M2 Amplitude Research Articles

PREDIKSI PASANG SURUT PERAIRAN SEKITAR KOTA BITUNG DENGAN PENAMBAHAN KONSTANTA HARMONIK PERAIRAN DANGKAL

Bitung is a city with the most intensive marine space utilization activities in North Sulawesi Province. Therefore, information regarding oceanographic conditions is very important in relation to development in this city. One of the important information regarding oceanographic conditions is tides. This study was conducted with the aim of assessing the accuracy of tidal prediction with the addition of shallow water harmonic constituents and examining the amplitude fluctuations of the main harmonic component due to the addition of the shallow water constituents. The least squares method was used to calculate the amplitude and phase of tidal harmonic constituents, followed by data prediction. The data used is the measurement data of BMKG Bitung City tidal station in January, February and March 2024. The results obtained are the addition of shallow water harmonic constituents to the tidal analysis around Bitung City increases the accuracy of the prediction data. The best RMSE value for January data is 3.8323 cm; February 4.1902 cm; and March 4.2558 cm. The correlation coefficient increases and the percentage of predicted data with differences < 3 cm and < 6 cm with the observation data becomes larger. It is also found that when processing with few harmonic constituents, the amplitudes of constants M2, S2, K1, and O1 fluctuate. The addition of harmonic constituents in the analysis results in more constant amplitudes of constituents M2, S2, K1, and O1.

Response of hydrodynamic environment to land reclamation in Sanmen Bay, China over the last half-century

Sanmen Bay (SMB) is one of the important harbors in Zhejiang Province. It is a semi-enclosed shallow bay that has undergone large-scale land reclamation activities. Long-term reclamation has changed the hydrodynamic conditions of SMB, such as tide, residual current, tidal prism, water exchange capacity, and tidal asymmetry. In this study, three typical periods of numerical models, based on historical charts and remote sensing, were established to investigate the influence of reclamation activities on the hydrodynamic conditions of SMB from 1971 to 2020. These model results reveal that the amplitude and phase of M2, the main tidal components in SMB, decreased by ~0.1–0.3 m and ~ 5°–15°, respectively, over the last half-century. Additionally, under the influence of ~200 km2 reclamation, many hydrodynamic conditions in SMB also changed. This includes the reduction of a residual current and tidal prism, an increase in residence time, and a change in tidal asymmetry characteristics. The residence time in nearby Xiayangtu exhibited a downward trend from 2003 to 2020, because land reclamation squeezed, and thus, enhanced the residual current eddy. The water-exchange capacity of the bay became weaker with the reduction of tidal prism to one-third and an increase in residence time. The tidal asymmetry characteristic of SMB changed from half of flood dominant to fully flood dominant by the influence of shoreline and bathymetry, which raised the flood risk. Research on the response of the hydrodynamic environment to reclamation activities in SMB reminds the local government to reassess the impact of land reclamation on the hydrodynamic environment.

Seasonal amplitude of principal tidal constituents in shallow shelf regions

Understanding the seasonal variation (annual cycle) of tides is crucial for coastal management and the planning of coastal infrastructure, as well as pivotal for navigation safety, prediction of extreme sea levels and so on. Specifically, seasonal variations of M2 amplitudes, larger in summer and smaller in winter, have attracted widespread attention. Nevertheless, investigations into other tidal constituents, such as S2, K1, O1, remain comparatively sparse. Here, seasonal variations of the four principal tidal constituents are studied in the shallow shelf regions of the China Seas and the North Sea, both acknowledged for exhibiting the most pronounced seasonal modulations of M2 amplitude, attributable to vertical stratification. We perform a segmented Harmonic Analysis with moving average filter, to remove semi-annual cycle, at 25 tide gauge data and find a synchronized seasonal modulation of M2 and O1 in China, which can be explained by stratification. Whereas S2 manifest an inverse seasonal modulation, larger in winter and smaller in summer, attributed to the sun's varying proximity to the earth. The K1 tide displays two very different seasonal cycle, also influenced by astronomical modulation. This observed pattern basically persists in the North Sea, while O1 shows a negative seasonal amplitude but still exists high correlation coefficient with M2. Consequently, this study provides a deep insight into 4 principal tidal constituents' seasonal modulation and underlying mechanisms, thereby enhancing our ability to comprehend and predict water level extremes and flooding with greater precision.

A likely role for stratification in long-term changes of the global ocean tides

Stratification—that is, the vertical change in seawater density—exerts a subtle control on the energetics and thus the surface elevation of barotropic (depth independent) flows in the ocean. Changes in stratification therefore provide a plausible pathway to explain some of the puzzling trends in ocean tides evident in tide gauge and, more recently, satellite altimetry data. Using a three-dimensional global ocean model, we estimate that strengthening of stratification between 1993 and 2020 caused open-ocean trends of order 0.1 mm yr−1 in the barotropic M2 tide, similar in structure and magnitude to long-term M2 changes deduced from satellite altimetry. Amplitude trends are predominantly negative, implying enhanced energy transfer to internal tides since the 1990s. Effects of stratification are also a relevant forcing of contemporary M2 trends at the coast, where they may modulate or even overprint the tidal response to sea level rise. Salient examples for such significant near-shore influence of stratification (≥ 95% confidence) include the Northwest Australian Shelf ( − 0.5 mm yr−1) and the coasts of western North America ( − 0.1 mm yr−1), commensurate with observed M2 amplitude trends at tide gauges.

The Latitudinal and Longitudinal Variation of the Semidiurnal Lunar Tide in the Ionosphere During 2013

AbstractThe present paper studies the latitudinal and longitudinal variation of ionospheric semidiurnal lunar tide (M2) perturbations in 2013 with total electron content (TEC) data. Results show that the latitudinal distribution of M2 exhibits longitudinal difference. At 100 ∼ 120°E and 80 ∼ 100°W longitudes, the strongest M2 perturbations in the northern and southern hemispheres roundly appeared at about 10° ∼ 20°N and 15° ∼ 25°S magnetic latitude (MLAT), respectively, which supports previous suggestion that the M2 maximized in the EIA crest regions. Moreover, the latitude range of M2 at 80 ∼ 100°W longitudes was wider than that at 100 ∼ 120°E longitudes, especially for southern hemisphere (SH). The M2 maximized at 20° ∼ 30°N MLAT in the northern hemisphere (NH) and was absent in the SH at 50°W ∼ 20°E longitudes, which was distinctly different than those at other longitudes. This abnormal behavior of latitudinal distribution of M2 may be caused by the large magnetic declination angle at 50°W ∼ 20°E longitudes. Meanwhile, the longitudinal variation and hemispheric asymmetry of M2 amplitude are different between NH winter (SSW period) and other seasons. The M2 amplitudes were stronger disturbances in the NH and in the American longitudinal sector in NH winter. However, in other seasons, the M2 amplitude was largest in Asian sector, and the M2 amplitude in NH was larger than SH at 0° ∼ 130°E longitudes, which was contrast to these at 0° ∼ 110°W longitudes. These differences of M2 variation between NH winter and other seasons may be related to the differences of the combined effects of the geomagnetic configuration and background conditions of the atmosphere and ionosphere.

Impacts of Reclamation on Hydrodynamic and Suspended Sediment Transport in the Bohai Sea

AbstractSince the 1970s, extensive land reclamation has been carried out in the Bohai Sea. However, reclamation activities were prohibited in 2018. Using the MIKE3 numerical model, this study selected 1976 and 2018 as representative years before and after reclamation, respectively, and developed a highly accurate three‐dimensional hydrodynamic and sediment transport model for the Bohai Sea. The model was employed to access the effects of reclamation on tidal waves, tidal prism, residual current, tidal flux (TF), and suspended sediment flux (SSF). The results revealed that after reclamation, the amphidromic points of M2 and S2 shifted southward near Qinhuangdao, and eastward near the Yellow River estuary. The M2 amplitude in the Bohai Bay exhibited a maximum increase of 30 cm. In the Bohai Sea, the tidal prism decreased by 3.95%, suggesting a decrease in water exchange capacity. The TF in the Bohai Sea and Laizhou Bay decreased by 0.7% and 12%, respectively, while increasing by 16% in Bohai Bay and 12% in Liaodong Bay, primarily due to the variations in tidal waves. Furthermore, SSF decreased by 6% in the Bohai Sea, but increased by 125% in Bohai Bay and 114% in Laizhou Bay. The significant increase in SSF in Bohai Bay and Laizhou Bay was mainly attributed to the higher suspended sediment concentrations and the stronger residual current velocities near the Yellow River estuary. This study provides comprehensive insights into the impacts of reclamation on TF and SSF in the Bohai Sea, which could contribute to marine resource development and the planning of environmental restoration projects.

Mode-1 N2 internal tides observed by satellite altimetry

Abstract. Satellite altimetry provides a unique technique for observing the sea surface height (SSH) signature of internal tides from space. Previous studies have constructed empirical internal tide models for the four largest constituents M2, S2, K1, and O1 by satellite altimetry. Yet no empirical models have been constructed for minor tidal constituents. In this study, we observe mode-1 N2 internal tides (the fifth largest constituent) using about 100 satellite years of SSH data from 1993 to 2019. We employ a recently developed mapping procedure that includes two rounds of plane wave analysis and a two-dimensional bandpass filter in between. The results show that mode-1 N2 internal tides have millimeter-scale SSH amplitudes. Model errors are estimated from background internal tides that are mapped using the same altimetry data but with a tidal period of 12.6074 h (N2 minus 3 min). The global mean error variance is about 25 % that of N2, suggesting that the mode-1 N2 internal tides can overcome model errors in some regions. We find that the N2 and M2 internal tides have similar spatial patterns and that the N2 amplitudes are about 20 % of the M2 amplitudes. Both features are determined by the N2 and M2 barotropic tides. The mode-1 N2 internal tides are observed to propagate hundreds to thousands of kilometers in the open ocean. The globally integrated N2 and M2 internal tide energies are 1.8 and 30.9 PJ, respectively. Their ratio of 5.8 % is larger than the theoretical value of 4 % because the N2 internal tides contain relatively larger model errors. Our mode-1 N2 internal tide model is evaluated using independent satellite altimetry data in 2020 and 2021. The results suggest that the model can make internal tide correction in regions where the model variance is greater than twice the error variance. This work demonstrates that minor internal tidal constituents can be observed using multiyear multi-satellite altimetry data and dedicated mapping techniques.

A framework for estuarine future sea-level scenarios: Response of the industrialised Elbe estuary to projected mean sea level rise and internal variability

In this study, we apply probabilistic estimates of mean sea level (MSL) rise and a sub-set of regional climate model ensemble simulations to force a numerical model of the southern North Sea, downscaling projected sea level variability to the Elbe estuary that serves as a prototype for an industrialised meso-tidal estuary. The specific forcing combination enables a localised projection of future estuarine hydrodynamics accounting for the spread of projected global sea level rise and the spread of the regional climate projection due to internal variability. Under the applied high-emission scenario, the Elbe estuary shows high decadal rates of mean water level (MWL) rise beyond 19 mm y-1, increase in the tidal range of up to 14 mm y-1 and increase in extreme water levels of up to 18 mm y-1. The bandwidth of the estuarine response is also high. For example, the range of average monthly extreme water levels is up to 0.57 m due to the spread of projected global sea level rise, up to 0.58 m due to internal variability whereas seasonal range attains 1.99 m locally. In the lower estuary, the spread of projected global sea level rise dominates over internal variability. Internal variability, represented by ensemble spread, notably impacts the range of estuarine water levels and tidal current asymmetry in the shallow upper estuary. This area demonstrates large seasonal fluctuations of MWLs, the M2 tidal amplitude and monthly extreme water levels. On the monthly and inter-annual time scales, the MWL and M2 amplitude reveal opposite trends, indicative of a locally non-linear response to the decadal MSL rise enforced at the open boundary. Overall, imposed by the climate projections decadal change and MSL rise enhance the horizontal currents and turbulent diffusivities whereas internal variability locally mitigates sea level rise–driven changes in the water column. This work establishes a framework for providing consistent regionalised scenario-based climate change projections for the estuarine environment to support sustainable adaptation development.

Interannual Changes in Tidal Conversion Modulate M2 Amplitudes in the Gulf of Maine

AbstractThe Gulf of Maine's lunar semidiurnal (M2) ocean tide exhibits spatially coherent amplitude changes of ∼1–3 cm on interannual time scales, though no causative mechanism has been identified. Here we show, using a specially designed numerical modeling framework, that stratification changes account for 32%–48% (Pearson coefficient 0.58–0.69) of the observed M2 variability at tide gauges from 1994 to 2019. Masking experiments and energy diagnoses reveal that the modeled variability is primarily driven by fluctuations in barotropic‐to‐baroclinic energy conversion on the continental slope south of the gulf's mouth, with a 1‐cm amplitude increase at Boston corresponding to a ∼7% (0.30 GW) drop in the area‐integrated conversion rate. Evidence is given for the same process to have caused the decade‐long M2 amplitude decrease in the Gulf of Maine beginning in 1980/81. The study has implications for nuisance flooding predictions and space geodetic analyses seeking highest accuracies.

Investigation of tidal evolution in the Bohai Sea using the combination of satellite altimeter records and numerical models

Understanding secular tidal variability is fundamental for many practical purposes such as navigation, coastal engineering and flood warning. Observational tidal evolution is primarily based on coastal tide gauge records whose location and number are extremely limited. Tides undergo significant changes in the Bohai Sea mainly due to coastline changes induced by human activities while there are no publicly accessible long-term hourly tide gauges in the Bohai Sea. In this paper, we investigate secular tidal evolution of four major constituents (M2, S2, O1, and K1) in the Bohai Sea based on multi-satellite altimeter records and the numerical model. Both satellite results and model results indicate consistent feature of tidal changes for M2 tide in the Bohai Sea: M2 amplitudes remarkably increase in the Bohai Bay and Liaodong Bay while noticeably decrease in the Laizhou Bay; M2 phases significantly decrease in the Bohai Bay and Laizhou Bay while substantially increase in the south of the Liaodong Bay. The tidal changes of S2, K1, and O1 constituents revealed by multi-satellite altimeters and the numerical model are contradictory in many parts in the Bohai Sea. Such inconsistency may be related to residual mesoscale ocean variability in satellite results and the ignored contribution of water depth changes to tidal evolution in model results.

Ionospheric Semidiurnal Lunitidal Perturbations During the 2021 Sudden Stratospheric Warming Event: Latitudinal and Inter‐Hemispheric Variations in the American, Asian‐Australian, and African‐European Sectors

AbstractThe ionosphere exhibits enhanced semidiurnal lunitidal (M2) perturbations during sudden stratospheric warming (SSW) events, of which the manifestation and mechanism are not well documented and understood. This paper studies the latitudinal and inter‐hemispheric variations of the ionospheric M2 perturbations during the 2021 major Arctic SSW with total electron content (TEC) data in the American, Asian‐Australian, and African‐European sectors. Results show that the M2 perturbations in the three sectors all enhanced during the SSW, with the strongest M2 perturbations in the Northern and Southern Hemispheres at about 10°∼15°N and 25°∼30°S geomagnetic latitudes, respectively. Also, clear M2 perturbations in the three sectors all extended to middle latitudes during the SSW only in the Southern Hemisphere. Comparatively, the inter‐hemispheric asymmetry of the magnitude of the M2 perturbations during the SSW was more complex. The inter‐hemispheric asymmetry of the crest locations of the low‐latitude M2 perturbations in the three sectors during the SSW was probably due to the F‐region meridional wind effect on the diffusion process of the low‐latitude plasma fountain. Meanwhile, the longitudinal differences were also obvious. The TEC M2 amplitude in the American sector was obviously larger than those in the Asian‐Australian and African‐European sectors during the SSW. The M2 characteristics in the southern middle latitude in the American sector were significantly different from those in the other two sectors during the SSW, which may be related to the differences of the combined effects of the local wind and geomagnetic configuration. In addition, a moderate geomagnetic activity occurred during the 2021 SSW, which brings difficulties to the analysis of the SSW‐related TEC variation.

Effects of Sea Level Rise on Tidal Dynamics in Macrotidal Hangzhou Bay

Sea level rise (SLR) due to climate change is expected to alter tidal processes and energy transport, disproportionately affecting coastal communities. Utilizing a nested hydrodynamics model, we provided an integrated investigation of tidal responses to SLR in the Hangzhou Bay (HZB). The scenarios of SLR in the next hundred years count for both non-uniform trends based on historical altimetry data and uniform trends from the latest IPCC projections. In a comparison of model results under different SLR scenarios, we found that the tidal range is amplified by SLR in HZB with stronger amplification at the shallow southern coast. Tidal range change generally increases with the SLR scale; however, neglecting the heterogeneities in the spatial distribution of SLR tends to overestimate the SLR effects. The harmonic analysis illustrates that SLR exaggerates the dominated semidiurnal tides (M2 and S2) but dampens their overtides and compound tides (M4, M6, and MS4), of which M2 amplitude amplification explains 71.2–90.0% of tidal range change. SLR tends to promote tidal energy entering HZB through the Zhoushan Archipelago (ZA) compared to the prototype, while dampened sea-bed roughness and reduced tidal velocity come with a less dissipative environment in HZB, resulting in 6–18% more tidal energy exported upstream. Numerical experiments indicate ZA has significant effects on tidal responses and energy flux generation, therefore, its quantitative influences and physical mechanism are also discussed in this paper.

Tidal variations associated with sea level changes in the Northern Bay of Bengal

Changing tides due to non-astronomical factors have been reported globally, with particular emphasis on estuaries and tidal rivers. Changes in tidal amplitude and range, coupled with altered mean sea level (MSL) has the potential to raise high water levels and aggravate the impacts of coastal flooding events. The current study used water level observations to analyze tidal changes over three decades and report its relationship to sea level variations along estuarine zones in the northern Bay of Bengal. The study highlights on the seasonal, inter-annual and long-term variations in tides, with special emphasis on the Ganga-Brahmaputra-Meghna (GBM) delta. The long-term trend analysis of M2 amplitude showed unusual shifts in magnitude and change in pattern after 1993, associated with similar changes in the MSL. This accelerated M2 amplitude (0.27 mm/y2) implied an association to accelerated RWL (Relative Water Level) changes in the Ganges floodplain, due to delta subsidence. These tidal changes were found to be inconsistent with astronomical forcing and the analysis of coherence between M2 and MSL showed 50–70% correlation, which was primarily due to a covarying seasonal signal. The coherence between sea level and tides varied regionally and is influenced by different processes at different timescales. The positive annual coherence between M2 and MSL directly stems from river forcing. The co-occurrence of strong El Niño and strong positive Indian Ocean Dipole events caused perturbations in both the water level and M2, but in a negatively coherent relationship. The study establishes a delayed response of M2 to inter-annual climate variability indices. The differential behavior of tidal constituents along the Chittagong coastal plain and the opposite nature of coherence between M2 and MSL there, relative to that at the GBM delta, imply the prevalence of spatially non-uniform response of tides to sea level fluctuations. The study emphasizes on the need to understand changing tides, for prediction of extreme sea levels and coastal flooding.

Tidal bore generation and transport mechanism in the Rokan River Estuary, Indonesia: Hydro-oceanographic perspectives

Studies related to the tidal bore phenomenon in the Rokan Estuary are limited. Therefore, revealing the physical characteristics and the environmental impact of a tidal bore is crucial. This study aims to determine the tidal regime in the estuarine area, the propagation of the tidal bore, and the transport mechanism throughout the Rokan River. A tidal harmonic analysis and a hydrodynamic model based on Saint-Venant equations were performed. The amplitude of M2 and S2 (semidiurnal co-tidal constituents) showed a sufficiently high magnitude upstream (around 0.5–1.4 m) with a shorter and more intense flood propagation (flood dominant). The tidal range peaked on average 5 m with a 6–6.5-hour displacement time during the full moon phase and lowered in amplitude and period during the first (third) quarter of the moon phase. A sudden increase in water level and velocity observed represented a hydraulic jump off a tidal bore. Of particular concern, the tidal bore height in the estuary was approximately 3 m and lowered by approximately 1 meter every 15 km upstream. The turbulent velocities showed a sharp increase in magnitude at the end of the ebb tide peaking at ＋0.5 m/s and significantly declined to approach −0.5 m/s during the passage of bore observed in the estuary (station R1), showing the reversal changes in the flow direction and magnitude. These mechanisms were not applicable upstream due to the irregular bottom morphology of the river channel affecting the transverse and vertical turbulent velocity. Some bed erosion-induced turbulence may affect the higher suspended sediment concentration (SSC), advected by the “whelps” beneath the first wave crest, transported upstream.

Variability in tidal harmonics of the coastal and estuarine waters of Goa during winter monsoon and spring inter-monsoon

Harmonic analysis was carried out with simultaneously observed tide elevation in the coastal and estuarine regions of Goa. The tide elevations were consistent along the coastal stretch of 80 km; however, significant variations were obtained inside the estuarine channels. In both estuaries, the increase in tide elevation was observed from mouth to head, also reflected in the major harmonic constituents. M2 was found to be the principal constituent, which was consistent in the coastal region. The maximum M2 amplitude of 66.27 cm was recorded at Pontemol (upstream of Zuari estuary) during spring inter-monsoon. The increase of M4 tide in both the estuarine channels confirms the non-linearity of the system. The phase lag of MS4 from the forcing tides of M2 and S2 indicated flood dominance of the system. The tidal current velocities were well-reflected by the tidal ellipse. Our datasets do not reflect significant changes in tidal dynamics in the two seasons. Instead, it was noticed that a small shift in bottom topography could lead to a considerable difference in the tidal dynamics of the system, which is reflected in the estimated form number and tidal skewness values. The study puts up an effort to compare the earlier harmonics with the present ones to infer the decadal evolution of the astronomical constituents in the estuarine stretch of Goa. Furthermore, the association between the larger wave with high water could indicate about the flood-risk of this region.

Influence of the semidiurnal lunar tide in the equatorial plasma bubble zonal drifts over Brazil

Abstract. Using OI6300 airglow images collected over São João do Cariri (7.4∘ S, 36.5∘ W) from 2000 to 2007, the equatorial plasma bubble (EPB) zonal drifts were calculated. A strong day-to-day variability was observed in the EPB zonal drifts, which is directly associated with the very complex dynamics of the nighttime thermosphere–ionosphere system near the Equator. The present work investigated the contribution of the semidiurnal lunar tide M2 for the EPB zonal drifts. The M2 presented an amplitude of 3.1 m s−1 in the EPB zonal drifts, which corresponds to 5.6 % of the average drifts. The results showed that the M2 amplitudes in the EPB zonal drifts were solar cycle and seasonally dependent. The amplitude of the M2 was stronger during the high solar activity, reaching over 10 % of the EPB zonal drift average. Regarding the seasons, during the Southern Hemisphere summer, the M2 amplitude was twice as large (12 %) compared to the equinox ones. The seasonality agrees with other observations of the M2 in the ionospheric parameters such as vertical drifts and electron concentration, for instance. On the other hand, the very large M2 amplitudes found during the high solar activity agree with previous observations of the lunar tide in the ionospheric E region.

Morphodynamic Equilibria in Double‐Inlet Systems: Existence and Stability

AbstractThe existence of morphodynamic equilibria of double‐inlet systems is investigated using a cross‐sectionally averaged morphodynamic model. The number of possible equilibria and their stability strongly depend on the forcing conditions and geometry considered. This is illustrated by considering a rectangular double‐inlet system forced by M2 tidal constituents only. Depending on the M2 amplitudes and phases at both entrances, no equilibrium, one equilibrium or multiple morphodynamic equilibria may exist. In case no equilibrium is found, the minimum water depth becomes zero somewhere in the system, reducing the double‐inlet system to two single‐inlet systems. In the other cases, the location of the minimum water depth and the direction of the tidally‐averaged sediment transport, as well as their actual values, depend strongly on the M2 tidal characteristics. Such parameter sensitivity is also observed when including the residual and M4 forcing contributions to the water motion, and when allowing for width variations. This suggests that, when considering a specific system, the number and stability of morphodynamic equilibria, as well as the characteristics of these quantities, can only be assessed by investigating that specific system in detail. As an example, the Marsdiep‐Vlie inlet system in the Dutch Wadden Sea is considered. It is found that, by using parameter values and a geometry characteristic for this system, the water motion and bathymetry in morphodynamic equilibrium are qualitatively reproduced. Also the direction and order of magnitude of the tidally‐averaged suspended sediment transport compare well with those obtained from a high‐complexity numerical model.

Numerical study of the circulation and water transport in Beibu Gulf: A short communication

The Beibu Gulf, located in the northwestern South China Sea, is rich in oceanic energy and biological resources. Based on the FVCOM model, a three-dimensional numerical model was adopted in the study, which was validated with the observation data. The results show that Beibu Gulf is a typical diurnal tidal area. The K1 and O1 tide have an amphidromic point near Hue, Vietnam, forming a rotary tidal system. From the amphidromic point, the amplitudes increase gradually, eventually reaching more than 80 cm and 100 cm, respectively. The maximum amplitudes of M2 and S2 tide are about 70 cm and 10 cm, respectively. Meanwhile the tidal current presents a typical rectilinear flow in Beibu Gulf, with strong the diurnal tidal currents. Seasonal changes in the circulation structure of Beibu Gulf are obvious. The Beibu Gulf is generally controlled by a non-closed anticlockwise circulation in winter. In summer, there are two counterclockwise eddies. The tidal residual current flowing through Qiongzhou Strait goes west all year round, with the largest average flow flux of 0.07 Sv in winter. Based on DYE module from FVCOM, pollutants from the Pearl River Estuary pass through Qiongzhou Strait, and reach Weizhou Island after 109 days. This is of great significance to the monitoring and control of red tide, and the protection of marine ecological environment in Beibu Gulf.

Projected Responses of Tidal Dynamics in the North Sea to Sea-Level Rise and Morphological Changes in the Wadden Sea

This study explores the projected responses of tidal dynamics in the North Sea induced by the interplay between plausible projections of sea-level rise (SLR) and morphological changes in the Wadden Sea. This is done in order to gain insight into the casual relationships between physical drivers and hydro-morphodynamic processes. To achieve this goal, a hydronumerical model of the northwest European shelf seas (NWES) was set-up and validated. By implementing a plausible set of projections for global SLR (SLRRCP8.5of 0.8 m and SLRhigh−endof 2.0 m) by the end of this century and beyond, the model was run to assess the responses of the regional tidal dynamics. In addition, for each considered SLR, various projections for cumulative rates of vertical accretion were applied to the intertidal flats in the Wadden Sea (ranging from 0 to 100% of projected SLR). Independent of the rate of vertical accretion, the spatial pattern of M2 amplitude changes remains relatively stable throughout most of the model domain for a SLR of 0.8 m. However, the model shows a substantial sensitivity toward the different rates of vertical accretion along the coasts of the Wadden Sea, but also in remote regions like the Skagerrak. If no vertical accretion is assumed in the intertidal flats of the Wadden Sea, the German Bight and the Danish west coast are subject to decreases in M2 amplitudes. In contrast, those regions experience increases in M2 amplitudes if the local intertidal flats are able to keep up with the projected SLR of 0.8 m. Between the different scenarios, the North Frisian Wadden Sea shows the largest differences in M2 amplitudes, locally varying by up to 14 cm. For a SLR of 2.0 m, the M2 amplitude changes are even more amplified. Again, the differences between the various rates of vertical accretion are largest in the North Frisian Wadden Sea (> 20 cm). The local distortion of the tidal wave is also significantly different between the scenarios. In the case of no vertical accretion, tidal asymmetry in the German estuaries increases, leading to a potentially enhanced sediment import. The presented results have strong implications for local coastal protection strategies and navigation in adjacent estuaries.

Dynamics of Tidal and Residual Currents Based on Coastal Acoustic Tomography Assimilated Data Obtained in Jiaozhou Bay, China

AbstractTo investigate the dynamics of tidal and residual currents in a shallow bay with narrow bay mouth and large tidal flat area, this study conducted three sets of coastal acoustic tomography (CAT) observations at 13 stations in deep area of Jiaozhou Bay (JZB), China, from December 6–18, 2015. Thereafter, travel time difference data observed via CAT were assimilated into a barotropic tidal model to realize an extrapolated study in tidal flats. The root mean square difference was 6.5 cm/s between assimilation and acoustic Doppler current profile data. Harmonic analyses indicated that the semidiurnal tide (M2) was predominant; however, relatively large nonlinear tidal currents (M4 and M6) were also found. Moreover, the spatial mean amplitudes of M2, M4, and M6 were 0.35 (excluding the tidal flat), 0.09, and 0.04 m/s, respectively. Dynamic analyses showed that continuity and advection terms were the main contributors to M4 in the bay mouth, whereas M4 in the tidal flats was generated by continuity and friction terms. Furthermore, bottom friction was the primary contributor to M6. In addition, residual currents with multi‐vortex structures appeared near the bay mouth, the maximum speed reached 0.60 m/s, and advection, continuity, and bottom friction contributed more to the generation of residual current eddies. Horizontal pressure gradient of residual elevation and advection of tidal and residual currents were the dominant factors maintaining the residual current near the bay mouth. The generation mechanisms of M4 and M6 were first discussed based on the assimilation of the current field data in JZB.