Semidiurnal Tide Research Articles

Seasonal to interannual variability of the tide in the Amazon estuary

The Amazon River exports the largest volume of fresh water to the ocean worldwide. Although previous studies have revealed the spatiotemporal tidal variability of the estuary, its hydrodynamics is still poorly understood. Here we evaluate the seasonal and interannual variability of the tide from Óbidos (800 km upstream) to the Atlantic Ocean and show how it is affected by the hydrological regime of the Amazon River. A high-resolution 2D hydrodynamic model was applied in this region at the scale of the whole estuary. The tide model is validated using data from 14 water level stations and shows an average complex error of 16 cm in the low flow season and 23 cm in the high flow season. The semi-diurnal tide is highly variable at seasonal timescales, and the seasonality of the discharge affects the tidal amplitude, the geographic extent of tidal influence, the tidal wave celerity, and the tidal flow reversal. Notably, the tidal influence on water level remains detectable up to Óbidos during the low flow season while during the high flow season it extends from the ocean to only 300 km downstream of Óbidos. On the other hand, the upstream limit of the domain where the tide induces a periodic flow reversal is different from the limit of tidal influence on the water level. The upstream limit of this flow reversal is shifted by 170 km (from 500 km to 670 km downstream of Óbidos) along the year due to the seasonality of the discharge. At interannual scale, anomalous hydrological discharges affect the tidal amplitude by up to 30% in the central reach of the estuary. Our findings open unprecedented opportunities to understand biogeochemical and geomorphological processes, help navigation, and assess flooding hazards.

The Stratosphere‐Ionosphere‐Protonosphere Coupling: Evidence From the Ion Composition Observations During the 2009 Sudden Stratospheric Warming

AbstractPrevious studies suggested that sudden stratospheric warmings (SSW) change the global atmosphere from troposphere to thermosphere/ionosphere. We report the low‐latitude O+ and H+ composition at 840‐km altitude during the 2009 SSW, with the DMSP satellite morning measurements. Our results indicate that the stratospheric variation around 30‐km altitude modulates the ion exchange between the ionosphere and protonosphere via the vertical and field‐aligned plasma drifts due to the enhanced lunar semidiurnal tides. The upward disturbance drift uplifts the ionospheric O+ into the protonosphere, and most O+ is changed to H+ via chemical coupling, while the O+/H+ transition height does not change under combined effects of the southward and upward disturbance drifts on 24–29 January. The disturbance drift turns downward, lowers the O+/H+ transition height, depletes the O+ density in the protonosphere, and the H+ at higher altitudes moves downward to supply the H+ at 840 km from 30 January to 5 February.

Energetics of semidiurnal internal tides near Madagascar in the Southwest Indian Ocean

The semi-diurnal internal tidal energetics near Madagascar Island in the Southwest Indian Ocean were analyzed based on a numerical simulation. The baroclinic energy budget revealed that internal tides radiating from the north of Madagascar Island (NMI) and the south of Madagascar Island (SMI), especially those from the continental slope, were the dominant energy sources of the baroclinic energy dissipation in the deep ocean of surrounding areas. In NMI and SMI, the most intense baroclinic energy dissipation and internal tide-induced diapycnal mixing occurred near ridges and seamounts, with depth-integrated energy dissipation of O(10-1-100) W·m−2, dissipation rates of O(10-7-10-6) W·kg−1 and diapycnal diffusivity of O(10-3-10-2) m2·s−1, about one order of magnitude larger than those in most areas of the Mozambique Channel near the 200-m isobath. These estimates based on the numerical simulation were comparable with previous results based on microstructure measurements or fine-scale parameterizations, and suggested that the remotely generated internal tides played an important role in the baroclinic energy dissipation in the deep ocean.

HIDRA2: deep-learning ensemble sea level and storm tide forecasting in the presence of seiches – the case of the northern Adriatic

Abstract. We propose a new deep-learning architecture HIDRA2 for sea level and storm tide modeling, which is extremely fast to train and apply and outperforms both our previous network design HIDRA1 and two state-of-the-art numerical ocean models (a NEMO engine with sea level data assimilation and a SCHISM ocean modeling system), over all sea level bins and all forecast lead times. The architecture of HIDRA2 employs novel atmospheric, tidal and sea surface height (SSH) feature encoders as well as a novel feature fusion and SSH regression block. HIDRA2 was trained on surface wind and pressure fields from a single member of the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric ensemble and on Koper tide gauge observations. An extensive ablation study was performed to estimate the individual importance of input encoders and data streams. Compared to HIDRA1, the overall mean absolute forecast error is reduced by 13 %, while in storm events it is lower by an even larger margin of 25 %. Consistent superior performance over HIDRA1 as well as over general circulation models is observed in both tails of the sea level distribution: low tail forecasting is relevant for marine traffic scheduling to ports of the northern Adriatic, while high tail accuracy helps coastal flood response. Power spectrum analysis indicates that HIDRA2 most accurately represents the energy density peak centered on the ground state sea surface eigenmode (seiche) and comes a close second to SCHISM in the energy band of the first excited eigenmode. To assign model errors to specific frequency bands covering diurnal and semi-diurnal tides and the two lowest basin seiches, spectral decomposition of sea levels during several historic storms is performed. HIDRA2 accurately predicts amplitudes and temporal phases of the Adriatic basin seiches, which is an important forecasting benefit due to the high sensitivity of the Adriatic storm tide level to the temporal lag between peak tide and peak seiche.

Remarkable changes in F region winds and plasma drifts during the stratospheric QBO disruption of 2019–2020

Analysis of the thermospheric wind and plasma drift data acquired respectively by the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) and Ion Velocity Meter (IVM) onboard Ionospheric Connection Explorer (ICON) satellite reveals remarkable changes during the disruption of the stratospheric quasi-biennial oscillation (QBO) due to high planetary wave activity in December 2019-January 2020. The thermospheric winds, in particular, are found to be unusually equatorward over the dip equator and northern geomagnetic low latitudes. Moreover, the plasma drifts are upward over the dip equator during midnight hours . Besides, the plasma drift exhibits a clear semi-diurnal variation. It is suggested to be due to the decrease in the diurnal tidal amplitude as a result of the non-linear interaction between the diurnal tide and the planetary waves. The relative increase of semi-diurnal tide may have imposed semi-diurnal variation in the meridional winds which could turn equatorward around midnight hours. The equatorward winds can increase the growth rate of the Rayleigh-Taylor instability to generate delayed F-region irregularities around midnight hours.

Observation of MLT region winds and tides by the USTC Mengcheng meteor radar

The atmospheric winds and waves in the mesosphere and lower thermosphere (MLT) region are essential for studying the dynamics and climate in the middle and upper atmosphere. The University of Science and Technology of China (USTC) meteor radar located at Mengcheng (33.36°N, 116.49°E) has been operating continuously since April 2014. More than 8 years of observation of mesospheric horizontal winds and tides are presented in this study. In addition, we present an intercomparison among the meteor radar observations and the Navy Global Environmental Model-High Altitude (NAVGEM-HA) analysis results. The meteor number at northern lower midlatitudes suffers from diurnal variations in meteor occurrence, with a high count rate in the local morning and a low rate during local afternoon-to-midnight. The meteor count rates show a clear annual variation, with a maximum in September–October and a minimum in February. The horizontal wind in the MLT region has dominant annual variations at lower midlatitudes, with the eastward wind during summer and the westward wind during winter above 84 km, and the eastward wind during winter and the westward wind during spring below 84 km. The meridional wind is northward during winter and southward during summer. The diurnal amplitude is dominant, followed by the semidiurnal tides at lower midlatitudes. The zonal and meridional diurnal tides show enhancements during spring (March) with amplitudes that can reach up to 40 m/s and 30 m/s and during autumn (September) with amplitudes that can reach up to 30 m/s and 25 m/s, respectively. The seasonal variations in diurnal tidal amplitude basically show characteristics that are strong during the equinox and weak during the solstice. The zonal and meridional semidiurnal tides are maximized during spring (April) and autumn (September) above 90 km.

Tidal Motion Enhancement on Spitsbergen Bank, Barents Sea

AbstractThe higher resolution numerical model investigates the semidiurnal and diurnal tides over the shallow Spitsbergen Bank. The tidal wave is trapped around Bear Island, Hopen Island, and Central Hill at the Bank in the semidiurnal range of oscillations. Semidiurnal waves around Bear and Hopen Islands generate a trapped dipole in the sea level distribution revealed by sea level maximum and minimum traveling on the opposite sides of the island. The dipole induces significant sea‐level differences and amplified velocity around the island perimeter. The role of the tidal currents over the Bank is connected through the mixing and transport to the generation of frontal regions. Tidal mixing directly relates to the local ice formation on Spitsbergen Bank as these areas coincide with the regions of strong tidal currents. The trapping of the semidiurnal tides leads to residual (permanent) circulation of up to 15 cm s−1. As this motion interconnects different bathymetric regions of the Bank, it influences the exchange of properties between these regions. The strength of residual currents shows that the large‐scale slow thermohaline motion cannot be complete without residual tidal motion. The diurnal tides show a different distribution since higher spatial resolution reveals sea‐level variability in many locations. These short‐wavelength shelf waves occur at seamounts, submarine peninsulas, and bays. We investigate in detail the wave dynamics in proximity to Central Hill and Hopen Island. We use observations of the buoys deployed on the ice floes to compare the drift velocity to the computed tidal velocity.

Socially-driven risk in a town of the Amazon coastal zone: the case of Vigia de Nazaré, in Pará state, Brazil

Vigia de Nazaré is a town located in the northeastern extreme of the Brazilian state of Pará, in the Pará Coastal Zone, a sub-region of the Amazon Coastal Zone. The town sits on the lower levels of the region’s terraces, and on fluviomarine plains. The different stages of its sociospatial evolution reflect the problem analyzed here, in particular, the occupation and urban development of the fluvial-marine plains, which was occupied spontaneously by large numbers of families, most of which are socially vulnerable. The present study investigated the physical and social processes that led this urban space’s vulnerability to hydrometeorological hazards, based on an integrated historical-geographic approach and the inherent precariousness of the environment. The data analysis revealed high levels of annual precipitation in the region, with a well-defined rainy season between January and May (mean monthly precipitation of over 300 mm), in an environment dominated by semidiurnal macro tides with amplitudes of up to 4.5 meters. In 2022, the high tide reached at least 4.2 meters in 60 occasions — the level of alert for possible rainfall-induced tidal surges. The recent occupation of the urban zone is concentrated on the low-lying areas of the fluviomarine plain, which recently account for one-third of the urban center. These areas are exposed to the hydrological dynamics of the local estuarine environment, which result in environmental degradation and hydro-morphodynamic processes alteration.

Effect of Estuary Urbanization on Tidal Dynamics and High Tide Flooding in a Coastal Lagoon

AbstractHigh tide floods (HTFs) are minor, shallow flooding events whose frequency has increased due to relative sea‐level rise (SLR) and secular changes in tides. Here we isolate and examine the role of historical landscape change (geomorphology, land cover) and SLR on tides and HTF frequency in an urbanized lagoonal estuary: Jamaica Bay, New York. The approach involves data archeology, historical (1870s) map digitization, as well as numerical modeling of the bay. Numerical simulations indicate that a century of landscape alterations (e.g., inlet deepening and widening, channel deepening, and wetland reclamation) increased the mean tidal range at the head of the bay by about 20%. The observed historical shift from the attenuation to amplification of semidiurnal tides is primarily associated with reduced tidal damping at the inlet and increased tidal reflection. The 18% decrease in surface area exerts a minor influence. A 1‐year (2020) water level simulation is used to evaluate the effects of both SLR and altered morphology on the annual number of HTFs. Results show that of 15 “minor flood” events in 2020, only one would have occurred without SLR and two without landscape changes since the 1870s. Spectral and transfer function analyses of water level reveal frequency‐dependent fingerprints of landscape change, with a significant decrease in damping for high‐frequency surges and tides (6–18 hr time scale). By contrast, SLR produced only minor effects on frequency‐dependent amplification. Nonetheless, the geomorphic influence on the dynamical response significantly increases the vulnerability of the system to SLR, particularly high‐tide flooding.

Long‐Term Characteristics of the Meteor Radar Winds Observed at King Sejong Station, Antarctica

AbstractThe characteristics of the horizontal wind in the mesosphere and lower thermosphere (MLT) at King Sejong Station (KSS) in the Antarctic Peninsula are investigated using approximately 15‐year (March 2007–November 2021) meteor radar (MR) observations. The 15‐year climatology and variability of the MLT winds at KSS are analyzed for the first time, and comparisons with the observations at other Antarctic stations are performed. The short‐period oscillations (periods between 2 hr and 10 days) of the winds, including tides and several planetary wave components, are examined for each month. Temporal and altitudinal variations in the amplitudes of the semidiurnal and diurnal tides, the two largest short‐period components, are analyzed. Annual and semiannual oscillations of the wind are distinct below and above z = 90 km, respectively. The long‐term trend of the zonal wind is estimated through a multiple linear regression analysis. Statistically significant trends of the zonal wind of ∼0.5–0.8 m s−1 yr−1 appear above z = 90 km in July. To understand observed MLT wind trends, the gravity‐wave momentum flux and gravity‐wave drag (GWD) in the MLT region are estimated from the MR observations at KSS for the 15 years. Similar seasonal variations are found between the trends of the zonal winds and zonal GWDs, especially above z = 94 km. In July where a strong positive zonal wind trend appears, positive correlation between the zonal wind and zonal GWDs is found to be statistically significant above z = 94 km.

Geodynamic, geodetic, and seismic constraints favour deflated and dense-cored LLVPs

Two continent-sized features in the deep mantle, the large low-velocity provinces (LLVPs), influence Earth's supercontinent cycles, mantle plume generation, and geochemical budget. Seismological advances have steadily improved LLVP imaging, but several fundamental questions remain unanswered, including: What is the true vertical extent of the buoyancy anomalies within these regions? And, are they purely thermal features, or are they also compositionally distinct? Here, we address these questions using a comprehensive range of geophysical observations. The relationship between measured geoid anomalies and long-wavelength dynamic surface topography places an important upper limit on the vertical extent of large-scale, LLVP-related density anomalies at ∼900 km above the core-mantle boundary (CMB). Instantaneous mantle flow modelling suggests that anomalously dense material must exist at their base to simultaneously reproduce geoid, dynamic topography, and CMB ellipticity observations. We demonstrate that models incorporating this dense basal layer are consistent with independent measurements of semi-diurnal Earth tides and Stoneley mode splitting functions. Our thermodynamic calculations indicate that the presence of early-formed, chondrite-enriched basalt in the deepest 100–200 km of the LLVPs is most compatible with these geodynamic, geodetic, and seismological constraints. By reconciling these disparate datasets for the first time, our results demonstrate that, although LLVPs are dominantly thermal structures, their basal sections likely represent a primitive chemical reservoir that is periodically tapped by upwelling mantle plumes.

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.

Internal Hydraulic Transition and Turbulent Mixing Observed in the Kuroshio over the I-Lan Ridge off Northeastern Taiwan

Abstract Complex small-scale processes and energetic turbulence are observed at a sill located on the I-Lan Ridge that spans across the strong Kuroshio off Taiwan. The current speed above the sill is strong (1.5 m s−1) and unsteady (±0.5 m s−1) due to the Kuroshio being modulated by the semidiurnal tide. Above the sill crest, isothermal domes, with vertical scales of ∼20 and ∼50 m during the low and high tides, respectively, are generated by turbulent mixing as a result of shear instability in the bottom boundary layer. Tidally modulated hydraulic character modifies the small-scale processes occurring on the leeward side of the sill. Criticality analysis, performed by solving the Taylor–Goldstein equation, suggests that the observed lee waves and intermediate layer sandwiched by two free shear layers are related to the mode-1 and mode-2 critical control between the sill crest and immediate lee, respectively. Around high tide, lee waves are advected further downstream, and only mode-1 critical control can occur, leading to a warm water depression. The shear instabilities ensuing from the hydraulic transition processes continuously mediate flow kinetic energy to turbulence such that the status of marginal instability where the Richardson number converges at approximately 0.25 is reached. The resultant eddy diffusivity Kρ is concentrated from O(10−4) to O(10−3) m2 s−1 and has a maximum value of 10 m2 s−1. The sill on the western flank of the Kuroshio is a hotspot for energetic mixing of Kuroshio waters and South China Sea waters.

Numerical simulations of generation and propagation of internal tides in the Andaman Sea

The generation and propagation of internal tides in the Andaman Sea are investigated using a three-dimensional high-resolution numerical model. Three categories of experiments, including driving the model with four main semidiurnal tides (M2, S2, N2, and K2), four main diurnal tides (K1, O1, P1, and Q1), and eight main tides (M2, S2, N2, K2, K1, O1, P1, and Q1), are designed to examine the effects of barotropic tides. The results show that the semidiurnal internal tides are dominant in the Andaman Sea, and the inclusion of diurnal barotropic tides negligibly modulates this result. That is partly due to the strength of the diurnal barotropic tides is generally one order smaller than that of the semidiurnal barotropic tides in this region. The sensitivity experiments put this on a firmer footing. In terms of the internal tidal energy, the experiments driven by the diurnal barotropic tides are three orders and one order smaller than those driven by the semidiurnal barotropic tides, respectively, during the spring and neap tides. In addition, the experiments result in total barotropic-to-baroclinic energy conversion rates over the Andaman Sea 29.15 GW (driven by the eight tides), 29.24 GW (driven by the four semidiurnal tides), and 0.05 GW (driven by the fourdiurnal tides) in the spring tidal period and 3.08 GW, 2.56 GW, and 0.31 GW in the neap tidal period, respectively. Four potential generation regions of internal tides are found, three of which are in the Andaman and Nicobar Islands and one in the northeastern Andaman Sea.

Assessing Metallic Pollution Using Taxonomic Diversity of Offshore Meiobenthic Copepods

The Gulf of Gabès, located on the south-east Tunisian coast, is an important maritime area, with great influence on the local economy and human welfare. The aim of the current study was to assess the response of meiobenthic copepod populations from this gulf to anthropogenic disturbances. Nine sampling sites, situated along the shores of the gulf were surveyed seasonally from winter of 2004 to autumn of 2005. Interestingly, this biotope has one of the highest semi-diurnal tides in the Mediterranean Sea. Despite the fact that the data being presented here are not that new, such a high maximum amplitude of tides reaching 2 m makes any information available on animals with diurnal dispersal cycles such as copepods extremely precious. Furthermore, the lack of knowledge on these crustaceans is obvious and lags behind that of numerous other meiobenthic taxa and planktonic calanoids. Actually, most publications do not reach the species level and are limited to determining densities. This is mainly due to the modest size of harpacticoids, and the lack of qualified taxonomic experts and global taxonomic databases. Sediment samples were analyzed for fluorine, carbohydrates and trace metals (i.e., Fe, Zn and Cd) content. A pollution index, based on the eigenvalues of the main ordination axis of a Principal Component Analysis, was applied. The highest values of chemicals were detected at the sites situated near an industrial complex, along with significant variations among seasons. The copepod community comprised 38 species, including five species new to science. Species richness, density and biomass of copepod communities varied among sites and seasons. These community-based indices were also analyzed separately for each season with conditional autoregressive models, revealing a significant negative response with the level of pollution. However, the results of partial Mantel tests showed positive correlations between species richness and pollution level, after controlling for the effect of geographic proximity in-between pairs of sites (winter: r = 0.927, p < 0.0001; spring: r = 0.935, p < 0.0001; summer: r = 0.918, p < 0.0001; autumn: r = 0.937, p < 0.0001). The overall pattern was that nearby sites were characterized by similar pollution levels and inhabited by similar species of copepods. Moreover, the copepod communities were significantly influenced by pollutants, mainly by trace metals. The results of the current survey emphasize the usefulness of meiobenthic copepods in biomonitoring programs not only for the gulf of Gabès, but also for other coastal areas from the Mediterranean Sea region.

Географическое и сезонное распределение поля приливной объемной силы в Охотском море в контексте динамики внутренних волн

Estimates of the barotropic tidal body force for diurnal and semidiurnal tides are obtained for the Sea of Okhotsk for the summer and winter periods. It is shown that in the study area, the tidal body force for diurnal tides is significantly greater than for semidiurnal ones. The maximum values of this quantity can reach about 2-8 m2s-2, and these values are typical for areas with a sharp bathymetric gradient. A comparison of the tidal body force for the two seasons showed noticeable differences. The features of the transformation of a barotropic tidal wave propagating in the zone of large values of the tidal body force for the K1, O1, M2 tidal constituents are demonstrated. Numerical simulations indicate that baroclinic tidal waves are effectively generated in this area, and intense short-period internal waves are likely to occur.

On the role of wind and tides in shaping the Gironde River plume (Bay of Biscay)

We study the Gironde River plume in the Bay of Biscay (North-East Atlantic) using a high resolution modelling approach of the estuary-mouth-shelf continuum. The Gironde River plume flows over a gently sloping shelf where semi-diurnal tides reach an amplitude of 1.3–1.5 m on average; the Gironde River discharge undergoes a significant seasonal variability with floods (>1000 m3/s) in winter and spring. From two years of simulation (2011–12), we document the behavior of the plume under varying forcing conditions. We then discuss the dynamical mechanisms at play, from the estimate of some mechanical energy budget terms and using sensitivity runs: run with no tides, run with filtered atmospheric forcing (no variability at time scales smaller than 30 days) and run with no river runoff. We first show that the plume's properties are determined by the processes occurring in a transition area at the estuary outlet between the Royan and Cordouan sections. The outflow passes through a deep and narrow channel both because of bathymetric constraints and tides. Indeed, tides generate a residual circulation made of anticyclonic and cyclonic vortices with diameters of 5–12 km. Tides also induce intense mixing that strongly modifies the stratification between the two sections. The mean conditions lead to classify the plume as a narrow outflow which is a favorable condition for a bulge to be formed according to the literature. We indeed observe the recurrent formation of a bulge with sharp salinity gradients and a narrow coastal current. The bulge develops in moderate to high discharge conditions, during the spring to neap tides transition and when winds are weak. When the intensity of tides and wind increases, the bulge breaks down and the plume spreads offshore under upwelling wind conditions or along the coast (both in northward and southward directions) under strong eastward wind conditions. Tides are the main driver at the estuary outlet (with a bottom friction term larger by two orders of magnitude than over the adjacent shelf) but winds and atmospheric pressure are the main forcing over the inner shelf. These results evidence the need to model accurately the whole estuary-shelf continuum when studying the fate of riverine waters over the shelf.

Characteristics of astronomical tides and their modulation on sea level extremes along the Indian coast

In this study, long-term hourly sea-level records from 18 tide gauge stations during 1972–2007 were analyzed to study the characteristics of astronomical tides, sea-level trends, and extremes around India's mainland. The de-tided signals were used to estimate these parameters and study tide and surge interaction. The observed sea level depicts significant variability in daily, seasonal, and inter-annual time scales. Semidiurnal tides are the most dominant among the high-frequency tides, with an amplitude of up to 2.5 m for M2 and 0.75 m for S2 tides in the northwestern and northeastern continental shelf and reduced to 0.25 m in the south. The amplitude of diurnal tides (O1 and K1)is relatively weak (<0.25m) at all the stations. The annual harmonics dominate the seasonal cycle, with an amplitude of 0.7 m at Garden Reach in the northeastern continental shelf, decreases to 0.15 m at Tuticorin in the south, and remains small (<0.1 m) along the west coast of India. The amplitude of lunar nodal and perigee tides are significantly high (up to 25 mm) at several stations compared to the long-term global mean sea level trend (∼3.3 mm/y). The sea level trend is significantly positive (up to 4 mm/y) for Sagar Island, Diamond Harbor, Haldia, and Mangalore; negative (up to −3 mm/y) for Garden Reach and Okha, and in-significant (0.5 mm/y) for Mumbai, Vishakhapatnam, and Paradip. Interaction between the semidiurnal tides and surges was intense at most stations, with a high probability of surge peaks during the fall-tide conditions in the northern continental shelf, at rising-tidal conditions for the south-eastern and western peninsula, and co-occurring at high tide for southern stations. The degree of tide-surge interaction increases from south to north with an increase in tidal range and significant nodal and perigee tidal modulation.

Changes of Hydraulic Transmissivity Orientation Induced by Tele‐Seismic Waves

AbstractWater level monitoring data from 5 deep wells on the North China Platform are used to study how two large earthquakes impact deep aquifers. After the passage of seismic waves from earthquakes, a subset of these wells shows changes in the phase shifts of water level responses to lunar diurnal (O1) and semidiurnal (M2) tides. We fit a model for the tidal responses of hydraulically conductive fractures that intersect the wells to explain the phase and amplitude of responses to both O1 and M2 tides. To explain changes after the earthquakes with this model, the apparent orientation of fractures must change. Because the stresses are small, we propose that changes in tidal response occur when the passage of seismic waves modifies the hydraulic connectivity of a network of fractures by unclogging or clogging flow paths, thus changing the apparent orientation of the fractures controlling tidal responses. The seismic energy density that results in changes appears to decrease as the dominant frequency of seismic waves increases.

Semidiurnal Internal Tides Observed on the Eastern Flank of Hanna Shoal in the Northeastern Chukchi Sea

AbstractThis paper investigates the role of semidiurnal tides in the Hanna Shoal region on the northeastern Chukchi Sea shelf to evaluate their impact on the regional shelf dynamics. The study is based on 2‐year long velocity time series from five oceanographic moorings. These records indicate the dominance of wind‐generated near‐inertial energy during the summer season with low ice cover. However, when the ocean is fully covered by sea ice, tides dominate the variability in the semidiurnal energy band. The records reveal considerable seasonal variability as well as regional differences, where barotropic tides dominate in the well‐mixed waters west of Hanna Shoal while bottom‐trapped internal (depth‐dependent) tides are observed east of Hanna Shoal, where stratification can persist year‐round. Resulting tide‐driven shear in winter east of Hanna Shoal under stratified conditions can occasionally reach the level of a Richardson number below 1 and can be as low as ∼0.25, implying the likelihood of shear instability and potentially eroding lower water column stability. This may lead to upward fluxes of near‐bottom nutrient‐rich Winter Water and thus carries ecosystem implications. Our study indicates that the internal tides east of Hanna Shoal are modulated by the spring‐neap cycle and result from the interaction of barotropic tides with local bathymetry and stratification.