ISSN 2096-5508

e-ISSN 2523-3521

CN 37-1518/P

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  • Physics

    Abstract：Marine heatwaves (MHWs), recognized for their significant impact on marine ecosystems, have become a key topic in physical oceanography. The current definition of MHWs, which solely depends on sea surface temperature while neglecting dynamical factors, may cause overlaps with other oceanic phenomena such as mesoscale eddies. However, the related research remains incomplete. The South China Sea (SCS), hosts a high frequency of both MHWs and mesoscale eddies, making it an ideal region for investigating the interaction between these phenomena. In the current investigation, we compared MHWs identified from OISSTV2.1 data with mesoscale eddies from the Meta 3.2 dataset. Our findings indicate that 37.79% of MHWs overlap with mesoscale eddies during their lifetime, with 3.50% remaining within mesoscale eddies throughout their entire duration. Despite cyclonic eddies (CEs) having a slightly larger spatial coverage, anticyclonic eddies (AEs) are more frequently associated with MHWs, indicating that they may better facilitate MHW occurrence and persistence. This can be explained by the dual role of AEs, which not only can themselves be identified as MHWs but also promote their formation by inducing seawater convergence. Additionally, 16.31% (17.51%) of CEs (AEs) contain MHWs, and 3.23% (3.12%) of CEs (AEs) are entirely identified as MHWs. This analysis contributed to a deeper understanding of MHWs and provided new insights into the relationship between MHWs and mesoscale eddies.  

    Wenjin SUN, Yindi WANG, Mingshen XIE, Yongyan PAN, Chunhui LI, Jinlin JI, Yang YU, Kenny Thiam Choy LIM KAM SIAN, Changming DONG

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  • Physics

    Abstract：Evaporation ducts, which originate at the air-sea boundary, significantly influence electromagnetic propagation, ship communication, radar ranging, and other related fields. However, the current understanding of the variability in evaporation ducts under the combined effect of atmospheric and oceanic processes remains unclear. Via shipboard observations, the changes in evaporation ducts at paired oceanic submesoscale fronts (OSFs) during and after successive passage of an atmospheric cyclone and anticyclone in the northwestern Pacific Ocean were investigated. The observations indicated that under the dominant influence of sea surface temperature, air temperature, and specific humidity changes, the average evaporation duct height above the OSFs during cyclone passage reached 9.76 m, which is 446% of the 2.19 m during the period when only OSFs occurred. During the anticyclone period, the evaporation duct height ranged from 2 to 3 m. Specific humidity variation was influenced by mainly evaporation, followed by advection and divergence flow. Differences in the influence mechanisms of sea surface temperature and wind speed on evaporation during different periods were explored.  

    Yuan GAO, Yongchui ZHANG, Xinyue XIA, Hongcheng LOU, Xiaofeng ZHAO, Mei HONG, Yang WANG, Shiyao CHEN, Zhijin QIU, Juli DING

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  • Physics

    Abstract：Based on wave data recorded in 2020 from 10 buoy stations in the northern part of the South China Sea, the synergistic effects of wave parameters such as wave steepness, spectral width, kurtosis, skewness and Benjamin-Feir index (BFI) on a rogue wave group in the northern waters of the South China Sea were investigated. The frequency and energy distributions of three types of rogue waves were recognized based on wavelet transform. Chaotic dynamics was introduced to study the chaotic phenomena in the wave series where rogue wave were located. Results show that wave steepening enhanced the nonlinear effect of waves, and a narrow spectrum could suppress the dispersion effect, which increased the BFI, triggered the modulation instability of waves, and promoted the generation of distorted waves. When kurtosis>3, the BFI increases as the degree of kurtosis deviation from the Gaussian distribution increases. In particular, when kurtosis>3 and H/Hs>2.2, the increase in kurtosis contributes significantly to the increase in the anomalous intensity of the rogue wave. Rogue waves induced by modulation instability and those induced by the superposition of wave clusters can be explained by phase and dispersion modulation as well as by energy distribution, whereas the generation mechanism of rogue waves that have not grown sufficiently is difficult to resolve from the time-domain waveform characteristics and the frequency-domain wavelet energy density. Qualitative analysis by the Poincaré cross section and quantitative analysis by the Lyapunov exponent verified that the nonlinear wave system, in which a rogue wave is located in the wave train, has weakly chaotic dynamic behavior.  

    Tao FENG, Tianyu ZHANG, Danchen YAN, Hailong WANG, Jian YANG, Xun LIN, Junping ZHANG

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  • Geology

    Abstract：The acoustic properties of seafloor sediments are crucial for accurate acoustic field prediction, seafloor resource exploration, and marine disaster prevention. However, traditional prediction equations, often based on laboratory-measured sound speeds, suffer from low precision and discrepancies with in situ measurements. To address these issues, we employed eXtreme Gradient Boosting (XGBoost) machine learning algorithms to develop high-precision in situ sound speed prediction models for seafloor sediments. The models were constructed using in situ sound speed and sediment physical property data (density, water content, porosity, median grain size, and grain group content) from 48 sites in the East China Sea shelf. Through feature parameter reduction and hyperparameter optimization, the optimal XGBoost model achieves training and validation R2 values of 0.989 and 0.977, respectively, having hyperparameters set at n_estimators=49 and max_depth=6. Compared to other machine learning models and empirical equations, the XGBoost model based on density, water content, sand content, and median grain size exhibited the lowest mean absolute error (MAE) and mean absolute percentage error (MAPE) at 5.603 m/s and 0.366%, respectively. This represents significant improvements over existing models, with MAE reductions ranging 2.165–118.903 m/s and MAPE reductions 0.137%–7.657%. This study thus provides an innovative and highly accurate method for predicting the in situ sound speed of seafloor sediments.  

    Mujun CHEN, Linqing ZHANG, Xinfeng HU, Xiangmei MENG, Guangming KAN, Siyou TONG, Jingqiang WANG, Guanbao LI, Baohua LIU, Qingfeng HUA, Siqi WU, Xiaobo ZHANG

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