Situated in the eastern coastal state of West Bengal, the Sundarbans Estuarine System (SES) is India’s largest monsoonal, macro-tidal delta-front estuarine system. It comprises the southernmost part ...of the Indian portion of the Ganga–Brahmaputra delta bordering the Bay of Bengal. The Sundarbans Estuarine Programme (SEP), conducted during 18–21 March 2011 (the Equinoctial Spring Phase), was the first comprehensive observational programme undertaken for the systematic monitoring of the tides within the SES. The 30 observation stations, spread over more than 3600 km
2
, covered the seven inner estuaries of the SES (the Saptamukhi, Thakuran, Matla, Bidya, Gomdi, Harinbhanga, and Raimangal) and represented a wide range of estuarine and environmental conditions. At all stations, tidal water levels (every 15 minutes), salinity, water and air temperatures (hourly) were measured over the six tidal cycles. We report the observed spatio-temporal variations of the tidal water level. The predominantly semi-diurnal tides were observed to amplify northwards along each estuary, with the highest amplification observed at Canning, situated about 98 km north of the seaface on the Matla. The first definite sign of decay of the tide was observed only at Sahebkhali on the Raimangal, 108 km north of the seaface. The degree and rates of amplification of the tide over the various estuarine stretches were not uniform and followed a complex pattern. A least-squares harmonic analysis of the data performed with eight constituent bands showed that the amplitude of the semi-diurnal band was an order of magnitude higher than that of the other bands and it doubled from mouth to head. The diurnal band showed no such amplification, but the amplitude of the 6-hourly and 4-hourly bands increased headward by a factor of over 4. Tide curves for several stations displayed a tendency for the formation of double peaks at both high water (HW) and low water (LW). One reason for these double-peaks was the HW/LW stands of the tide observed at these stations. During a stand, the water level changes imperceptibly around high tide and low tide. The existence of a stand at most locations is a key new finding of the SEP. We present an objective criterion for identifying if a stand occurs at a station and show that the water level changed imperceptibly over durations ranging from 30 minutes to 2 hours during the tidal stands in the SES. The tidal duration asymmetry observed at all stations was modified by the stand. Flow-dominant asymmetry was observed at most locations, with ebb-dominant asymmetry being observed at a few locations over some tidal cycles. The tidal asymmetry and stand have implications for human activity in the Sundarbans. The longer persistence of the high water level around high tide implies that a storm surge is more likely to coincide with the high tide, leading to a greater chance of destruction. Since the stands are associated with an amplification of the 4-hourly and 6-hourly constituents, storm surges that have a similar period are also likely to amplify more during their passage through the SES.
The recently upgraded barotropic tidal model TiME is employed to study the influence of fundamental tidal processes, the chosen model resolution, and the bathymetric map on the achievable model ...accuracy, exemplary for the M2 tide. Additionally, the newly introduced pole‐rotation scheme allows to estimate the model’s inherent precision (open ocean rms: 0.90 cm) and enables studies of the Arctic domain without numerical deviations originating from pole cap handling. We find that the smallest open ocean rms with respect to the FES14‐atlas (3.39 cm) is obtained when tidal dissipation is carried out to similar parts by quadratic bottom friction, wave drag, and parametrized eddy‐viscosity. This setting proves versatile to obtaining high accuracy values for a diverse ensemble of additional partial tides. Using the preferred model settings, we show that for certain minor tides it is possible to obtain solutions that are more accurate than results derived with admittance assumptions from data‐constrained tidal atlases. As linear admittance derived minor tides are routinely used for de‐aliasing of satellite gravimetric data, this opens the potential for improving gravity field products by employing the solutions from TiME.
Plain Language Summary
We introduce the upgraded computer model TiME, that simulates ocean tides originating from the gravitational attraction of the sun and moon. The model relies on the physics of relevant processes without incorporating actual observations of water level variations. Formerly unconsidered effects that strongly impact tidal dynamics are now included. We discuss the individual impact of these effects on the model accuracy, which is estimated relatively to local measurements from tide gauges. We further compare our results to external tidal models, that employ satellite observations for increased accuracy. Here we find that the upgraded model performs well in the open ocean, and has a reduced accuracy in shallow and coastal waters. The final model setting can simulate tides that recur once or twice per day at a similar level of accuracy. This weak dependence on the excitation amplitude renders TiME especially suited for studying minor tides. Due to their low amplitudes, these tides make up a smaller part of tidal dynamics and are hard to determine with satellite data, thus rendering solutions by our model being free of data constraints valuable. Comparing our solutions with routinely used, empirically motivated estimates of minor tides we show that an increased accuracy is obtained.
Key Points
A new barotropic tidal model with rotated numerical pole and full self‐attraction and loading potential is introduced
The solution accuracy is nearly constant over a wide range of frequencies and only weakly dependent on the potential amplitude
Two partial tide solutions exhibit a smaller deviation from tide gauge data than solutions built on altimetric data and admittance assumptions
Tide–surge interaction plays an important role in the distribution of surges along the coast of China. A comprehensive understanding of tide–surge interaction will provide more accurate estimates of ...extreme sea levels and storm surges. This study applied a statistical method to hourly tide–surge data from 12 tide gauges located along the coast of China to examine the dependency of residual maxima on tidal phases. Statistical significance has been tested quantitatively for each site utilizing the chi-square test. Results show that significant tide–surge interaction exists at all 12 tide gauge sites, but the strength of the interactions differs according to location. The distributions of peak residuals are significantly different from the uniform distribution and can be divided into three types. In the first type, the residual generally reaches its peak value during the rising tide (2–4 h before high tide), whereas in the second type, the residual reaches its peak mostly during the ebb tide (2–4 h after the high tide). In the third type, the residual reaches peaks during both ebb and rising tides. The main causes of tide–surge interactions are the tidal phase alteration caused by surge and the modulation of surge due to tides at the tide gauge. Tide–surge interactions at Lianyungang and Xiamen are more significant than those at other gauge sites. At Kanmen and Keelung, the tide–surge interactions are weakest. Variations of the tide–surge interaction at Xiamen and Quarry Bay show that the interaction has not varied significantly during the past 40 years and that tide–surge interactions tend to be more stable with the strengthening of the interaction.
Due to the continuous intensification of human activities in the ocean, the frequent outbreaks of red tide have caused great harm to the marine environment and ecology. Thus, the rapid detection and ...monitoring of red tide become particularly important. At present, the main monitoring methods depend on artificial and buoy data, as well as optical satellite remote sensing. However, these methods may not be able to effectively deal with the characteristics of red tide bloom, such as suddenness and unpredictability. The global navigation satellite system-reflectometry (GNSS-R) is an emerging technology that makes use of navigation signals as a remote sensing opportunity to obtain Earth surface information. GNSS-R has already been proved to be capable of retrieving sea surface parameters (e.g., dielectric constant and sea surface roughness) closely related to the outbreak of a red tide. In this article, we proposed a new method to estimate red tide density, which utilizes an all-new model associating GNSS-R observations with sea surface red tide density. This method can remove the weather influence and greatly decrease the revisit period, which is much longer for optical red tide remote sensing methods. The Landsat-8 near-infrared data and TechDemoSat-1 (TDS-1) GNSS-R data of a red tide outbreak in the sea off the Tsingtao coast in China are used to build and test the proposed method. The results demonstrate that the correlation coefficient is 0.73, and the root mean square error of retrieved red tide density is 2.84%, which shows that the GNSS-R technology shows great potential to perform the rapid and preliminary red tide monitoring and judgment.
The outbreaks of green tide have caused severe harm to the marine environment and human society. Synthetic Aperture Radar (SAR) plays an important role in green tide monitoring by virtue of its high ...resolution and cloud-free nature. The existing green tide extraction methods still face challenges in identifying multi-scale green tide patches due to noise interference, uneven greyscale and blurred boundaries in SAR images. Meanwhile, the practical application of deep learning methods with high precision is limited due to the complexity of the model and the large amount of computation. Therefore, we propose a multi-scale context-aware and batch-independent lightweight green tide extraction network called MBL-Net. A novel lightweight heterogeneous backbone is designed to extract multi-scale discriminative features and improve segmentation efficiency by using multi-scale selection kernel (MSK) modules and lightweight stages. Meanwhile, Triplet attention module is introduced to improve the internal consistency of the green tide region and suppress the effect of speckle noise. Then, the mixed pooling-based channel prior module (MCPM) is used to expand the receptive field of the network and extract the fine green tide structure by fusing multi-scale features. In addition, Filter Response Normalisation (FRN) is innovatively applied for feature normalization in the decoding stage, eliminating batch dependency. In order to verify the effectiveness of the proposed method, a dataset is built using the Sentinel-1 images of the Yellow Sea, China, from 2019 to 2021. The experimental results show that the proposed method achieves an overall accuracy of 98.59% with 0.970 G FLOPs and 3.525 M parameters, which ensures high precision and improves green tide detection efficiency. Compared with several representative networks, this method can capture more details of green tide with fewer parameters and faster calculation speed.
Vertical mixing is often regarded as the Achilles' heel of ocean models. In particular, few models include a comprehensive and energy‐constrained parameterization of mixing by internal ocean tides. ...Here, we present an energy‐conserving mixing scheme which accounts for the local breaking of high‐mode internal tides and the distant dissipation of low‐mode internal tides. The scheme relies on four static two‐dimensional maps of internal tide dissipation, constructed using mode‐by‐mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three‐dimensional map of dissipation which compares well with available microstructure observations and with upper‐ocean finestructure mixing estimates. This relative agreement, both in magnitude and spatial structure across ocean basins, suggests that internal tides underpin most of observed dissipation in the ocean interior at the global scale. The proposed parameterization is therefore expected to improve understanding, mapping, and modeling of ocean mixing.
Plain Language Summary
When tidal ocean currents flow over bumpy seafloor, they generate internal tidal waves. Internal waves are the subsurface analog of surface waves that break on beaches. Like surface waves, internal tidal waves often become unstable and break into turbulence. This turbulence is a primary cause of mixing between stacked ocean layers—a key process regulating ocean currents and biology and a key ingredient of computer models of the global ocean. In this article, a three‐dimensional global map of mixing induced by internal tidal waves is presented. This map incorporates a large variety of energy pathways from the generation of tidal waves to turbulence, accounting for the conservation of energy. The map is compared to available observations of turbulence across the globe and found to reproduce with good fidelity the main patterns identified in observations. This relatively good agreement suggests that internal tidal waves are the main source of turbulence in the subsurface ocean and implies that the map may serve a range of applications. In particular, the three‐dimensional map provides an efficient and realistic means to represent mixing by internal tidal waves in global ocean models.
Key Points
A global three‐dimensional map of mixing induced by internal tides is presented
The map can serve as a comprehensive and energy‐constrained tidal mixing parameterization in global ocean models
The map compares well to available microstructure and upper‐ocean finestructure mixing estimates
Mass loss from the Antarctic ice sheet is sensitive to conditions in ice shelf grounding zones, the transition between grounded and floating ice. To observe tidal dynamics in the grounding zone, we ...moored an ocean pressure sensor to Ross Ice Shelf, recording data for 54 days. In this region the ice shelf is brought out of hydrostatic equilibrium by the flexural rigidity of ice, yet we found that tidal pressure variations at a constant geopotential surface were similar within and outside of the grounding zone. This implies that the grounding zone ocean cavity was overpressurized at high tide and underpressurized at low tide by up to 10 kPa with respect to glaciostatic pressure at the ice shelf base. Phase lags between ocean pressure and vertical ice shelf motion were tens of minutes for diurnal and semidiurnal tides, an effect that has not been incorporated into ocean models of tidal currents below ice shelves. These tidal pressure variations may affect the production and export of meltwater in the subglacial environment and may increase basal crevasse heights in the grounding zone by several meters, according to linear elastic fracture mechanics. We find anomalously high tidal energy loss at the K1 constituent in the grounding zone and hypothesize that this could be explained by seawater injection into the subglacial environment at high tide or internal tide generation through interactions with topography. These observations lay the foundation for improved representation of the grounding zone and its tidal dynamics in ocean circulation models of sub–ice shelf cavities.
Plain Language Summary
One of the challenges for sea level rise prediction is understanding how the Antarctic ice sheets and the Southern Ocean interact. Ocean tides are an important component of this interaction, influencing ice shelf melting and the flow rate of grounded ice toward the coast. We report new observations relevant to this interaction: tidally varying ocean pressures where the ice first goes afloat to become an ice shelf. These tidal ocean pressure variations influence tidal currents below the ice shelf, and we propose that they also push seawater beneath the ice inland of the ice shelf and extend fractures at the ice shelf base. This study identifies tidal processes that may affect melt and fracture near the inland edge of ice shelves, a highly sensitive zone for ice dynamics.
Key Points
We present the first concurrent observations of ocean pressure and ice flexure in the grounding zone of an Antarctic ice shelf
Peak ocean pressure in the grounding zone at high tide exceeded glaciostatic pressure and preceded the peak ice shelf tidal deflection
These pressure variations may enhance basal crevassing and influence subglacial hydrology near the grounding line
Utilizing atmospheric temperature observed from Mars Years 33–36 by the imaging ultraviolet spectrograph (IUVS) onboard the Mars atmosphere and volatile evolution (MAVEN) and Mars climate sounder ...(MCS) onboard Mars Reconnaissance Orbiter (MRO), we derive the diurnal and semidiurnal thermal tides from 30 to 160 km. Vertical phase velocities of the migrating tides indicate their upward propagation above 100 km during the dust season (solar longitude, Ls 240°–300°). During the non‐dust season (Ls 30°–150°), the diurnal eastward wavenumber 2 (DE2) and wavenumber 3 (DE3) tides can propagate upward from the lower atmosphere to ∼140 km. The seasonal variation of DE2 and DE3 amplitudes in the thermosphere corresponds well to their counterparts in the lower atmosphere, primarily controlled by their Hough (1, 1) modes. The upward propagation of these tides could potentially impact the vertical coupling between the Martian lower and upper atmosphere.
Plain Language Summary
Atmospheric thermal tides are perturbations caused by the absorption of solar radiation in the atmosphere, a phenomenon in Earth's and other planetary atmospheres. The vertical propagation of tides plays a crucial role in transporting energy and momentum vertically within the atmosphere. Observing atmospheric temperatures at different local times makes it possible to determine various tidal components' amplitude and propagation characteristics. Based on data from different satellites, this study investigates the characteristics of tidal amplitude and propagation on Mars. The migrating tides, which move westward in sync with the sun's motion, can propagate upward in the region above 100 km during the dust season. During the non‐dust season on Mars, the diurnal eastward propagating wavenumber 2 (DE2) and wavenumber 3 (DE3) tide exhibits apparent upward propagation from the atmosphere below 40 km into the thermosphere. As Mars has weaker gravity and a lower exobase altitude than Earth, the vertical propagation of tides may directly transmit energy from lower atmospheric activities to the upper layers of the Martian atmosphere, influencing the atmospheric escape rate.
Key Points
The thermal tides are derived from temperature observations by two satellites with different altitude ranges
Migrating tides are strong in the mesosphere and thermosphere and can propagate upward from 100 km during the Martian dust season
During the non‐dusty season, the diurnal eastward wavenumber 2 (DE2) and 3 (DE3) tides propagate from the troposphere to the thermosphere
Improved determinations of the oft-ignored third-degree ocean tides can yield better accuracy for tidal predictions, numerical model solutions, and geodesy. While only a small part of tidal range, ...these components can be larger at certain coastal locations due to shelf resonances and other effects. Here, we discuss observations of the M
3
lunar terdiurnal tide using 9-year windowed tidal harmonic analyses at 157 tide gauges compares to a global assimilation model (TPXO9v5a), with a focus on the Western Pacific and the European Shelf. TPXO9v5a does well in estimating the observed M
3
amplitudes and phase lags in most regions, though determinations in coastal zones and in morphologically complex areas are coarse and often inaccurate. We also employ a shallow-water model (MARS) on the European Shelf, which can yield localized improvement over TPXO. In five subregions of the European Shelf, regional root-mean-squared-errors (RMSEs) are lower (and thus a better fit) at three locations for TPXO for amplitudes, and three for phase lags, with MARS simulations being a better fit in the other subregions. We also show that some locations have experienced significant long-term increases and/or decreases in the M
3
amplitude over time, likely related to resonance changes under sea level rise (SLR) which can modulate the oceanic response to astronomical forcing. This hypothesis is explored for Europe using the MARS model by applying various sea level rise scenarios, showing that the directionality (positive or negative) of the long-term changes in M
3
amplitudes over time match the model results for more than half of our validation stations.
Ocean circulation strongly influences how internal tides radiate and break and stimulates the spatial inhomogeneity and temporal variation of internal tidal mixing. Qualitative and quantitative ...characterizations of interactions between internal tides and general circulation are critical to multi‐scale circulation dynamics. Based on significant progress in regional circulation simulation, we obtain an observation‐supported internal tide energy field around Luzon Strait by deterministically resolving the dynamics of the radiating paths of the internal tide energy. These paths are created when the known most powerful internal tide of Luzon Strait interacts with the Kuroshio Current. We found that the radiating tidal pattern, local dissipation efficiency, and energy field respond differently to the leaping, looping, and leaking Kuroshio paths within Luzon Strait. Our new insights into the dynamics and our clarifying the controlling refraction mechanism within the general circulation create the potential for internal tides to be represented better in climate models.
Plain Language Summary
Internal tides drive oceanic mixing that varies spatially and temporally. This mixing crucially impacts how different ocean layers are maintained and drives overturning circulation of the global ocean's conveyor belt. Furthermore, the ocean's general circulation itself strongly influences how the internal tides radiate and break, which, in turn, stimulate variable turbulent mixing. Luzon Strait, which is between Taiwan and the Philippines, features the most powerful internal tides in the world. The strait also lies in the path of the strong western boundary Kuroshio Current as the current flows northeastward. In the last decade, Luzon Strait has been the global benchmarking site for internal wave studies. By numerically simulating the various paths and the variability of the Kuroshio Current that flows within Luzon Strait, we obtained an internal tide energy field that we validated with observations. Lastly, we proposed an evaluation criterion to theoretically clarify the refraction mechanism that controls how the internal tide radiates within general oceanic circulation. These new insights help us understand and represent internal tide energetics, tidal mixing, and the circulation energy spectrum in climate‐scale models.
Key Points
Interaction between the internal tide in Luzon Strait and Kuroshio greatly affects internal tidal energy field
We obtain an unprecedented observation‐supported internal tidal energy field around Luzon Strait
Radiating tidal pattern, dissipation efficiency, and energy field respond differently to the leaping, looping, and leaking Kuroshio paths
PDF
