Subtidal Sea Level Fluctuations Research Articles

Alongshore Propagation of Subtidal Sea Level Fluctuations Around the Korean Peninsula Over Varying Stratification and Shelf Topography

Understanding subtidal (a period band of 3–20 days) coastal sea level fluctuations is of increasing importance for the sustainable use of coastal areas under challenging conditions resulting from regional and global sea-level rise. The wind-forced coastal-trapped waves (CTW) theory often accounts for subtidal sea level fluctuations observed off global coasts with a considerable range of propagation speeds. Here, the propagation speeds of subtidal sea level fluctuations observed at seven coastal tide-gauge stations around the Korean Peninsula (KP) from 1997 to 2017 were compared with phase speeds modeled for 10 segments of realistic bottom topography around the KP and four seasons of density stratification using a wind-forced CTW model. Alongshore variations in the modeled phase speed (2.0–10.0 m s–1, increasing with the bottom depth, shelf width, and vertical density difference) were consistent with those of the observed propagation speed (4.7–18.9 m s–1), despite systematic underestimation by 50%, i.e., the mean speed of 6.0 vs. 11.8 m s–1. This underestimation is discussed considering subtidal sea level fluctuations of non-CTW origin that were not incorporated into the CTW model, such as (1) Barometric sea level response to atmospheric pressure disturbances, and (2) Upwelling/downwelling response to local alongshore wind stress. This study suggests that subtidal coastal sea level fluctuates around the KP within and beyond CTW dynamics.

Sea level fluctuations in central California at subtidal to decadal and longer time scales with implications for San Francisco Bay, California

Sea level elevations from near the mouth of San Francisco Bay are used to describe the low-frequency variability of forcing of the coastal ocean on the Bay at a variety of temporal scales. About 90% of subtidal fluctuations in sea level in San Francisco Bay are driven by the sea level variations in the coastal ocean that propagate into the Bay at the estuary mouth. We use the 100-year sea level record available at San Francisco to document a 1.9 mm/yr mean sea level rise, and to determine fluctuations related to El Nino-Southern Oscillation (ENSO) and other climatic events. At time scales greater than 1 year, ENSO dominates the sea level signal and can result in fluctuations in sea level of 10–15 cm. Alongshore wind stress data from central California are also analyzed to determine the impact of changes in coastal elevation at the mouth of San Francisco Bay within the synoptic wind band of 2–30 days. At least 40% of the subtidal fluctuations in sea level of the Bay are tied to the large-scale regional wind field affecting sea level variations in the coastal ocean, with little local, direct wind forcing of the Bay itself. The majority of the subtidal sea level fluctuations within the Bay that are not related to the coastal ocean sea level signal are forced by an east–west sea level gradient resulting from tidally induced variations in sea level at specific beat frequencies that are enhanced in the northern reach of the Bay. River discharge into the Bay through the Sacramento and San Joaquin River Delta also contributes to the east–west gradient, but to a lesser degree.

Sea level changes along the Turkish coasts of the Black Sea, the Aegean Sea and the Eastern Mediterranean

Short, tidal, subtidal, seasonal, secular sea-level variations, sea-level differences and interactions between the basins have been studied, based on the data collected at some permanent and temporary tide gauges located along the Turkish coasts, mostly along the Straits connecting the Marmara Sea to outer seas. Even though the deficiency of sufficient information prevented us to reach the desired results, many pre-existed studies have been improved. Short-period oscillations were clearly identified along the Turkish Strait System and related to their natu-ral periods. The tidal amplitudes are low along the Turkish coasts, except northern Aegean and eastern Mediterranean. The stability of harmonic constants of Samsun and Antalya were examined and most of the long period constituents were found to be unstable. Even the Marmara Sea is not affected from the tidal oscillations of Black and Aegean Seas, some interactions in low frequency band have been detected. Subtidal sea level fluctuations (3-14 day) have relations with the large-scale cyclic atmospheric patterns passing over the Turkish Straits System. Short-term effects of wind on sea level are evident.Seasonal sea-level fluctuations along the Turkish Straits System are in accord with Black Sea's hydrological cycle. The differential range of the monthly mean sea levels between the Black Sea and the Marmara Sea is highly variable; high during spring and early summer and low during fall and winter.On the average, there is a pronounced sea-level difference (55 cm) along the Turkish Straits System. However, the slope is nonlinear, being much steeper in the Strait of Istanbul. This barotrophic pressure difference is one of the most important factors causing the two-layer flow through the system. The topography and hydrodynamics of the straits, the dominant wind systems and their seasonal variations make this flow more complicated. For secular sea level changes, a rise of 3.2 mm/a was computed for Karsiyaka (1935-71) and a steady trend (-0.4 mm /a) has been observed for annual sea levels at Antalya (1935-77). The decreasing trend (-6.9 mm/a) at Samsun is contrary to the secular rising trend of the Black Sea probably because of its rather short monitoring period (1963-77).

On the Low-Frequency Transport Processes in a Shallow Coastal Lagoon

The low-frequency transport processes in a small, shallow coastal lagoon (Indian River Bay, Delaware) are examined based on a set of data derived from tide gauges, near-bottom current meter measurements, and drifter releases. The subtidal sea-level fluctuations in the interior of the lagoon are forced primarily by the coastal sea-level fluctuations off the mouth of the inlet, which connects the lagoon with the coastal ocean. The effect of local wind plays a secondary role in modifying the coastally forced sea level inside the lagoon. Given the continuity constraint which links sea-level fluctuations to the depth and laterally integrated barotropic transport, the coastal pumping effect would be expected to be the dominant factor in controlling the subtidal barotropic exchange within the bay. However, the dominance of the coastal pumping effect on the barotropic exchange does not readily translate into the dominance of this effect on the transport and distribution of waterborne material in the bay at subtidal frequencies. The observed nearbottom subtidal current fluctuations are not coherent with the coastal sea-level fluctuations. The observed current is also much stronger than the barotropic current inferred from the continuity constraint. This suggests the presence of a depth-dependent flow field, with current in the upper layer fluctuating in opposite direction to that at depth. Furthermore, the observed near-bottom current also shows significant spatial variability within the bay. As for the mean current, the residual flow field shows distinctly different patterns between the surface and the bottom. The residual current at the surface exhibits a consistent mean flow out of the bay. At the bottom, the residual current shows a mean flow into the estuary in the upper part of the lagoon and a spatially variable flow in the lower part of the lagoon. A competition between gravitational circulation and tidally rectified current may contribute to the observed vertical and horizontal variabilities in the residual flow field.

On the relative importance of the remote and local wind effects to the subtidal variability in a coastal plain estuary

A set of month‐long sea level and current measurements is used to examine the subtidal variability in the Delaware estuary and assess the relative importance of remote and local wind effects on the observed subtidal variability. The evidence indicates that the remote wind effect, through the impingement of coastal sea level at the mouth of the estuary, is more important than the local wind effect in producing the subtidal sea level fluctuations in the interior of the estuary. On the other hand, the local wind effect dominates over the remote wind effect in producing the observed subtidal current fluctuations at the mooring site. It appears that the remote wind effect is important to the sectionally averaged subtidal transport into or out of the estuary. However, the local wind effect may be more important than the remote wind effect in producing the subtidal currents at any given point along the estuary's cross section. The spatial structure associated with the local wind‐induced circulation is very important to the long term transport and distribution of waterborne material.

Numerical Simulation of the Exchange Process within a Shallow Bar-Built Estuary

Great South Bay is the largest of a series of interconnected bar-built estuaries on the south shore of Long Island, New York. The depth-averaged barotropic motions in the bay were simulated by using a finite element two-dimensional numerical model. The barotropic motions were driven with astronomical tides, subtidal coastal sea level fluctuations induced by longshore wind stress over the adjacent continental shelf, and local wind stress over the surface of the bay. There was vigorous exchange at tidal frequencies between the western part of Great South Bay and the surrounding waters, but the tidal exchange was heavily damped in the eastern part of the bay. At subtidal frequencies the volume exchange did not exhibit significant attenuation in the interior of the bay. In the eastern part of Great South Bay, the magnitude of the subtidal volume exchange could exceed that of the tidal exchange. The principal mode of subtidal volume exchange was found to be associated with subtidal sea level fluctuations along the coast, which characteristically caused a filling or emptying of the system from all open boundaries of Great South Bay.

On the Contribution of Subtidal Volume Fluxes to Algal Blooms in Long Island Estuaries

A bloom of a new picoplankter species Aureococcus anophagefferens occurred in several estuaries in the northeast coast of the U.S.A. in 1985. The phenomenon is believed to be the result of a complex chain of physical-chemical-biological factors. Based on 9 years (1980-88) of tidal data taken at three oceanic locations (Nantucket, Montauk, Atlantic City) and four impacted estuaries on Long Island (Peconic, Shinnecock, Moriches and Great South Bays) it was found that the variance of the sea level fluctuations at subtidal frequencies was much higher than that of the tidal frequencies and highly dependent upon the season. These subtidal variances reached an absolute minimum in the spring of 1985, corresponding to the lowest subtidal volume exchange between shelf waters and the estuaries. Subtidal flushing times were computed taking into account an estimated recirculation parameter. These times were at an absolute high in the spring of 1985; increases of 16, 24 and 12% were determined for Great South Bay, Moriches and Shinnecock respectively. Along with reduced freshwater inflows due to the existing drought, the increased flushing times may have supported a longer permanence of available inorganic nutrient compounds contributing to a favourable environment for the ecological dominance of Aureococcus. The low subtidal sea level fluctuations of 1985 are related to a minimum energy content of the regional wind stress, particularly the alongshore (East-West) component which is responsible for the Ekman pumping into and out of the estuaries.

Subtidal sea level fluctuations in a large fjord system

A cross‐spectral analysis is made of subtidal (0.01–0.5 cpd) sea level (SSL), atmospheric pressure and wind fluctuations observed during 1966–1988 in the southern reaches of the fjord system formed by the Straits of Juan de Fuca/Georgia and Puget Sound. It shows that observed SSL can be largely explained by quasi‐steady inverse barometer compensation to atmospheric pressure and the propagation of adjusted sea level fluctuations from the nearby continental shelf, with some local weather forcing in summer. Variations in coherence, amplitudes and relative phase between seasons, locations and frequencies support previous work that both barotropic and baroclinic processes have important influences on SSL within the fjord system.

Sea Level Variability in Long Island Sound

Sea level variability in Long Island Sound is examined at both tidal and subtidal frequencies over a 1-yr period. The sound is found to be decoupled effectively from the lower Hudson Estuary at tidal frequencies. The predominantly semidiurnal tides in the sound are forced by the oceanic tides transmitted from the mouth. There is a near fourfold amplification of the semi-diurnal tides within the sound due to resonance. Diurnal tides are much weaker in the sound, and there is also no evidence of significant amplification in the interior. At subtidal frequencies, the pressure-adjusted sea level in the interior of the sound is forced by a combination of co-oscillation with coastal sea level at the mouth and direct setup induced by local wind forcing over the surface of the sound. Because the longitudinal axis of Long Island Sound is roughly aligned with the open coast from Montauk Point to Sandy Hook, these two mechanisms work in concert to produce larger subtidal sea level fluctuations in the western sound relative to those in the eastern sound. A linearized, frequency-dependent analytical model is developed to aid the interpretation of field observations.

The current and sea level variability in the Chesapeake and Delaware Canal

Current and sea level observations in the Chesapeake and Delaware (C&D) Canal are used to examine the frequency‐dependent spatial distributions of tidal and subtidal variability along the canal. To first order of approximation, the C&D Canal is shown to be a linear system. A linearized, frequency‐dependent analytical model is developed to examine details in the tidal and subtidal variability along the canal, and results agree well with observations. The principal semidiurnal tides (M2) at the two ends of the canal are roughly in phase but differ substantially in amplitude. The amplitude of the M2 tide increases monotonically from the Chesapeake end of the canal toward the Delaware end. The amplitude of the M2 current is of the order 60 cm s−1, and it is higher in the western part of the canal than in the eastern part of the canal. The amplitudes of the principal diurnal tides (K1) at the two ends of the canal are much smaller than those for M2, but the K1 tides at the ends are nearly 180° out of phase with each other. The amplitude of the K1 tide decreases toward the interior of the canal from both ends. Relatively strong K1 current of the order 20 cm s−1 exists in the canal, and its amplitude shows only slight variation along the canal. There are substantial differences in the subtidal sea level fluctuations along the canal, resulting in strong subtidal currents which may exceed 70 cm s−1. The importance of the C&D Canal on the upper Delaware Bay is assessed by comparing the volume flux through the canal relative to that in the upper Delaware. It is shown that the influence of the canal on the Delaware depends strongly on the time scales involved. It appears that the C&D Canal is not important to the overall tidal response in upper Delaware Bay, but the subtidal volume flux through the canal must be included for a proper assessment of the dynamics of upper Delaware Bay at subtidal frequencies.

Subtidal volume exchange through the Chesapeake and Delaware Canal

The Chesapeake and Delaware Canal is a man‐made waterway connecting the head of the Chesapeake Bay and the upper reaches of the Delaware estuary. We found significant subtidal variability along the canal during a 1‐month survey in 1975. There were significant differences between subtidal sea level fluctuations at the two ends of the canal, reflecting the different response characteristics of the two adjoining estuaries to atmospheric forcing. The resulting hydraulic head along the canal forced energetic subtidal volume flux through the canal which was of the same order of magnitude as the Delaware River discharge during the spring freshet period.

Tidal and Subtidal Variability in Delaware's Inland Bays

Sea level observations made during a three-month period in late 1984 at four stations in and near Delaware's inland bays are used for the examination of tidal and subtidal variability in the bays. Five tidal Constituents are found to be active in the bays, With M2 being the most important one. The inland bays, with their narrow inlets and shallow depths, behave like low-pass filters for oceanic disturbances propagating into the interior of the bays. Overall, tidal variance decreases sharply inside the bays, with the semidiurnal tides suffering significantly higher attenuation than the diurnal tides. At subtidal frequencies, sea level fluctuations in the interior of the bays are primarily forced by coastal sea level fluctuations generated by atmospheric forcing on the adjacent shelf. Local wind forcing within the bays appears to play only a secondary role in modifying the coastally forced sea level. The active subtidal sea level fluctuations induced by the bay-shelf coupling effect experience no appreciable attenuation within the bays. As a consequence, sea level variance at subtidal frequencies is even greater than that at tidal frequencies in one of the inland bays.

Sea-level fluctuations in a coastal lagoon

Sea-level observations made during December, 1979, at six stations in Great South Bay (which is a coastal lagoon on the south shore of Long Island, New York) reveal that there were significant subtidal fluctuations in addition to the tidal oscillations. Harmonic analysis of the tidal oscillations of sea level indicates that M 2 is the dominant tidal constituent. The M 2 amplitude, however, suffered a more than 50% reduction in the interior of the Bay due largely to the narrow inlet. The subtidal sea level fluctuations within the Bay were forced primarily by the low-frequency fluctuations of the adjacent shelf water. The active subtidal exchange induced by this Bay-shelf coupling appeared to have suffered only minor attenuation within the Bay. As a consequence, the variance associated with subtidal sea level fluctuations was greater than that associated with the tidal oscillations over most of Great South Bay.

Observations of wind‐induced, subtidal variability in the Delaware estuary

Sea level and current observations made in the Delaware estuary during autumn of 1982 were examined for evidence of wind‐forced subtidal variability. The large subtidal sea level fluctuations at the mouth of the Delaware were found to be forced primarily by the wind stress component over the continental shelf nearly parallel to shore, indicating coastal Ekman transport to the right of the wind. Local wind forcing within the estuary is shown to be insignificant. The subtidal sea level in the interior of the estuary was found to be driven by the wind through a combination of two remote forcing mechanisms: one acting at the mouth of the Delaware through direct estuary‐shelf coupling and a second acting locally over the Chesapeake Bay and being transmitted from the upper Chesapeake through the Chesapeake and Delaware Canal, which links the two estuaries. In the upper Delaware estuary we found predominantly barotropic subtidal current fluctuations superimposed on a much weaker two‐layer gravitational circulation. These current fluctuations were produced primarily by the local subtidal surface slope generated by the difference between the two remote forcing mechanisms. The evidence strongly suggests that the Chesapeake and Delaware Canal plays a critical role in the subtidal circulation of the Delaware estuary.

Subtidal Sea Level Fluctuations and Their Relation to Atmospheric Forcing along the North Carolina Coast

Coastal sea level fluctuations in Onslow Bay are selectively coupled with local atmospheric forcing variables. The coupling is strongest in period bands of 2.5–3.5 and ≳10 days, which span absolute zero group speed, barotropic continental shelf wave periods. The phase of the sea level disturbances propagated upstream from Beaufort to Wilmington, N. C., as expected for stable discrete shelf waves, consistent with earlier results by Mysak and Hamon (1969). The “barometric function”, when corrected for coherent wind stresses, indicated selective, super barometric atmospheric-pressure-to-sea-level coupling in the zero group speed period bands. Stochastic models of atmospheric cold front wind stress and wind stress curl fields were found to selectively force barotropic shelf wave responses near zero group speed periods and wavelengths. A strong Onslow Bay response to the model cold front occurred near the second harmonic forcing frequency, the wind stress curl contributed importantly to the shape and the amplitude of the response spectrum. The results suggest that shelf waves forced by the atmosphere contribute to the Gulf Stream meander field off North Carolina.