Physical Oceanography is a fundamental discipline for gaining an in-depth understanding of the ocean and its role in the Earth system. It primarily studies the dynamic and physical properties, processes, and mechanisms of the ocean by analyzing movements, distributions, variation of physical and biogeochemical properties of seawater. Over the past century, physical oceanography has evolved from observing, discovering, and understanding regional-scale phenomena into a science that systematically reveals the ocean's variability and its role in the Earth system from a global perspective, relying on observations, theories, and numerical models. Driven by the needs of human social development, continuous innovations in observational technologies, rapid improvements in computing capabilities, and the widespread application of artificial intelligence theories and methods, physical oceanography has become an interdisciplinary science integrating theories, technologies, and engineering, which equally emphasizes ocean exploration and ocean governance and is driven by multiple application scenarios. Ocean plays a unique role in global climate change. In the past century, global climate has been continuously warming under the forcing of greenhouse gas emissions from human activities. During this process, the ocean fundamentally mitigates the rate of global anthropogenic warming, by absorbing more than 90% of the excess heat in the entire climate system and over 30% of the greenhouse gases emitted by human activities. Obviously, a better understanding of the dynamic and physical properties, processes, and mechanisms of the ocean is the core foundation for further clarifying the ocean's role in global change and the impact of human activities on the Earth's climate system. In this context, we systematically reviewed developments and trends of physical oceanography, summarized grand challenges of physical oceanography from three dimensions-research paradigms, spatiotemporal scales, and interface interactions, as follows: (1) Research paradigm: it encompasses observation, numerical simulation, theoretical analysis, and artificial intelligence. Developing new observational technologies, enhancing the construction of ocean observation systems, and acquiring ocean observational data have always been the foundation of the development of physical oceanography. Meanwhile, since no observation system can achieve full and real-time coverage of the global ocean, it is necessary to rely on computer technologies, including computational fluid dynamics and artificial intelligence, to simulate and fully understand the variation processes of the entire ocean. (2) Spatiotemporal scale: global ocean experiences multi-scales temporal and spatial variability, ranging from seconds, minutes to thousands or even tens of thousands of years, and from centimeters, meters to thousands or even tens of thousands of kilometers. Oceanic changes involve energy exchange and interaction between movements across the aforementioned spatiotemporal scales. (3) Interface interaction: it includes not only connections between estuarine and coastal areas and the deep ocean, and between seawater and ice, but also interactions between the ocean and the atmosphere, the ocean and the land (including the seabed), and the ocean and the biosphere. Based on these three dimensions, we refined our understanding into Ten priorities for Physical Oceanography. They are: (1) construction of a cross-scale, real-time global ocean observation system; (2) development of ultra-high-resolution global ocean numerical models and ocean digital twin; (3) meso-and small-scale dynamic processes in the ocean, and their material and energy transport and climatic effects; (4) mechanisms of multi-scale air-sea interaction and extreme climate events; (5) marine energy and material cycles regulated by complex seabed topography; (6) key processes and climatic effects of rapid polar changes; (7) structure, causes, and driving mechanisms of deep ocean circulation; (8) tipping point of thermohaline circulation and its predictability; (9) resilience regulation of estuarine-coastal systems under compound stress; (10) impacts of marine dynamic processes on carbon-nitrogen-oxygen biogeochemical cycles Ten priorities for Physical Oceanography outlines the core development directions of physical oceanography in the coming period and will promote a deeper understanding of the ocean and its role in the Earth system.

Atmospheric rivers (ARs) are long, narrow corridors of intense water vapor transport that can cause extreme precipitation, particularly upon landfall in mountainous regions. Most existing AR projections rely on low-resolution (LR;-0.5 degrees-1 degrees) climate models, which show systematic biases in AR simulation, casting doubt on the accuracy of AR projections. This study evaluates AR simulations and projections using both high-resolution (HR) and LR Community Earth System Model (CESM) across boreal winter and summer seasons on a global scale. We fi find that LR-CESM systematically underestimates AR frequency, intensity, integrated vapor transport (IVT), and precipitation by 20%-40%, primarily due to the underrepresentation of extreme ARs (CAT5) and the overrepresentation of nonextreme events (CAT1-4). HR-CESM demonstrates substantial improvements in simulating historical AR characteristics, particularly for extreme events. Both HRand LR-CESM project more frequent and intense ARs with a poleward shift under warming. While the relative increases are similar, LR-CESM underestimates absolute values due to weaker historical baselines. HR-CESM projections feature a-50% increase in extreme ARs and reduced nonextreme events, whereas LR-CESM shows increases in both, continuing to underestimate the contribution of extreme ARs. Thermodynamic effects primarily drive AR intensification under warming, but dynamic processes modulate regional responses. Winter projections are more consistent between HR and LR-CESM, while summer reveals larger divergence in dynamic responses and precipitation. These fi findings highlight the critical importance of resolving mesoscale ocean-atmosphere interactions in climate models to accurately simulate and project AR behavior under climate change. SIGNIFICANCE STATEMENT: Atmospheric rivers (ARs) are characterized by intense moisture transport and cause extreme rainfall. Comparing high-resolution (HR) and low-resolution (LR) Community Earth System Model (CESM) simulations reveals substantial biases in simulated ARs for most intensity categories in LR-CESM, which are significantly reduced in HR-CESM. Under anthropogenic warming, both models project increased AR frequency (-30%), intensity (integrated vapor transport,-40%), and precipitation (-30%) at similar rates. However, LR-CESM underestimates absolute values due to weaker historical baselines. Projections for extreme (CAT5) versus nonextreme (CAT1-4) ARs differ between resolutions, with LR-CESM substantially underestimating extreme event increases. Thermodynamic effects primarily drive AR intensification, while dynamic processes modulate regional responses.

In the past decade, large‐scale and persistent warm sea surface temperature anomalies (SSTA) occurred frequently in the mid‐latitude Northeast Pacific (NEP), offering a valuable opportunity to further illuminate the oceanic feedback to the atmosphere, which has been a complex issue for mid‐latitude ocean‐atmosphere interaction. Previous studies reported that the prolonged warm SSTAs in the NEP are usually passively triggered by atmospheric high‐pressure anomalies. In this study, results show that the early winter positive SSTAs can actively reverse the high‐pressure anomalies into low‐pressure anomalies within the whole troposphere through diabatic heating and transient eddy vorticity forcing. However, when the positive SSTAs appear in mid‐winter, the atmospheric preconditioning of warm air temperature anomalies related to the high‐pressure anomalies prevents oceanic thermodynamic and dynamics feedback onto the atmosphere, which results in no reversal of the high‐pressure anomalies. Therefore, the atmospheric state is an important factor in initiating the mid‐latitude oceanic feedback to the atmosphere.

The intensifying hot extreme events under anthropogenic warming severely affect human health and the natural environment, yet the factors driving their heterogeneous geographical distribution remain unclear. Here we utilize an eddy-resolving high-resolution climate model alongside multiple simulations from Coupled Model Intercomparison Project Phase 6, we find baseline temperature variability as a key factor shaping the global distribution of projected hot extremes, with over 80% of the global increase in hot extremes anticorrelated with baseline temperature variability, a relationship interpretable within the signal-to-noise ratio framework. We further demonstrate that the baseline temperature variability is anchored by persistent land-atmosphere coupling, which endures over century timescales and sustains the spatial heterogeneity of future hot extremes. Our findings suggest that baseline temperature variability could serve as a potential indicator for future hot extreme distribution, offering valuable insights for developing targeted adaptation strategies and improving regional resilience.

Tropical cyclone (TC) plays a critical role in driving hydrological extremes. However, current generation climate models often fail to capture TC inner-core dynamical structures, leading to substantial underestimation (>50%) of TC rainfall (TCR). Here, using a set of eddy-resolving high-resolution (HR) simulations from the Community Earth System Model (CESM), we show that simulated TCR closely aligns with observations, primarily due to the improved TC upward motion. Under the Representative Concentration Pathway 8.5 warming scenario, projected TCR increases in non-eddy-resolving models reach only 15%-50% of those in HR CESM, likely reflecting their large historical TCR biases. Owing to suppressed TC upward motion, the projected TCR fractional increase by non-eddy-resolving models remain comparable to or below the Clausius-Clapeyron (C-C) scaling. In contrast, HR CESM projects a much larger TCR increase rate (similar to 12.0% K-1), exceeding the C-C rate and driven by a strengthened TC upward motion.

Subtropical western boundary currents (WBCs) refer to swift narrow oceanic currents that flow along the western edges of global subtropical ocean basins. Earlier studies indicated that the WBCs are extending poleward under a warming climate. However, owing to limited observations and coarse resolution of climate models, how greenhouse warming may affect the zonal structure of the WBCs remains unknown. Here, using seven high-resolution climate models, we find an onshore intensification of the WBCs in a warming climate. The multimodel ensemble mean of onshore acceleration ranges from 0.10 +/- 0.08 to 0.51 +/- 0.24 cm s(-1) per decade over 1950-2050. Enhanced oceanic stratification associated with fast surface warming induces an uplift of the WBCs, leading to the projected change. The onshore intensification could induce anomalous warming that exacerbates coastal marine heatwaves, reduces ability of the coastal oceans to absorb anthropogenic carbon dioxide and destabilizes methane hydrate stored below the sea floor of shelf regions.

"Imbalance" and "injustice" within the society are the key concerns during heat waves, which are considered a typical phenomenon of climate change. However, there is a limited understanding of the precise ways of measuring and respond to environmental justice needs. Here we developed an equity index to examine the environmental justice problems in respect to the provision of social resource in response to heat wave. By integrating multi-source data and synthetic population algorithmic, a method of measuring environmental equity indexes with high spatial resolution is developed. The integrated equity index map provides a comprehensive picture of the urgency of heat wave related social resources provision: Despite the higher heat wave related demand in the central urban area, some suburban areas generally have lower equality level due to the lack of social resources supply, especially in some emerging cross-boundary functional zone. The social-media data-based heat wave perception map is found to be correlated with environmental equity level graph, which indicate the prospect of using heat wave perception to verify the equity index.

The influence of greenhouse warming on mesoscale air-sea interactions, crucial for modulating ocean circulation and climate variability, remains largely unexplored due to the limited resolution of current climate models. Additionally, there is a lack of theoretical frameworks for assessing changes in mesoscale coupling due to warming. Here, we address these gaps by analyzing eddy-resolving high-resolution climate simulations and observations, focusing on the mesoscale thermal interaction dominated by mesoscale sea surface temperature (SST) and latent heat flux (LHF) coupling in winter. Our findings reveal a consistent increase in mesoscale SST-LHF coupling in the major western boundary current regions under warming, characterized by a heightened nonlinearity between warm and cold eddies and a more pronounced enhancement in the northern hemisphere. To understand the dynamics, we develop a theoretical framework that links mesoscale thermal coupling changes to large-scale factors, which indicates that the projected changes are collectively determined by historical background wind, SST, and the rate of SST warming. Among these factors, the large-scale SST and its warming rate are the primary drivers of hemispheric asymmetry in mesoscale coupling intensification. This study introduces a simplified approach for assessing the projected mesoscale thermal coupling changes in a warming world.

Oceanic mesoscale eddies are important dynamical processes in the Southern Ocean. Using high-resolution (similar to 0.1 degrees for the ocean) Community Earth System Model (CESM-HR) simulations under a high-carbon emission scenario, we investigate the role of mesoscale eddies in regulating the response of the Subantarctic Front (SAF) to global warming. The CESM-HR simulates more realistic oceanic fronts and mesoscale eddies in the Southern Ocean than a coarse-resolution (similar to 1 degrees for the ocean) CESM. Under global warming, the SAF is projected to intensify. The mean flow temperature advection intensifies the front, whereas the mesoscale-eddy-induced temperature advection and atmospheric dampening play primary (similar to 67%) and secondary (similar to 28%) roles in counteracting the effect of mean flow temperature advection. Our study suggests the importance of mesoscale eddies on inhibiting the SAF intensification under global warming and necessity of mesoscale-eddy-resolving simulations for faithful projection of future climate changes in the Southern Ocean.

The ocean is a magnificent reservoir of kinetic energy possessed by currents at diverse spatio-temporal scales. These currents transport heat and material, regulating the regional and global climate. It is generally thought that large-scale ocean circulations should become more energetic under global warming, especially in the ocean's upper layer. However, using high-resolution global climate simulations, here we demonstrate that the total ocean kinetic energy is projected to be significantly reduced in a warming climate, despite overall acceleration of large-scale ocean circulations in the upper layer. This reduction is primarily attributed to weakened ocean mesoscale eddies in the deep ocean. Enhanced vertical stratification under global warming reduces the available potential energy stored in large-scale ocean circulations, diminishing its conversion into eddy kinetic energy. Our findings reveal a more quiescent deep ocean under global warming and suggest a crucial role of mesoscale eddies in determining the anthropogenic change of total ocean kinetic energy. Studies show climate change will alter the ocean, with increased surface layer kinetic energy. This work, using full ocean depth and high-resolution projections with a high-emission scenario, shows an overall ocean kinetic energy decrease due to a calmer deep ocean with weaker mesoscale eddies.

Extreme warm water events, known as marine heatwaves, cause a variety of adverse impacts on the marine ecosystem. They are occurring more and more frequently across the global ocean. Yet monitoring marine heatwaves below the sea surface is still challenging due to the sparsity of in situ temperature observations. Here, we propose a statistical learning method guided by ocean dynamics and optimal prediction theory, to detect subsurface marine heatwaves based on the observable sea surface temperature and sea surface height. This dynamics-guided statistical learning method shows good skills in detecting subsurface marine heatwaves in the oceanic epipelagic zone over many parts of the global ocean. It outperforms both the classical ordinary least square regression and popular deep learning methods that do not effectively exploit ocean dynamics, with clear dynamical interpretation for its outperformance. Our study provides a useful statistical learning method for near real-time monitoring of subsurface marine heatwaves at a global scale and highlights the importance of exploiting ocean dynamics for enhancing the efficiency and interpretability of statistical learning. Subsurface marine heatwaves in the oceanic epipelagic zone can be detected based on satellite-measured sea surface temperature and height anomalies, by using a statistical learning method guided by ocean dynamics.

Near-inertial waves (NIWs) play an important role in diapycnal processes and energy dissipation. A mooring observation deployed on the continental shelf in the East China Sea captured anomalously intensified subsurface near-inertial kinetic energy (NIKE) during the passage of Typhoon Danas (2013). An early study has investigated the role of Parametric Subharmonic Instability (PSI) induced by internal tides in the intensification of the subsurface intensified near-inertial velocity. However, results based on regional numerical simulations reveal that strong subsurface near-inertial velocity persists even in the absence of tidal effects, implying the existence of additional sources of NIWs. Our analyses showed that after excluding the effect of PSI, approximately 30% of the remaining subsurface NIKE can be attributed to another Typhoon Fitow (2013), which occurred a week prior to Typhoon Danas. Constrained by the Kuroshio current and the continental shelf, the NIKE generated by Typhoon Fitow propagates northward and reaches the mooring location, leading to the intensified subsurface NIW signal. Our simulation, together with the observations, suggests complicated NIW dynamics in continental shelf regions, involving interactions between successive typhoons, topography and background current, and differing from the open ocean. These interactions will further influence vertical mixing on the continental shelf along the pathway of NIW.

The Gulf Stream (GS) ocean front exhibits intense ocean–atmosphere interaction in winter, which has a significant impact on the genesis and development of extratropical cyclones in the North Atlantic. The atmospheric rivers (ARs), closely related with the cyclones, transport substantial moisture from the North Atlantic towards the Western European coast. While the influence of the GS front on extratropical cyclones has been extensively studied, its effect on ARs remains unclear. In this study, two sets of ensemble experiments are conducted using a high-resolution global Community Atmosphere Model forced with or without the GS sea surface temperature front. Our findings reveal that the inclusion of the GS front leads to approximately 25% enhancement of water vapor transport and precipitation associated with ARs in the GS region, attributed to changes in both AR frequency and intensity. Furthermore, this leads to a more pronounced downstream response in Western Europe, characterized by up to 60% (40%) precipitation increases (reductions) around Spain (Norway) for the most extreme events (exceeding 90 mm/day). The influence of the GS front on ARs is mediated by both thermodynamic and dynamic factors. The thermodynamic aspect involves an overall increase of water vapor in both the GS region and Western Europe, promoting AR genesis. The dynamic aspect encompasses changes in storm tracks and Rossby wave train, contributing to downstream AR shift. Importantly, we find the co-occurrence of ARs and the GS front is crucial for inducing deep ascending motion and heating above the GS front, which perturbs the deep troposphere and triggers upper-level Rossby wave response. These findings provide a further understanding of the complex interaction between the oceanic front in the western boundary current regions and extratropical weather systems and the associated dynamics behind them.

Abstract Air‐sea exchanges across oceanic fronts are critical in powering cloud formation, precipitation, and atmospheric storms. Oceanic submesoscale fronts of scales 1–10 km are characterized by strong sea surface temperature (SST) gradients. However, it remains elusive how submesoscale fronts affect the overlying atmosphere due to a lack of high‐resolution observations or models. Based on rare high‐resolution in situ observations in the Kuroshio Extension region, we quantify the air‐sea exchanges across an oceanic submesoscale front. The cross‐front SST and turbulent heat flux gradients reaches 2.4°C/km and 47 W/m2/km, respectively, far stronger than that typically found in mesoscale‐resolving products. The stronger SST gradient drives substantially stronger air‐sea fluxes and vertical mixing than mesoscale fronts, enhancing cloud formations. The intense air‐sea exchanges across submesoscale fronts are confirmed in idealized model simulations, but not resolved in mesoscale‐resolving climate models. Our finding provides essential knowledge for improving simulations of cloud formation, precipitation, and storms in climate models.

Extreme atmospheric rivers (EARs) are responsible for most of the severe precipitation and disastrous flooding along the coastal regions in midlatitudes. However, the current non-eddy-resolving climate models severely underestimate (~50%) EARs, casting significant uncertainties on their future projections. Here, using an unprecedented set of eddy-resolving high-resolution simulations from the Community Earth System Model simulations, we show that the models' ability of simulating EARs is significantly improved (despite a slight overestimate of ~10%) and the EARs are projected to increase almost linearly with temperature warming. Under the Representative Concentration Pathway 8.5 warming scenario, there will be a global doubling or more of the occurrence, integrated water vapor transport and precipitation associated with EARs, and a more concentrated tripling for the landfalling EARs, by the end of the 21st century. We further demonstrate that the coupling relationship between EARs and storms will be reduced in a warming climate, potentially influencing the predictability of future EARs.

The North Atlantic Ocean hosts the largest volume of global subtropical mode waters (STMWs) in the world, which serve as heat, carbon and oxygen silos in the ocean interior. STMWs are formed in the Gulf Stream region where thermal fronts are pervasive and result in feedback with the atmosphere. However, their roles in STMW formation have been overlooked. Using eddy-resolving global climate simulations, we find that suppressing local frontal-scale ocean-to-atmosphere (FOA) feedback leads to STMW formation being reduced almost by half. This is because FOA feedback enlarges STMW outcropping, attributable to the mixed layer deepening associated with cumulative excessive latent heat loss due to higher wind speeds and greater air-sea humidity contrast driven by the Gulf Stream fronts. Such enhanced heat loss overshadows the stronger restratification induced by vertical eddies and turbulent heat transport, making STMW colder and heavier. With more realistic representation of FOA feedback, the eddy-present/rich coupled global climate models reproduce the observed STMWs much better than the eddy-free ones. Such improvement in STMW production cannot be achieved, even with the oceanic resolution solely refined but without coupling to the overlying atmosphere in oceanic general circulation models. Our findings highlight the need to resolve FOA feedback to ameliorate the common severe underestimation of STMW and associated heat and carbon uptakes in earth system models.