the external inputs is especially important for light REEs, such as neodymium (Nd). The linear increase in Nd concentration in the North Pacific Ocean cannot be sufficiently reproduced by the reversible scavenging alone; rather, a combination of the reversible scavenging and the external inputs is necessary. On the other hand, the distribution of heavy REEs, such as lutetium (Lu), can be broadly reproduced without the external inputs, suggesting that Lu has similarity with conservative nutrient-like tracer. When compared with REE observations compiled from both the recently obtained GEOTRACES dataset and pre-GEOTRACES reported data, our simulations successfully reproduced the overall features of these observations. Observational data suggested that the vertical profiles of REEs are not the same among the basins; our model simulations demonstrate that this feature can be clearly reproduced by considering both the reversible scavenging and the external REE inputs from the coastal regions. [ABSTRACT FROM AUTHOR]

Deep cycle turbulence (DCT) is a diurnally oscillating turbulence that penetrates into a stratified shear layer below the surface mixed layer, which is often observed in the eastern Pacific and Atlantic above the Equatorial Undercurrent (EUC). Here we present the simulation of DCT by a global ocean general circulation model (OGCM) for the first time. As the k‐ε vertical mixing scheme is used in the OGCM, the simulation of observed DCT structure based on in situ microstructure measurements can be explicitly demonstrated. The simulated DCT is found in all equatorial ocean basins, and its characteristics agree very well with observations. Zonal and meridional variations of DCT in the entire equatorial Pacific and Atlantic are described through constructing the composite diurnal cycle. In the central Pacific where the maximum shear associated with EUC is deep, the separation of DCT from the surface mixed layer is much more prominent than other areas. Plain Language Summary: Deep cycle turbulence (DCT) is a nighttime intensified turbulence that develops in the stratified layer below the base of the surface mixed layer. It is often observed below the equatorial Pacific and Atlantic cold tongue regions above the Equatorial Undercurrent (EUC). Mixing caused by DCT is essential in modulating sea surface temperature (SST), which could have a large impact on air‐sea interaction and thus global climate variability. However, simulations of DCT in global ocean models have not been demonstrated so far, and the spatial variation of DCT characteristics in the entire equatorial oceans is not well known. This study presents the first global ocean general circulation model simulation of DCT, demonstrated by the comparison of simulated turbulence with that derived from in situ observations. The simulated DCT is found in all equatorial ocean basins, and its characteristics agree very well with observations. Large‐scale spatial variability of DCT in the equatorial Pacific and Atlantic is described through the analysis of model output. The DCT layer completely separated from surface mixed layer is found at locations where the EUC is deep, such as the central equatorial Pacific near the dateline. Key Points: Deep cycle turbulence is well simulated for the first time by a global ocean general circulation model in all equatorial ocean basinsLarge‐scale zonal and meridional variations of the deep cycle turbulence in equatorial Pacific and Atlantic are fully describedSeparation of deep cycle turbulence from the surface mixed layer is most prominent in the central Pacific near the dateline [ABSTRACT FROM AUTHOR]

The formation mechanism as well as its temporal change of the North Pacific subtropical mode water (NPSTMW) is investigated using a 50‐year (1960–2009) ocean general circulation model hindcast. The volume budget analysis suggests that the formation of the NPSTMW is mainly controlled by the air‐sea interaction and ocean dynamics, but there is a regime shift of the relative importance between the two around late‐1980s. While the local air‐sea interaction process is a main driver of the NPSTMW formation prior to late‐1980s, ocean dynamics including the vertical entrainment become dominant since then. The NPSTMW formation is affected by the North Pacific Oscillation simultaneously in the early period, but with a few years lag in the later period. The interdecadal change of the driving mechanism of the interannual variability of the NPSTMW is probably due to the stronger (weaker) influence of local atmospheric forcing in the western North Pacific and unfavorable (favorable) wind stress curl condition for the remote oceanic forcing from the central North Pacific during the former (later) period. This regime shift may be related to the change of centers of the actions of the wind stress curl since the late‐1980s. Plain Language Summary: Using a three‐dimensional ocean model, we conduct a numerical simulation for 1960–2009. The ocean model reproduces well the feature of the North Pacific subtropical mode water (NPSTMW), which is characterized by a thick subsurface layer of the relatively uniform temperature and potential density, to the east and south of Japan. During cooling seasons (from December to following March), the NPSTMW is formed by the atmospheric cooling and the oceanic processes, such as the deepening of the mixed layer depth and advective flux. Our study found that the relative contributions from the two factors differ in two different epochs, before and after late‐1980s. Before late‐1980s, the atmospheric cooling mainly controls the NPSTMW formation, while the oceanic processes driven by remote forcing significantly contribute the NPSTMW formation after late‐1980s. The variability of the NPSTMW formation is related to the basin‐scale climate variability, particularly the North Pacific Oscillation. Key Points: The air‐sea interaction primarily drives the formation of the North Pacific subtropical mode water prior to late‐1980sSince late‐1980s, the formation of the subtropical mode water is mainly controlled by the ocean dynamics driven by remote forcingThe variability of the formation rate is associated to the North Pacific Oscillation [ABSTRACT FROM AUTHOR]

High-resolution global ocean general circulation models (OGCMs) play a key role in accurate ocean forecasting. However, the models of the operational forecasting systems are still not in high resolution due to the subsequent high demand for large computation, as well as the low parallel efficiency barrier. Good scalability is an important index of parallel efficiency and is still a challenge for OGCMs. We found that the communication cost in a barotropic solver, namely, the preconditioned conjugate gradient (PCG) method, is the key bottleneck for scalability due to the high frequency of the global reductions. In this work, we developed a new algorithm—a communication-avoiding Krylov subspace method with a PCG (CA-PCG)—to improve scalability and then applied it to the Nucleus for European Modelling of the Ocean (NEMO) as an example. For PCG, inner product operations with global communication were needed in every iteration, while for CA-PCG, inner product operations were only needed every eight iterations. Therefore, the global communication cost decreased from more than 94.5% of the total execution time with PCG to less than 63.4% with CA-PCG. As a result, the execution time of the barotropic modes decreased from more than 17,000 s with PCG to less than 6000 s with CA-PCG, and the total execution time decreased from more than 18,000 s with PCG to less than 6200 s with CA-PCG. Besides, the ratio of the speedup can also be increased from 3.7 to 4.6. In summary, the high process count scalability when using CA-PCG was effectively improved from that using the PCG method, providing a highly effective solution for accurate ocean simulation.

• Formulations of variable sea ice salinity and melt pond processes in an OGCM. • Observed sea ice salinity distributions are broadly reproduced in both hemispheres. • Satellite-based Arctic pond area variations are well reproduced with new scheme. • Ice/snow albedo reductions in summer become consistent with satellite observations. • Impacts on a large-scale sea ice area and thickness fields are discussed quantitatively. Enhanced representation of sea ice processes in ocean modeling studies is required for advancing our understanding and prediction of the climate variability. In the present study, we improved the sea ice thermodynamics in an OGCM by introducing processes for variable sea ice salinity and melt ponds on sea ice. The former affects the latent heat and conductivity of sea ice as well as the salt budget in the ocean–sea ice system. We formulated salinification and desalination processes based on previous observational studies. Melt ponds enhance summertime surface melting by reducing the surface albedo. In our formulation of the melt pond thermodynamics, the shortwave radiation absorbed in the pond was partly released to the air and partly conducted to the underlying sea ice. By adopting these parameterizations, seasonal cycles of the sea ice salinity distributions in both hemispheres were generally reproduced. Simulated seasonal and interannual variations in the Arctic melt pond area and associated surface albedo reductions were basically consistent with observational estimates. The melt pond effect reduced the Arctic sea ice volume by about 1.5 × 103 km3 in the summer; half of this impact remained throughout the year. Our results suggest that the new parameterizations have the potential to improve the seasonal evolution of the sea ice thickness and concentration distributions, which would contribute to prediction studies. [ABSTRACT FROM AUTHOR]

Radioactive cesium (137Cs) is distributed in the world's oceans as a result of global fallout from atmospheric nuclear weapons tests, releases from fuel reprocessing plants, and inputs from nuclear power plant accident. In order to detect future radionuclide contamination, it is necessary to establish a baseline global distribution of radionuclides such as 137Cs and to understand the ocean transport processes that lead to that distribution. In order to aid in the interpretation of the observed database, we have conducted a suite of simulations of the distribution of 137Cs using a global ocean general circulation model (OGCM). Simulated 137Cs radioactivity concentrations agree well with observations, and the results were used to estimate the changes in inventories for each ocean basin. 137Cs activity concentration from atmospheric nuclear weapons tests are expected to be detectable in the world ocean until at least 2030. [Display omitted] • The global 137Cs distribution was simulated by multiple sources and compared to available observations. • Simulated results were generally in good agreement with the database. • Changes in inventories and flux between ocean basins are summarized. • The impact of global fallout will continue to be detectable and could be a useful water mass tracer for ocean circulation and mixing studies. [ABSTRACT FROM AUTHOR]

Outputs from a high-resolution ocean general circulation model (OGCM) for the period of 1990-2003 indicate an interannual modulation of intraseasonal Kelvin waves along the equatorial Indian Ocean. During normal conditions without IOD event, the first mode explains about 30-40% of the total variance in the western (60-65ºE) and central (75-80ºE) basin, while the second mode contributes up to 45% to the total variance in the central basin around the longitude of 82ºE. In contrast, during the 1997/98 IOD event, the fourth mode caused about 40% of the total variance in the central and eastern basin. During the 1994 IOD event, the contribution from the fourth baroclinic mode in the eastern basin caused 45% of the total variance. In the central basin, the second and the fourth baroclinic mode caused almost the same variance (~40%). The variations in the characteristics of the intraseasonal Kelvin waves are related to variations in the vertical stratification. During the IOD event, the pycnocline in the eastern basin was raised by about 50 m and the stratification at the upper level is strengthened, while it is weakened at lower levels. These changes lead to an increase in the contribution of higher-order baroclinic modes.

Radioactive cesium ( 137 Cs) is distributed in the world's oceans as a result of global fallout from atmospheric nuclear weapons tests, releases from fuel reprocessing plants, and inputs from nuclear power plant accident. In order to detect future radionuclide contamination, it is necessary to establish a baseline global distribution of radionuclides such as 137 Cs and to understand the ocean transport processes that lead to that distribution. In order to aid in the interpretation of the observed database, we have conducted a suite of simulations of the distribution of 137 Cs using a global ocean general circulation model (OGCM). Simulated 137 Cs radioactivity concentrations agree well with observations, and the results were used to estimate the changes in inventories for each ocean basin. 137 Cs activity concentration from atmospheric nuclear weapons tests are expected to be detectable in the world ocean until at least 2030., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Highlights • New methods for online isolation of near-inertial internal waves in numerical models are proposed. • The proposed methods require the velocity records to be stored only at two or three time steps. • The proposed methods perform reasonably well in isolating near-inertial internal waves outside the tropical region. Abstract Near-inertial internal waves (NIWs) play an important role in furnishing ocean diapycnal mixing. However, parameterization of NIW-induced mixing in high-resolution ocean general circulation models (OGCMs) remains a challenging problem partially due to the lack of appropriate tools for isolating NIWs during the model integration. In this study, we propose two new methods feasible for online isolation of NIWs in high-resolution OGCMs, i.e., the two-step filter and three-step filter. The two-step filter only requires the velocity records to be stored at two time steps. It performs reasonably well in isolating NIWs poleward of 30° if the time step is set as 6 h. By utilizing the velocity information at one additional time step, the three-step filter with a time step of 6 h generally outperforms the two-step filter in the global ocean. Specifically, the three-step filter results in reasonable isolation of NIWs poleward of 15° where the NIW-induced mixing is strongest. [ABSTRACT FROM AUTHOR]

Tropical instability waves (TIWs) are a major source of internally-generated oceanic variability in the equatorial Pacific Ocean. These non-linear phenomena play an important role in the sea surface temperature (SST) budget in a region critical for low-frequency modes of variability such as the El Niño-Southern Oscillation (ENSO). However, the direct contribution of TIW-driven stochastic variability to ENSO has received little attention. Here, we investigate the influence of TIWs on ENSO using a 1/4∘ ocean model coupled to a simple atmosphere. The use of a simple atmosphere removes complex intrinsic atmospheric variability while allowing the dominant mode of air−sea coupling to be represented as a statistical relationship between SST and wind stress anomalies. Using this hybrid coupled model, we perform a suite of coupled ensemble forecast experiments initiated with wind bursts in the western Pacific, where individual ensemble members differ only due to internal oceanic variability. We find that TIWs can induce a spread in the forecast amplitude of the Niño 3 SST anomaly 6-months after a given sequence of WWBs of approximately ±45% the size of the ensemble mean anomaly. Further, when various estimates of stochastic atmospheric forcing are added, oceanic internal variability is found to contribute between about 20% and 70% of the ensemble forecast spread, with the remainder attributable to the atmospheric variability. While the oceanic contribution to ENSO stochastic forcing requires further quantification beyond the idealized approach used here, our results nevertheless suggest that TIWs may impact ENSO irregularity and predictability. This has implications for ENSO representation in low-resolution coupled models. [ABSTRACT FROM AUTHOR]

The algorithm for splitting k-ω turbulence equations is used to parameterize viscosity and diffusion coefficients in the ocean general circulation model. The k-ω equations are split into stages describing the transport-diffusion and generation-dissipation of the turbulent kinetic energy and frequency function ω. At the generation-dissipation stage, the equations are solved analytically. Calculations of circulation in the North Atlantic-Arctic Ocean for 1948-2009 have been carried out. The experiments demonstrate an adequate reproduction of hydrophysical characteristics and high efficiency of the algorithm. It is shown that considering the climatic annual mean buoyancy frequency in the turbulence equations at the generation-dissipation stage is an important factor in improving the accuracy of simulated fields. [ABSTRACT FROM AUTHOR]

Previous investigations of the large-scale deployment of Ocean Thermal Energy Conversions (OTEC) systems are extended by allowing some atmospheric feedback in an ocean general circulation model. A modified ocean-atmosphere thermal boundary condition is used where relaxation corresponds to atmospheric longwave radiation to space, and an additional term expresses horizontal atmospheric transport. This produces lower steady-state OTEC power maxima (8 to 10.2 TW instead of 14.1 TW for global OTEC scenarios, and 7.2 to 9.3 TW instead of 11.9 TW for OTEC implementation within 100 km of coastlines). When power production peaks, power intensity remains practically unchanged, at 0.2 TW per Sverdrup of OTEC deep cold seawater, suggesting a similar degradation of the OTEC thermal resource. Large-scale environmental effects include surface cooling in low latitudes and warming elsewhere, with a net heat intake within the water column. These changes develop rapidly from the propagation of Kelvin and Rossby waves, and ocean current advection. Two deep circulation cells are generated in the Atlantic and Indo-Pacific basins. The Atlantic Meridional Overturning Circulation (AMOC) is reinforced while an AMOC-like feature appears in the North Pacific, with deep convective winter events at high latitudes. Transport between the Indo-Pacific and the Southern Ocean is strengthened, with impacts on the Atlantic via the Antarctic Circumpolar Current (ACC). [ABSTRACT FROM AUTHOR]

The results of simulations performed by the CORE-II scenario using the two Russian OGCMs, INMOM and INMIO, are presented. The models use different coordinate systems in the basic set of primitive equations and different numerical techniques. Both models are used as oceanic components of the INM RAS coupled models. Simulations have shown that reproducing ocean circulation using both models agrees with observations and simulations by other models. In general, the INMOM slightly underestimates the meridional heat transport in the ocean when compared to the INMIO model and climatic estimations. However, the INMIO yields a higher bias in temperature than the INMOM. [ABSTRACT FROM AUTHOR]

Previous modeling and observational studies have established that it is possible to accurately monitor the Atlantic Meridional Overturning Circulation (AMOC) at 26.5°N using a coast-to-coast array of instrumented moorings supplemented by direct transport measurements in key boundary regions (the RAPID/MOCHA/WBTS Array). The main sources of observational and structural errors have been identified in a variety of individual studies. Here a unified framework for identifying and quantifying structural errors associated with the RAPID array-based AMOC estimates is established using a high-resolution (eddy resolving at low-mid latitudes, eddy permitting elsewhere) ocean general circulation model, which simulates the ocean state between 1978 and 2010. We define a virtual RAPID array in the model in close analogy to the real RAPID array and compare the AMOC estimate from the virtual array with the true model AMOC. The model analysis suggests that the RAPID method underestimates the mean AMOC by ∼1.5 Sv (1 Sv = 10 6  m 3  s −1 ) at ∼900 m depth, however it captures the variability to high accuracy. We examine three major contributions to the streamfunction bias: (i) due to the assumption of a single fixed reference level for calculation of geostrophic transports, (ii) due to regions not sampled by the array and (iii) due to ageostrophic transport. A key element in (i) and (iii) is use of the model sea surface height to establish the true (or absolute) geostrophic transport. In the upper 2000 m, we find that the reference level bias is strongest and most variable in time, whereas the bias due to unsampled regions is largest below 3000 m. The ageostrophic transport is significant in the upper 1000 m but shows very little variability. The results establish, for the first time, the uncertainty of the AMOC estimate due to the combined structural errors in the measurement design and suggest ways in which the error could be reduced. Our work has applications to basin-wide circulation measurement arrays at other latitudes and in other basins as well as quantifying systematic errors in ocean model estimates of the AMOC at 26.5°N. [ABSTRACT FROM AUTHOR]

Oceanic general circulation models have become an important tool for the study of marine status and change. This paper reports a numerical simulation carried out using LICOM2.0 and the forcing field from CORE. When compared with SODA reanalysis data and ERSST.v3b data, the patterns and variability of the tropical Pacific-Indian Ocean associated mode (PIOAM) are reproduced very well in this experiment. This indicates that, when the tropical central-western Indian Ocean and central-eastern Pacific are abnormally warmer/colder, the tropical eastern Indian Ocean and western Pacific are correspondingly colder/warmer. This further confirms that the tropical PIOAM is an important mode that is not only significant in the SST anomaly field, but also more obviously in the subsurface ocean temperature anomaly field. The surface associated mode index (SAMI) and the thermocline (i.e., subsurface) associated mode index (TAMI) calculated using the model output data are both consistent with the values of these indices derived from observation and reanalysis data. However, the model SAMI and TAMI are more closely and synchronously related to each other. [ABSTRACT FROM AUTHOR]

The treatment of vertical mixing in ocean general circulation models is reviewed and its future direction discussed, especially concerning the reproducibility of the Pacific deep circulation. Internal tides are thought to be the principal source of mixing in the deep ocean, and there have been many studies focusing on generation of internal tides over rough bathymetry and their dissipation in the vicinity of generation. A great challenge currently is how to adequately parameterize the dissipation process of far-propagating internal tides. [ABSTRACT FROM AUTHOR]

Using outputs from an ocean general circulation model, the relative importance of the processes contributing to the development of the Indian Ocean Dipole (IOD) is examined systematically based on two metrics. One metric quantifies the relative importance of the surface heat flux term against the oceanic terms, while the other metric compares the contribution from the horizontal advection and vertical diabatic terms. It is revealed that the anomalous cooling in the eastern tropical Indian Ocean associated with the positive IOD varies with events and four representative events are investigated in more detail. During the 1991 IOD, the horizontal advection term made the largest contribution to the eastern cooling, and the vertical diabatic term was equally important in the early stage of the development. In the 1994 event, negative SST anomalies were generated by the surface heat flux term at first, and then matured by the vertical diabatic term. Anomalous cooling during the 1997 IOD was mainly produced by the vertical diabatic term. In 2012, anomalous surface heat flux and horizontal advection played the crucial role in the development of the eastern pole, but the vertical diabatic term opposed to the anomalous cooling. Furthermore, the dependence of the seasonal prediction skill by a global ocean-atmosphere coupled general circulation model on the generation mechanisms was examined. It is demonstrated that events with the vertical diabatic term playing a more important role in the development of the eastern pole are better predicted than those with the vertical diabatic term making relatively small contribution or opposing the occurrence. [ABSTRACT FROM AUTHOR]

Intercomparison and evaluation of the global ocean surface mixed layer depth (MLD) fields estimated from a suite of major ocean syntheses are conducted. Compared with the reference MLDs calculated from individual profiles, MLDs calculated from monthly mean and gridded profiles show negative biases of 10-20 m in early spring related to the re-stratification process of relatively deep mixed layers. Vertical resolution of profiles also influences the MLD estimation. MLDs are underestimated by approximately 5-7 (14-16) m with the vertical resolution of 25 (50) m when the criterion of potential density exceeding the 10-m value by 0.03 kg m is used for the MLD estimation. Using the larger criterion (0.125 kg m) generally reduces the underestimations. In addition, positive biases greater than 100 m are found in wintertime subpolar regions when MLD criteria based on temperature are used. Biases of the reanalyses are due to both model errors and errors related to differences between the assimilation methods. The result shows that these errors are partially cancelled out through the ensemble averaging. Moreover, the bias in the ensemble mean field of the reanalyses is smaller than in the observation-only analyses. This is largely attributed to comparably higher resolutions of the reanalyses. The robust reproduction of both the seasonal cycle and interannual variability by the ensemble mean of the reanalyses indicates a great potential of the ensemble mean MLD field for investigating and monitoring upper ocean processes. [ABSTRACT FROM AUTHOR]

This paper examines the difference in the Atlantic Meridional Overturning Circulation (AMOC) mean state between free and assimilative simulations of a common ocean model using a common interannual atmospheric forcing. In the assimilative simulation, the reproduction of cold cores in the Nordic Seas, which is absent in the free simulation, enhances the overflow to the North Atlantic and improves AMOC with enhanced transport of the deeper part of the southward return flow. This improvement also induces an enhanced supply of North Atlantic Deep Water (NADW) and causes better representation of the Atlantic deep layer despite the fact that correction by the data assimilation is applied only to temperature and salinity above a depth of 1750 m. It also affects Circumpolar Deep Water in the Southern Ocean. Although the earliest influence of the improvement propagated by coastal waves reaches the Southern Ocean in 10-15 years, substantial influence associated with the arrival of the renewed NADW propagates across the Atlantic Basin in several decades. Although the result demonstrates that data assimilation is able to improve the deep ocean state even if there is no data there, it also indicates that long-term integration is required to reproduce variability in the deep ocean originating from variations in the upper ocean. This study thus provides insights on the reliability of AMOC and the ocean state in the Atlantic deep layer reproduced by data assimilation systems. [ABSTRACT FROM AUTHOR]

Coastal set-net fisheries have been frequently damaged by the occurrence of sudden current (known as kyucho) in the Japan Sea. In this study, a high-resolution coastal ocean model is developed to provide a means to predict this stormy current. The 1.5 km-mesh model nested in a regional ocean data assimilation system is driven by mesoscale atmospheric conditions at 1-hour intervals. The modeled results show rapid changes of the coastal current along the San-in Coast, on the eastern side of the Tango Peninsula, and around the Noto Peninsula and Sado Island, mostly associated with strong wind events. These modeled coastal water responses are consistent with in-situ velocity measurements. The simulation also shows that the vortex separated from the Tango Peninsula frequently grows to a bay-scale anticyclonic eddy in Wakasa Bay. Evidently, the coastal branch of the Tsushima Warm Current becomes unstable due to a strong meteorological disturbance resulting in the generation of this harmful eddy. [ABSTRACT FROM AUTHOR]

To clarify the characteristics and mechanism of mesoscale variability in the deep ocean, we analyzed historical mooring observations conducted at Site R (30°N, 147°E) south of the Kuroshio Extension during 1978–85 and the 10-year output of an eddy-resolving ocean general circulation model. The power spectral density (PSD) of current velocity at a depth of 5000 m had a peak at periods of 45–75 days at two mooring stations that were zonally 98 km apart and at 54 days at Site R in the model. In the model, the PSD at 54 days at 5000 m had a horizontal maximum 100 km north of Site R as well as in the Kuroshio Extension region 500 km north of Site R. Within the high-PSD region near Site R, variability in the velocity was coherent, and its phase propagation was characterized by zonal and meridional wavenumbers of −2.9×10 −5 and 0.1×10 −5 rad m −1 , respectively, the former of which was comparable to the value of −2.6×10 −5 rad m −1 obtained from the mooring observations. Such wavenumbers matched well with the dispersion relation of barotropic topographic Rossby waves (TRWs). Backward ray tracing and PSD distribution suggested that the barotropic TRWs at Site R were generated in the Kuroshio Extension region, and their energy propagated southward. [ABSTRACT FROM AUTHOR]

The surface roughness of sea ice is highly variable because of the diversity of discrete obstacles to the flow present on the sea ice surface. These obstacles result in form drag, an effect poorly accounted for in the calculation of surface fluxes over sea ice in climate models. In this study, we implement in the ocean–sea ice model NEMO-LIM3 a variable formulation of the atmospheric and oceanic neutral drag coefficients over sea ice that includes the form drag effect. We examine the impact of this formulation on the ice cover and ocean surface properties over the past decades in the Arctic and Antarctic. Including form drag in the simulations leads to 10–50 cm thinner sea ice in the peripheral Arctic seas and ∼ 10 cm thicker sea ice in the Antarctic ice pack on annual means. Form drag significantly impacts summer sea surface temperatures in the marginal ice zone (+0.5 to +1.5 °C in the Arctic and -0.5 °C in the Antarctic) and sea surface salinities under the ice cover (+0.52 to +0.89 g/kg in both polar regions). The seasonality of the ocean surface stress under sea ice is reversed, with a maximum (minimum) ocean surface stress under sea ice in summer (winter) when including form drag. However, this seasonal reversal is likely too intense in our simulations due to the underestimation of the prescribed mean floe lengths in summer. In the Arctic, the long-term declines in sea ice volume and multi-year ice are associated with decreasing neutral drag coefficients and negative trends in ocean surface stress. In the Antarctic, the trends in the neutral drag coefficients control the increase (decrease) in ocean surface stress in the Weddell and Ross (Bellingshausen and Amundsen) Seas. Form drag results in a 22–38 % deeper summer mixed layer under polar sea ice and a year-long 10–25 m deeper mixed layer in the Central Arctic. All these impacts on the ocean surface properties demonstrate the relevance of form drag for polar ocean modelling. • Form drag controls the seasonality and trends in ocean surface stress under sea ice. • The mixed layer is deepened by 22–38 % in summer with form drag. • Form drag increases the summer sea surface salinity by 0.52–0.89 g/kg under sea ice. • Arctic MIZ warms (1.0 °C) and Antarctic MIZ cools (−0.5 °C) in summer with form drag. [ABSTRACT FROM AUTHOR]