Coastal Sea Level Fluctuations Research Articles

Incidence and Reflection of Offshore Subinertial and Barotropic Pressure Signals in Wide Shelf Seas

Abstract The response of a wide shelf to subinertial and barotropic offshore pressure signals from the shelf edge was investigated. By relaxing the semigeostrophic approximation, an elliptical wave structure equation was formulated and solved with the integral transform method. It was found that when the imposed offshore signal has an along-shelf length scale similar to the shelf width, it can efficiently break the potential vorticity barrier and propagate toward the coast, producing a significant coastal sea level setup. Thereafter, the pressure signal reflects from the coast or the sloping topography, producing a transient eddy and propagates to the downshelf. The intensities of the coastal setup and the eddy increase as the along-shelf scale of the subinertial signal decreases or when its time scale is close to the inertial period. For a signal with longer time scale, the eddy is insignificant. The nature of the shelf response is controlled by the shelf conductivity κ ≡ r/(fsB), in which r is the Rayleigh friction coefficient, f is the Coriolis parameter, s is the shelf slope, and B is the shelf width, respectively. For a given offshore signal, coastal setup increases with κ. For large κ, the eddy energy is concentrated at low modes, producing a large eddy, whereas a small κ produces a small eddy. The proposed theory can explain coastal sea level fluctuations under eddy impingement in the Mid-Atlantic Bight or other similar areas. Significance Statement Coastal sea level and shelf circulation are greatly affected by offshore pressure signals, e.g., mesoscale eddy impingements or boundary current fluctuations. It is often assumed that the along-shelf length scale of the forcing is much larger than the shelf width, i.e., the semigeostrophic approximation. Here in this study, we found this approximation significantly underestimates the shelf–ocean interaction. A general shelf wave equation was developed that relaxed the semigeostrophic approximation and was solved analytically with a novel mathematical method. The solution can characterize the shelf response to subinertial offshore forcing at arbitrary spatiotemporal scales. It was found that for a subinertial signal with scale close to or smaller than the shelf width, significant coastal sea level setup and transient eddy can be formed, which was consistent with realistic phenomena. The new theory could promote the understanding of coastal sea level variations and along-/cross-shelf transports at synoptic and intermediate scales.

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.

Quasi-biweekly oscillation in sea level along the western Bay of Bengal

This study investigates the variability and sources of Quasi-Biweekly (QB) oscillation (10–20 days) in coastal sea level along the western boundary of the Bay of Bengal (BoB) using tide-gauge data and simulations from a very high-resolution regional model. Observations show most significant spectral energy in the QB band (∼12 days) exists in the northernmost location (Paradeep) with an amplitude of about 5–10 cm in sea level and energy in this band decreases equatorward along the coast. The QB oscillations in sea level contribute up to 16%–36% of the total subtidal sea-level variability in the western BoB and these oscillations are more pronounced during June–November. It is observed that sea-level oscillations in the QB band propagate equatorward along the east coast of India as coastal-trapped waves with a phase speed of about 2–4 ms−1 and this speed is consistent with the theoretical estimates. Our study revealed that the strong QB winds found in the northern BoB are one of the forcing factors that excite QB oscillation in sea level in the western BoB. Further analysis using numerical simulations shows that the QB sea level propagating from the equatorial Indian Ocean and QB oscillation generated by strong winds associated with the tropical cyclones formed in the BoB also contribute to QB coastal sea-level fluctuations in this region.

Mid-Holocene to present circum-Arabian sea level database: Investigating future coastal ocean inundation risk along the Arabian plate shorelines

The Arabian Peninsula has a unique setting to resolve how proximity to tectonically diverse plate margins, eustacy, and glacial isostatic adjustment (GIA) influences coastal sea level fluctuations. New 14C AMS and U series dating of coastal sediments from the western, southern, and eastern plate margins (Al-Wajh and Thuwal, Saudi Arabia; Bar Al-Hikman, Oman; Abu Dhabi, U.A.E) have been integrated with an archive of dated coastal sediments (n = 145, 31 locations). Each dated sample is recalibrated for its elevation based on a high-resolution coastal DEM dataset, tidal and tectonic correction, and reservoir correction, to develop a new-suite of relative sea level (RSL) plots, separated into six tectonically distinct zones. A mid-Holocene highstand has been identified in most of the zones (1, 2, 4, 6), however, with complex spatial and temporal variability. No data is available for zone 3, whereas no Holocene highstand was observed in zone 5. The results suggest that eustacy, varying rates of vertical tectonic change (max 1 mm/year) and glacial isostatic adjustment (max 0.5 mm/year) were the major drivers of sea level fluctuations since Mid-Holocene albeit in different proportions along the different sections of Arabian shorelines. Furthermore, the topography of a shoreline, tidal range, and presence/absence of a coastal barrier also significantly influences the coastal inundation. Based on our findings, eastern Arabia will in the near future experience the largest coastal flooding (largest 0–10 m elevation area, > 2 m tidal amplitude, no barriers), followed by western (1–2 m tidal amplitude, discontinuous coral reef barriers) and southern Arabia shorelines (>3 m tidal amplitude, no barriers) shorelines. The presented insights are critical in supporting communities along the coasts of the Arabian Peninsula, who will have to cope with flooding from rising oceans over the next few decades due to climate change.

Regional influences on reconstructed global mean sea level

AbstractReconstructions of global mean sea level (GMSL) based on tide gauge measurements tend to exhibit common multidecadal rate fluctuations over the twentieth century. GMSL rate changes may result from physical drivers, such as changes in radiative forcing or land water storage. Alternatively, these fluctuations may represent artifacts due to sampling limitations inherent in the historical tide gauge network. In particular, a high percentage of tide gauges used in reconstructions, especially prior to the 1950s, are from Europe and North America in the North Atlantic region. Here a GMSL reconstruction based on the reduced space optimal interpolation algorithm is deconstructed, with the contributions of individual tide gauge stations quantified and assessed regionally. It is demonstrated that the North Atlantic region has a disproportionate influence on reconstructed GMSL rate fluctuations prior to the 1950s, notably accounting for a rate minimum in the 1920s and contributing to a rate maximum in the 1950s. North Atlantic coastal sea level fluctuations related to wind‐driven ocean volume redistribution likely contribute to these estimated GMSL rate inflections. The findings support previous claims that multidecadal rate changes in GMSL reconstructions are likely related to the geographic distribution of tide gauge stations within a sparse global network.

Coastal eolian landforms and sea level fluctuations

This paper is dedicated to correlation of the formation of coastal marine landforms with sea level fluctuations and climate changes in the last millennium using the southeastern Baltic region as an example. The morphological analysis of spits and historical evidence reveal three sea level oscillations in its evolution. It is shown that the sea level rise against the background of the sandy material excess in the coastal zone and optimum angle of the prevailing land-directed wind are the main factors responsible for accumulation of the thickest sandy formations. The recent climate warming and related sea level rise provoke global destabilization of coastal dune massifs.

Coastal upwelling in summer 2000 in the northeastern South China Sea

Using a combination of hydrographic, tide‐gauge, near‐bottom mooring, and satellite observations; and a numerical circulation model, we investigate the coastal upwelling in the northeastern South China Sea (NSCS) off the coast of Fujian and Guangdong Provinces, China, in the summer of 2000. Subsurface upwelling phenomenon exists mainly near the bottom boundary in the whole region investigated. It is closely related to the coastal sea level fluctuations, which are evidently modulated by both the local wind‐forcing and the large‐scale circulation. The northeastward interior flow following the bathymetry is accelerated by the drop of coastal sea level and leads to onshore transport and subsequent cooling in the bottom boundary layer (BBL) over the shelf west of Shantou. To the east of Shantou, the near‐bottom flow veers more eastward, parallel to the coastline, and transports the nearshore cold water mass farther to the southern Fujian coast. The cross‐shelf advected cold water does not always penetrate through the stratification and reach the surface. The local wind exhibits considerable synoptic variability. The decrease in sea surface temperature (SST) is mostly significant near Dongshan‐Shantou, intermittent in time and intensifies preferably during weather events that bring southwesterly alongshore wind. To the west a freshwater tongue originating from the Pearl River forms a barrier layer, which results in high surface temperature in the freshwater plume. The observational evidences and modeled results shown in this study provide important information for further understanding the ecological effects associated with the upwelling processes in the NSCS.

Japan Tsunami Current Flows Observed by HF Radars on Two Continents

Quantitative real-time observations of a tsunami have been limited to deep-water, pressure-sensor observations of changes in the sea surface elevation and observations of sea level fluctuations at the coast, which are essentially point measurements. Constrained by these data, models have been used for predictions and warning of the arrival of a tsunami, but to date no detailed verification of flow patterns nor area measurements have been possible. Here we present unique HF-radar area observations of the tsunami signal seen in current velocities as the wave train approaches the coast. Networks of coastal HF-radars are now routinely observing surface currents in many countries and we report clear results from five HF radar sites spanning a distance of 8,200 km on two continents following the magnitude 9.0 earthquake off Sendai, Japan, on 11 March 2011. We confirm the tsunami signal with three different methodologies and compare the currents observed with coastal sea level fluctuations at tide gauges. The distance offshore at which the tsunami can be detected, and hence the warning time provided, depends on the bathymetry: the wider the shallow continental shelf, the greater this time. Data from these and other radars around the Pacific rim can be used to further develop radar as an important tool to aid in tsunami observation and warning as well as post-processing comparisons between observation and model predictions.

Ocean Currents and Sea Surface Heights Estimated across the West Florida Shelf

Abstract The across-shelf structures of the ocean circulation and the associated sea surface height (SSH) variability are examined on the west Florida shelf (WFS) for the 3-yr interval from September 1998 to December 2001. Five sets of characteristic circulation patterns are extracted from 2-day, low-pass-filtered data using the self-organizing map: extreme upwelling and downwelling structures with strong currents, asymmetric upwelling and downwelling structures with moderate currents, and a set of transitional structures with weak currents. The temporal variations of these structures are coherent with the local winds on synoptic weather time scales. On seasonal time scales they are related to both the local winds and the water density variations. The circulation is predominantly upwelling during autumn to spring months (October–April) and downwelling during summer months (June–September). Coastal sea level fluctuations are related to both the dynamical responses of the inner shelf circulation to meteorological forcing and the offshore SSH. On long time scales, the offshore SSH variations appear to dominate, whereas on synoptic weather time scales, the inner shelf wind-driven circulation responses are largest. The across-shelf distribution of SSH is estimated from the velocity, hydrography, wind, and coastal sea level data, and the results are compared with satellite altimetry data, thereby providing a means for calibrating satellite altimetry on the shelf.

Alongshore Wind Forcing of Coastal Sea Level as a Function of Frequency

Abstract The amplitude of the frequency response function between coastal alongshore wind stress and adjusted sea level anomalies along the west coast of the United States increases linearly as a function of the logarithm (log10) of the period for time scales up to at least 60, and possibly 100, days. The amplitude of the frequency response function increases even more rapidly at longer periods out to at least 5 yr. At the shortest periods, the amplitude of the frequency response function is small because sea level is forced only by the local component of the wind field. The regional wind field, which controls the wind-forced response in sea level for periods between 20 and 100 days, not only has much broader spatial scales than the local wind, but also propagates along the coast in the same direction as continental shelf waves. Hence, it has a stronger coupling to and an increased frequency response for sea level. At periods of a year or more, observed coastal sea level fluctuations are not only forced by the regional winds, but also by joint correlations among the larger-scale climatic patterns associated with El Niño. Therefore, the amplitude of the frequency response function is large, despite the fact that the energy in the coastal wind field is relatively small. These data show that the coastal sea level response to wind stress forcing along the west coast of the United States changes in a consistent and predictable pattern over a very broad range of frequencies with time scales from a few days to several years.

Wind‐forced dynamics at the estuary‐shelf interface of a large coastal plain estuary

Wind‐current relationships at the mouth of a large coastal plain estuary are examined using seasonal current, wind, and sea level records. Observations indicate a small portion of subtidal current is forced by along‐shelf (AL) wind‐induced coastal sea level fluctuations (remote forcing). However, subtidal fluctuations on the order of 10 cm/s appear to be largely forced by along‐estuary wind stress (local forcing). Local forcing dominates the wind‐driven subtidal current during principal AL and along‐estuary wind conditions, but along‐estuary wind stress accentuates the local effect while diminishing the remote effect. A simple two‐dimensional analytical model helps clarify the competition between remote and local wind forcing of current near the estuary mouth, supporting the existence of a strong bi‐directional current that is locally wind‐driven, regardless of principal wind forcing. Both observed and model results indicate that the relative orientation of the wind stress with respect to the estuary and the adjacent shelf are important in determining the roles of remote and local forcing. Findings from this study challenge the application of traditional coastal pumping models for estuary‐shelf exchange and have implications for larval recruitment and exchange of material between estuaries and inner shelves.

On the wind-induced exchange between Indian River Bay, Delaware and the adjacent continental shelf

The structure of the wind-induced exchange between Indian River Bay, Delaware and the adjacent continental shelf is examined based on current measurements made at the Indian River Inlet which represents the only conduit of exchange between the bay and the coastal ocean. Local wind measurements and sea levels from stations located along the open coast and in the interior of the bay are also obtained. Coastal sea level fluctuations are coherent with winds over periodicities of 2–10 days, with the highest response corresponding to winds aligned along two bands, one broadly centered around the general orientation of the Middle Atlantic Bight coastline between New Jersey and Virginia (040°T), and the other narrowly focused on the direction perpendicular to the local coastline (090°T). Currents in the inlet are also highly coherent with the wind along these two directions, but the high coherence is limited to time scales between 2 and 5 days. Within those time scales the currents in the inlet show largely depth-independent fluctuations which are highly coherent and in quadrature phase with the coastal sea level fluctuations, indicating that the subtidal exchanges in those time scales are principally accomplished by the remote wind effects on the shelf through the impingement of coastal sea level fluctuations at the entrance to the inlet. This coastal pumping effect produces unidirectional inflow throughout the water column under coastal set-up conditions, and outflow occurs under coastal set-down conditions. There is indication that a weaker, but still significant mode of bi-directional exchange also exists in the inlet, with currents in the upper layer fluctuating in opposite directions relative to those in the lower layer.

Monitoring a shelf edge current using bottom pressures or coastal sea-level data

Low-frequency fluctuations in ocean currents can be monitored by their effects on water level and bottom pressure gradients due to geostrophy. A predominantly barotropic current along the Norwegian continental slope is studied to show how routine monitoring of coastal sea-level, air pressure and coastal hydrography can be used to monitor fluctuations of an important source of salt and heat to arctic regions. These data are also needed as forcing boundary conditions for running models of the adjacent shelf seas. The first attempt showed a promising validation to monthly average transports measured 1300 km upstream. The evidence suggests that changes in the slope current are established on the time scale of a few days along the entire continental slope, apparently forced by sea-level changes in the northeast Atlantic. Correcting for atmospheric pressure and the buoyancy of the coastal water near the tide gauge at Bodø, we obtain five-day running average coastal sea-level fluctuations that correlate to within 15% of observed current and bottom pressure gradient fluctuations 300 km away. The extension of the derived relation for monitoring this flow over large distances, with a time resolution of 5 days is demonstrated. Improved quality of the measurements is needed to reduce the inherent sources of noise. Part of the noise is due to mesoscale variability in the coastal current. This can be reduced by averaging over longer times. The method fails at higher time resolutions due to atmospheric variability, basin modes and other processes that are unaccounted for in the simple geostrophic model.

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.

Analysis of Subtidal Coastal Sea Level Fluctuations Using Wavelets

Subtidal coastal sea level fluctuations affect coastal ecosystems and the consequences of destructive events such as tsunamis. We analyze a time series of subtidal fluctuations at Crescent City, California, during 1980–1991 using the maximal overlap discrete wavelet transform (MODWT). Our analysis shows that the variability in these fluctuations depends on the season for scales of 32 days and less. We show how the MODWT characterizes nonstationary behavior succinctly and how this characterization can be used to improve forecasts of inundation during tsunamis and storm surges. We provide pseudocode and enough details so that data analysts in other disciplines can readily apply MODWT analysis to other nonstationary time series.

Preliminary annual estimates of regional nitrate supply in the southern Benguela using coastal sea level fluctuations as a proxy for upwelling

First-order estimates of annual potential new production are presented for the southern Benguela upwelling region for the decade of the 1980s. Using satellite images of sea surface temperature and a 10-year record of sea level, preliminary estimates of potential new production for 12-month periods between May and June inclusive were made for the period 1980/81 to 1989/90. The annual period was selected so as to encompass the upwelling season (austral spring and summer) and to optimize the interaction between upwelling and biological response. The range of these estimates was 5.16 × 1013 to 6.19 × 1013 gC·year−1, with a mean of 5.60 × 1013 gC·year−1. The trend in the estimates was found to be consistent with related variables and their validity, in terms of order of magnitude, was tested against 15NO3-N uptake rates measured in the southern Benguela and scaled to a regional level. In addition, they were subjected to an intra- and interregional comparison.

A numerical study of the continental shelf circulation of the U.S. South Atlantic Bight during the autumn of 1987

The autumn circulation on the inner- and mid-shelf of the U.S. South Atlantic Bight (SAB) is examined numerically. Using data collected in 1987 during the Fall Experiment (FLEX) the alongshore structure of the currents and the coastal sea level fluctuations were found to be correlated to local winds which were strong and persistently northeasterly. The observed inshore distribution of freshwater during FLEX, characterized by the presence of a coastal front confined to the coast inside the 25 m isobath, reflects the local autumn discharge subjected to strong and persistent downwelling winds. The freshwater signal found outside the 25 m isobath is suggested to be the previous summer's discharge advected northward by the summer winds subsequently returning south forced by the autumn winds.

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.

The effect of coastal sea level forcing on Indian River Bay and Rehoboth Bay, Delaware

Previous studies have suggested that sea level and current variability in Indian River Bay and Rehoboth Bay, Delaware are primarily forced by coastal sea level fluctuations. A linearized frequency-dependent pumping mode model is developed to (1) examine the response characteristics of the Indian River Bay-Rehoboth Bay system, and (2) assess the relative importance of the coastal forcing from the Indian River Inlet and the Lewes-Rehoboth Canal. The results indicate that the pumping mode model can adequately address the first-order response of the two bays. The results further indicate that sea level variabilities in the two bays are almost entirely forced by coastal forcing from the Indian River Inlet at both tidal and subtidal frequencies. The coastal forcing from the inlet also dominates the volume flux through the system at all frequencies. but the coastal forcing conveyed through the Lewes and Rehoboth Canal can generate up to 20% of the total volume transport at very low frequencies. The lowfrequency volume flux through the canal, however, generates a flow through the entire system and produces minimal sea level response. The overall response of the two bays to coastal forcing depends strongly on the degree to which the two bays are coupled. The ditch connecting the two bays acts as an effective low-pass filter to preferentially damp out high-frequency tidal motions in Rehoboth Bay. Because of the coupled nature of the response, the ditch also exerts substantial influence on the response characteristics of Indian River Bay.

The Madden‐Julian oscillation and the angular momentum balance in a barotropic ocean model

Surface wind stresses associated with the Madden‐Julian oscillation transfer angular momentum between the atmosphere and the underlying ocean. The resulting anomalies in oceanic angular momentum (related to either changes in zonal currents or latitudinal redistribution of mass) and the torques coupling the ocean and the solid Earth are examined for simple zonal stress patterns representing idealized 40‐ to 50‐day fluctuations of Pacific winds. Because the contribution of baroclinic variability to the angular momentum budget is in general small when vertically integrated quantities are considered (Ponte, 1990), only the barotropic response is studied. Numerical solutions of the shallow water equations with linear bottom friction are found for a constant depth basin centered on the equator and extending over 140° in longitude. The angular momentum exchanges between the ocean and solid Earth in the model involve the continental torque (i.e., body forces on eastern and western walls due to coastal sea level anomalies) and the bottom friction torque. Solutions yield a dominant balance between the wind and continental torques, with the bottom friction torque being negligible. The amplitude of the total torque acting on the ocean and the corresponding angular momentum fluctuations are very small compared with the amplitude of the applied wind torque. The ocean delivers to the solid Earth, with no significant time delays, the angular momentum anomalies received from the atmosphere. In general, sea level anomalies are nearly uniform along the eastern boundary, and the respective meridionally integrated torque is much larger than the one exerted on the western wall. Solutions also suggest that coastal (barotropic) sea level fluctuations as small as 0.5–1.5 cm are sufficient to cause variations in the length of day comparable to observed values in the 40‐ to 50‐day band.