Internal wave energetics modulated by Indonesian Throughflow at Lombok Strait

<p>The interaction between the energetic internal waves in the Indonesian Seas and the Indonesian Throughflow (ITF) is not well known. Here we conduct a series of high-resolution numerical simulations surrounding the Lombok Strait, Indonesia, which is an important exit channel for the ITF, to explore the influences of the ITF on the spatiotemporal variations of M2 internal tides and associated internal solitary waves from the Strait. The ITF enhances the north-south asymmetry of internal tide propagation from the Lombok Strait, thus resulting in the spatial variability of northward and southward internal solitary waves. Interannual variability of internal tide generation and dissipation are due to ITF and air-sea freshwaterflux induced stratification variations associated with El Niño-Southern Oscillation. The local dissipation efficiency q exhibits substantial seasonal and interannual variations, which may provide effective feedback to the climate processes in the low-latitude equatorial oceans.</p>

The impact of channel deepening and sand mining on estuarine tidal dynamics

<p>Lingding Bay (LDB) is a part of the Pearl River Estuary in south China. It is surrounded by large growing cities and has been subjected to heavy waterways traffic over the past 5 decades. The tide propagation pattern has been greatly modified ever since. It has widespread morphological and ecological impacts on the LDB system. However, a systematic study on the response of tide propagation pattern to channel deepening and sand mining is currently lacking to provide future management guidelines for the Lingding Bay. Based on a state-of-the-art modeling tool (Delft3D Flexible Mesh), we explore the tidal propagation pattern (tidal amplitude, tidal phase, residual current, and tidal-energy flux) of the LDB over the last century (1901-2016). Three stages of bathymetry and tidal dynamics variation are divided by our investigation. Stage I (1901-1964) is a natural evolution process, the LDB is manifested as gradually filling by fluvial sediment, the tidal amplitude decline, and tidal dynamics decrease. Stage II (1964-1989) is characterized by a slow increase in water depth and tidal dynamics, which affected by channel dredging activities. While stage III (1989-2016) is influenced by channel deepening and sand mining, shown an abrupt increase of water depth in a short time and the rapid enhancement of tidal dynamics.</p><p>The investigation indicating that the channel deepening and sand mining activities amplified the tidal dynamics distribution difference between the channel and shoal. The increased tidal dynamics in the channels may increase saline intrusion and coastal flooding risks. Spatially, these two activities may also lead to contrasting morphodynamic patterns between the inner and outer LDB. The morphology state of erosion in the inner LDB and deposition in the outer LDB reported by other studies are consistent with the hydrodynamic variation in our study. Most likely, channel deepening and sand mining in inner LDB cause the sea bed to appear to lose，lead to larger SSC levels in the vicinity. This resulting in the inner LDB formed a new sediment source, under the stronger runoff in inner LDB, the depocenter is moved southward, and outer LDB developed to a sedimentary area. Our findings conclude that channel deepening and sand mining can greatly change the tidal dynamics distribution in an estuary, thereafter affect the sediment transport pattern. We suggest that coastal engineering planning should pay more attention to sand mining activities. The insights obtained from this study are of value to the future management of the LDB and other estuaries that are also under similar stress.</p>

Low-Mode Internal Tide Propagation in a Turbulent Eddy Field

Understanding and predicting how internal tides distort and lose coherence as they propagate through the ocean has been identified as a key issue for interpreting data from the upcoming wide-swath altimeter mission Surface Water and Ocean Topography (SWOT). This study addresses the issue through the analysis of numerical experiments where a low-mode internal tide propagates through a quasigeostrophic turbulent jet. Equations of motion linearized around the slower turbulent field are projected onto vertical modes and assumed to describe the dynamics of the low-mode internal tide propagation. Diagnostics of the terms responsible for the interaction between the wave and the slow circulation are computed from the numerical outputs. The large-scale change of stratification, on top of eddies and jet meanders, contributes significantly to these interaction terms, which is shown to be consistent with an independent scaling analysis. The sensitivity of interaction terms to a degradation of the slow field spatial and temporal resolution indicates that present-day observing systems (Argo network, altimetry) may lack the spatial resolution necessary to correctly predict internal tide evolution. The upcoming SWOT satellite mission may improve upon this situation. The number of vertical modes required to properly estimate interaction terms is discussed. These results advocate development of a simplified model based on solving a modest number of the linearized equations subject to a prescribed mesoscale field and internal tide sources.

The Impact of Subtidal Circulation on Internal Tide Generation and Propagation in the Philippine Sea

This study examines the effects of the subtidal circulation on the generation and propagation of the M2 internal tide in the Philippine Sea using a primitive equation model. Barotropic to baroclinic conversion at the Luzon Strait is found to vary due to the background circulation changes over the generation site and the changing influence of remotely generated internal tides from the Mariana Arc. The varying effect of remotely generated waves results from both changing generation energy levels at the Mariana Arc and variability in the propagation of the internal tides across the Philippine Sea. The magnitude and direction of the depth-integrated baroclinic energy fluxes vary temporally, due to a combination of changing generation, propagation, and dissipation. Spatial patterns of internal tide propagation near the Luzon Strait are influenced by the locations of mesoscale eddies to the east and west of the strait. The results provide insight into the mechanisms of variability of the baroclinic tides and highlight the importance of considering both the remotely generated internal tides and the subtidal dynamics to estimate internal tide energetics.

Incoherent Nature of M2 Internal Tides at the Hawaiian Ridge

Moored current, temperature, and conductivity measurements are used to study the temporal variability of M2 internal tide generation above the Kaena Ridge, between the Hawaiian islands of Oahu and Kauai. The energy conversion from the barotropic to baroclinic tide measured near the ridge crest varies by a factor of 2 over the 6-month mooring deployment (0.5–1.1 W m−2). The energy flux measured just off the ridge undergoes a similar modulation as the ridge conversion. The energy conversion varies largely because of changes in the phase of the perturbation pressure, suggesting variable work done on remotely generated internal tides. During the mooring deployment, low-frequency current and stratification fluctuations occur on and off the ridge. Model simulations suggest that these variations are due to two mesoscale eddies that passed through the region. The impact of these eddies on low-mode internal tide propagation over the ridge crest is considered. It appears that eddy-related changes in stratification and perhaps cross-ridge current speed contribute to the observed phase variations in perturbation pressure and hence the variable conversion over the ridge.