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This study examines data collected with an autonomous underwater glider during a period of vigorous radiatively driven convection (RDC) and low winds in deep, unstratified Lake Superior. Conductivity, temperature and depth (CTD) measurements reveal distinct convective plumes of warm downwelling water with temperature anomalies of ∼0.1°${\\sim} 0.1{}^{\\circ}$ C and width scales on the order of 10−100$10-100$m, consistent with theoretical scalings for the unstratified convective regime. Shear and temperature microstructure measurements indicate turbulent kinetic energy (TKE) dissipation (ε)$(\\varepsilon )$and temperature variance dissipation rates χT$\\left({\\chi }_{T}\\right)$orders of magnitude greater in thermal plumes than laterally adjacent waters. Decay timescales of ε$\\varepsilon $indicate highly efficient mixing is sustained throughout the night. Energetics, mixing efficiency, and constraints on convective plume scales are also discussed. These observations demonstrate that RDC can dominate vertical mixing dynamics even in deep ice‐free systems, and these systems can serve as a real‐scale laboratory for investigation of convective dynamics. Plain Language Summary A counterintuitive property of freshwater is that below 4 °^{\\circ}$ C it becomes denser when heated. As a result, sufficiently cold freshwater lakes become stratified with colder water on top and warmer water below during winter. In the spring, solar radiation increases the density of surface waters and drives convective mixing. An autonomous underwater vehicle was used to observe the properties of laterally distinct structures that develop during this process. The mixing associated with this process is highly efficient and maintained throughout the daily cycle. Key Points Laterally distinct convective plumes dissipate kinetic energy and temperature variance orders of magnitude faster than surrounding waters Turbulence decays slowly overnight (e‐folding time ∼${\\sim} $ 4–5 hr), sustaining active mixing throughout the entire diel cycle Open water radiatively driven convective mixing is highly efficient η ≈ 0.72, consistent with previous observations of under‐ice systems

The Gulf Stream transports heat from the tropics poleward and is an integral part of how the planet redistributes heat. Studies of the variability of the Gulf Stream have suggested that the seasonal cycle of ocean mixed layer heat content in the Gulf Stream may be driven by the local net atmospheric heat flux or by oceanic advection of heat in the strong western boundary current. Here we use sustained underwater glider observations of the Gulf Stream from 80°^{\\circ}$ W to 67°^{\\circ}$ W during 2015–2023 to show that oceanic advection heats the ocean mixed layer in spring when the upper ocean switches from cooling to warming. Estimated terms of the mixed layer temperature budget demonstrate that the net atmospheric heat flux cools the upper ocean in all seasons except summer and that mixed layer warming in spring is due to ocean advection of heat from farther south. Plain Language Summary Over the North Atlantic Ocean, the seasonal cycle of upper ocean temperature is influenced by seasonal variations in heating from the sun. However, the seasonal variation of upper ocean temperature in the Gulf Stream may also depend on the amount of heat transported in the fast‐moving, warm, ocean current. Using an estimate of the seasonal cycle of the upper ocean of the Gulf Stream and estimates of the heat exchanged with the atmosphere, we demonstrate that heat carried by the ocean current warms the Gulf Stream at the start of spring. The ocean heat transport is an important component of how the Gulf Stream's upper ocean temperature evolves seasonally. Key Points Observations collected by underwater gliders in 2015–2023 were used to estimate the seasonal cycle of the Gulf Stream Ocean advection is responsible for warming the Gulf Stream mixed layer during the spring The net air‐sea heat flux moves heat into the atmosphere over the Gulf Stream for all seasons except summer

Navigation planning and execution for underwater gliders operating in thermocline-affected environments is challenging due to the coupled influence of energy constraints, spatially distributed environmental disturbances, and limited control authority. Spatially varying thermocline structures act as structured environmental disturbances that degrade motion efficiency and tracking accuracy, and therefore must be explicitly considered in both path planning and control design. This paper proposes a hierarchical control-oriented decision framework for underwater glider navigation in thermocline regions. At the planning layer, a thermocline-aware multi-objective optimization problem is formulated to regulate the trade-off between navigation efficiency and cumulative environmental disturbance, characterized by total path length and cumulative thermocline exposure, respectively. A multi-objective artificial bee colony (MOABC) algorithm is employed to generate a set of Pareto-optimal reference trajectories that explicitly reveal this trade-off. At the execution layer, pitch angle regulation is formulated as a stochastic tracking control problem under environmental uncertainty. A Markov Decision Process (MDP) is constructed to model the coupled effects of pitch control on energy consumption and trajectory deviation, and a deep deterministic policy gradient (DDPG) algorithm is adopted to synthesize a feedback control policy for adaptive pitch regulation during path execution. Simulation results demonstrate that the proposed framework effectively reduces cumulative thermocline exposure and overall energy consumption while maintaining improved trajectory consistency compared with representative benchmark methods. These results indicate that integrating multi-objective planning with learning-based control provides an effective control-oriented solution for constrained underwater glider navigation in thermally stratified environments.

The vibration caused by the movement of internal actuating components within an acoustic underwater glider can interfere with onboard sensors. However, as a new vibration-damping material, phononic crystals can effectively reduce this impact. Using simulation and an underwater test, this work studied the vibration-damping mechanism of the phononic crystal suspension (PCS) designed by Tianjin University, China. The bandgaps and the modes of PCS were calculated first, which offered basic data for the following simulation. Then, the relationship between the modes and attenuation zones (AZs) were broadly considered to reveal the variation law of the AZs with the change in modes, both in the air and under water. Finally, an underwater test was carried out to verify the good vibration-damping effect of the PCS. The results show that the cutoff frequency of the AZs could be predicted by finding the relevant modes. The PCS showed a good vibration-damping effect from 170 Hz to 5000 Hz in the underwater test, with a maximum decrease of 6 dB at 2000 Hz. Finally, the damping of the PCS could suppress the overlap of modes that resulted from Bragg scattering. This work will also provide theoretical guidance for further study on the optimization of phononic crystal mechanisms for vibration damping.

Buoyancy-driven underwater gliders are highly efficient winged underwater vehicles driven by modifying the net buoyancy and internal shape. Many advantages, such as wide cruise range, less power consumption, low noise, and no pollution, make the underwater glider an important platform for marine environment observation and ocean resource exploration. For the wide cruise range, attitude control of underwater glider becomes the core technology. In this paper, the underwater glider named OUC-III has been developed for marine observation. To control the attitude of glider, the kinematic and dynamic models of it have been calculated by mathematical analysis. Furthermore, a novel control algorithm is proposed to control the attitude of glider. The algorithm is combined reinforcement learning with Active Disturbance Rejection Control (ADRC) and compared with classical ADRC by simulation based on the dynamic model of OUC-III. The simulation experimental results indicate that the proposed algorithm compensates well for the ocean current disturbances on OUC-III attitude control mission and it obtains high-precision and high-adaptive control ability.

Unlike traditional propeller-driven underwater vehicles, blended-wing-body underwater gliders (BWBUGs) achieve zigzag gliding through periodic adjustments of their net buoyancy, enhancing their cruising capabilities while minimizing energy consumption. However, enhancing gliding performance is challenging due to the complex system design and limited design experience. To address this challenge, this paper introduces a model-based, multidisciplinary system design optimization method for BWBUGs at the conceptual design stage. First, a model-based, multidisciplinary co-simulation design framework is established to evaluate both system-level and disciplinary indices of BWBUG performance. A data-driven, many-objective multidisciplinary optimization is subsequently employed to explore the design space, yielding 32 Pareto optimal solutions. Finally, a model-based physical system simulation, which represents the design with the largest hyper-volume contribution among the 32 final designs, is established. Its gliding performance, validated by component behavior, lays the groundwork for constructing the entire system’s digital prototype. In conclusion, this model-based, multidisciplinary design optimization method effectively generates design schemes for innovative underwater vehicles, facilitating the development of digital prototypes.

Long-range underwater gliders (LRUGs) have emerged as essential platforms for sustained and autonomous observation in deep and remote marine environments. This paper provides a comprehensive review of their developmental status, performance characteristics, and application progress. Emphasis is placed on two critical enabling technologies that fundamentally determine endurance: lightweight, pressure-resistant hull structures and high-efficiency buoyancy-driven propulsion systems. First, the role of carbon fiber composite pressure hulls in enhancing energy capacity and structural integrity is examined, with attention to material selection, fabrication methods, compressibility compatibility, and antifouling resistance. Second, the evolution of buoyancy control systems is analyzed, covering the transition to hybrid active–passive architectures, rapid-response actuators based on smart materials, thermohaline energy harvesting, and energy recovery mechanisms. Based on this analysis, the paper identifies four key technical challenges and proposes strategic research directions, including the development of ultralight, high-strength structural materials; integrated multi-mechanism antifouling technologies; energy-optimized coordinated buoyancy systems; and thermally adaptive glider platforms. Achieving a system architecture with ultra-long endurance, enhanced energy efficiency, and robust environmental adaptability is anticipated to be a foundational enabler for future long-duration missions and globally distributed underwater glider networks.

This paper addresses the aperture limitation problem faced by array-equipped underwater gliders (UGs) in direction-of-arrival (DOA) estimation. A passive synthetic aperture (PSA) method for DOA estimation using a single hydrophone mounted on a UG is proposed. This method uses the motion of the UG to synthesize a linear array whose elements are positioned to acquire the target signal, thereby increasing the array aperture. The dead-reckoning method is used to determine the underwater trajectory of the UG, and the UG’s trajectory was corrected by the UG motion parameters, from which the array shape was adjusted accordingly and the position of the array elements was corrected. Additionally, array distortion caused by movement offsets due to ocean currents underwent linearization, reducing computational complexity. To validate the proposed method, a sea trial was conducted in the South China Sea using the Haiyi 1000 UG equipped with a hydrophone, and its effectiveness was demonstrated through the processing of the collected data. The performance of DOA estimation prior to and following UG trajectory correction was compared to evaluate the impact of ocean currents on target DOA estimation accuracy.

The accurate prediction of the surfacing position of underwater gliders (UGs) is critical for mission success and cost-effective retrieval. However, current state-of-the-art (SOTA) methods often rely on complex multi-model integrations or large volumes of ocean current data, thereby increasing operational costs and system complexity. In this study, we systematically introduce—for the first time—a coordinate-transformation-based prediction framework, originally applied in other navigation contexts, into the UG surfacing-position-prediction task. By projecting both the glider’s entry and surfacing positions into a Universal Transverse Mercator (UTM) planar coordinate system and treating the resulting displacement as the prediction target, we avoid dependence on heavily parameterized current models, simplify the training process, and maintain robust predictive accuracy. Our approach combines common machine learning predictors (e.g., AdaBoost, LGBM, gradient boosting, random forest, decision trees) instead of advanced deep learning architectures, thus reducing computational overhead. Experiments on two real-world sea trial datasets (containing 2159 and 1456 profiles, respectively) show that, compared with direct regression approaches, this method improves positioning accuracy by up to 50% within a 500-meter range, yet requires minimal multi-source data. Overall, this study integrates the concept of coordinate transformation into the task of predicting the surfacing position of underwater gliders, effectively streamlining the method without sacrificing accuracy. The result is a highly flexible and cost-effective approach, providing theoretical support for future optimizations of underwater glider navigation systems.

Mesoscale eddies are important for transporting oceanic energy and matter. We investigated the three-dimensional structure of an irregularly shaped warm eddy using three Chinese underwater gliders and satellite data during May 2015 in the northern South China Sea. The warm eddy lasted for 2 months, remained quasi-steady, and had a mean radius of ~ 70 km from May 10 to May 31. The heat contents observed along the two glider tracks differed markedly, by 2 × 10 9 J/m 2 , which reflected an imbalance in the geostrophic and tangential velocity distributions of the eddy. The geostrophic/tangential velocity decreased/increased with depth within the warm eddy. The maximum tangential velocities calculated using the datasets from the two gliders were 0.8 and 0.25 m/s, respectively, confirming that the shape of the warm eddy was horizontally asymmetrical. Large errors can arise when the heat, energy, and matter transport for an irregularly shaped eddy are estimated using a regular circular model. We suggest that more intersecting glider tracks should be used to retrieve the three-dimensional eddy structure, and that those tracks should be better designed. The irregular shape of the warm eddy was likely induced by oceanic currents such as the wind-induced Ekman current. Further study is needed to elucidate the eddy–current interactions and the mechanisms thereof.