Abstract In forced ocean general circulation models (OGCMs), regional sea‐level interannual variability arises from internal ocean dynamics and external forcings, among which freshwater fluxes from Greenland and rivers are typically prescribed as a seasonal climatology rather than with their full variability. This study examines the impact of fully time‐varying rivers and Greenland fluxes on forced and internal sea‐level variability over 1980–2018, using ensemble‐based sensitivity experiments of an eddy‐permitting OGCM. We introduce an improved, closed ensemble variance decomposition by adding a biased variance term which accounts for ensemble variance in the long‐term time mean. The impact of continental freshwater fluxes variability on sea‐level variance is highest in the tropical South Atlantic, the maritime continent, and the Kuroshio region. The first two regions are mainly impacted by rivers, primarily through a forced halosteric response, occurring over the full ocean depth in the tropical South Atlantic and mostly over the first 50 m in the maritime continent. In the Kuroshio region, sea‐level variance is impacted by both rivers and Greenland through an internally driven response of thermosteric origin. The biased variance enables to correctly evaluate the sources of halosteric sea‐level variability in that region as it reveals that its long‐term variability is not well resolved by our experiment length. These results emphasize the need for accurately representing time‐varying freshwater fluxes in OGCMs to better capture sea‐level variability, and the value of the newly closed decomposition for correctly attributing changes in internal variance of any variable, especially in regions of high variability and for short‐timescale experiments.

Numerical simulations of oceanic variations at submesoscale spatial scales are essential for understanding dynamic characteristics of the Northern South China Sea (NSCS). Currently, the spatial resolution and accuracy of available reanalysis and simulation datasets are still inadequate to comprehensively capture submesoscale processes, leading to an incomplete depiction of the region's detailed dynamic characteristics. In this study, a regional oceanic simulation dataset at (1/90) ° × (1/90) ° spatial and daily temporal resolutions for the year 2019 is produced based on a non-hydrostatic ocean general circulation model (OGCM). Assessments from an idealized experiment suggest that non-hydrostatic calculations can improve the simulation accuracy of high-resolution OGCMs for small-scale oceanic features, such as internal tides or submesoscale phenomena. Comparisons with the observations demonstrate that simulations from the non-hydrostatic OGCM are generally more accurate than those from hydrostatic OGCM. Along with its good performance in simulating dynamic processes in the NSCS, this dataset can enhance understanding of the dynamical patterns and interactions among multi-scale processes, including large-scale circulation, mesoscale eddies, and submesoscale phenomena.

ABSTRACT The Rio Negro Frog ( Atelognathus nitoi ) is an amphibian species endemic to Patagonia, shared between Argentina and Chile. Currently, it is classified as Vulnerable (IUCN) and is mainly threatened by anthropogenic pressures such as habitat alteration and climate change. Due to its life history and habitat requirements, primarily determined by temperature and precipitation, climate change may particularly influence the species' distribution. We evaluated how climate change may affect geographic range by producing a Species Distribution Model (SDM), generating a habitat suitability map and calculating its Extent of Occurrence (EOO). Projections were made for the period 2041–2060 under the Shared Socioeconomic Pathways SSP1‐2.6 and SSP5‐8.5, representing optimistic and pessimistic climate change scenarios, respectively. A set of five atmosphere–ocean general circulation models was used to reduce uncertainty. We evaluated the degree of connectivity of the lagoons with records of the species by applying Lin's functional linkage index, allowing comparisons in terms of their relative importance for the system's connectivity. Our results indicate an increase in suitability area over time, with a westward distribution shift in future scenarios. However, local extinctions are projected at the northern and southern margins of the species' range. Therefore, although the EOO is expected to expand, the species' life‐cycle characteristics, limited dispersal capacity and low connectivity between the populations analysed should be taken into account when planning conservation actions.

This study uses the ocean general circulation model COCO4.9 to examine for the first time the influence of climate conditions and horizontal resolutions on the spatial and temporal distributions of anthropogenic tritium released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) site into the ocean. For the tritium activities, the accidental discharge of March 2011 and the treated water release from TEPCO Scenario A (i.e., "largest amount of tritium" scenario) were used as inputs in COCO4.9 to perform global ocean tritium simulations extending up to the year 2099. Simulated tritium concentrations in the Pacific Ocean reveal similar spatial distributions and are below the detection limit, except for a tritium peak near the FDNPP discharge site during the accident. Under the SSP5-8.5 climate scenario (the highest CO2 emission case scenario), the shifting of the Kuroshio extension northward and the associated enhanced eastward transport affect the temporal variability of the tritium signal and increase the tritium concentration at the south of Japan, but still below the detection limit. In the high-resolution experiment, the Kuroshio current and its extension are narrower and stronger in the marginally eddy-resolving simulation and the transport of tritium is strengthened, allowing it to reach the western US or the Asian coast from the release point in a shorter time. However, except near the FDNPP discharge site, tritium concentration values are only slightly affected by the horizontal resolution, showing that the long-term safety threshold in terms of tritium concentration is not exceeded with the currently planned treated water release.

A parallel algorithm for an Ocean General Circulation Model based on a unified dynamics framework

Abstract Kuroshio frontal eddies in the East China Sea (ECS) have been observed and investigated for a long time; however, the energetics of frontal eddy‐mean flow interactions remain unclear. In this study, we revisit the Kuroshio frontal eddies in the ECS from the perspective of the eddy energy budget using an eddy‐resolving ocean general circulation model (OGCM) named OFES2. By decomposing the variables into frontal eddy (5 < T < 20 days) and background components ( T > 20 days), we diagnose each term in the eddy energy budget equation to examine their contributions and spatiotemporal variations. The results demonstrate that the frontal eddy energy increases downstream along the Kuroshio in the ECS, with high values in the upper 500 m. Baroclinic instability is the primary source of frontal eddy energy in the entire ECS Kuroshio region, while barotropic and shear instabilities also contribute to one‐third of the total energy conversion. In contrast, the work done by wind to eddy energy is insignificant. Further instability analysis suggests that these eddies can be generated either locally within the ECS or come from the upstream region east of Taiwan Island, with a faster growth rate in the ECS. The temporal variation of the frontal eddy energy is largely controlled by baroclinic instability, which is further modulated by the Kuroshio volume transport. The results of this study provide new insights into the Kuroshio frontal eddies, which may be extended to other western boundary current regions where the frontal eddies exist.

Abstract Long‐lead seasonal forecasts (>12 months) of the Indian summer monsoon rainfall (ISMR) are crucial for adaptive planning and damage minimization against climate change‐induced increasing threats of higher frequency of hydrological disasters in the coming decades. However, the growth of initial errors and drift of forecast climatology with lead month drive the seasonal forecast skill of ISMR by Atmosphere–Ocean General Circulation Models (AOGCMs) to decrease with lead time, making them useless beyond an approx. six‐month lead. Hope of overcoming the challenge is rekindled from recent advances in the application of deep‐learning models to weather and climate prediction that extend the skill of weather prediction beyond the limit of the best numerical weather prediction (NWP) models. Simultaneous to this advance, we have established the physical basis of high‐potential predictability of seasonal forecast of ISMR up to 24‐month leads. Here, we develop a physics‐guided deep‐learning (PGDL) model‐based ‘long‐lead forecast system’ for ISMR trained on the relationship between ISMR and the depth of the 20 °C isotherm in the tropics from a large ensemble of AOGCM historical simulations and past observations to overcome the challenge of poor forecast skill of ISMR at long leads. In contrast to the initialized physical AOGCMs, our model makes increasingly skillful seasonal forecasts up to 24‐month leads in accordance with potential predictability while demonstrating superior skill in predicting extreme excess/deficient ISMR between 1980 and 2023 at 18‐ and five‐month leads. Operational feasibility of the model is demonstrated by making an experimental 18‐month lead forecast of ISMR for 2024. Our findings establish a physical basis and methodology for long‐lead seasonal prediction of ISMR and a building block for three‐dimensional tropical seasonal predictions.

ABSTRACTThe Japanese sardine, Sardinops melanostictus, is a major fishery resource in the Sea of Japan (SOJ) and East China Sea (ECS); however, recent research on suitable habitats and migration patterns of this fish in these seas is lacking. We aimed to identify the oceanographic conditions (temperature, chlorophyll‐a concentration, and sea surface height anomaly) suitable for Japanese sardines and predict migration patterns based on temporal variation in areas with suitable oceanographic conditions. We evaluated suitable oceanographic conditions using generalized linear models and presence/absence data from Japanese purse seine fishery operations. Based on the ocean general circulation models, monthly habitat suitability index (HSI) values were calculated and mapped. Areas with high HSI values indicated two possible migration patterns: (1) From January to June (winter–spring), sardines move from the northern SOJ to the Japanese coastal area and then migrate northward again. This pattern matches temporal variations in egg distribution, potentially reflecting the migration of age‐1+ sardines. The HSI maps suggest that age‐1+ sardines inhabit the northeastern edge of the SOJ and its adjacent areas during summer. (2) From July to December (summer–autumn), sardines migrate from the southern part of the SOJ to the coastal area. We consider this a possible age‐0 sardine migration pattern because mainly age‐0 sardines are caught in the southern part of the SOJ in summer–autumn. Thus, we suggest that the two age cohorts have different migration patterns and that sardine stocks in the SOJ and ECS comprise a mix of these two migration groups.

The three-dimensional energetics of deep mesoscale eddies is investigated in a time-dependent theoretical framework using 36-year output of a 1/10° eddy-resolving ocean general circulation model. Composite analyses are conducted based on 29 anticyclonic eddies in the Deep Western Boundary Current (DWBC) at 11°S. The DWBC constitutes a key component of the lower limb of the Atlantic Meridional Overturning Circulation, crucial for comprehending climate change dynamics. Energetics analyses reveal that within the DWBC isolated eddies, advection is the main source of eddy kinetic energy (EKE) and eddy potential energy (EPE). Near the coast, energy conversion terms show that the anticyclones drain energy from the mean DWBC, enhancing the eddy field. Depth-integrated analysis found direct and inverse energy cascades within the average DWBC eddy with similar magnitude and area coverage size. The eddy–mean flow interaction terms were analysed in an Eulerian frame of reference along the eddy area, depicting their variability while migrating at a fixed point. Overall, direct energy cascade dominates the barotropic energy conversions during eddy migration, with enhanced conversion rates associated with peaks of eddy velocity. By contrast, the baroclinic energy conversions presented alternate direct-inverse energy cascades during eddy migration. At the eddy core depth, the barotropic (baroclinic) energy pathway contributes to the growth (decay) of EKE (EPE) at a rate of ~1.0 J m−3 day−1 (5.4 × 10−6 J m−3 day−1). This study seeks to extend our knowledge of the energy budgets within deep mesoscale eddies, a key factor for understanding ocean dynamics and circulation.

Abstract. The atmospheric forcing and the heat exchanges between the ocean and the atmosphere represent one of the major sources of uncertainty for numerical ocean reconstructions and predictions, together with inaccuracies in vertical mixing and solar radiation penetration. Air–sea heat fluxes may suffer from inaccuracies in meteorological fields, sea surface variables, and bulk formulations, which have a strongly nonlinear dependence on the ocean state. Here, state-dependent errors in heat fluxes are learned by artificial neural networks (ANNs) from a dataset of heat flux correction terms, derived in turn from previous sea surface temperature nudging experiments. The pre-trained model predictors include stationary fields, atmospheric forcing data, ocean state, and stratification indices. Variable importance scores emphasize the dependence of air–sea heat flux errors on wind forcing. The pre-trained heat flux correction model is then used to adaptively correct fluxes online, in a series of global ocean experiments performed with the NEMO version 4 (Nucleus for European Modelling of the Ocean) ocean general circulation model, augmented with ANN inference capabilities in Fortran90. Results indicate the positive impact of the correction procedure, beyond the training period, e.g. in independent observation–poor and –rich periods, leading to the same dynamic and subsurface signature as in nudging experiments. Prediction experiments also indicate the method's potential for use in operational forecast applications. The method may also be adopted in coupled long-term reanalyses, long-range predictions, and projections.

Closed parameterizations (aka turbulence closures) are needed for representing the effects of unresolved oceanic mesoscale eddies in non-eddy-resolving and eddy-permitting oceanic general circulation models, such as those used for climate modeling studies. One of the most significant difficulties for parameterizing eddy effects is eddy backscatter, which largely maintains eastward jet extensions of the western-boundary currents and their adjacent recirculation zones. In this paper, we focus on the classical wind-driven, quasigeostrophic double-gyre ocean dynamics and propose and test a novel data-driven eddy closure. For this, the eddy effects are defined as the coarse-grid model errors arising from the approximation of the given eddy-resolving reference solution containing an energetic and coherent eastward jet. Without the eddy effects being taken into account, the coarse-grid non-eddy-resolving version of the model yields no eastward jet at all. These missing eddy effects are restored approximately by the implemented eddy closure that interactively corrects the dynamically resolved potential vorticity field. The closure is data-driven because it utilizes some important information about the actual eddies in the reference solution, which is treated as a substitute for the oceanic observational data. The systematically assessed closure skills are significant because the eddy-parameterized solutions qualitatively correctly recover the eastward jet, which is completely missed otherwise. First, our results serve as a proof of concept for implementing a closure extension into the primitive equations, which are used routinely in comprehensive oceanic general circulation models. Second, our results emphasize the fundamental importance of representing the key eddy/large-scale correlations by any parameterization of the eastward jet eddy backscatter.

When modeling processes in the ocean, the issue of describing turbulent exchange inevitably arises. Today, there are numerous methods for parameterizing turbulence in the upper layer of the ocean. The most common and established closure methods for hydrodynamic equations are considered by introducing turbulent kinetic energy and turbulent mixing length, and a formulation of the ocean general circulation model is provided. A series of experiments were conducted, each using different combinations of equations for turbulence parameterization, which also utilized data from The Copernicus Global 1/12° Oceanic and Sea Ice GLORYS12 Reanalysis and HYCOM + NCODA Global 1/12° Reanalysis to describe the advective components of scalar quantities. The comparison of model data was made with observational data obtained from automatic marine stations of the Pacific Marine Environmental Laboratory. It is shown that using more complex forms of the turbulent kinetic energy equation, as well as additional equations for calculating the turbulent mixing length, does not lead to unambiguous improvements in results. It is also shown that the same combinations of equations can yield opposite results in terms of quality.

Abstract Intraseasonal variability in zonal velocity at middepths (between 450 and 800 m) of the equatorial Indian Ocean is investigated using in situ velocity measurements for the period from 2014 to 2019 at 0°, 83°E and the output of a wind-forced ocean general circulation model (OGCM). The spectral analysis of observed zonal velocity indicates that the highest energy peak at middepths on intraseasonal time scales is at the period of about 57 days between 450- and 800-m depths. The OGCM is able to simulate the spectral peak qualitatively. The statistical analysis using model output shows that the meridional structure, zonal wavelength, and vertical wavelength of 57-day variability in zonal velocity are consistent with those of a free, first meridional mode Rossby wave in an ocean at a state of rest. Two possibilities are discussed as the energy source of middepth 57-day variability. The first is that variability near the surface propagates to middepths as a wind-forced linear Rossby wave. This possibility is partly supported by the analysis of wave rays, which connect the spectral peak of surface zonal velocity with the analysis region at the middepths. The second possibility is the supply of energy by nonlinear advection, which is supported by the results obtained from the zonal kinetic energy budget. It is found that the forcing by advection propagates to the west at a similar phase speed to that of zonal velocity, suggesting a resonant forcing. Significance Statement Previous studies examined intraseasonal variability in zonal velocity near the surface in the equatorial Indian Ocean; reported elevated energy at the periods of about 30, 60, 90, and 180 days; and proposed their generation mechanisms. In contrast, intraseasonal variability in zonal velocity at middepths has not been fully investigated. The current study examines zonal velocity variability in the central equatorial Indian Ocean below the pycnocline using in situ observations and model output and found that the highest energy peak on intraseasonal time scales at middepths is at the bimonthly period (about 57 days). The horizontal and vertical structures of detected variability are examined, and their energy source is discussed.

Abstract The spatiotemporal characteristics and generation mechanisms of submesoscale processes (SMPs) in the low-latitude western Pacific Ocean are investigated based on 1/30° Ocean General Circulation Model for the Earth Simulator (OFES) outputs and mooring measurements. Energetic submesoscale activities with the spatial and temporal scales shorter than 80 km and 20 days, respectively, are detected southeast of the Mindanao Island and north of the Halmahera Island along the paths of the Mindanao Current and Halmahera Eddy. Mooring observations indicate that the SMPs are intensified near the surface and can penetrate the thermocline down to 150 m. SMPs exhibit a significant seasonal cycle with high submesoscale kinetic energy (SKE) values in summer and autumn and relatively small SKE values in winter and spring. Barotropic instability associated with current–island interactions is the mechanism responsible for the generation and seasonal modulation of SMPs. This stands in contrast to the midlatitudes, where SMPs extract available potential energy through mixed layer baroclinic instability. Further energetic diagnostic analysis indicates that in the barotropic energy transfer chain, the mean kinetic energy is the main energy source for SKE. Mean kinetic energy contributes 65.6% of the kinetic energy to submesoscale processes, while the contribution of eddy kinetic energy is only 35.4%. Furthermore, the SKE budget results imply a route to energy dissipation via the forward energy cascade, which emphasizes the effects of SMPs on the diapycnal mixing in the low-latitude western tropical Pacific Ocean.

The simulation of the South China Sea by the variable resolution version of the global ocean general circulation model LICOM3.0

Calcium carbonate dissolution is the dominant negative feedback in the ocean for neutralizing the acidity from rising atmospheric carbon dioxide. Mimicking this natural process, the accelerated weathering of limestone (AWL) can store carbon as bicarbonate in the ocean for tens of thousands of years. Here, we evaluate the potential of AWL on ships as a carbon sequestration approach. We show a successful prediction of laboratory measurements using a model that includes the most recent calcite dissolution kinetics in seawater. When simulated along a Pacific shipping lane in the Estimating the Circulation and Climate of the Ocean–Darwin ocean–general circulation model, surface alkalinity and dissolved inorganic carbon increase by <1.4% after 10 years of continuous operation, leaving a small pH and partial pressure of carbon dioxide impact to the ocean while reducing 50% carbon dioxide emission in maritime transportation.

In marine ecosystems, cetaceans are large mobile predators that depend on maximizing foraging efficiency. Their presence within a habitat can therefore be strongly related to the modulation of local prey by oceanographic conditions. Understanding how cetaceans are impacted by prey responses to the physical environment is challenging due to the difficulty of collecting presence data of cetaceans and their prey over long, comparable time periods. We used passive and active acoustic recordings collected from moorings within the San Diego Trough, along with physical oceanographic sampling (i.e. in situ, satellite-derived, and ocean general circulation model measurements), to elucidate relationships between cetaceans, their prey, and the physical environment. Our results show that the predator-prey dynamics of some cetaceans within the San Diego Trough are influenced by seasonal changes in the physical oceanographic conditions and processes that shape their prey resources. Specifically, common dolphin Delphinus delphis foraging activity increased during conditions associated with increased presence of diel vertically migrating fish prey. Blue whale Balaenoptera musculus foraging-associated acoustic activity increased during periods with increased presence of mid-water crustacean zooplankton and was replaced with breeding-associated acoustic activity during conditions associated with the waning of mid-water crustacean zooplankton. Fin whale B. physalus foraging-associated calling activity was more complex to model, most likely because these animals have a generalist diet and occupy this area year-round. Our results highlight environmental conditions and features relevant to cetaceans inhabiting this region and may aid in developing better spatially explicit management actions.

Abstract The frequency of multiyear La Niña (MLN) events is increasing under global warming, exerting significant impacts on marine ecosystems through various ocean dynamic processes. However, the characteristics and physical mechanisms underlying the response of ocean chlorophyll to MLN events remain poorly understood. In this study, using observational and reanalysis data, we show that surface chlorophyll in the eastern and western equatorial Pacific exhibits distinct responses in individual years during the MLN events. In the first year, enhanced vertical mixing induced by intensified trade winds facilitates a rapid increase in both large and small phytoplankton in the eastern and western equatorial Pacific, respectively, leading to an overall increase in surface chlorophyll. In the second year, the zonal advection process plays a key role in determining the decrease and increase in chlorophyll during the boreal spring and winter in the western equatorial Pacific, respectively. In contrast, a notable decrease in chlorophyll in the eastern equatorial Pacific is associated with ocean wave adjustments during the boreal spring. Sensitivity experiments using an ocean general circulation model confirm that the reduction in chlorophyll in the eastern equatorial Pacific is driven by easterly wind anomalies over the northwestern equatorial Pacific, along with westerly wind anomalies associated with the negative phase of the North Pacific Meridional Mode. These anomalies generate eastward downwelling Kelvin waves along the equator, which deepen the thermocline and nutricline, further contributing to the reduction in chlorophyll through weakened upwelling in the eastern equatorial Pacific. These findings suggest that marine ecosystems exhibit complex regional responses to MLN events, which are closely associated with ocean dynamic processes.

Abstract The Indonesian Throughflow (ITF) regulates heat and freshwater distributions over the Indo‐Pacific Oceans and fundamentally affects the climate. The past decade has witnessed acute interannual variations in the volume transport within the Makassar Strait—the main ITF inflow passage—such as a decrease of ∼4 Sv (1 Sv ≡ 10 6 m 3 s −1 ) in 2015–2016 boreal winter and an enhancement of ∼3 Sv in 2017 autumn, relative to a mean transport of ∼12 Sv. The Pacific Ocean dynamics, dictated largely by El Niño‐Southern Oscillation (ENSO), cannot fully explain these variations, and a quantitative understanding of the Indian Ocean (IO) dynamics involved in the ITF transport variability remains lacking. Here, by performing regional forcing experiments with a 0.1° ocean general circulation model, we reveal that the wind‐driven IO dynamics have operated as a buffering effect for ∼56% of the time and a reinforcing effect for ∼44% of the time during the past decade. Notably, the IO dynamics buffered the weakened ITF by ∼2 Sv in 2015–2016 winter and contributed to the enhanced ITF by ∼0.5 Sv in 2017 autumn. The buffering effect of IO winds is commonly seen during strong ENSO events, while the reinforcing effect arises from Indian Ocean Dipole (IOD) events independent of ENSO. Our study aids in the prediction of the ITF strength under the amplifying ENSO and IOD variabilities expected in a warming climate.

Abstract High‐frequency wind and wave variability influence air‐sea gas fluxes by modulating the gas transfer velocity at the interface. Traditional gas transfer velocity formulations scale solely with wind speed and neglect wave effects, including wave breaking and bubble‐mediated transfer. In this study, we quantify the influence of wave effects on the air‐sea flux and ocean carbon storage using a wind‐wave‐bubble gas transfer velocity formulation in an ocean general circulation model (MOM6‐COBALTv2). Wave effects introduce strong variability in global air‐sea fluxes at high‐frequency and seasonal timescales (+15–40%). Compared to a traditional wind‐dependent formulation, local fluxes can be modified by 2–20 mmol (i.e., 20–50% flux difference), with the largest differences occurring during storms. The wind‐wave‐bubble formulation yields a modest global increase in ocean carbon storage (+0.07 PgC , 3%) due to regional and seasonal compensations, as well as the p feedback that limits the flux response to a faster exchange velocity. Yet, wave effects lead to an enhancement of carbon storage within the ocean interior, with the largest gain in mode and intermediate waters and a wave‐induced hemispheric asymmetry in carbon storage. Notably, the southern hemisphere, where wave activity is consistently high, gains more carbon than the more sheltered northern hemisphere. These results highlight the need to account for wave‐induced variability to capture local and seasonal carbon dynamics, which are essential, for instance, to high‐frequency in situ observational deployments and regional marine carbon dioxide removal assessment efforts.