Non-tidal Component Research Articles

Variability in Water Temperature Vertical Distribution and Advective Influences: Observations from Early Summer 2021 in the Central Yellow Sea

To analyze variations in the vertical distribution of water temperatures and the impact of advection in the central Yellow Sea, multi-layer water temperature and current observations were conducted from 31 May to 8 June 2021. Water temperatures exhibited a typical three-layer summer structure, with a uniform deep-layer temperature averaging 8.23 ± 0.05 °C. The current field was dominated by northeast–southwest tidal currents, but residual current characteristics indicated that non-tidal components significantly influenced circulation. Water temperature changes lagged tidal changes by about 3 h, with strong correlations (R > 0.7), especially in deep layers. Residual currents showed significant correlations with water temperature variations, which were attributed to advective displacement or baroclinic currents. Empirical orthogonal function (EOF) and complex EOF analyses revealed that thermocline variations (T1, explaining approximately 75% of total variance) were driven by strong northward (C1, approximately 34%) and cyclonic (C2, approximately 32%) advection. In deep layers, slight temperature changes were caused by southward Yellow Sea Cold Water Mass (C1) and northward Yellow Sea Warm Current Water (C2) propagation. This study confirms that vertical water temperature variations result from a complex interaction between various advection patterns, with southward tide-induced residual currents (C3, approximately 12%) playing a key dynamic role.

Sea Level Tidal Attributes in Port Said Harbor, Eastern Egyptian Mediterranean Coast

Observed sea level data attained from a pressure sensor tide gauge, installed inside Port Said harbor in Egypt, was utilized to modernize tidal characteristics and surges in the study area. Five years of hourly sea level dataset (June 2010 to June 2015) were analyzed using the harmonic analysis approach of Delft-3D Tide suit. The results of the analysis revealed that the semi-diurnal constituents (M2, S2), together with the solar annual (Sa) dominate tide behavior in the area. Furthermore, a semidiurnal tidal regime by a ratio of 0.21 is based on the form factor (FF) equation. Moreover, tidal asymmetry (Ar.) shows a flood tide with a short period of a value equal to 0.01068 > 0.01, which reflects a tide wave distortion. From power spectral analysis, the sea-level change is controlled by both the significant tidal and non-tidal (storm surge) components by almost an equal percentage of 50.63 % and 49.37 % respectively. Additionally, the residual from sea level harmonic analysis was annually correlated with the meteorological parameters of wind, temperature, and atmospheric pressure to estimate their effect on Sea-Level Rise (SLR). A small positive trend line of SL was distinguished in the years from 2011 to 2015 by approximately (2 mm/yr.) due to the weak correlation of meteorological parameters, in conjunction with a conventional relationship with atmospheric pressure and temperature. In 2010, a slight negative linear tendency was noticed of 0.3 mm/yr., which can be related to the direct proportional relation of atmospheric pressure and surge component, besides an inversely proportional relation of air temperature and residual component, regardless of the weak correlation with wind vector. Overall, the research provides insights into the tidal characteristics, surges, and sea-level rise in Port Said Harbor. Understanding these attributes serves the objective of this paper to assess the impact of sea-level rise and develop appropriate adaptation strategies in coastal areas

Semidiurnal nonmigrating tides in low-latitude lower thermospheric NO: A climatology based on 20 years of Odin/SMR measurements

The Sub-Millimetre Radiometer (SMR) on board the Odin satellite provides almost 20 years of nitric oxide (NO) measurements in the mesosphere and lower thermosphere (MLT) at equatorial crossing local solar times (LSTs) of 6AM and 6PM. In this study, we use Odin/SMR observations to estimate how lower thermospheric NO mixing ratios at low latitudes are affected by solar nonmigrating tides. Most of the previous studies based on satellite data have focused on the signatures of diurnal tides in the MLT and above, while we concentrate here on nonmigrating semidiurnal tides. To study the contribution of these tides to NO mixing ratio variations, we average pairs of NO measurements along ascending and descending orbital tracks at 107 km altitude over latitudes between −40°and +40°. We consider monthly climatologies of these pair-averages and analyse residuals with respect to their zonal mean. In this way, it is possible to study the effect of nonmigrating even-numbered tidal components, albeit there is a non-tidal component arising largely from quasi-stationary planetary waves. Spectral wave amplitudes are extracted using a Fourier transform as function of (apparent) zonal wavenumber with a focus around −30°, −20°and 30°latitudes. From our analysis, it appears that the semidiurnal (apparent) zonal wavenumber 4 arising from the SW6 and SE2 tides is dominant close to the equator (e.g., at −20°), except during some boreal summer months (June, July, August). On the other hand, wave-1 plays a more prominent role at subtropical latitudes, especially in the southern hemisphere, where it surpasses wave-4 during 7 months (March and May-to-October) at −30°. There is little observational evidence to date documenting the presence of the semidiurnal nonmigrating tides in NO in the low-latitude MLT. Our results hence provide one of the first evidences of the climatological signature of these tides in NO, in an altitude range that remains poorly observed.

A method of constructing a dynamic chart depth model for coastal areas.

The depth is important for vessel navigation at sea. Currently, most vessels use electronic navigation charts to navigate at sea. In coastal areas, especially close to shallow water areas, the dynamic change of the water level is very important to safe navigation. Ships calculate the change of water level by using up-to-date tide tables, to obtain the dynamic water depth in the channels. However, the depth caused by the tide and non-tidal components may reach several meters in some seas, causing the dynamic depth below the safety depth, which can easily lead to grounding of vessels stranding accidents. The channel is regularly dredged to achieve navigational depth. Without regular dredging, the offshore non-channel area becomes the common area of ship grounding. The dynamic chart depth model studied in this article can provide real-time depth, which serves the ships navigation in the non-channel. The model incorporates the chart depth and the dynamic water levels on the same reference datum. The chart depth is from the electronic navigational chart depth. The dynamic water levels are constructed by the simulated tidal levels and continuous series of nontidal residual. We then designed a deviation correction method to reduce the discrepancy of the simulated tidal level with the actual water level, including datum offset correction and residual water level correction. Finally, by merging the revised dynamic water levels with the electronic navigational chart depth, we obtained the dynamic chart depth model of the study region.

Water Depth Dependence of Long‐Range Correlation in Nontidal Variations in Seafloor Pressure

AbstractIsolating the source of nontidal oceanographic noise in seafloor pressure data is critical for improving the use of these data for seafloor geodetic applications. Residuals between nearby bottom pressure records have typically been used to remove the nontidal components, as these are largely common‐mode. To evaluate the similarities between pairs of observed bottom pressure records at a range of water depths, we calculate the standard deviations of the time series of residuals between data from all site pairs, installed in the Hikurangi subduction zone offshore New Zealand. We find that the magnitude of the standard deviation depends more on relative water depth than the distance between sites. This confirms the result of previous studies from Cascadia that nontidal components are more similar along isobaths even if the distance between sites is large. Furthermore, in order to reduce noises, the required depth difference between site pairs also varies with site depths.

Complex Extreme Sea Levels Prediction Analysis: Karachi Coast Case Study.

In this study, the analysis of the extreme sea level was carried out by using 10 years (2007–2016) of hourly tide gauge data of Karachi port station along the Pakistan coast. Observations revealed that the magnitudes of the tides usually exceeded the storm surges at this station. The main observation for this duration and the subsequent analysis showed that in June 2007 a tropical Cyclone “Yemyin” hit the Pakistan coast. The joint probability method (JPM) and the annual maximum method (AMM) were used for statistical analysis to find out the return periods of different extreme sea levels. According to the achieved results, the AMM and JPM methods erre compatible with each other for the Karachi coast and remained well within the range of 95% confidence. For the JPM method, the highest astronomical tide (HAT) of the Karachi coast was considered as the threshold and the sea levels above it were considered extreme sea levels. The 10 annual observed sea level maxima, in the recent past, showed an increasing trend for extreme sea levels. In the study period, the increment rates of 3.6 mm/year and 2.1 mm/year were observed for mean sea level and extreme sea level, respectively, along the Karachi coast. Tidal analysis, for the Karachi tide gauge data, showed less dependency of the extreme sea levels on the non-tidal residuals. By applying the Merrifield criteria of mean annual maximum water level ratio, it was found that the Karachi coast was tidally dominated and the non-tidal residual contribution was just 10%. The examination of the highest water level event (13 June 2014) during the study period, further favored the tidal dominance as compared to the non-tidal component along the Karachi coast.

Tidal and non-tidal components of water level and currents in the Sundarbans ecosystem

The present study is an effort to understand the complex tidal characteristics and flow conditions at Kaikhali station located in the Matla River of the Indian Sundarbans ecosystem. The study region is located south of the low-lying Ganges–Brahmaputra deltaic environment that is highly vulnerable to the impact of sea level rise. The maximum recorded tidal range at Kaikhali location is about 6.35 m, and the current speed varied between 0.03 to 1.18 m s−1 with an average speed of 0.48 m s−1 during the observation period. The currents closely followed the flood and ebb phases of the tidal cycle with a maxima recorded during the spring high tide conditions. The predominant current direction was northeast (NE) and southwest (SW) during the flood and ebb phases of the tidal cycle, respectively. The tidal characteristics are semidiurnal in nature, and among the five major tidal constituents M2, S2, K1, O1 and N2, the relative influence of principal lunar (M2) and principal solar (S2) components contributes 58% and 25%, respectively. The circulation characteristic in this ecosystem is a result of the combined effects of freshwater discharge from riverine tributaries, creeks and tidal propagation from the open sea. A detailed investigation performed in the present study using the measured data signify that the tidal currents dominate as compared to the non-tidal components in this region. The study has practical applications in understanding the estuarine hydrodynamics over this region.

Seafloor Crustal Deformation on Ocean Bottom Pressure Records With Nontidal Variability Corrections: Application to Hikurangi Margin, New Zealand

AbstractOcean bottom pressure (OBP) observations are a powerful tool for determining vertical crustal displacements, especially due to earthquakes and slow earthquakes, with centimeter‐level resolution. In these studies, removal of oceanographic noise (tens of centimeters) is required to identify centimeter‐level crustal deformation. We undertake barotropic modeling to remove oceanographic signals from data from an OBP array deployed offshore New Zealand in 2014/2015. We show that removing the nontidal component calculated from a barotropic ocean model reduces the variance in the data by about 66% and provides a feasible means to resolve pressure changes due to crustal deformation during the slow slip events. We also discuss the vertical displacements from slow slip events that occurred in late September to mid‐October 2014, and we outline our procedure for processing OBP data.

The Effects of Non-TIDAL Components, Depth of Measurement and the Use of Peak Delays in the Application of Tidal Response Methods

The efficacy and applicability of tidal response methods (TRMs) were assessed in terms of the techniques used, the, data used in the analysis, and the implementation of the methods under different conditions. The tidal efficiency (TE) and time lag (TL) methods were applied to directly measured groundwater head (GWH) values in the Motril-Salobrena coastal aquifer and compared with the same pre-filtered time series after eliminating the non-tidal signals through the continuous wavelet transform (CWT) procedure. The use of maximum and minimum groundwater peaks and sea-level peaks in combination to obtain different delay values and the effects of asymmetries in tidal fluctuations were assessed. Application of the TE method yields different D values when a complete groundwater head time series is considered, instead of the net induced tidal oscillation. Moreover, when the asymmetry of tidal oscillations is not taken into account, the application of TL may yield a higher uncertainty. In unconfined coastal aquifers, hydraulic diffusivity can be overestimated by the TE method if the non-tidal components are not removed from the measured time series of groundwater head. However, prior filtering provides better diffusivity results when the TE method is applied. The depth of the water head measurements leads to different D values when applying the TE method as a consequence of the changes in the specific storage with depth, which result from changes in pressure. The results of the application of the TL method depend on whether maximum or minimum peaks are used. Since the use of maximum peak delays can overestimate D, minimum peak delays are recommended, as they yield D values that are closer to the values obtained using the TE method.

Non-Tidal Ocean Loading Correction for the Argentinean-German Geodetic Observatory Using an Empirical Model of Storm Surge for the Río de la Plata

The Argentinean-German Geodetic Observatory is located 13 km from the Rio de la Plata, in an area that is frequently affected by storm surges that can vary the level of the river over ±3 m. Water-level information from seven tide gauge stations located in the Rio de la Plata are used to calculate every hour an empirical model of water heights (tidal + non-tidal component) and an empirical model of storm surge (non-tidal component) for the period 01/2016–12/2016. Using the SPOTL software, the gravimetric response of the models and the tidal response are calculated, obtaining that for the observatory location, the range of the tidal component (3.6 nm/s2) is only 12% of the range of the non-tidal component (29.4 nm/s2). The gravimetric response of the storm surge model is subtracted from the superconducting gravimeter observations, after applying the traditional corrections, and a reduction of 7% of the RMS is obtained. The wavelet transform is applied to the same series, before and after the non-tidal correction, and a clear decrease in the spectral energy in the periods between 2 and 12 days is identify between the series. Using the same software East, North and Up displacements are calculated, and a range of 3, 2, and 11 mm is obtained, respectively. The residuals obtained after applying the non-tidal correction allow to clearly identify the influence of rain events in the superconducting gravimeter observations, indicating the need of the analysis of this, and others, hydrological and geophysical effects.

Surface currents in the Porsanger fjord in northern Norway

Abstract We describe surface currents in the Porsanger fjord (Porsangerfjorden) located in the European Arctic in the vicinity of the Barents Sea. Our analysis is based on surface current data collected in the summer of 2014 using High Frequency (WERA, Helzel Messtechnik GmbH) radar system. One of our objectives was to separate out the tidal from the nontidal components of the currents and to determine the most important tidal constituents. Tides in the Porsanger fjord are substantial, with tidal range on the order of about 3 m. Tidal analysis attributes to tides about 99% of variance in sea level time series recorded in Honningsvaag. The most important tidal component in sea level data is the M2 component, with amplitude of ~90 cm. The S2 and N2 constituents (amplitude of ~20 cm) also play a significant role in the semidiurnal sea level oscillations. The most important diurnal component is K1 with amplitude of about 8 cm. The most important tidal component in analyzed surface currents records is the M2 component. The second most important component is the S2. Our results indicate that in contrast to sea level, only about 10-30% of variance in surface currents can be attributed to tidal currents. This means that about 70-90% of variance is due to wind-induced and geostrophic currents.

Assessing changes in extreme sea levels along the coast of China

AbstractHourly tide‐gauge data along the coast of China are used to evaluate changes in extreme water levels in the past several decades. Mean sea level, astronomical tide, nontidal component and the tide‐surge interaction was analyzed separately to assess their roles in the changes of extreme sea levels. Mean sea level at five tide gauges, Kanmen, Keelung, Zhapo, Xiamen and Quarrybay, show significant increasing trends during the past decades (1954–2013) with a rate of about 1.4–3.5 mm/yr. At Keelung, Kaohsiung and Quarrybay the mean high waters increased during 1954–2013 with a rate from 0.6 to 1.8 mm/yr, while the annual mean tidal range rose at the same time by 0.9 to 3.8 mm/yr. In terms of storm surge intensities, there is interannual variability and decadal variability but five tide gauges show significant decreasing trends, and three gauges, at Keelung, Xiamen and Quarrybay, exhibited significant increases of extreme sea levels with trends of 1.5–6.0 mm/yr during 1954–2013. Significant tide‐surge interactions were found at all 12 tide gauges, but no obvious change was found during the past few decades. The changes in extreme sea levels in this area are strongly related to the changes of mean sea levels (MSL). At gauges, where the tide‐surge interaction is large, the astronomic tides are also an important factor for the extreme sea levels, whereas tide gauges with little tide‐surge interaction, the changes of wind driven storm surge component adds to the change of the extreme sea levels.

内湾域の表層流が海上風より受ける影響の把握

The transport and diffusion of drifter litters are affected by surface current dominantly. Therefore, it is very important to predict the surface current in order to collect these litters quickly. By using the HFRs surface current data and ocean wind data, we analyzed the characteristics of surface current in Tokyo Bay. By comparison with the non-tidal component of sea surface current and ocean wind, the non-tidal components of sea surface current are strongly influenced by ocean wind. Especially, the magnitude of the influence in summer is larger than in winter and the affected currents have 20-40° deflection angle from wind direction. Furthermore, we revealed that the sea current circulation is influenced by ocean wind by comparison with both surface current and ocean wind vorticities.

Characterization and implications of intradecadal variations in length of day

Variations in Earth's rotation (defined in terms of length of day) arise from external tidal torques, or from an exchange of angular momentum between the solid Earth and its fluid components. On short timescales (annual or shorter) the non-tidal component is dominated by the atmosphere, with small contributions from the ocean and hydrological system. On decadal timescales, the dominant contribution is from angular momentum exchange between the solid mantle and fluid outer core. Intradecadal periods have been less clear and have been characterized by signals with a wide range of periods and varying amplitudes, including a peak at about 6 years (refs 2-4). Here, by working in the time domain rather than the frequency domain, we show a clear partition of the non-atmospheric component into only three components: a decadally varying trend, a 5.9-year period oscillation, and jumps at times contemporaneous with geomagnetic jerks. The nature of the jumps in length of day leads to a fundamental change in what class of phenomena may give rise to the jerks, and provides a strong constraint on electrical conductivity of the lower mantle, which can in turn constrain its structure and composition.

Joint modelling of vertical profiles of large ocean currents

We present an empirical approach to modelling the vertical (vector) profile of large ocean currents, based on a conditional extremes model (Heffernan and Tawn, 2004) from the statistics literature. Observed vector currents at each of a number of water depths are expressed as components with respect to major- and minor-axis of current variation at that depth using Principal Components Analysis. Each current component is then independently decomposed into the sum of (deterministic periodic) tidal and (random) non-tidal currents using a local harmonic model. The marginal and dependence structure of extremes of hourly maxima and minima of non-tidal components is characterised using the conditional extremes model. We simulate under this model to estimate characteristics of extreme current profiles corresponding to arbitrary return periods, and quantify the uncertainty of those estimates. For a sample collected over a 2.5-year period in 250m water on the outer shelf of North Western Australia, the model predicts monthly instantaneous extreme conditional profiles well. We also estimate marginal and conditional current profiles corresponding to a 10-year return period.

A comparison of the main methods for estimating probabilities of extreme still water levels

Sea-level return periods are estimated at 18 sites around the English Channel using: (i) the annual maxima method; (ii) the r-largest method; (iii) the joint probability method; and (iv) the revised joint probability method. Tests are undertaken to determine how sensitive these four methods are to three factors which may significantly influence the results; (a) the treatment of the long-term trends in extreme sea level; (b) the relative magnitudes of the tidal and non-tidal components of sea level; and (c) the frequency, length and completeness of the available data. Results show that unless sea-level records with lengths of at least 50 years are used, the way in which the long-term trends is handled in the different methods can lead to significant differences in the estimated return levels. The direct methods (i.e. methods i and ii) underestimate the long (> 20 years) period return levels when the astronomical tidal variations of sea level (relative to a mean of zero) are about twice that of the non-tidal variations. The performance of each of the four methods is assessed using prediction errors (the difference between the return periods of the observed maximum level at each site and the corresponding data range). Finally, return periods, estimated using the four methods, are compared with estimates from the spatial revised joint probability method along the UK south coast and are found to be significantly larger at most sites along this coast, due to the comparatively short records originally used to calibrate the model in this area. The revised joint probability method is found to have the lowest prediction errors at most sites analysed and this method is recommended for application wherever possible. However, no method can compensate for poor data.

Assessing changes in extreme sea levels: Application to the English Channel, 1900–2006

A recently extended and spatially rich English Channel sea level dataset has been used to evaluate changes in extreme still water levels throughout the 20th century. Sea level records from 18 tide gauges have been rigorously checked for errors and split into mean sea level, tidal and non-tidal components. These components and the interaction between surge and tide have been analysed separately for significant trends before determining changes in extreme sea level. Mean sea level is rising at 0.8–2.3 mm/year, depending on location. There is a small increase (0.1–0.3 mm/year) in the annual mean high water of astronomical tidal origin, relative to mean sea level, and an increase (0.2–0.6 mm/year) in annual mean tidal range. There is considerable intra- and inter-decadal variability in surge intensity with the strongest intensity in the late 1950s. Storm surges show a statistically significant weak negative correlation to the winter North Atlantic Oscillation index throughout the Channel and a stronger significant positive correlation at the boundary with the southern North Sea. Tide–surge interactions increase eastward along the English Channel, but no significant long-term changes in the distribution of tide–surge interaction are evident. In conclusion, extreme sea levels increased at all of the 18 sites, but at rates not statistically different from that observed in mean sea level.

Concurrent simulation of the eddying general circulation and tides in a global ocean model

This paper presents a five-year global simulation of HYCOM, the HYbrid Coordinate Ocean Model, that simultaneously resolves the eddying general circulation, barotropic tides, and baroclinic tides with 32 layers in the vertical direction and 1/12.5° (equatorial) horizontal grid spacing. A parameterized topographic wave drag is inserted into the model and tuned so that the surface tidal elevations are of comparable accuracy to those in optimally tuned forward tide models used in previous studies. The model captures 93% of the open-ocean sea-surface height variance of the eight largest tidal constituents, as recorded by a standard set of 102 pelagic tide gauges spread around the World Ocean. In order to minimize the impact of the wave drag on non-tidal motions, the model utilizes a running 25-h average to approximately separate tidal and non-tidal components of the near-bottom flow. In contrast to earlier high-resolution global baroclinic tide simulations, which utilized tidal forcing only, the simulation presented here has a horizontally non-uniform stratification, supported by the wind- and buoyancy forcing. The horizontally varying stratification affects the baroclinic tides in high latitudes to first order. The magnitude of the internal tide perturbations to sea surface elevation amplitude and phase in a large box surrounding Hawai’i is quite similar to that observed in satellite altimeter data, although the exact locations of peaks and troughs in the modeled perturbations differ from those in the observed perturbations.

Extreme sea levels, coastal flooding and climate change with a focus on Atlantic Canada

Estimation of the probability distribution of extreme sea levels, for the present time and the next century, is discussed. Two approaches are described and their strengths and weaknesses are compared. The first approach is based on dynamics and uses a storm surge model forced by tides, winds and air pressure fields. The second approach is based on the statistical analysis of observed hourly sea level records using a new first-order Markov process that can capture non-Gaussian characteristics (such as skewness) in the non-tidal component of the observed sea level record. It is shown that both approaches can provide good estimates of present day flooding probabilities for regions with relatively strong tides. The limitations of both approaches in terms of assessing the effect of global sea level rise, glacial-isostatic adjustment of the land, and changes in the frequency and severity of storms and hurricanes, are illustrated using recent results for the Northwest Atlantic. Some sensitivity studies are carried out to transform uncertainty in climate change projections into uncertainties in the probability of coastal flooding.

Principal component analysis of tsunami buoy record: Tide prediction and removal

Principal component or Empirical Orthogonal Function (EOF) analysis is applied to tsunameter records by treating them as two-dimensional signals, where the second dimension is created by breaking a single time series into cycles and treating the cycle number as a second dimension. Under certain conditions, principal components calculated from different records are shown to determine the same functional space. Signal decomposition into pre-calculated principal components is used to predict or extract the tidal component of a record. This work shows that EOF processing allows for short-term tidal predictions at tsunami buoy locations with the precision of more advanced methods and with minimal a priori knowledge about tidal dynamics. Also shown is that filtering in EOF domain is sensitive to the non-tidal component of a record and therefore presents a tool for early tsunami detection and quantification.