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1.
In tide-dominated environments, residual circulation is the comparatively weak net flow in addition to the oscillatory tidal current. Understanding the 3D structure of this circulation is of importance for coastal management as it impacts the net (longer term and event-scale) transport of suspended particles and the advection of tracer quantities. The Dee Estuary, northwest Britain, is used to understand which physical processes have an important contribution to the time-varying residual circulation. Model simulations are used to extract the time-varying contributions of tidal, riverine (baroclinicity and discharge), meteorological, external and wave processes, along with their interactions. Under hypertidal conditions, strong semi-diurnal interaction within the residual makes it difficult to clearly see the effect of a process without filtering. An approach to separate the residual into the isolated process contribution and the contribution due to interaction is described. Applying this method to two hypertidal estuarine channels, one tide dominant and one baroclinic dominant, reveals that process interaction can be as important as the sub-tidal residual process contributions themselves. The time variation of the residual circulation highlights the impact of different physical process components at the event scale of tidal conditions (neap and spring cycles) and offshore storms (wind, wave and surge influence). This gives insight into short-term deviation from the typical estuarine residual. Both channels are found to react differently to the same local conditions, with different short-term change in process dominance during events of high and low energy.  相似文献   

2.
Suspended sediment transport processes in a short tidal embayment with a simple geometry are investigated using analytic and numerical models. On the basis of numerical results, the horizontal gradient of depth-averaged suspended sediment concentration can be parameterized with a combination of the first harmonic and mean. Using the parameterization, the solution of the analytic model is obtained. Evaluation of the major terms from the solution of the analytic model shows that a quarter-diurnal frequency is significant near the mouth while a semidiurnal component dominates the interior area. The settling lag consists of local and nonlocal components. The local phase lag is a function of the ratio between tidal period and settling time. The nonlocal phase lag is determined by the phase difference between tidal velocity and the horizontal gradient of sediment concentration and by the strength of erosion and horizontal advection.  相似文献   

3.
Idealized experiments using linear (LM) and nonlinear (NM) multilevel global spectral models have been carried out to investigate and understand the impact of nonlinearities on the stationary wave response in the tropical atmosphere and its sensitivity to the vertical profile of heating. It is found that nonlinearities exert a dominant influence on the low-latitude stationary Kelvin and Rossby waves particularly in the vicinity of the forcing region. Our study shows that nonlinear effects on the upper tropospheric response produce prominent eastward displacement of the anticyclonic vorticity and horizontal shifts of the maximum equilibrium divergence relative to the prescribed heating. These changes due to nonlinear terms are found to be quite sensitive to the vertical structure of diabatic heating. The strongest nonlinear effects are found to occur when the vertical level under consideration is strongly forced from below. Detailed vorticity budget calculations indicate that stronger nonlinear contributions from stretching and horizontal advection of relative vorticity favour the generation of upper tropospheric anticyclonic circulation and its eastward displacement. Larger vertical advection and twisting terms appear to oppose the generation of upper tropospheric anticyclonic vorticity. It is found that the nonlinear terms which affect the vorticity generation in the upper levels are crucially controlled by the vertical profile of heating. The mid-tropospheric response due to deep convective heating in the NM is characterized by anomalous equatorial westerlies in the low-latitude Rossby regime and exhibits prominent ageostrophic motions. Such nonlinear effects appear probably because of a vertical shift of the low level circulation anomalies in the NM. In the case of shallow convective heating the occurrence of anomalous zonal flows and ageostrophic motions in the low latitude regions of the NM takes place near the level of the maximum heating. Our study shows that large heating amplitudes and small vertical gradient of heating at a given vertical level together favour generation of anomalous zonal flows and ageostrophic motions in the near equatorial regions. These anomalous basic flows in the low-latitudes have implications on the propagation of transients from the tropics to midlatitudes. Non-linear effects on the lower tropospheric stationary waves are prominently seen in the case of strong low level heating which produces a large strengthening of the lower tropospheric cyclonic anomalies that exhibit distinct eastward shifts in the NM relative to the LM.  相似文献   

4.
The interaction between extratropical transition process and the mid-latitude jet system stimulates the downstream development. In this paper, three typhoon cases were selected to study their downstream development mechanism through the analysis of the eddy kinetic energy budget and the idealized simulations. The results of Chen’s work to the Pacific region were examined. The results were consistent with the results of Chen’s Atlantic hurricane Case. ET downstream at the upper levels generated first eddies, and the disturbances triggered the low level eddy development. Then the upper and the lower coupled and formed a deep cyclone system throughout the whole troposphere. The ageostrophic geopotential flux promoted the formation and development of the downstream ridge from the typhoon. Vertical ageostrophic geopotential flux transferred energy from upper downward that convergence happened in lower, which stimulated the lower-level cyclone development. Simulation results showed that, in the process of ET, TC outflow transported low potential vorticity to mid-latitude jet, which enhanced the PV gradient and the baroclinic. Then, it is inspired the Rossby wave in the jet and propagated downstream. The formation of downstream ridge-trough couple and development of the further wave was the spread to the downstream through the Rossby wave.  相似文献   

5.
Large-scale human-built infrastructure is shown to alter the salinity and subtidal residual flow in a realistic numerical simulation of hydrodynamic circulation in a coastal plain estuary (Tampa Bay). Two model scenarios are considered. The first uses a modern bathymetry and boundary conditions from the years 2001–2003. The second is identical to the first except that the bathymetry is based on depth soundings from the pre-construction year 1879. Differences between the models' output can only result from changes in bay morphology, in particular built infrastructure such as bridges, causeways, and dredging of the shipping channel. Thirty-day means of model output are calculated to remove the dominant tidal signals and allow examination of the subtidal dynamics. Infrastructure is found to steepen the mean axial salinity gradient $ \partial \overline{s}/ dx $ by ~40% when there is low freshwater input but flatten $ \partial \overline{s}/ dx $ by ~25% under more typical conditions during moderate freshwater inflow to the estuary. Deepening of the shipping channel also increases the magnitude of the residual Eulerian circulation, allowing for larger up-estuary salt transport. Local bathymetry and morphology are important. Some regions within the estuary show little change in residual circulation due to infrastructure. In others, the residual circulation can vary by a factor of 4 or more. Major features of the circulation and changes due to infrastructure can be partially accounted for with linear theory.  相似文献   

6.
Kinetic energy exchange equations (Saltzman 1957) in wave number domain are partitioned into standing, transient and standing-transient components following Murakami (1978, 1981). These components are computed for the 1991 summer monsoon using dailyu andv grid point data at 2.5° latitude-longitude interval between the equator and 40°N at 200 hPa and 850 hPa levels for the period June through August. The data are obtained from NCMRWF, New Delhi. The study shows that at 200 hPa wave number 1 over Region 3 (30°N to 40°N), wave number 2 over Region 2 (15°N to 30°N) and wave number 3 over Region 1 (equator to 15°N) dominate the spectrum of transport of momentum and wave to zonal mean flow interaction. Wave number 1 over Region 1 and Region 3 and wave number 2 over Region 2 are the major sources of kinetic energy to other waves via wave-to-wave interaction. At 850 hPa wave number 1 over Region 3 has maximum contribution in the spectrum of transport of momentum and kinetic energy and more than 90% of its contribution is from the standing component. This indicates that standing wave number 1 over Region 3 plays a very important role in the dynamics of monsoon circulation of the lower troposphere. The study further shows that although the circulation patterns at 200 hPa and 850 hPa levels are opposite in character, a number of energy processes exhibit a similar character at these levels. For example, (i) transport of momentum by most of the waves is northward, (ii) small scale eddies intensify northward, (iii) eddies are sources of kinetic energy to zonal mean flow over Region 1 and (iv) standing eddies are sources of kinetic energy to transient eddies. Besides the above similarities some contrasting energy processes are also observed. Over Region 2 and Region 3 standing and transient eddies are sources of kinetic energy to zonal mean flow at 200 hPa, while at 850 hPa the direction of exchange of kinetic energy is opposite i.e. zonal mean flow is a source of kinetic energy to standing as well as transient eddies. L(n) interaction indicates that at 200 hPa waves over R2 maintain waves over R1, while at 850 hPa waves over R1 maintain waves over R2. It has been found that the north-south gradient of zonal mean of zonal wind is the deciding factor of wave to zonal mean flow interaction.  相似文献   

7.
Tidal marsh functions are driven by interactions between tides, landscape morphology, and emergent vegetation. Less often considered are the diurnal pattern of tide extremes and seasonal variation of solar insolation in the mix of tidal marsh driver interactions. This work demonstrates how high-frequency hydroperiod and water temperature variability emerges from disparate timescale interactions between tidal marsh morphology, tidal harmonics, and meteorology in the San Francisco Estuary. We compare the tidal and residual flow and temperature response of neighboring tidal sloughs, one possessing natural tidal marsh morphology, and one that is modified for water control. We show that the natural tidal marsh is tuned to lunar phase and produces tidal and fortnight water temperature variability through interacting tide, meteorology, and geomorphic linkages. In contrast, temperature variability is dampened in the modified slough where overbank marsh plain connection is severed by levees. Despite geomorphic differences, a key finding is that both sloughs are heat sinks in summer by latent heat flux-driven residual upstream water advection and sensible and long-wave heat transfer. The precession of a 335-year tidal harmonic assures that these dynamics will shift in the future. Water temperature regulation appears to be a key function of natural tidal sloughs that depends critically on geomorphic mediation. We investigate approaches to untangling the relative influence of sun versus tide on residual water and temperature transport as a function of system morphology. The findings of this study likely have ecological consequences and suggest physical process metrics for tidal marsh restoration performance.  相似文献   

8.
This research investigates the dynamics of the axial tidal flow and residual circulation at the lower Guadiana Estuary, south Portugal, a narrow mesotidal estuary with low freshwater inputs. Current data were collected near the deepest part of the channel for 21 months and across the channel during two (spring and neap) tidal cycles. Results indicate that at the deep channel, depth-averaged currents are stronger and longer during the ebb at spring and during the flood at neap, resulting in opposite water transport directions at a fortnightly time scale. The net water transport across the entire channel is up-estuary at spring and down-estuary at neap, i.e., opposite to the one at the deep channel. At spring tide, when the estuary is considered to be well mixed, the observed pattern of circulation (outflow in the deep channel, inflow over the shoals) results from the combination of the Stokes transport and compensating return flow, which varies laterally with the bathymetry. At neap tide (in particular for those of lowest amplitude each month), inflows at the deep channel are consistently associated with the development of gravitational circulation. Comparisons with previous studies suggest that the baroclinic pressure gradient (rather than internal tidal asymmetries) is the main driver of the residual water transport. Our observations also indicate that the flushing out of the water accumulated up-estuary (at spring) may also produce strong unidirectional barotropic outflow across the entire channel around neap tide.  相似文献   

9.
The upper ocean has complex and variable temperature stratification, and the surface layers in the northwest Bay of Bengal in winter indicate the presence of transient thermal inversions that wane with the advancement of the season. During winter, the sea surface loses heat and the surface waters of the coastal regions of the east coast of India are fairly stratified with the residual freshwater atop from the preceding southwest monsoonal discharge. The vertical stability favors the formation and sustenance of temperature inversions. To investigate the mechanism and the influence of ubiquitous internal waves that thrive on stability, a three-dimensional Princeton Ocean Model is configured for the east coast of India and is applied to study the process in the surface layers in association with the internal waves. The model domain constitutes a variable curvilinear grid, and the input fields comprise bathymetry, initial temperature and salinity, wind stress, air-sea heat fluxes and tidal forcing at the open boundaries. The numerical experiments demonstrate that vertical stability alone cannot cause, support or augment the internal wave oscillations, if the stratification is attributed to salinity only. Internal waves may therefore be perceived in stable layers, essentially from temperature-induced stratification. Despite stratification and enough vertical density gradient in the upper ocean, the conditions may not suit for the occurrence of internal waves due to thermal diffusive processes that overpower the salinity gradients. The vertical spreading of heat due to double diffusion is believed to be transparent to tidal forcing as the generation of internal waves is subdued even under density stratification. The model simulations indicate that the horizontal convergence/divergence motions, required for the manifestation of internal waves at the surface are inhibited in the presence of temperature inversion. The available SAR imageries in winter endorse the model simulations to this effect.  相似文献   

10.
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11.
Linkages among density, flow, and bathymetry gradients were explored at the entrance to the Chesapeake Bay with underway measurements of density and flow profiles. Four tidal cycles were sampled along a transect that crossed the bay entrance during cruises in April–May of 1997 and in July of 1997. The April–May cruise coincided with neap tides, while the July cruise occurred during spring tides. The bathymetry of the bay entrance transect featured a broad Chesapeake Channel, 8 km wide and 17 m deep, and a narrow North Channel, 2 km wide and 14 m deep. The two channels were separated by an area with typical depths of 7 m. Linkages among flows, bathymetry, and water density were best established over the North Channel during both cruises. Over this channel, greatest convergence rates alternated from the left (looking into the estuary) slope of the channel during ebb to the right slope during flood as a result of the coupling between bathymetry and tidal flow through bottom friction. These convergences were linked to the strongest transverse shears in the along-estuary tidal flow and to the appearance of salinity fronts, most markedly during ebb periods. In the wide channel, the Chesapeake Channel, frontogenesis mechanisms over the northern slope of the channel were similar to those in the North Channel only in July, when buoyancy was relatively weak and tidal forcing was relatively strong. In April–May, when buoyancy was relatively large and tidal forcing was relatively weak, the recurrence of fronts over the same northern slope of the Chesapeake Channel was independent of the tidal phase. The distinct frontogenesis in the Chesapeake Channel during the increased buoyancy period was attributed to a strong pycnocline that insulated the surface tidal flow from the effects of bottom friction, which tends to decrease the strength of the tidal flow over relatively shallow areas.  相似文献   

12.
Touboul  Julien  Kharif  Christian 《Natural Hazards》2016,84(2):585-598
The kinematic and dynamic of steep two-dimensional focusing wave trains on a shearing flow in deep water are investigated analytically and numerically. In the absence of waves, the vorticity due to the vertical gradient of the horizontal current velocity is assumed constant. A linear kinematic model based on the spatio-temporal evolution of the frequency is derived, predicting the focusing distance and time of a chirped wave packet in the presence of constant vorticity. Furthermore, a linear model, based on a Fourier integral, is used to describe the evolution of the free surface on shearing current. To compute the fully nonlinear evolution of the wave group in the presence of vorticity, a new numerical model, based on a BIEM approach, is developed. On the basis of these different approaches, the role of constant vorticity on rogue wave occurrence is analysed. Two main results are obtained: (1) the linear behaviour expected in the presence of constant vorticity is significantly different from what is commonly expected in the presence of constant current and (2) the nonlinear effects are found to be of significant influence in the case at hand.  相似文献   

13.
Residual Exchange Flows in Subtropical Estuaries   总被引：1，自引：0，他引：1  
Observations of residual exchange flows at the entrance to four subtropical estuaries, two of them semiarid, indicate that these flows are mainly tidally driven, as they compare favorably with theoretical patterns of tidal residual flows. In every estuary examined, the tidal behavior was that of a standing or near-standing wave, i.e., tidal elevation and tidal currents were nearly in quadrature. The pattern of exchange flow that persisted at every estuary exhibited inflow in the channel and outflow over the shoals. Curiously, but also fortuitously, this pattern coincides with the exchange pattern driven by density gradients in other estuaries. The tidal stresses and the residual elevation slopes should be the dominant mechanisms that drive such tidal residual pattern because the Stokes transport mechanism is negligible for standing or near-standing waves. Time series measurements from the semiarid estuaries showed fortnightly modulation of the residual flow by tidal forcing in such a way that the strongest net exchange flows developed with the largest tidal distortions, i.e., during spring tides. This modulation is opposite to the modulation that typically results in temperate estuaries, where the strongest net exchange flows tend to develop during neap tides. The fortnightly modulation on tidal residual currents could be inferred from previous theoretical results because residual currents arise from tidal distortions but is made explicit in this study. The findings advanced herein should allow the drawing of generalities about exchange flow patterns in subtropical estuaries where residual flows are mainly driven by tides.  相似文献   

14.
Drying estuarine sandbanks experience only that part of the tidal cycle around high water. In a partially progressive tidal wave, this means that the duration of the flood over the sandbank will be greater than that of the ebb: a process of tidal rectification. In this paper, we propose the hypothesis that this leads to flood-directed tidal residual currents over drying sandbanks. The hypothesis is tested by observation and a 2-D hydrodynamical model in the Conwy estuary, a vertically well-mixed macrotidal estuary in North Wales. The observations include tide gauge data, tidal cycle boat surveys, and fixed current meter data. The data show weak flood-directed residual currents over a drying sandbank and much stronger ebb-directed residuals in the channels along the sides of the sandbank. The model reproduces the observations in the vicinity of the sandbank and shows that the tidal rectification mechanism produces a general pattern of residual circulation in the estuary, with flood-directed flow in the drying areas and ebb-directed flow in the channels. The flood residuals are most marked near the estuary mouth where the tidal wave is most progressive in nature. The main application of this mechanism is believed to be in the transport of bedload sediment. The flood-directed residuals will tend to move the tops of the sandbanks upstream.  相似文献   

15.
Observations of current velocity profiles and hydrography over and near a tall sill in a Chilean glacial fjord are used to illustrate the interactions between barotropic and baroclinic tides. The character of the barotropic tide in the glacial fjord is mixed with semidiurnal dominance. The ratio of sill height to water column depth at the study site is ca. 0.95. Water column stratification appeared only in the upper 5 m of the water column. Current velocity variations in the stratified surface layer were quite different to those underneath. Below the pycnocline, nonlinear interactions between semidiurnal M2 and diurnal K1 oscillations yielded a third-diurnal distortion MK3. Most interesting, surface layer currents were distortedby the superposition of semidiurnal M2 and sixthdiurnal M6 oscillations. The oscillations with M6 variability were identified, through wave superposition approaches, as reflected internal tides linked to M2 tidal variations. This was confirmed by theoretical results of stratified barotropic tidal flows interacting with abrupt bathymetry. Under the predominantly tidally mixed regime of the study area, the distortion to surface currents caused by the reflected wave was nearly symmetric during the large tidal ranges of the diurnal cycle. Nearly symmetric distortions resulted as the phase lag between incident and reflected wave-inducted currents was small (reflected currents developing a few minutes after maximum tidal flows). During the small ranges of the diurnal cycle, distortions were asymmetrical because of the relatively larger phase lags of the reflected signal (reflected currents developing tens of minutes after maximum tidal flows).  相似文献   

16.
A tidally-induced frontal system regularly develops in a small area off Newport News Point in the lower James River, one of the tributaries of the Chesapeake Bay. In conjunction with the front, a strong counter-clockwise eddy develops on the shoals flanking the northern side of the channel as the result of tidal interaction with the local bathymetry and estuarine stratification. A three-dimensional hydrodynamic model was applied to simulate the eddy evolution and front development, and to investigate time-varying circulation and material transport over a spring-neap tidal cycle. The model results show that variation of tidal range, together with periodic stratification-destratification of the estuary, has a significant impact on the residual circulation of the lower James River. The net surface water circulation, which takes the form of a counterclockwise eddy on the Hampton Flats, is stronger during neap tide than during spring tide. Strong stratification and weak flood current during neap tide results in a dominant ebb flow at the surface, which delays flooding within the channel and advances the phase lead of flood tide on shoals adjacent to the channel, thus increasing both period and intensity of the eddy. Front development in the area off Newport News Point provides a linkage between shoal surface water and channel bottom water, producing a strong net upriver bottom transport. The existence of the vertical transport mechanism was independently demonstrated through tracer experiments. The impact of the dynamics on larval dispersion was investigated through a series of model simulations of the movement of shellfish larvae over multiple tidal cycles following their release at selected bottom sites. These results show that eddy-induced horizontal circulation and vertical transport associated with the frontal system are important mechanisms for the retention of larval organisms in the James River.  相似文献   

17.
Analysis of current and salinity time series from early 1982 over the inner shelf near the Texas-Louisiana border indicates a rapid freshening of the coastal current waters in response to the discharge of the Mississippi River. Coherence between records is surprisingly poor, although cross-shelf advection of the salinity front by tidal advection appears to be an important local process.  相似文献   

18.
A local, one-dimensional, depth-dependent model is used in conjunction with a one-dimensional, longitudinal, hydrodynamical model to examine the mechanisms affecting yertical profiles of longitudinal residual current in the macrotidal (tidal range typically exceeds 4 m during spring tides), partly-mixed Tamar Estuary. Residual currents are simulated at a deep (15m) station in the lower reaches, which possesses a small tidal amplitude to depth ratio and a nonzero salinity throughout the tidal cycle, as well as at a shallow station in the upper reaches, which varies in depth from 1 m at low water, when salinity is zero, to 5 m at high water. A slow, up-estuary current dominates the residual circulation just beneath the high-water level at the deeper station. Further down the water column a down-estuary residual current develops which is the near-surface component of a two-layer gravitational circulation. The up-estuary component of this gravitational circulation occurs deeper in the column and extends to the bed at the deep station, whereas at the shallow station it is eventually dominated by a down-estuary current in the bottom 1 m. Simulated residual currents are fairly insensitive to estuary-bed slope and to observed depth variations in longitudinal density gradient. Residual current profiles of the observed form can only be generated by a longitudinal density gradient. The reduction in vertical eddy viscosity by water column stability due to stratification is an essential requirement for producing a strong gravitational circulation of the observed magnitude. Stratification at the shallow station is much higher during the ebb than during the flood and this asymmetry enhances the gravitational circulation in the upper reaches. The formation of residual flows at both stations is illustrated by showing time-series data over a tidal cycle for the simulated current profiles.  相似文献   

19.
An 18-yr chlorophyll time series for Narragansett Bay based on weekly samples collected without regard to tidal phase revealed a long-term decrease in mean annual levels. The potential influence of neglecting tidal phase in the sampling strategy on measured chlorophyll and its apparent long-term decrease is evaluated. A two year data set (1995–1996) is used as a proxy for the 1973–1990 time series together with an observed relationship between continuous measurements of in situ chlorophyll fluorescence and accompanying tidal phase. The deviations in chlorophyll from long-term means relative to deviations from mean low water at the time of sample collection are also analyzed, as is the potential influence of tidally-induced advective increases or dilution on measured chlorophyll levels. The analyses, which compare the magnitude and trends in tidally adjusted and directly measured chlorophyll, indicate that semi-diurnal intratidal variations in chlorophyll had little apparent effect on the long-term and seasonal patterns and trends deduced from the chlorophyll measurements. Neither tidal advection of the chlorophyll gradient, nor bloom magnitude appear to compromise application of the model. The 18-yr decline in annual mean chlorophyll observed between 1973–1990 in narragansett Bay is considered to be a bonafide portrayal of actual events, and not an artifact of failure to consider tidal phase in the weekly sampling strategy. The results also suggest that intratidal variability in chlorophyll does not seriously confound its meaningful measurement and usefulness as a representative index of phytoplankton abundance at the permanent monitoring station established for Narragansett Bay. Nonetheless, there is need to refine and to incorporate temporal sampling strategies more closely attuned to the tempo of growth, grazing, and nutrient recycling which accompany estuarine phytoplankton dynamics.  相似文献   

20.
南海东北部受黑潮入侵、季风等动力因素的影响,背景剪切流场复杂,涡旋众多,水体垂向层结季节性变化明显,同时又因吕宋海峡的复杂底地形和强潮流的影响,内潮、内孤立波现象显著。但是,以往关于内潮、内孤立波的研究很少考虑到背景剪切流场和涡旋对其影响,因而难以揭示内波的生成、传播和演变规律。主要概述了南海东北部的剪切流、涡旋和内波等多种中尺度物理现象及其之间的相互作用的研究进展,进而提出未来关于南海东北部剪切流场对内波生成、传播和演变影响研究中的一些问题和研究思路。  相似文献   

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