西沙群岛西北深水海域的内潮特征分析

Characteristics of internal tides in deep water northwest of the Xisha Islands

基于海南岛至西沙群岛之间深水海域一长达5年的锚定潜标测流资料,采用谱分析、调和分析和动力模态分解等方法主要分析了局部海域内潮的基本特征.结果表明：研究海域的正压和斜压潮均以全日振荡为主,500～900m的海洋中层全日等密度线垂向振幅可达40m;全日内潮主要沿垂直于陆坡方向传播,与天文潮锁相的全日内潮可占全日内潮总能量的41%;海洋上层,O1内潮垂向平均振幅与局地海面高度呈显著正相关关系,K1内潮则表现为夏、冬季增强的半年循环特征;超过70%的O1分潮能量集中于第一、二斜压模态上,K1分潮在第三斜压模态上亦有相当能量.

Internal tides are internal gravity waves with quasi-tidal periodicities, which are ubiquitous in the ocean. Their characteristics in deep water areas between the Hainan Island and Xisha Islands in the northwestern South China Sea （SCS） are investigated based on 5-year moored current observations. The methods used in this study include power spectral analysis, harmonic analysis and dynamical mode decomposition are performed. The results show that the barotropic and baroclinic tidal currents in the study region are both dominated by diurnal oscillations. The vertical isopycnal diurnal displacement can reach 40 m at a depth of 500-900 m. Diurnal internal tidal currents mainly propagate across the continental slope, approximately 41% of which is phase locked with astronomical tides. The ratio is nearly double of that in the northeast SCS. In the upper ocean, it is found that the variability of internal O1 amplitude is significantly positively correlated with the local sea level height, which suggests a strong modulation by the upper oceanic density field. On the other hand, the K1 internal tide shows a semi-annual variation with intensification in summer and winter due to the superposition of barotropic forcing from K1 and P1 constituents. Furthermore, dynamical mode decomposition shows that the two baroclinic modes together account for more than 70% of O1 constituent energy, while the contribution of the third mode in the K1 cannot be neglected, which has 25% of total K1 energy in someone deployment.