Sea-surface Fluctuations Research Articles

Characteristics of the Observed 2003 Tokachi-Off Earthquake Tsunami Profile

Observed profiles of tsunami caused by the 2003 Tokachi-off Earthquake at 10 offshore wave gauges and 23 coastal tide stations were compiled and analyzed. Comparison of offshore wave and coastal tide data was conducted in wave-to-wave analysis bases and spectrum analysis bases. The tsunami amplification ratio between offshore station and coastal tide station varied due to seabed topography induced natural frequency difference. The frequency spectrum response was obtained at those harbors. The correlation analysis among the sea-surface fluctuation, on-off-shore current and long-shore current was also conducted at offshore wave stations.

Comparisons of High-Frequency Acoustic Fluctuations With Ocean-Temperature and Sea-Surface Fluctuations in Shallow Water

Dynamic ocean processes produce small thermal variations that induce spatial and temporal variability in the ocean's index of refraction and in the spatial scale along an acoustic propagation path. This paper reports measurements and analysis of thermal microstructure effects on ping-to-ping amplitude and phase variability of shallow-water direct-path acoustic propagation in the 20-200 kHz frequency range. These measurements were conducted during a joint experiment conducted by the Naval Research Laboratory and the North Atlantic Treaty Organization Supreme Allied Commander Atlantic (SACLANT) Undersea Research Centre, La Spezia, Italy, in 8 m of water off American Beach, located between Pisa and Livorno, Italy. Experimental observations are compared with predictions for isotropic and anisotropic turbulence, as well as for sea-surface swell. Measured phase and log-amplitude variances coincide with predictions and are relatively insensitive to weak water-column stability. The sea-surface swell dominates phase variances for this data and turbulence dominates log-amplitude variances. These results provide a reasonable lower limit on high-frequency ping-to-ping amplitude and on phase variability produced by benign shallow-water thermal fluctuations.

Abnormal historic sea-surface fluctuations, SW England

Abstract This paper presents a series of documentary accounts of abnormal sea surface changes that have affected the coastline of SW England during historic times. An attempt is made to consider possible origins for individual events although it is recognised that certain episodes of coastal flooding appear to have no simple explanation. However, certain coastal floods appear to have been generated by tsunamis triggered by far-field offshore earthquakes.

Decadal Buoyancy Forcing in a Simple Model of the Subtropical Gyre

The decadal response of the ventilated thermocline to surface buoyancy forcing is investigated, using simplified mixed layer thermodynamics and two-layer planetary geostrophic dynamics. The model is forced by a stationary Ekman pumping and a fluctuating sea surface density, while the associated surface buoyancy flux is diagnosed from the buoyancy budget of ventilated columns. The decadal forcing is represented as a slow modulation of the seasonal cycle, rather than a slow periodic forcing as in Liu and Pedlosky. It is found that the amplitude and the degree of nonlinearity of the interior response depend on the anomalies of the yearly averaged surface buoyancy flux, but that omitting the seasonal cycle leads to an overestimation of the thermocline variability by a factor of 2. At periods longer than 10 years, the interior response becomes linear and frequency independent so that in the presence of stochastic buoyancy forcing the spectrum of potential vorticity or interface displacement is white at low frequency and red at high frequency. A statistical signature of the buoyancy forcing is the asymmetric shape of the cross-covariance function between sea surface density and interface displacement anomalies in lead and lag conditions.

Island-generated internal waves at Scott Reef, Western Australia

Oceanographic field studies were carried out, in August–October 1993, of the water circulation around Scott Reef, a 39 km wide island rising nearly vertically in 500 m depth on the outer continental shelf slope of Western Australia. Macro-tide prevailed and generated 0.6 m s -1 tidal currents. Internal waves 60 m peak-to-trough occurred around Scott Reef mainly at the semi-diurnal frequencies. A numerical model and the field data suggested that the internal waves were locally generated by the interaction of the tidal currents and the bathymetry. Also these waves were predicted to rotate counterclockwise around the island with maximum amplitude along the island slopes, while radiating energy as free internal waves propagating freely away in the open ocean. They were generated apparently without eddies forming in the lee of the island. The wave frequencies were not restricted to the diurnal and semi-diurnal tidal frequencies which dominated the sea surface fluctuations, indeed higher frequencies were important. The energy at the inertial frequency was negligible. It appears that oceanic islands are internal wave generators and may contribute to the oceanic variability even far away.

On the satellite altimeter crossover problem

Mean sea surface heights and residual radial orbit errors are estimated simultaneously in a single global crossover adjustment of multiple cycles of satellite altimetry data. The rank defect inherent in the estimation problem is explicitly identified and treated in various ways to give solutions that minimise (in norm) either orbit errors or mean sea surface heights. The rank defect gives rise to geographically correlated orbit error, consisting of those components of the orbit error or those components of the map of sea surface heights which fall within the nullspace of the estimation problem and which cannot be distinguished as orbit error or ocean signal. We show that, in the case of Topex/Poseidon data, the geographically correlated error consists largely of long-wavelength and long-period sea surface fluctuations, which in the past has often been assigned as orbit error.

Suspended-sediment transport in the surf zone: response to breaking waves

A field experiment designed to investigate the influence of wave breaking on suspended-sediment transport was conducted at Duck, NC, from 6 to 9 September 1985. Arrays of optical backscatter sensors, electromagnetic current meters and pressure sensors were deployed at five positions on a shore-normal transect that spanned the surf zone. At each position measurements were made of cross-shore and longshore velocity, sea-surface fluctuations, and suspended sediment at five levels above the bed. Experimental data runs were conducted when incident swell waves ( H s = 0.5m, T= 10–12s) broke (primarily plunging) within the experimental transect. This paper describes the spatial characteristics of the plunge-to-bore tranformation region and describes (1) the cross-shore variability of sediment resuspension, including the mean concentrations and mean suspended load; (2) the net longshore and cross-shore flux across the surf zone; (3) mean suspended-sediment profiles as a function of wave type, e.g. plunging, spilling and bore, and unbroken at four positions across the surf zone; and (4) discusses the relative contribution of each wave type to the net longshore and cross-shore sediment flux.

Effects of Sea-Surface Fluctuations on Response of Offshore Structures

The magnitudes of the effects of the sea-surface fluctuations on the response of wave-excited offshore structures are assessed. Such fluctuations are accounted for by using the linear wave theory in combination with two distinct approaches: “stretching” and “vertical extrapolation.” Structural responses are calculated for water-particle kinematics obtained by applications of the unmodified linear-wave theory, linear-wave theory in combination with the stretching approach, and linear-wave theory in combination with the vertical extrapolation technique. These responses are examined by separating the associated static and dynamic effects for wide ranges of natural frequencies. It is shown that responses obtained for the stretching approach are almost the same as those calculated for the unmodified wave-theory model. It is further shown that applications of the vertical-extrapolation technique lead to the evaluation of peak responses that are generally larger than those obtained for the unmodified wave model, particularly for drag-force dominated systems. These larger responses are primarily induced, under conditions of practical importance, by increases in the associated static effects. Additional relevant information is also reported that is responsive to the need that exists at the present time for improved appreciations of the dynamics of offshore platforms. The results of the study apply to offshore platforms operating in water depths in the approximate range of 300–1,000 m.

Suspended sediment transport in the surf zone: Response to incident wave and longshore current interaction

This paper reports the results of a field experiment carried out in the inner surf zone of a dissipative beach at San Marine, Oregon, in 1984. Observations were made at a location 70 m seaward of the mean high water line (mean depth 1.3 m) during a period when significant offshore wave heights were 3–5 m and the surf zone was 500 m wide. Instruments were deployed in a closely spaced array and measurements included cross-shore and longshore velocity, sea surface fluctuations, and sediment concentration profiles. The results reported in this paper are from a particular 44-min data run characterized by high waves and strong longshore currents. In the first section of this paper the field measurements are summarized to provide insight into the resuspension process and observed sediment concentrations and fluxes on time scales including incident and infragravity bands, and mean values for the entire data set. In the second section a numerical model is presented that incorporates the nonlinear interaction of waves and currents and calculates velocity profiles, boundary shear stress, suspended sediment profiles, and longshore and cross-shore particle flux over 0.5 s intervals throughout the time series. Finally, model calculations of mean suspended load and net longshore and cross-shore flux of sediment are compared to observations. The results show that the model calculates mean suspended load and net longshore and cross-shore flux of sediment within a factor of two of the observations. The model suggests that, for this data set, incident waves and longshore currents contribute equally to the suspended load. For the longshore particle flux, the current contribution is approximately 1.7 times the wave contribution (63% vs 37%), primarily because currents carry suspended sediment higher in the water column where it is advected by stronger longshore currents. Additionally, model results suggest that wave/current interactions enhance suspended sediment load and longshore sand transport by approximately 50–60% over the transport forced by waves alone.

Internal surges in coastal waters

A multiple element thermistor chain was installed at the end of Scripps pier in La Jolla, California, in 5 m of water along with a pressure sensor to record sea surface fluctuations. Temperature fluctuations throughout the water column are characterized by the occurrence of events, times during which a significant thermal gradient, of the order of or greater than those associated with the seasonal thermocline in deeper waters, exists in the water column. Temperature differences between the bottom and the surface of up to 5°C have been measured in events that are similar to those reported by Cairns (1967). The contributions of this work are to report (1) that the onset of events can be very rapid, occurring during times of the order of 1 s, (2) that motions exist in the water column at frequencies much higher than the buoyancy frequency that are not directly coupled to the surface wave field, and finally (3) that events are likely to be strongly three-dimensional in such shallow waters. Extrapolations of observations at Scripps pier on a worldwide scale indicate that such events could be responsible for an energy flux into shoal waters ranging between 2×105 kW and 2×107 kW, an estimate which is to be compared with the 2.5×106 kW proposed by Wunsch and Hendry (1972) on the basis of estimates of the power, in the form of internal waves, incident on the coastline of the world.

Model tropical cyclone behaviour in experiments related to modification attempts

A numerical model of tropical cyclone development (Sundqvist, 1970?) has been used to study and compare the effects on the cyclone intensity by cloud seeding and spatial variations of sea surface temperature. The cloud seeding is simulated by taking into account heat released by freezing for temperatures below 0° C at selected radii of the convective area. The movement of a cyclone over a sea surface with a non-homogeneous temperature distribution is simulated in this axi-symmetric model by changing the sea temperature from one period of time to another inside a certain radius. The results indicate that the maximum winds of the symmetric model tropical cyclone rather have a tendency to intensify than to decrease when cloud seeding is employed. The results of the experiments simulating a fluctuating sea surface temperature show that even rather small scale (even smaller than the size of the convective region) variations significantly affect the intensity of a tropical cyclone. DOI: 10.1111/j.2153-3490.1972.tb01527.x