Upwelling and ocean structures off Algoa Bay and the south-east coast of South Africa

As the Agulhas Current flows along the south-east coast of South Africa, a number of processes operate that bring cold, deep water up onto the narrow shelf. As a consequence, upwelling along the coastline is enhanced farther southward and downstream. This situation is investigated off Algoa Bay and along the south-east coast to Port Alfred, where measurements demonstrate that marked temperature variability occurs at the coastline, particularly in summer when temperature structures are more intense and easterly-component winds more common. There is no indication that upwelling is more prevalent at Port Alfred; increasing variability farther south is evident at Woody Cape/Cape Padrone, where the coastline veers westwards, forming the eastern boundary of Algoa Bay. Here it is found that, after a wind change to north-easterly, cold water is upwelled along the shoreline between 19 hours and 2.5 days later. Such upwelling progresses north-eastwards with the movement of the wind and weather systems, although colder water also moves south-westwards into Algoa Bay. Winds, currents, sea level and sea temperatures are highly correlated, with fluctuations in sea level measuring >50 cm being associated with coastal trapped waves (CTWs). Such barotropic wind-driven CTWs are frequently active during upwelling, although it is unclear whether there is any interaction between the two phenomena.     

High precision fiber SINS with spin technology

Fiber strapdown inertial navigation system （FSINS） is presently used in several applications related to marine navigation. However, the absolute position from FSINS contains the error that increases with time, which prevents its long-term use for the ship cruise. In order to improve the performance of FSINS based on our present inertial sensors, the spin technology was proposed in the system to mitigate the navigation errors and a prototype of the proposed system was developed in Navigation Lab. The prototype contains the IMU, temperature controller, rotating configuration, navigation and I/O electronics group, control and display, power supply subsystem and other modules. In the proposed spin technology, the IMU is rotated back and forth in azimuth through four orthogonal positions relative to the ship’s longitudinal axis. Experimental testing was conducted for the prototype in the laboratory and the results showed that the RFSINS’s navigation performance is improved 10 times.     

Multiple equilibria of cross-equatorial Inertial jets

Based on the developed Anderson and Moore's theory about cross-equatorial inertial jets and a nonlinear equivalence shallow water model, new universal functions are determined by the characters of the vortical large-scale air flow (atmosphere) or ocean current (ocean) related to the jet, then the potential vorticity and energy conservation equations along the streamline in the cross-equatorial in-ertial jets can be obtained. Because the governing equations are nonlinear, some limited multiple equi-libria of cross-equatorial inertial jets may exist. According to the character of large-scale air flow or ocean current outside the jets, the existent criterion for multiple eqnilibria in cross-equatorial inertial jets is discussed, and two examples for multiple equilibia of nonlinear governing equations are given.     

日本东北M_W9.0地震海啸在港池及邻近区域诱发的涡流危险性计算与评估分析

海啸造成的灾害与损失并非都与淹没有关,特别是港口中海啸诱导的强流会对船只及海事设施产生重要的影响及损害.由于海啸流观测数据稀缺及海啸诱导涡流机制的不确定性,过去60年海啸科学主要集中于对海啸波特征及淹没过程的研究与分析,海啸流模拟及验证工作开展较少,导致对海啸流基本特征及其造成灾害现象的曲解.开展海啸诱导的涡流研究及预警服务显得尤为重要及紧迫.考虑快速海啸预警需要,综合对比海啸诱导涡流的物理框架及模型方法,探索兼顾效率与计算精度的海啸流模拟方法是本文的核心工作及出发点.通过分析浅层湍流相干结构(TCS)产生的主要物理耗散机制,确定了考虑2D水平耗散机制的非线性浅水方程可用于海啸涡流的模拟分析.基于高精、高分辨率有限体积模型Geoclaw建立了三个精细化的港口海啸流模型,模型分辨率为5m.利用基于海啸浮标反演的海啸源模型作为初始条件,模拟分析了日本东北地震海啸在远场的海啸波流特征.海啸波流特征模拟结果与观测吻合较好,结果可信.对比发现:波驱动的自由表面流,小的位相或波幅误差就会导致大的流速误差,流的模拟和预报相对波幅来说更具挑战性.研究了海啸波流能量在港池中的分布特征,得到:港池入口及防波堤两端常被强流控制,具有极高的危险性;相对于波幅的空间变化,海啸流具有更强的空间敏感性;所建立的高分辨率海啸模型模拟再现了日本海啸在近场的涡旋结构,给出了与观测基本一致的涡流特征.最后,引入海啸流危险等级标准,分析了港口海啸流危险性等级分布、船只疏散的安全深度及回港的时间周期.针对港口、海湾同时考虑海啸波流特征的海啸预警与评估对于港口应急管理者科学决策具有重要意义.     

Modelling the tides and their impacts on the vertical stratification over the Sofala Bank,Mozambique

The Sofala Bank, a wide shelf located along the central coast of Mozambique, hosts tides with high amplitudes. The Regional Ocean Modelling System (ROMS) was used to analyse the tidal currents on the bank and to investigate their effects on the stratification and generation of tidal fronts. During spring tides, barotropic tidal currents with maximum values ranging from 40 cm s^–1 to 70 cm s^–1 are found on the central bank. The major axis of the tidal ellipses for M₂ and S₂ follow a cross-shelf direction with mainly anticlockwise rotation. Similar to observations, three distinct regimes occur: (i) a warm well-mixed region on the inner shelf where the depths are <30 m; (ii) a wellmixed colder region above the shelf edge; and (iii) a stratified region offshore. The model shows that the tides lead to cooling where two criteria are satisfied: the Simpson and Hunter parameter log₁₀(h/U³) <3.2 and the depth h >30 m. The shelf edge of the bank is important for internal tide generation. Two frontal structures result, one offshore between cooler mixed waters and warmer stratified waters and the other in shallow inshore waters, between cooler mixed waters and solar heated mixed waters.     

A numerical model of beach morphological evolution due to waves and currents in the vicinity of coastal structures

A numerical model was developed of beach morphological evolution in the vicinity of coastal structures. The model includes five sub-models for random wave transformation, surface roller development, nearshore wave-induced currents, sediment transport, and morphological evolution. The model was validated using high-quality data sets obtained during experiments with a T-head groin and a detached breakwater in the basin of the Large-scale Sediment Transport Facility at the Coastal and Hydraulics Laboratory in Vicksburg, Miss, USA. The simulations showed that the model reproduced well the wave conditions, wave-induced currents, and beach morphological evolution in the vicinity of coastal structures. Both salient and tombolo formation behind a T-head groin and a detached breakwater were simulated with good agreement compared to the measurements.     

Assessing SARAL/AltiKa Data in the Coastal Zone: Comparisons with HF Radar Observations

We present an initial assessment of SARAL/AltiKa data in the coastal band. The study focuses on the Ibiza Channel where the north-south water exchanges play a key role in controlling the circulation variability in the western Mediterranean. In this area, the track 16 of SARAL/AltiKa intercepts the domain covered by a coastal high-frequency (HF) radar system, which provides surface currents with a range up to 60 km. We evaluate the performance of the SARAL/AltiKa Ssalto/Duacs delayed-time along-track products compared to the HF radar surface velocity fields. SARAL/AltiKa data are retrieved at a distance of only 7 km from the coast, putting in evidence the emerging capabilities of the new altimeter. The derived velocities resolved the general features of the seasonal mesoscale variability with reasonable agreement with HF radar fields (significant correlations of 0.54). However, some discrepancies appear, which might be caused by instrumental hardware radar errors, ageostrophic velocities as well as inaccurate corrections and editing in the altimeter data. Root mean square (rms) differences between the estimated SARAL/AltiKa and the HF radar velocities are about 13 cm/s. These results are consistent with recent studies in other parts of the ocean applying similar approaches to Topex/Poseidon and Jason-1 missions and using coastal altimeter corrections.     

Analytical study of the transverse distribution of along-channel and transverse residual flows in tidal estuaries

We present an analytical model to decompose complex along-channel and transverse residual flows into components induced by individual mechanisms. The model describes the transverse distribution of residual flows in tidally dominated estuaries. Scaling and perturbation techniques are used to obtain analytical solutions for residual flows over arbitrary across-channel bed profiles. The flows are induced by horizontal density gradients, tidal rectification processes, river discharge, wind, channel curvature and the earth's rotation. These rectification processes induce residual flows that are up-estuary to the right and down-estuary to the left of an estuarine channel (looking up-estuary in the northern hemisphere). The tidal rectification processes fundamentally change the transverse structure of along-channel residual flows in many tidal estuaries, as these processes cause the flows to be internally asymmetric about the mid-axis of the channel for relatively large tidal velocities, steep channels or narrow estuaries. In addition, velocity scales are derived from the analytical solutions to estimate the relative importance of the various residual flow mechanisms from estuarine parameters. A case study of a transect across the Upper Chesapeake Bay showed that important features of the residual flow observed in that transect are reproduced and explained by the analytical model. The velocity scales were able to identify the relevant residual flow mechanisms as well. The tidal rectification processes considered here result from advection of along-channel tidal momentum by Coriolis-induced transverse tidal currents.     

Horizontal distribution and transport processes of bloom-forming Microcystis in a large shallow lake (Taihu, China)

The horizontal distribution of bloom-forming Microcystis in a specific lake area and the transport of Microcystis by wind-driven lake currents between Meiliang Bay and the open water of Taihu Lake were measured during continuous field observations from August 21 to 25, 2006. During the observations, the horizontal distributions of Microcystis, represented by Chlorophyll a, showed a clear concentration toward downwind lake areas. According to the lake currents and the Chl a concentrations on the boundary line between the Meiliang Bay and the open water, the transported Chl a was less than 2% of the total weight of Chl a in Meiliang Bay on August 22, 24 and 25. The results suggest that the horizontal distribution of the bloom-forming Microcystis was strongly affected by the wind conditions, and the wind-driven Microcystis accumulation was mainly determined by surface drift; the transport of Microcystis by lake currents was much less important in this large, shallow lake.