Sea Level Difference Research Articles

The Effect of Boundary Sea Level Height on the Salinity Distribution in the Seto Inland Sea

The Seto Inland Sea, which 21 1st grade rivers are flowing into, is a semi-enclosed shelf sea connected to the Pacific Ocean through two open boundaries. It is considered that current is controlled by the difference in sealevel between two boundaries accompanied by the variation of Kuroshio path. And the sea level height is variedby the distribution of salinity formed by the current characteristic. In this study, we investigated the relation ofsalinity distribution and boundary sea level height in the whole area of the Seto Inland Sea using field observationdata and numerical calculation in order to analyze the sea level variation characteristics.

鹿児島湾への外洋水の流入に及ぼす大隅分枝流の影響について

Properties of the current velocity index of the Osumi Branch Current (OBC) in the Osumi Strait south of Kyushu were re-examined using sea-level data during 1971-2003. The high correlation with the current velocity measured in the Osumi Strait confirmed that the sea-level difference between Nishino-omote and Odomari is a good index to monitor the current velocity of the OBC. Using daily values of the index and water temperature measured in Kagoshima Bay in 1999, it is found that the current velocity fluctuations of the OBC with periods less than about 8 days significantly contribute to oceanic water inflow into Kagoshima Bay. Moreover, the oceanic water is shown to reach to the central part of Kagoshima Bay about 1.3 day after it enters into the bay along the Osumi Peninsula.

Annual variation of throughflow in the Seto Inland Sea

The latitude of the Kuroshio stream axis is closely related to the intrusion of the Kuroshio water mass and the throughflow offshore from two open boundaries. The water densities at entrances depend on the intrusion difference of the Kuroshio water mass. A new data processing method, which accounts for various data of water quality measured at different observation points under different river discharge rates, is proposed to let the model be applicable to long-term and wide-area simulations. The throughflow depends on the boundary sea level difference and the density gradient between boundaries caused by the intrusion of the Kuroshio water.

Extracting the 2004 Indian Ocean tsunami signals from sea surface height data observed by satellite altimetry

Altimeter‐equipped satellites flew over the propagating area of the Indian Ocean tsunami caused by the 2004 Sumatra‐Andaman earthquake on 26 December 2004. Tsunami signals can be detected as differences in sea level changes of multiple tracks. However, observed changes in sea level differences involve not only tsunami signals but also effects from various ocean phenomena and errors due to observation technology and data processing. A multisatellite time‐spatial interpolation method is performed to define reference sea surface heights. After applying the method to the products on sea level anomalies along tracks of altimeter‐equipped satellites, quality tsunami‐height profiles with only 4‐ to 5‐cm root mean square errors were obtained for the Indian Ocean tsunami along five tracks from four satellites (Jason‐1, TOPEX/POSEIDON, ENVISAT, and Geosat Follow‐On). Maximum tsunami height in the open ocean for the 2004 Indian Ocean tsunami as observed from satellite altimetry was 1.1 m trough‐to‐crest 115 min after the main shock. Reliable tsunami‐height profiles from satellite altimetry were extracted for the first time. The method employed in this study has the potential to extract tsunami signals of 0.1 m or greater trough‐to‐crest height from satellite altimetry observation data on the deep sea by ongoing satellite missions.

Atlantic to Mediterranean Sea Level Difference Driven by Winds near Gibraltar Strait

Abstract Observations and numerical simulations show that winds near Gibraltar Strait cause an Atlantic Ocean to Mediterranean Sea sea level difference of 20 cm peak to peak with a 3-cm standard deviation for periods of days to years. Theoretical arguments and numerical experiments establish that this wind-driven sea level difference is caused in part by storm surges due to alongshore winds near the North African coastline on the Atlantic side of Gibraltar. The fraction of the Moroccan coastal current offshore of the 284-m isobath is deflected across Gibraltar Strait, west of Camarinal Sill, resulting in a geostrophic surface pressure gradient that contributes to a sea level difference at the stationary limit. The sea level difference is also caused in part by the along-strait wind setup, with a contribution proportional to the along-strait wind stress and to the length of Gibraltar Strait and adjoining regions and inversely proportional to its depth. In the 20–360-day band, average transfer coefficients between the Atlantic–Alboran sea level difference and surface wind stress at 36°N, 6.5°W, estimated from barometrically corrected Ocean Topography Experiment (TOPEX)/Poseidon data and NCEP–NCAR reanalysis data, are 0.10 ± 0.04 m Pa−1 with 1 ± 5-day lag and 0.19 ± 0.08 m Pa−1 with 5 ± 4-day lag for the zonal and meridional wind stresses, respectively. This transfer function is consistent with equivalent estimates derived from a 1992–2003 high-resolution barotropic simulation forced by the NCEP–NCAR wind stress. The barotropic simulation explains 29% of the observed Atlantic–Alboran sea level difference in the 20–360-day band. In turn, the Alboran and Mediterranean mean sea level time series are highly correlated, ρ = 0.7 in the observations and ρ = 0.8 in the barotropic simulation, hence providing a pathway for winds near Gibraltar Strait to affect the mean sea level of the entire Mediterranean.

A Near-Uniform Basin-Wide Sea Level Fluctuation of the Mediterranean Sea

Abstract A new basin-wide oscillation of the Mediterranean Sea is identified and analyzed using sea level observations from the Ocean Topography Experiment (TOPEX)/Poseidon satellite altimeter and a numerical ocean circulation model. More than 50% of the large-scale, nontidal, and non-pressure-driven variance of sea level can be attributed to this oscillation, which is nearly uniform in phase and amplitude across the entire basin. The oscillation has periods ranging from 10 days to several years and has a magnitude as large as 10 cm. The model suggests that the fluctuations are driven by winds at the Strait of Gibraltar and its neighboring region, including the Alboran Sea and a part of the Atlantic Ocean immediately to the west of the strait. Winds in this region force a net mass flux through the Strait of Gibraltar to which the Mediterranean Sea adjusts almost uniformly across its entire basin with depth-independent pressure perturbations. The wind-driven response can be explained in part by wind setup; a near-stationary balance is established between the along-strait wind in this forcing region and the sea level difference between the Mediterranean Sea and the Atlantic Ocean. The amplitude of this basin-wide wind-driven sea level fluctuation is inversely proportional to the setup region’s depth but is insensitive to its width including that of Gibraltar Strait. The wind-driven fluctuation is coherent with atmospheric pressure over the basin and contributes to the apparent deviation of the Mediterranean Sea from an inverse barometer response.

Effect of the change of bottom depth on the penetration of Kuroshio water onto the East China Sea shelf

A theoretical problem of the penetration of a western boundary current, driven by a cross‐stream sea level difference, onto a shallow basin to the west of the current is put forth to account for the penetration of the Kuroshio onto the East China Sea (ECS) shelf and formation of the Tsushima Warm Current. Different from previous studies, this study assumes that the change of bottom depth is the primary factor determining the penetration. For a given depth of the shallow basin, the penetration depends on two external parameters: speed of the western boundary current, V0, and surface elevation of the current at the edge of the shallow basin, Z0. Two penetration modes emerge, depending on the magnitude of the inertia of penetration. For sufficiently large inertia, which is favored by large Z0 and hindered by large V0, penetration occurs in the form of a free jet, mode “J.” Otherwise, it is in the form of a coastal current, mode “C.” Volume transport of the penetration is maximized in mode “J.” In general, volume transport, width, and speed of the penetrating current increase with Z0, although the latter two are slightly more dependent on V0 in mode “C.” The model considered in this study predicts that the penetration of the Kuroshio onto the ECS shelf is of mode “J,” as indicated by observations. The volume transport of the penetration is somewhat overestimated, possibly because bottom friction is ignored in this model. Although this model is highly idealized, it allows one to gain insight into the behavior of the Kuroshio penetrating onto the shallow ECS shelf.

Formation mechanism of the cold-water belt formed off the soya warm current

The cold-water belt (CWB) is frequently formed off the Soya Warm Current (SWC) during summer and autumn. The detailed distribution of the flow and temperature fields observed by the R/V Sinyo-maru in the summer of 2001 captured the structures of the SWC and the CWB. The temperature and density distributions showed that the vertical distribution of the CWB is associated with the upwelling formed off the SWC. Numerical experiments using a two-layer model with realistic bottom topography have been performed to understand the formation mechanism of CWB and the upwelling structure off the current. In the experiment, the sea level difference between the Japan Sea and the Okhotsk Sea, and baroclinic flow assuming the Tsushima Warm Current were given along the open boundary. The numerical model well reproduces the current system of the SWC and upwelling region off it. The upwelling region is formed at the Soya Strait first, and then it spreads on the offshore side along the SWC as a developing current system. Analysis of the model data indicated that the geostrophic balance mainly dominates in the current system, while convergence of the bottom Ekman transport due to the SWC forms the upwelling region as the secondary circulation. In addition, the advection effect due to the SWC is found to strengthen the upwelling.

Precambrain Fault Systems as Control on Regional Differences in Relative Sea Level Along the Early Ordovician Platform of Eastern North America

Abstract This study proposes that there was regional variation in Early Ordovician subsidence histories along eastern Laurentia controlled by broad differences in the susceptibility of inherited Precambrian crustal structure to reactivation with changing ocean-basin tectonism. Two continental-scale Phanerozoic fault systems (Southern Oklahoma Fault System, Ottawa–Bonnechere Graben) extend into the craton interior from the apices of the Ouachita and Quebec embayments or reentrants, respectively. Their traces identify reactivated Proterozoic fault zones. Previous studies show that rates of subsidence for Early Ordovician sedimentation along the axis of the Southern Oklahoma Fault System are elevated compared to regional rates beyond this structural corridor. A similar history applies to Early Ordovician sedimentation within the Ottawa Embayment (eastern Ontario), along the present axis of the Ottawa–Bonnechere Graben. The Theresa Formation (lower Beekmantown Group) contains a third-order peritidal (siliciclastic-dominated) sequence. Evidence for local tectonism prior to and during its development within the embayment includes: (1) Late Cambrian (pre-Theresa) preferential uplift, differential erosion, and subsidence along the eastern structural boundary of the embayment; (2) differential intrabasinal subsidence during Theresa deposition; (3) an apparent offset in timing of peak flooding within the embayment to that along the St. Lawrence Promontory, ~ 1500 km distant; and (4) absence of a subaerial unconformity in the embayment that otherwise separates eustatic cycles on the promontory, and defines a likely sea-level fall below the shelf margin. In comparison, subsidence within the embayment appears to have matched (at least) the rate of the eustatic fall. Regional differences in Early Ordovician subsidence histories along this margin may illustrate that the Precambrian fault zones were structurally more sensitive than promontories to the developing reconfiguration of oceanic lithospheric elements that would lead to the Taconic orogeny.

Observation of the Soya Warm Current using HF ocean radar

Three High Frequency (HF) ocean radar stations were installed around the Soya/La Perouse Strait in the Sea of Okhotsk in order to monitor the Soya Warm Current (SWC). The frequency of the HF radar is 13.9 MHz, and the range and azimuth resolutions are 3 km and 5 deg., respectively. The radar covers a range of approximately 70 km from the coast. The surface current velocity observed by the HF radars was compared with data from drifting buoys and shipboard Acoustic Doppler Current Profilers (ADCPs). The current velocity derived from the HF radars shows good agreement with that observed using the drifting buoys. The root-mean-square (rms) differences were found to be less than 20 cm s−1 for the zonal and meridional components in the buoy comparison. The observed current velocity was also found to exhibit reasonable agreement with the shipboard ADCP data. It was shown that the HF radars clearly capture seasonal and short-term variations of the SWC. The velocity of the Soya Warm Current reaches its maximum, approximately 1 m s−1, in summer and weakens in winter. The velocity core is located 20 to 30 km from the coast, and its width is approximately 40 km. The surface transport by the SWC shows a significant correlation with the sea level difference along the strait, as derived from coastal tide gauge records at Wakkanai and Abashiri.

Subinertial to interannual transport variations in the Korea Strait and their possible mechanisms

The voltage‐derived transport in the Korea Strait from March 1998 to April 2002 reveals various temporal variations such as subinertial, monthly, seasonal, and interannual variations. The driving mechanisms of these temporal variations in the flows through the straits and the mean sea level of the East Sea (Sea of Japan) are investigated using a simple analytical barotropic model. The East Sea is simplified as a flat‐bottomed semienclosed basin, and it is forced by the atmospheric pressure, the along‐strait wind stress, and the sea level differences along the straits. Despite its simplicity this model explains most variations and helps us understand dominant driving forces on each timescale. At the subinertial periods of 2–10 days the atmospheric pressure dominantly drives the flows into or out of the East Sea due to a Helmholtz resonance, whose period is about 3 days, and the mean sea level cannot respond isostatically. The effects of the atmospheric pressure on the transport variations become weak at long periods because there is enough time for the East Sea to be drained or filled through the strait flows as its mean sea level responds isostatically. On the other hand, the changes in the adjusted sea levels outside the straits of the East Sea cause the pressure gradient forces along the straits and induce most of variations in the strait flows and the mean sea level at the monthly to interannual periods.

Variability of the Mindanao Current: Mooring observation results

Mooring observations were conducted from October 1999 to July 2002 near the east coast of Mindanao Island, the Philippines, (6°50′N, 126°43′E) to observe current variability at the axis of the Mindanao Current (MC). The MC was a strong current with a subsurface velocity maximum exceeding 1.3 m s−1 at approximately 100 m depth. The MC flows shallower than 700 m, and there was no evidence of a steady northward current (the Mindanao Undercurrent) at the study location. Compared with the large average velocity, MC variability was low (standard deviation <0.2 m s−1 for all directions). All the observed interannual, seasonal, and intraseasonal variations in the MC were of comparable amplitude. The MC was strong during boreal summers and during the onset of the 2002–03 El Niño. The core velocity of the MC at approximately 100 m was correlated with the sea level difference between Cebu, Philippines and Malakal, Palau.

Volume Transport through the Tsushima Straits Estimated from Sea Level Difference

The relations between the volume transport and the sea level difference across the Tsushima Straits have been investigated using current data provided by ADCP mounted on the ferry Camellia, plying between Hakata and Pusan. Empirical formulas to deduce the volume transports using the sea level differences across the eastern and western channels are proposed, considering the seasonal variation of the vertical current structure. The interannual variation of volume transport through the Tsushima Straits for 37 years from 1965 to 2001 is estimated using the empirical formulas. The total volume transport through the Tsushima Straits, averaged for 37 years, is 2.60 Sv and those of the eastern and western channels are 1.13 Sv and 1.47 Sv, respectively. The total volume transport through the Tsushima Straits tends to decrease with a roughly 15 year variation until 1992, then begins to increase.

The Influence of James Bay River Discharge on Churchill, Manitoba Sea Level

The Hudson Bay region faces the possible impact of two anthropogenic modifications, hydro-electric damming and climate change. Both of these have the potential to influence sea level in Hudson and James Bay waters. To gain a better understanding of the interplay between sea level and runoff into this water system, the impact of James Bay spring runoff on sea level variation at Churchill, Manitoba, is assessed. James Bay discharge data for 1963 to 1983 are used with corresponding sea level data recorded at Churchill. A significant correlation between spring (May and June) discharge from James Bay and a secondary peak in sea level in October and November at Churchill was found. The coefficient of determination (R2) increased substantially from 26% to 47% when an outlier year, 1963, was removed from the analysis. This is justified due to the limited data available for approximating discharge for that year. A lower limit of 5 cm/s advection speed is estimated for propagation of discharge from James Bay to Churchill consistent with previous estimates. Using the difference in sea level arising from high and low discharge years, it was estimated that 35-50% of the discharge from James Bay does not exit directly from Hudson Bay but recirculates in western Hudson Bay. It can be extrapolated that hydroelectric regulation of the rivers draining into James Bay will potentially affect the seasonal variation of sea level throughout Hudson Bay.

The Fast Fourier Transform for Experimentalists

In part one of this series, we discussed several basic properties of the fast Fourier transform (FFT). In addition to fundamental elements, we treated zero padding, aliasing, the relationship to a Fourier series, and ended with an introduction to windowing. In part II, we continue our discussion with a more general approach to spectrum estimation, including the periodogram, the autocorrelation function, autoregressive spectral estimation, and maximum entropy spectral estimation. In addition, we include brief descriptions of treating convolution, filtering, and detrending. In the final installment, as we treat several applications, we draw on a number of the ideas discussed in the first part dealing with concepts and then apply some of the methods. The particular applications we consider include the spectral analysis of a bat chirp, atmospheric sea-level pressure differences, and perhaps even take a look, at atmospheric CO/sub 2/.

Model simulations of the Gulf of Maine response to storm forcing

The storm response of the Gulf of Maine/Georges Bank region was investigated using the Dartmouth three‐dimensional, nonlinear, finite element coastal ocean circulation model called QUODDY, with uniform density and quadratic bottom stress. A suite of model hindcast experiments were conducted for the 4–8 February 1987 period that featured a strong nor'easter. The model was forced at the open ocean boundaries by the M2 semidiurnal tidal sea level and in the interior by realistic surface wind stresses and, in some cases, atmospheric pressure. The reference model run, with just tidal and wind stress forcing and a time‐space constant bottom drag coefficient Cd of 0.005, captured 65% of the observed detided, low‐pass filtered or “subtidal” sea level variance of a representative array of coastal and bottom pressure stations in the Gulf. However, there remained large unexplained model/observed sea level differences, particularly during the storm. Model sensitivity testing to a realistic range of constant Cd values did not improve model results significantly. However, results of a model run in which atmospheric pressure forcing, derived through linear interpolations of regional measurements, was added to wind stress forcing captured 82% of the observed subtidal sea level variance.

Taiwan strait current in winter

Four bottom-mounted current profilers were deployed across the Taiwan Strait from September 28 to December 14 of 1999 to monitor the current velocity when the northeast monsoon was strong. Results indicate both diurnal and semidiurnal tidal currents were primarily barotropic. The barotropic diurnal tide might be explained by a single Kelvin wave propagating along the Mainland China coast from north to south. However, the barotropic semidiurnal tide manifested as a more complicated form in the Taiwan Strait. The subtidal current generally fluctuated with the northeast winds. When the northeast wind was weak, the along- and cross-strait subtidal current flowed primarily against the wind and toward Taiwan, respectively. As the northeast wind intensified, the along-strait current flowed downwind, brought the cold China coastal water southward, and formed a baroclinic velocity front in the western portion of the Taiwan Strait. The Ekman effect forced the cross-strait current toward Mainland China in the upper water column and toward Taiwan in the lower water column, respectively. The along-strait volume transport, estimated from interpolated current velocity, varied from −5 to 2 Sv with a mean value of 0.12±0.33 Sv. Similar transport was also estimated from the sea level difference across the Taiwan Strait. Although the local wind played a dominant role for the fluctuations of current velocity and transport in the Taiwan Strait, it could be not the only important factor. The current or transport directed frequently against the wind could be related to the northward current, which was consistently observed in the Penghu Channel.

Wind driven upwelling along the African coast of the Strait of Gibraltar

Regular remote sensing data from various sensors are used here for the study of the wind driven upwelling phenomenon along the African coast of the Strait of Gibraltar. It is shown for an extended summer period (May 15 till September 15, 2003) that sea surface temperature (SST) data in the strait are correlated with NCEP winds, each westward wind increase being followed by a clear surface temperature decrease. Local surface temperature of about 22°C at that time drops down to 15°C, value corresponding to the 80–120 m depth conditions. The analysis of subsequent images indicates that the cold upwelling plume typically moves first to the Atlantic during wind forcing, and then to the Mediterranean after the wind event. The presence of the northern coast of the strait is taken as responsible for a rise of a cross‐strait sea level gradient and the enhancement of the associated westward geostrophic current that explains the first stage of the plume deployment. Sea level difference measured between Tarifa (European coast) and Ceuta (African coast), well described by a linear equation in term of the westward wind component, supports this idea as well as the subsequent remotely sensed SST distributions.

Catch fluctuations of the diamond squid Thysanoteuthis rhombus in the Sea of Japan and models to forecast CPUE based on analysis of environmental factors

Abstract This paper describes recent catch fluctuations of the diamond squid Thysanoteuthis rhombus in the Sea of Japan and the development of models used to forecast catch per unit effort (CPUE) off Hyogo Prefecture based on statistical analyses of environmental indices measured 600 km upstream in the Tsushima Current. Annual catches during 1989–2002 fluctuated widely, and prefectural annual catches were closely related to each other, especially among the western prefectures. Four indices in June were closely related with CPUE during the fishing season (September–November) off Hyogo: (1) water temperature in the Tsushima Strait, (2) salinity in the Tsushima Strait, (3) sea level at Izuhara (Tsushima Island) and (4) sea level difference between Izuhara and Hakata (Kyushu Island). Using these indices as independent variables, simple and multiple regression analyses were conducted, and CPUE was accurately estimated by both regression and extrapolation, indicating that the CPUE off Hyogo can be forecasted 2 months before the fishery starts using the described models. The strong correlation among catches in different prefectures suggests the indices discussed here affect the catches over a large area.

Currents, water levels, and mass transport over a modern Caribbean coral reef: Tague Reef, St. Croix, USVI

We evaluated the relationships of cross-reef velocity and volume flux to waves, tides, and sea-level differences (SLDs) measured over multiple tidal cycles at Tague Reef, off St. Croix, USVI. A wave-driven model of cross-reef currents and setup incorporating bottom friction provided improved insight into the complex and dynamic reef setting. Velocity varied at two time scales associated, respectively, with surface gravity waves and tides. We observed a wave-velocity asymmetry, offshore at the forereef and lagoonward at the crest and backreef. Mean velocities were ⩽−7 cm s −1 (lagoonward) and varied with the local tides. At the forereef, wave heights and mean velocities were uncorrelated. At the crest, mean velocities correlated weakly with wave heights and SLDs, but not with wave setup. At the backreef, mean velocities correlated moderately with SLDs, but weakly with wave heights and setup. Based on these relationships it is suggested that under small wave conditions SLDs dominate the dynamics of cross-reef velocities, except at the crest, where both SLDs and wave heights dominate. The volume flux was lagoonward at high tides, but decreased to near zero or reversed at low tide. Across-reef wave-induced volume flux ranged from −0.01 to −0.02 m 2 s −1, exhibited small tidal variations, and contributed ∼45–60% of the total volume flux. Mean flux during the study was −0.07 m 2 s −1 at the forereef, −0.01 to −0.02 m 2 s −1 at the crest, and −0.02 m 2 s −1 at the backreef and varied diurnally and semi-diurnally. Uncertainties around the flux estimates masked any divergence between the forereef and backreef estimates, suggesting a constant cross-reef water flux. The cross-reef velocity model showed that waves and across-reef water flux played significant roles in generating sea-level changes over the coral reef. Onshore reef transport created a set-down that increased the apparent setup between the forereef and reef crest, a condition that holds even for offshore transport less than 0.01 m 2 s −1. The modeled velocity over the reef crest was strongly proportional to the SLDs, and water depth, and indirectly to the wave height. This result agreed with the observations. The setup over the reef crest was proportional to distance, transport, and the friction coefficient, but inversely proportional to the square of the water depth. Model predictions of setup between the forereef and the reef crest and SLDs between the forereef and lagoon were both highly correlated with observations ( r∼0.61). Positions of the water level gauges over the reef were critical for proper interpretation of water level changes.