Observation of Internal Tides in the Qiongzhou Strait by Coastal Acoustic Tomography

In this study,power spectral density and inverse analyses were performed to obtain the frequency characteristics and spatial distribution of temperature in the Qiongzhou Strait using reciprocal sound transmission data obtained in a coastal acoustic tomo-graphy experiment conducted in 2013.The results reveal three dominant types of internal tides(diurnal,semidiurnal,and terdiurnal).Spectral analysis of the range-average temperature deviation along the northern and southern transmission paths shows that along the northern path,the energy of the diurnal internal tides was significantly larger than that of the semidiurnal tides.The semidiurnal internal tides,in contrast,were more pronounced along the southern path.A terdiurnal spectrum with an energy level equivalent to that of the semidiurnal internal tide was discernable for both the northern and southern paths.These three types of internal tides can also be recognized in the time variation of the zonal-average temperature deviation.The diurnal internal tides were strengthened along the northern coast,implying their westward propagation and the existence of coastally trapped effects.The other two types of internal tides,which have smaller wavelengths than the diurnal internal tides,were less resolved over the entire tomographic domain due to the insufficient resolution of the inversion.The data quality was verified to be satisfactory by error estimation.

Nontidal sea level changes in Hiroshima Bay,Japan

Nontidal sea level changes generated in Hiroshima Bay of the Seto-Inland Sea in Japan are studied over various time scales, from the sub-tidal （2 d to 1 month） to inter-annual scales （〉2 years）. The total sea level variation produces a standard deviation （STD） of 12.5 cm. The inter-annual component of the sea level variation in Hiroshima Bay oscillates with a STD of 3.4 cm, forming a long-term trend of 4.9 mm/a. The STD of the sea level variation is 9.8 cm for the seasonal component （8 months to 2 years） and 4.7 cm for the intra-seasonal one （1 month to 8 months）. Significant sea level variations with a STD of 4.2 cm also occur in the sub-tidal range. Special attention is paid to the sub-tidal sea level changes. It is found that the upwelling and associated transient sea level changes generated along the north coast of Hiroshima Bay （opened southward） by the strong northerly wind, play a significant role in sub-tidal sea level changes. The transient sea level changes are over 10 cm in most cases when caused by typhoons that pass through the Pacific Ocean offthe Kii Peninsula, located at about 400 km east of Hiroshima Bay. Reasonable sea level changes are evaluated by the balance of pressure forces at the onshore and offshore boundary of the study domain.

吕宋海峡ADCP观测的450m以浅水层内潮特征分析

应用2008年4月25日至9月26日的声学多普勒流速剖面仪(ADCP)的锚定测流资料,研究分析吕宋海峡上层海洋的内潮特征.谱分析和能量计算均显示在观测海区全日潮和半日潮为主要的能量频带,深度积分的全日潮频带总能量为31 kJ.m,在半日潮频带和高频带深度积分的总能量分别为6.9和3.4 kJ.m.谱分析结果显示:近惯性频率峰值仅出现在斜压分量.在66 m以浅深度,显著分潮(O1,K1,M2,MK3,M4)和近惯性频率分量都基本符合线性内波理论关系式E+(ω)/E-(ω)=(ω-f)2/(ω+f)2;然而,在其他观测深度,并非所有的主要分潮均满足上述线性内波能量一致性关系式.分析潮流的垂直结构和时间变化,结果表明:正压潮和内潮均为混合潮,潮日不等现象明显,显示了约14天的大潮/小潮周期变化.平行与垂直于陆架方向的K1和O1分潮的分量几乎相等,而M2和S2分潮则主要以波束形式向南海北部陆架传播.内潮幅度和相位均随时间变化,M2和S2垂向结构主要表现为第1模态,而K1和O1则接近于第2模态结构;在主温跃层附近,它们的短轴与长轴之比与f/ω的比值接近.

Analysis of internal tidal characteristics in the layer above 450 m from acoustic Doppler current profiler observations in the Luzon Strait

The characteristics of internal tides in the upper layer of the Luzon Strait are investigated on the basis of direct-observation current data recorded on April 25 and September 26, 2008 by an acoustic Doppler current profiler. Spectral analysis and energy estimation show that the diurnals and semidiurnals carry most of the energy of internal tides. Values of the depth-integrated total energy E for the three frequency bands of diurnal, semidiurnal, and high frequencies are 31, 6.9, and 3.4 kJ. m, respectively. Near-inertial peaks are only present in the baroclinic component. The behavior of typical tidal frequencies （i.e., O1, K1, M2, MK3, and M4） and the near-inertial frequency is basically consistent with linear internal wave theory, which predicts E＋（ω）/E_（ω）=（ω-f）2/（ω＋f）2 at depths above 66 m, while not all prominent tidal components coincide well with the relation of the linear internal wave field at other depths. Examinations of depth structures of the baroclinic tides and temporal variations show that the surface tides and internal tides are both of mixed type, having diurnal inequality and spring-neap fortnight periods. The K1 and O1 tides have comparable cross- and along-shelf components, while the M2 and S2 tides propagate toward the shelf in the northern South China Sea as wave beams. The amplitude and phase of internal tides vary with time, but M2 and S2 tides appear to have structures dominated by the first mode, while the K1 and O1 tides resemble second-mode structures. The minor to major axis ratios are close to expected values of flω in the thermocline.