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.

Ocean general circulation model simulations of anthropogenic tritium releases from the Fukushima Daiichi nuclear power plant site.

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.

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.

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.

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.

Abstract The spatiotemporal characteristics and generation mechanism of submesoscale processes (SMPs) in the low-latitude western Pacific Ocean are investigated based on 1/30° Ocean General Circulation Model for 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 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-islands interactions is the mechanism responsible for generation and seasonal modulation of SMPs. This stands in contrast to mid-latitudes, 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% 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.

Geological observations informed by climate dynamics imply that the oceans were 99.9% covered by light-blocking ice shelves during two discrete, self-reversing Snowball Earth epochs spanning a combined 60 to 70 Myr of the Cryogenian Period (720 to 635 Ma). The timescale for initial ice advances across the tropical oceans is ~300 y in an ice−atmosphere−ocean general circulation model in Cryogenian paleogeography. Areas of optically thin oceanic ice are usually invoked to account for fossil marine phototrophs, including macroscopic multicellular eukaryotes, before and after each Snowball, but different taxa. Ecosystem relocation is a scenario that does not require thin marine ice. Assume that long before Cryogenian Snowballs, diverse supra- and periglacial biomes were established in polar−alpine regions. When the Snowball onsets occurred, those biomes migrated in step with their ice margins to the equatorial zone of net sublimation. There, they prospered and evolved, their habitat areas expanded, and the cruelty of winter reduced. Nutrients were supplied by dust (loess) derived from cozonal ablative lands where surface winds were strong. When each Snowball finally ended, those biomes were mostly inundated by the meltwater-dominated and rapidly warming lid of a nutrient-rich but depauperate ocean. Some taxa returned to the mountaintops while others restocked the oceans. This ecosystem relocation scenario makes testable predictions. The lineages required for post-Cryogenian biotic radiations should be present in modern polar−alpine biomes. Legacies of polar−alpine ancestry should be found in the genomes of living organisms. Examples of such tests are highlighted herein.

Abstract The North Pacific subtropical mode water (NPSTMW) is a vertically homogeneous water mass located between the seasonal and permanent thermoclines in the subtropical northwestern Pacific Ocean. It plays a critical role in oceanic heat and carbon storage, significant for understanding climate change. However, coarse-resolution models tend to overestimate NPSTMW formation compared to observations. In this study, we show that the observed NPSTMW formation is well reproduced by a 1/30° submesoscale-permitting Ocean General Circulation Model. Our findings reveal that submesoscale restratification significantly reduces NPSTMW formation. During late winter to early spring—when wintertime convective cooling deepens the mixed layer to its annual maximum—submesoscale effects associated with mixed layer instability (MLI) are mostly energetic. The MLI induces surface buoyancy gain, shallowing the mixed layer, narrowing the outcrop zone, and reducing the ventilation time of NPSTMW. Additionally, submesoscale effects indirectly reduce the NPSTMW formation by modulating the large-scale deep mixed layer pattern in the Kuroshio Extension region. These results underscore the importance of incorporating submesoscale effects for an accurate estimation of the NPSTMW formation, with broader implications for improving climate models. Significance Statement This study evaluates the submesoscale effects on the formation of the North Pacific subtropical mode water (NPSTMW). This is important because the formation and associated strength of NPSTMW plays a significant role in shaping the North Pacific Ocean circulation and regulating the oceanic heat and carbon storage. Our results highlight that explicitly resolving submesoscale processes is essential for accurately simulating mode water. Alternatively, appropriate parameterization of submesoscale effects is needed for climate models, which are often run over hundreds if not thousands of years.

Abstract Near-surface measurements of meridional velocity suggest that wind forcing excites equatorial waves in the biweekly band in the Indian Ocean. The characteristics of these waves in the deep ocean are poorly constrained, and it is unclear how well models capture the deep variability. In this work, biweekly temperature variations in a few low vertical modes in the deep east Indian Ocean are observed using seismically generated sound waves. These so-called T waves are generated by earthquakes off Sumatra and received by a hydrophone station off Diego Garcia. Changes in their travel times reflect temperature-induced sound speed variations in the intervening ocean. Regression analysis indicates that these variations are caused by westward-propagating Yanai waves. A comparison between T-wave data and model output shows generally good consistency in biweekly variations dominated by the first three vertical modes, although the biweekly variance differs by up to a factor of 2 between the data and the models. A similar degree of discrepancy appears in the comparison between the models and deep mooring measurements. These results highlight the potential of using T-wave data to study biweekly Yanai waves in the deep equatorial ocean and to calibrate numerical simulations of the variability they cause. Significance Statement Biweekly Yanai waves are an important mode of variability in the tropical Indian Ocean, affecting the Indian and Australian–Indonesian monsoons. This study examines biweekly Yanai waves in the east Indian Ocean using sound waves generated by natural repeating earthquakes. The sound waves sample the deep structure of Yanai waves, which has been poorly constrained by previous observations. The seismic data generally show good consistency with output from two ocean general circulation models, although quantitative differences are manifested and can help improve the representation of Yanai waves in numerical models.

Abstract The ice shelves of the Amundsen Sea are in a phase of rapid melting with intruded Circumpolar Deep Water (CDW) from outside the continental shelf contributing most of the heat. Using a coupled sea ice—ice shelf—ocean general circulation model, the cross‐shelf break heat flux and the mechanism of eastward undercurrent deflection are studied. Model results show higher cross‐shelf break heat transfer during winter months regulated by both the barotropic and baroclinic geostrophic flow. The vorticity budget along the continental shelf break is examined using the depth‐averaged vorticity budget equation based on the model's outputs. Results show that the advection of planetary vorticity (APV) and the joint effect of baroclinicity and relief (JEBAR) dominate the vorticity balance at the CDW intrusion sites on the shelf break, and the JEBAR effect is considered an effective indicator of CDW intrusion. The CDW intrusion is mainly regulated by the southward deflection of the undercurrent on the Amundsen Sea slope. Pre‐deflection, the undercurrent's core lies on the southern edge of the shelf break, enabling it to modulate downstream density transport through its vertical distribution variations. Concurrent increases in undercurrent velocities and vertical extent are captured upstream of intrusion sites, supporting more CDW intrusions per unit time and altering the horizontal density gradient, thereby amplifying the JEBAR effect. Additionally, spectral analysis reveals a semiannual cycle in the JEBAR amplitude and heat flux across the Amundsen Sea slope.