Ocean Tidal Currents Research Articles

Wave Modulation in a Strong Tidal Current and Its Impact on Extreme Waves

Abstract Accurate estimates of extreme waves are central for maritime activities, and stochastic wave models are the best option available for practical applications. However, the way currents influence the statistics of space–time extremes in spectral wave models has not been properly assessed. Here we demonstrate impacts of the wave modulation caused by one of the world’s strongest open ocean tidal currents, which reaches speeds of at least 3 m s−1. For a bimodal swell and wind sea state, we find that most intense interactions occur when the wind sea opposes the tidal current, with an increase in significant wave height and spectral steepness up to 45% and 167%, respectively. The steepness modulation strengthens the second-order Stokes contribution for the normalized extreme crests, which increases between 5% and 14% during opposing wind sea and current. The normalized extreme wave heights have a strong dependence on the narrow-bandedness parameter, which is sensitive to the variance distribution in the bimodal spectrum, and we find an increase up to 12% with currents opposing the wind sea. In another case of swell opposing a tidal jet, we find the spectral steepness to exceed the increase predicted by a simplified modulation model. We find support in single-point observations that using tidal currents as forcing in wave models improves the representation of the expected maximum waves, but that action must be taken to close the gap of measurements in strong currents. Significance Statement The purpose of this study is to investigate how a very strong tidal current affects the surface wave field, and how it changes the stochastic extreme waves formulated for a space–time domain. Our results suggest that the expected maximum waves become more realistic when tidal currents are added as forcing in wave models. Here, the expected extremes exceed traditional model estimates, i.e., without current forcing, by more than 10%. These differences have implications for maritime operations, both in terms of planning of marine structures and for navigational purposes. However, there is a significant lack of observations in environments with such strong currents, which are needed to further verify our results.

Impact of strong easterly trade winds on carbonate petroleum exploration - Relationships developed from Caicos Platform, southeastern Bahamas

Brisk, persistent easterly trade wind influences define Holocene patterns of carbonate sedimentation across the Caicos Platform (southeastern Bahamas). Resultant predictive sedimentary facies models based on trade wind influences are more widely applicable to the exploration for subsurface carbonate plays than are existing models based on the northern Bahamas facies patterns, which are characterized by gentle trade winds and strong platform-margin-related oceanic processes (swells and tidal currents). The Caicos Platform relationships may be more applicable because many ancient shallow carbonate depositional environments were within the trade wind belts and commonly within broad intracratonic seas that were little influenced by oceanic processes.The grainstone-dominated Caicos Platform exhibits reservoir potential over much of its surface, in contrast to northern Bahamian platforms, where oceanic tidal currents or swells and gentle easterly trade winds confine higher energy environments with reservoir potential to platform margins. Strong easterly trade winds across Caicos Platform promote widespread Holocene platform-interior oolitic, skeletal and grapestone grainstone bodies on this platform. Orientations of ooid sand bodies vary depending on preexisting topography, water depth and bottom energy. Shallow subtidal ooid sand shoals orient parallel to these winds. Ooid sands developed along older shorelines orient parallel to the shorelines but prograde perpendicular into these winds. Deeper platform-interior oolitic sands exist as widespread, sheet-like deposits. These trade winds allow reefs and ooids to coexist in many settings, permit isolated linear reefs to flourish on certain leeward platform margins, and promote effective off-bank transport of carbonate sands that create onlapping grainstone wedges. These relationships are very applicable to the rock record.Strong trade wind influences, such as seen on Caicos Platform, better explain the occurrence of Cretaceous reef and/or oolitic grainstone reservoirs developed well in from platform margins (Fairway Field in East Texas; Black Lake Field in Louisiana) than do existing northern Bahamian models. Depositional models based on conditions in the northern Bahamian models would predict low-energy facies in these platform-interior settings. A trade-wind-driven depositional model, characterized by strong persistent easterlies, also better explains the origin of the onlapping wedge of skeletal grainstones at Poza Rica oilfield in Mexico.The Caicos Platform easterly trade-wind-driven depositional model should generally apply to any ancient shallow carbonate environment that developed 5–22° north or south of the paleoequator, whether or not a platform margin was near by. Future carbonate exploration or exploitation should always factor in not only geological age, but also physiographic, latitudinal and climatic setting at a global and local scale.

Extracting tidal variability of sea ice concentration from AMSR‐E passive microwave single‐swath data: a case study of the Ross Sea

AbstractThe periodic divergence of stress applied by ocean tidal currents to sea ice affects the time‐averaged ice concentration (Cice) and heat and freshwater fluxes at the ocean surface. We demonstrate that, at sufficiently high latitudes, tidal variability in Cice can be extracted from single‐swath data from the Advanced Microwave Scanning Radiometer–EOS (AMSR‐E) satellite passive microwave sensor, although time intervals between swaths are irregular. For the northwest Ross Sea where tidal currents are large, tidal divergence is the dominant cause of Cice variability in winter, with a range of ±0.2 about a mean of ~0.8. Daily‐averaged Cice values vary from >0.9 at neap tides to ~0.7 at spring tides. Variability at the fundamental tidal periods is about half that expected from an inverse barotropic tide model for the Ross Sea, suggesting that the measured tidal signal in Cice may be used to diagnose sea ice mechanical properties and ice/ocean coupling.

Aggregation and spawning of the humphead wrasse Cheilinus undulatus (Pisces: Labridae): general aspects of spawning behaviour

The humphead wrasse Cheilinus undulatus formed resident spawning aggregations daily after high tide at specific locations along the seaward edge of the Palau barrier reef. The location and extent of one aggregation site remained consistent for 6 years with no physical features distinguishing it from adjacent areas. Spawning was documented most months and probably occurred year round with possible seasonal and lunar variation. Spawning males arrived first at the site, followed by females and potentially small primary males. The aggregation female to male sex ratio was estimated to be between 6:1 and 10:1. A maximum of 15 males and 100–150 females were observed at the site. A male courtship posture with the anal fin pointed, the caudal fin folded down and the dorsal fin folded against the body was maintained while swimming a few metres off the bottom in view of females. When ready to spawn females rose up as the posturing male passed and the pair released gametes in a relatively sedate fashion near the surface along the shelf break. No attempted predation on spawning adults was seen. Egg predation after spawning was uncommon. On days with early to mid‐day high tides the spawning period started 2·0–2·5 h after high tide when the speed of lagoon–ocean tidal currents peaked and lasted c. 1 h. On days with later afternoon high tides, spawning occurred sooner after high tide and before current speeds peaked. Other fishes with planktonic eggs spawned at the site as pairs or small groups in a rough succession after high tide with C. undulatus, the last species to spawn.

Tides of the Ross Sea and Ross Ice Shelf cavity

Two new ocean tide models for the Ross Sea including the ocean cavity under the Ross Ice Shelf, are described. The optimum model for predicting ice shelf surface height variability is based on assimilation of gravimetry-derived tidal constituents from the Ross Ice Shelf. Synthetic aperture radar interferograms provide an independent test of model performance. The standard deviation of tide height variability is largest under the eastern ice shelf along the Shirase and Siple Coasts, where it can exceed 0.8 m. The maximum peak-to-peak tidal range in this region is ∼3 m. The best predictor for ocean tidal currents north of the ice front is a dynamics-based model that solves the depth-integrated shallow water equations with a linear representation of benthic friction rather than the more usual quadratic form. Tidal currents over the open Ross Sea are dominated by diurnal, topographically trapped vorticity waves. The strongest modelled currents exceed 1 m s−1 at spring tide in a narrow band along the upper continental slope in the north-western Ross Sea. Typical tidal currents in the central continental shelf area of the Ross Sea are 10–20 cm s−1. Under the ice shelf the typical currents are ∼5 cm s−1.

Inversion of oceanic tidal currents from measured elevations

The problem of deducing tidal current velocities from measured tidal elevations is considered. Emphasis is given to global scales for which accurate cotidal charts based on satellite altimetry are readily available. Barotropic tidal currents are deduced from a recent altimetric tide solution by solving the two-dimensional momentum equations and the continuity equation in a least squares fashion. This approach represents a middle ground between commonly used data assimilation schemes and simple local solutions of the momentum equations. The estimated velocities are compared with those of other available global models, with acoustic velocity measurements, and with current meter measurements. Agreement with acoustic measurements is especially good; RMS discrepancies are consistently less than 2 mm s −1. The method appears to be a useful tool for investigating various aspects of tidal dynamics.

High‐frequency ice motion and divergence in the Weddell Sea

We describe the spatial variability of high frequency ice velocity in the Weddell Sea using satellite‐tracked ice‐mounted buoys. Ice motion is analyzed separately for “diurnal” (1/36–1/18 cph) and “semidiurnal” (1/18–1/6 cph) bands. Ice motion in both bands is caused by a combination of ocean tidal currents and wind stress. Monthly mean diurnal band ice speeds over the deep basin range from 2 to 4 cm s−1 depending on wind stress variance and ice concentration (Cice). Higher speeds (∼10 cm s−1) are found in the semidiurnal band in regions of low Cice, notably the northern Weddell Sea, where the ice velocity is dominated by the inertial response to wind stress variations. Monthly mean tidal band ice speeds over the continental slope and shelves often exceed 10 cm s−1. We use comparisons between buoy velocities, moored current meter data, and an ocean tidal model to demonstrate that ice motion is frequently a good indicator of ocean tidal currents in strongly tidal regions. The standard deviation of the divergence of ocean tidal currents estimated from an ocean‐only tidal model is small (< 0.1×10−6 s−1) over most of the Weddell Sea but has values of 1‐5×10−6 s−1 along the Ronne Ice Front and the continental shelf break. High frequency ice divergence is dominated by ice response to wind stress rather than by tides except along the shelf break and ice fronts. In these tidally dominated regions the periodic divergence maintains a mean lead (open water) area of ∼2–5%. This increased lead fraction implies an increase in area‐averaged winter ocean‐to‐atmosphere heat exchange rate of ∼4–10 W m−2 and an increase in salt flux into the upper ocean as new ice forms in the leads.

Lunar and solar torques on the oceanic tides

A general framework for calculating lunar and solar torques on the oceanic tides is developed in terms of harmonic constituents. Axial torques and their associated angular momentum and Earth rotation variations are deduced from recent satellite‐altimeter and satellite‐tracking tide solutions. Torques on the prograde components of the tide produce the familiar secular braking of the rotation rate. The estimated secular acceleration is approximately −1300 arcseconds/century2 (less 4% after including atmospheric tides); the implied rate of change in the length of day is 2.28 milliseconds/century. Torques on the retrograde components of the tide produce periodic rotation variations at twice the tidal frequency. Interaction torques (for example, solar torques on lunar tides) generate a large suite of rotation‐rate variations at sums and differences of the original tidal frequencies. These are estimated for periods from 18.6 years to 6 hours. At subdaily periods the angular momentum variations are 5 to 6 orders of magnitude smaller than the variations caused by ocean tidal currents.

NASA-GSFC's geodetic VLBI program: a twenty-year retrospective

Our geodetic VLBI group at the Goddard Space Flight Center was constituted in 1974 and has been active ever since. The earliest VLBI data in our archives were taken in 1972 with the Mark I VLBI system. In the analysis of these data the formal errors in the length of 4000-km baselines spanning the U.S. are typically 70 mm. Today - ∼25 years later - with the Mark IIIA VLBI system we are obtaining formal errors under 1 mm for the best comparable baselines. There have been similar improvements in the estimates of UT1 and polar motion based on VLBI. These improvement came about as a result of continuous improvements in data acquisition hardware, correlators, experiment planning and data analysis. Geodetic VLBI has produced numerous important scientific results. It has verified that the motions of the stable plate interiors are in very good agreement with motions predicted by plate motion models based on data sensitive to effects on the geologic time scale. It has shown that there are broad regions of deformation along the plate boundaries and it has detected discontinuous motions associated with earth-quakes. The Earth orientation results showed that space geodetic techniques could completely replace classical techniques for monitoring Earth orientation. Earth orientation results from VLBI have been used to study the Earth's core/mantle interface and ocean tidal currents. Starting in 1999 our group will be a part of an international consortium measuring Earth orientation on a continuous basis with 24-hour observing sessions scheduled every day and using the Mark IV VLBI system.

The effect of ocean tides on the Earth's rotation as predicted by the results of an ocean tide model

The published ocean tidal angular momentum results of Seiler [1991] are used to predict the effects of the most important semidiurnal (M2, S2, N2), diurnal (K1, O1, P1), and long period (Mf, Mf′, Mm, and Ssa) ocean tides on the Earth's rotation. The separate, as well as combined, effects of ocean tidal currents and sea level height changes on the length‐of‐day, UT1, and polar motion are computed. The predicted polar motion results reported here account for the presence of the free core nutation and are given in terms of the motion (within the rotating, body‐fixed terrestrial reference frame) of the celestial ephemeris pole so that they can be compared directly to the results of observations. Outside the retrograde diurnal tidal band, the summed effect of the semidiurnal and diurnal ocean tides studied here predict peak‐to‐peak polar motion amplitudes as large as 2 mas. Within the retrograde diurnal tidal band, the resonant enhancement caused by the free core nutation leads to predicted polar motion (or, equivalently, nutation) amplitudes as large as 9 mas (at the unobservable retrograde K1 tidal frequency).

Combined hydrodynamical and empirical modeling of ocean tides

The paper presents a detailed analysis of the author’s hydrodynamical interpolation technique, which was developed and tested to compute a realistic eleven-mode ocean tide model in the real world oceans. Since ocean tidal currents are distinguished from other general ocean and atmospheric circulations by a massive number of available empirical tide data, advantage was taken of this unique opportunity to search systematically for realistic eddy-dissipation and bottom-friction laws. Those laws and their scale factors were determined in trial-and-error computer experiments to assure their proper representation of the real 1°-macroscopic nature of turbulent tidal currents. The quality of the representation was measured by the smoothness with which the hydrodynamically computed tidal field integrated thousands of empirical tide data uniformly over the world-wide oceans.

Sand waves and early Earth-Moon history

The geologic record may provide a test for recent suggestions that the moon became an earth satellite within the last two billion years, at an initial distance one-half to two-thirds of the present distance. Oceanic tidal currents are influenced by the distance between the earth and the moon and by length of day; therefore, tidal current velocities at the time of the origin of the earth-moon system were 4 to 14 times the velocities of today. Sand waves develop on sandy bottoms where current velocities are 1–3 knots and are preserved in sandstones as large-scale cross strata. In today's seas, sand waves occur in geographically restricted areas where tidal currents are increased by topography. At the time of the moon's closest approach, tidal currents of greater than 1 knot were normal for the continental shelves; therefore, a prolific development of sand waves should have occurred in the neritic environment. Cross strata produced by migrating sand waves should be abundant in marine sandstones and calcarenites formed directly after the origin of the earth-moon system and should be less abundant throughout the remainder of geologic time. The presence of abundant large-scale cross strata and other characteristics of the late Precambrian marine shelf sediments of eastern California could be explained by increased tidal currents. Marine shelf sediments of this age throughout the world should show similar evidence of a high-energy environment if the earth-moon system originated in the late Precambrian.