Subtidal Sea Level Research Articles

Moana Ocean Hindcast – a > 25-year simulation for New Zealand waters using the Regional Ocean Modeling System (ROMS) v3.9 model

Abstract. Here we present the first open-access long-term 3D hydrodynamic ocean hindcast for the New Zealand ocean estate. The 28-year 5 km×5 km resolution free-running ocean model configuration was developed under the umbrella of the Moana Project, using the Regional Ocean Modeling System (ROMS) version 3.9. It includes an improved bathymetry, spectral tidal forcing at the boundaries and inverse-barometer effect usually absent from global simulations. The continuous integration provides a framework to improve our understanding of the ocean dynamics and connectivity, as well as identify long-term trends and drivers for particular processes. The simulation was compared to a series of satellite and in situ observations, including sea surface temperature (SST), sea surface height (SSH), coastal water level and temperature stations, moored temperature time series, and temperature and salinity profiles from the CORA5.2 (Coriolis Ocean database for ReAnalysis) dataset – including Argo floats, XBTs (expendable bathythermographs) and CTD (conductivity–temperature–depth) stations. These comparisons show the model simulation is consistent and represents important ocean processes at different temporal and spatial scales, from local to regional and from a few hours to years including extreme events. The root mean square errors are 0.11 m for SSH, 0.23 ∘C for SST, and <1 ∘C and 0.15 g kg−1 for temperature and salinity profiles. Coastal tides are simulated well, and both high skill and correlation are found between modelled and observed sub-tidal sea level and water temperature stations. Moreover, cross-sections of the main currents around New Zealand show the simulation is consistent with transport, velocity structure and variability reported in the available literature. This first multi-decadal, high-resolution, open-access hydrodynamic model represents a significant step forward for ocean sciences in the New Zealand region.

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.

Response of sea level to tide, atmospheric pressure, wind forcing and river discharge in the Kitimat Fjord System

The response of sea level to tide, atmospheric pressure, wind forcing, and river discharge in the Kitimat Fjord System (KFS) is investigated by using observations from tide gauges and results from a high-resolution ocean circulation model. Tidal harmonic analysis of the observed hourly sea level at the head and mouth of Douglas Channel shows that the largest semidiurnal tide (M2) alone accounts for 79% of the total sea level variance, and the top four major tidal constituents (M2, S2, K1, and O1) account for 94% of the total variance. In addition, the amplitude of M2 increases slightly from 1.586 to 1.647 m between the mouth and the head. For the observed subtidal sea level, the local inverse barometer response due to the variation of atmospheric pressure accounts for 59% of the variance. The variation of subtidal sea level difference between the mouth and the head of Douglas Channel is observable and can be largely attributed to the along-channel wind (54%) with an additional 13% of the variation due to the daily-averaged and seasonal-cumulative river discharges.A fjord-shelf three-dimensional ocean circulation model for KFS is developed to simulate the sea level variability. The model performance in simulating the sea level is assessed by comparing model results with observed harmonic constituents of ~70 tidal stations inside the model domain and time series recorded at the two tide gauges of KFS. The modelled tides are in good agreement with observations with a domain-averaged root-mean-square error estimate of ~0.04 m for M2. For the subtidal sea level, the model is able to capture the observed synoptic variations. Further analysis suggests that the underestimates of subtidal sea level in the ocean circulation model are mainly due to the weaker-than-observed wind forcing taken from the atmospheric model especially during the peak of strong down-fjord katabatic wind events.

Subtidal variability in the Taiwan Strait induced by combined forcing of winter monsoon and topography

As an important channel connecting the East and South China Seas, circulations in the Taiwan Strait are strongly influenced by the East Asian monsoon and the topography of the strait, especially the Taiwan Bank (TWB), which is a remarkable topographic feature located at the southern entrance to the strait. Based on a series of pressure gauges deployed roughly 40 km offshore along the western Strait, subtidal sea-level variability under the combined impact of winter monsoon and topography was studied. The analyses show significant along-strait coherences of subtidal sea levels and their coherences with the large-scale monsoon wind for periods from 2 to 14 days. It is suggested that these fluctuations are mainly forced waves driven by the large-scale winds. In addition to the normal cross-shore wind setup, a sea-level setup in the along-strait direction is confirmed, which is induced by the combined forcing of the fluctuating winter monsoon and the blocking of the TWB. A southward current surge driven by a northerly wind event will cause a rising sea level over the TWB inducing a southward along-shore slope anomaly to the north of the TWB and a reversed slope anomaly to the south. The subtidal current through the channel to the west of the TWB is found to be influenced by the reversed slope anomalies generated via the along-shore setup.

Coastal sea level variability in the US West Coast Ocean Forecast System (WCOFS)

Sea level variability along the US West Coast is analyzed using multi-year time series records from tide gauges and a high-resolution regional ocean model, the base of the West Coast Ocean Forecast System (WCOFS). One of the metrics utilized is the frequency of occurrences when model prediction is within 0.15 m from the observed sea level, F. A target level of F = 90% is set by an operational agency. A combination of the tidal sea level from a shallow water inverse model, inverted barometer (IB) term computed using surface air pressure from a mesoscale atmospheric model, and low-pass filtered sea level from WCOFS representing the effect of coastal ocean dynamics (DYN) provides the most straightforward approach to reaching levels F>80%. The IB and DYN components each add between 5 and 15% to F. Given the importance of the DYN term bringing F closer to the operational requirement and its role as an indicator of the coastal ocean processes on scales from days to interannual, additional verification of the WCOFS subtidal sea level is provided in terms of the model-data correlation, standard deviation of the band-pass filtered (2–60 days) time series, the annual cycle amplitude, and alongshore sea level coherence in the range of 5–120-day periods. Model-data correlation in sea level increases from south to north along the US coast. The rms amplitude of model sea level variability in the 2–60-day band and its annual amplitude are weaker than observed north of 42 N, in the Pacific Northwest (PNW) coast region. The alongshore coherence amplitude and phase patterns are similar in the model and observations. Availability of the multi-year model solution allows computation and analysis of spatial maps of the coherence amplitude. For a reference location in the Southern California Bight, relatively short-period sea level motions (near 10 days) are incoherent with those north of the Santa Barbara Channel (in part, due to coastal trapped wave scattering and/or dissipation). At a range of periods around 60 days, the coastal sea level in Southern California is coherent with the sea surface height (SSH) variability over the shelf break in Oregon, Washington, and British Columbia, more than with the coastal SSH at the same latitudes.

Tidal and sub-tidal sea level variability at the northern shelf of the Brazilian Northeast Region.

A characterization of the sea level variability at tidal and sub-tidal frequencies at the northern shore of the Brazilian Northeast shelf for the period 2009-2011 is presented. The sea level data used was obtained from the Permanent Geodetic Tide Network from the Brazilian Institute of Geography and Statistics for the Fortaleza gauge station. Local wind data was also used to assess its effects on the low-frequency sea level variability. The variability of the sea level was investigated by classical harmonic analysis and by morphology assessment over the tidal signal. The low frequencies were obtained by low-pass filtering. The tidal range oscillated with the highest value of 3.3 m during the equinox and the lowest value of 0.7 m during the solstice. Differences between the spring and neap tides were as high as 1 m. A total of 59 tidal constituents were obtained from harmonic analysis, and the regional tide was classified as semi-diurnal pure with a form number of 0.11. An assessment of the monthly variability of the main tidal constituents (M2, S2, N2, O1, and K1) indicated that the main semi-diurnal solar S2 presented the highest variability, ranging from 0.21 to 0.41 m; it was the main element altering the form number through the years. The low frequency sea-level variability is negligible, although there is a persistent signal with an energy peak in the 10-15 day period, and it cannot be explained by the effects of local winds.

Propagation of subtidal sea level oscillations in the river channel: A case study of the St. Johns River, Florida, USA.

Analysis of water level and river discharge time series collected at three locations in the St. Johns River, FL reveals that subtidal variability with periods of several days is associated with long waves propagating from the ocean into the river channel. These dynamics are similar to tidal wave propagation: both tidal and subtidal frequency bands have the same ratio of free surface-to-discharge standard deviations, which is not the case on oceanic shelves. However, important differences also emerge: as waves pass through the river mouth, tidal oscillations exhibit much stronger attenuation, while subtidal oscillations propagate at a lower speed. Further upstream, where the channel cross-sectional area contracts (between Palatka and Buffalo Bluff), the waves in two frequency bands adjust differently: tidal waves are amplified and continue upstream, while a significant fraction of subtidal energy is reflected. The amplification of tidal waves occurs mostly through the generation of overtides. Also, tidal wave attenuation in the river relative to the mouth is nearly constant over the observation period, while the attenuation of subtidal waves exhibits strong changes. Variations in subtidal attenuation are linked to the influence of the river discharge: higher discharge (relative to the subtidal water level variability) causes stronger attenuation of subtidal waves.

Nontidal sea level changes in Hiroshima Bay, Japan

Nontidal sea level changes generated in Hiroshima Bay of the Seto-Inland Sea in Japan are studied over various time scales, from the sub-tidal (2 d to 1 month) to inter-annual scales (>2 years). The total sea level variation produces a standard deviation (STD) of 12.5 cm. The inter-annual component of the sea level variation in Hiroshima Bay oscillates with a STD of 3.4 cm, forming a long-term trend of 4.9 mm/a. The STD of the sea level variation is 9.8 cm for the seasonal component (8 months to 2 years) and 4.7 cm for the intra-seasonal one (1 month to 8 months). Significant sea level variations with a STD of 4.2 cm also occur in the sub-tidal range. Special attention is paid to the sub-tidal sea level changes. It is found that the upwelling and associated transient sea level changes generated along the north coast of Hiroshima Bay (opened southward) by the strong northerly wind, play a significant role in sub-tidal sea level changes. The transient sea level changes are over 10 cm in most cases when caused by typhoons that pass through the Pacific Ocean off the Kii Peninsula, located at about 400 km east of Hiroshima Bay. Reasonable sea level changes are evaluated by the balance of pressure forces at the onshore and offshore boundary of the study domain.

Altimeter Data Evaluation in the Coastal Gulf of Maine and Mid-Atlantic Bight Regions

Satellite altimeter sea surface height anomaly (SSHA) data accuracy and applications are rapidly maturing over the open ocean, but more attention is required where continental shelves and marginal seas are concerned. Refined radar range measurement correction approaches are needed to enhance the quality of coastal altimeter SSHA data and to enhance the potential impact that altimeter data can play in improved ocean circulation modeling and prediction. This study provides a regional assessment of satellite altimeter SSHA data from the Jason-2, Jason-1, and TOPEX/Poseidon data sets collected over the Northwest Atlantic coastal region of the United States. The validation approach follows recent studies in numerous shelf regions to address changes from standard or default data quality flagging scheme that can enhance satellite altimeter SSHA data recovery, optimal choice of de-aliasing corrections in deriving subtidal sea level time series, and validation of altimeter data using in situ tide gauge and current measurement data sets. Spatial analyses show the impact of data filtering and corrections from the coastline out onto the slope. Revised handling of the data provides a 5–10% increase in availability (from about 70% up to 80% for waters beyond 50 km from shore). The RMS differences between tide gauge and surface layer current measurements at the 60 day time scale are 3–4 cm and 6 cm/s, respectively, and of magnitude consistent with several recent coastal altimeter data validation efforts.

Alternative Estimators of Wavelet Variance

The wavelet variance is a scale-based decomposition of the process variance that is particularly well suited for analyzing intrinsically stationary processes. This decomposition has proven to be useful for studying various geophysical time series, including some related to sub-tidal sea level variations, vertical shear in the ocean and variations in soil composition along a transect. Previous work has established the large sample properties of an unbiased estimator of the wavelet variance formed using the non-boundary wavelet coefficients from the maximal overlap discrete wavelet transform (MODWT). The present work considers two alternative estimators, one of which is unbiased, and the other, biased. The new unbiased estimator is appropriate for asymmetric wavelet filters such as the Debaucheries filters of width four and higher and is obtained from the non-boundary coefficients that result from running a wavelet filter through a time series in both a forward and a backward direction. The biased estimator is constructed in a similar fashion, but utilizes all wavelet coefficients that result from filtering a time series in forward and backward directions. While the two alternative estimators have the same asymptotic distribution as the original unbiased estimator (with some restrictions in the case of the biased estimator), they can have substantially better statistical properties in small sample sizes. Formula for evaluating the mean squared errors of the usual unbiased estimator and the two alternative estimators are developed and verified for several fractionally differenced processes via Monte Carlo experiments.

Temporal and spatial variability of sea level and volume flux in the Murderkill Estuary

The instantaneous sea level determined at two sites in the Murderkill Estuary, a tributary of Delaware Bay, results from the superposition of temporal variability operating over different time and spatial scales. Over the relatively short tidal time scales, the semidiurnal tides that represent the dominant tidal constituents in lower Delaware Bay show a modest increase in tidal amplitudes from the bay mouth (Lewes, Delaware), up to Bowers Beach (the mouth of the Murderkill Estuary). However, as the tides propagate into the Murderkill Estuary, the semidiurnal constituents undergo heavy attenuation, resulting in a 48% reduction in tidal amplitude from Bowers to Frederica (approximately the extent of saline intrusion). The diurnal tide, on the other hand, experiences only a 25% reduction in amplitude. The limited tidal asymmetry that is observed may be a result of interaction between flows in the tidal channel and the adjacent salt marsh. At longer time scales, the subtidal sea level experiences no attenuation. The Murderkill Estuary thus behaves like a low pass filter to preferentially damp out high frequency sea level forcing from lower Delaware Bay. The subtidal volume flux in the Murderkill is highly coherent with the time rate of change of sea level, indicating that the Murderkill basically co-oscillates with Delaware Bay in a standing wave fashion over the subtidal time scale. This remote coupling controls more than 90% of the variance in subtidal sea level in the estuary. The surface slopes in the lower bay and the Murderkill Estuary are closely correlated with winds along the orientation of the two waterways, consistent with the effect of local wind on subtidal sea level.

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.

Subtidal sea level variability in a shallow Mississippi River Deltaic estuary, Louisiana

The relative roles of river, atmospheric, and tidal forcings on estuarine sea level variability are examined in Breton Sound, a shallow (0.7 m) deltaic estuary situated in an interdistributary basin on the Mississippi River deltaic plain. The deltaic landscape contains vegetated marshes, tidal flats, circuitous channels, and other features that frictionally dissipate waves propagating through the system. Direct forcing by local wind stress over the surface of the estuary is minimal, owing to the lack of significant fetch due to landscape features of the estuary. Atmospheric forcing occurs almost entirely through remote forcing, where alongshore winds facilitate estuary-shelf exchange through coastal Ekman convergence. The highly frictional nature of the deltaic landscape causes the estuary to act as a low-pass filter to remote atmospheric forcing, where high-frequency, coastally-induced fluctuations are significantly damped, and the damping increases with distance from the estuary mouth. During spring, when substantial quantities of controlled Mississippi River inputs (−q = 62 m3 s-1) are discharged into the estuary, upper estuary subtidal sea levels are forced by a combination of river and remote atmospheric forcings, while river effects are less clear downestuary. During autumn (−q = 7 m3 s-1) sea level variability throughout the estuary is governed entirely by coastal variations at the marine boundary. A frequency-dependent analytical model, previously used to describe sea level dynamics forced by local wind stress and coastal forcing in deeper, less frictional systems, is applied in the shallow Breton Sound estuary. In contrast to deeper systems where coastally-induced fluctuations exhibit little or no frictional attenuation inside the estuary, these fluctuations in the shallow Breton Sound estuary show strong frequency dependent amplitude reductions that extend well into the subtidal frequency spectrum.

Tide and wind control of megalopal supply to estuarine crab populations on the Portuguese west coast

Physical processes that force transport of planktonic larvae of invertebrates are responsible for some of the spatial and temporal variability in recruit- ment. We investigated the influence of tide- and wind- driven circulation on intra-year variability of megalopal supply to populations of the crab Carcinus maenas in 2 estuaries on the Portuguese west coast. Daily data on physical variables and on supply and settlement of megalopae were subjected to time series analysis and mul- tiple regression techniques in order to identify periodicity in the variables as well as time lags between larval supply and physical variables. Relaxation of northerly winds, which favor upwelling, was associated with temperature increase and subtidal sea level rise at the coast, which are indicative of coastal convergence of the surface layer. Supply of megalopae to the estuaries, as measured with passive plankton nets, followed a fortnightly cycle with maximum larval supply at the time of maximum tidal am- plitude. Supply was enhanced by southerly winds, with delays of 0 to -2 d. Settlement of megalopae on artificial settlement substrates deployed on the bottom was uncou- pled to supply at both estuaries. Therefore, transport of C. maenas megalopae to the nearshore is accomplished by onshore advection following downwelling winds, and supply to the estuaries occurs by selective tidal stream transport. Involvement of internal waves and internal tidal bores cannot be be ruled out, but very particular periodicities of the generating mechanisms would have to be assumed to account for the observed time lags.

Across-sill circulation near a tidal mixing front in a broad fjord

The Clyde Sea is a broad fjord located on the southwest coast of Scotland. Cross-boundary circulation processes are investigated using observations over the shallow sill that separates the stratified Clyde Sea from the well-mixed North Channel of the Irish Sea. A simple method for determining the amplitude and phase of the M 2 tidal constituent and the spatial residual current field from shipboard acoustic Doppler profiler (ADCP) data is presented. Tidal results using this method compare well with established model results, and also agree sufficiently well with nearby moored tidal observations. Using new, high-resolution cross-sill sections observed with an undulating conductivity, temperature, depth profiler, the presence of a high-gradient tidal mixing front is confirmed, indicating steeply sloping (1:250) isopycnals near the surface expression of the front. Repeated alongsill sections throughout a tidal period show a consistent temperature and salinity structure, with a sloping interface (1:700) separating heavy, high-salinity water in the lower water column toward the southeastern sill from less-dense water over the northwestern sill. A geostrophic interpretation of the observed density field supports the existence of a northwestward directed, alongsill jet near the frontal outcrop, and a residual across-sill exchange modified by the earth's rotation. However, residual across-sill currents, measured both by the shipboard ADCP and moored instruments, indicate flows additional to the across-sill circulation predicted by the density structure. Subtidal currents from three sill moorings reveal barotropic motions, which are likely responsible for frontal displacements, and depth-dependent exchanges that are partially wind-driven. The barotropic flows are of unknown origin and are not spatially correlated between mooring locations, but appear periodic in nature and are an order of magnitude larger than flows required to account for the observed variations in subtidal sea level at the sill. The depth-dependent across-sill exchange resembles a classical density-driven circulation, which is enhanced when the wind is directed parallel to the sill toward the southeast, and reduced when the wind is directed into the bay. The substantial sampling effort provides clear evidence for the spatial complexity of the circulation over the sill; however, temporal undersampling prevented conclusive quantification of forcing mechanisms responsible for the observed across-sill circulation.

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.

Physical forcing of onshore transport of crab megalopae in the northern Portuguese upwelling system

Wind-driven circulation is a mechanism that may be involved in the shoreward transport of invertebrate larvae. This study investigated the temporal variability of the supply of crab megalopae to an estuary located in the northwest coast of Portugal, which is affected by seasonal upwelling, and related variations in supply to several upwelling indices. Megalopae of the European common shore crab Carcinus maenas (L.). were collected daily, using artificial settlement substrates, during a 3-month period, from March 15 to June 15, 2000. Two settlement episodes were detected, which were consistent with the variation in abundance and size of juvenile crabs. Variations in subtidal sea level and sea surface temperature anomaly were correlated with the strength of along-shore wind stress and consistent with upwelling theory. Contingency analyses showed positive significant associations between increasing intensity of southerly winds and megalopal supply, for time lags of 0–6 d, indicating that shoreward transport occurred during relaxation or reversal of upwelling favourable winds. Associations between megalopal supply and tidal range or the duration of nocturnal flood were not detected. Compared to other studies of the supply of crab megalopae to coastal habitats that used a similar methodology, very low numbers of megalopae were collected during this study. The paucity of C. maenas megalopae in the Ria de Aveiro is in agreement with the low densities of this larval stage in the plankton, and indicates that maintenance of the population is achieved despite a low supply of megalopae.

Effect of wind and local density on the subtidal circulation of the inner Scotian Shelf

A comprehensive oceanographic data set was collected over the inner Scotian Shelf in February 1996. Hydrographic observations reveal that the water properties had typical wintertime distributions characterized by a two-layer system: relatively cold and fresh surface water overlying a warmer and saltier bottom water. The moored current measurements imply a persistent southwestward surface-intensified flow, consistent with previous studies. We follow Thompson and Sheng (J. Geophys. Res. 102 (1997) 24 987) and use a linear statistical model to hindcast the time-varying shelf response to local wind and remotely generated free waves propagating along the shelf. The wind is assumed to be spatially uniform and equal to that observed at Sable Island. The remote forcing is represented by subtidal sea level observations at Halifax after removal of local wind and density effects. Sable Island stress, on average, accounts for about 25% of the subtidal current variance. Sable Island wind and Halifax sea level together account for about 50% of the variance. The vertical shears in the mean alongshore currents agree reasonably well with the thermal wind relationship, indicating the importance of baroclinic effects during this period

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).

Tidal and subtidal fluctuations in temperature, salinity and pressure for the winter 1996 larval ingress experiment—Beaufort Inlet, NC

A multidisciplinary field experiment was conducted to compare water properties, larval abundances, and transport and retention processes at Beaufort Inlet and two channels leading to the estuarine nursery grounds. Temperature, salinity and subsurface pressure were monitored in situ for a six‐week period during March and April 1996 in each channel. Intensive sampling was performed during two neap‐tide periods when water mass conditions in the estuary were significantly different. Currents were stronger in the eastern channel during both experiments. Ebb currents were stronger than flood in both channels. Decreasing subtidal sea level appeared to account for the stronger ebb currents. Subtidal sea level in the inlet responded optimally to north–south (along the inlet axis) wind stress and along a line 15° clockwise of north–south. This direction closely parallels the channel axis of Core Sound and may provide an efficient conduit to carry large volumes of low‐salinity Pamlico Sound water into the estuarine complex when winds blow south in this sector. The tidal stream in Beaufort Inlet sets up strong cross‐inlet gradients by the advection on the east side of higher salinity shelf water and advection on the west side of Beaufort Inlet plume water. The axial fronts produced by differential advection of these two water masses might play some role in redistributing larvae present in one tidal stream to another.