Strait Flows Research Articles

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

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

Response of the southwestern Japan/East Sea to atmospheric pressure

Abstract The response of the southwestern Japan/East Sea (JES) to atmospheric pressure ( P atm ) and wind-stress ( τ ⇒ ) forcing is investigated by analyzing 2-year bottom pressure ( P bot ) data and coastal tide-gauge records. Coherence analyses between P bot data reveal that the response of the southwestern JES is nearly uniform at frequencies lower than 0.6 cycles per day (cpd). The Ulleung Basin (UB) average P bot ( P ¯ bot ) departs significantly from inverted-barometer (IB) response to the basin average P atm ( P ¯ atm ) at frequency bands from 0.2 to 0.7 cpd. The coherence between P ¯ atm and P ¯ bot has maximum value at 0.2 cpd. Multiple coherence analysis, applied with P ¯ atm and UB average τ ⇒ ( τ ⇒ ¯ ) as inputs and P ¯ bot as output, reveals that P ¯ atm is the most significant forcing, with a peak at frequencies between 0.2 and 0.3 cpd. A simple model [Garrett, 1983. Variable sea level and strait flows in the Mediterranean: a theoretical study of the response to meteorological forcing. Oceanologica Acta 6, 79–87; Lyu et al., 2002. Atmospheric pressure-forced subinertial variations in the transport through the Korea Strait. Geophysical Research Letters 29, 10.1029/2001GL014366] is used to investigate the limiting role of the three straits on the JES response to P atm . Coastal sea-level ( η ) data inside the JES as well as outside the straits demonstrate that the JES responds with a damped Helmholtz-like resonance. The resonance frequency predicted by this simple model is near the frequency of maximum coherence between P ¯ atm and η ¯ d , estimated from P ¯ bot by the hydrostatic approximation. Phase relations and response function gain between these variables confirm the applicability of this simple model to the JES for low-frequency bands below the Helmholtz-like resonance frequency. At higher frequencies, the response relaxes back toward IB, which suggests the mass field adjusts internally within the JES.

Local “Mean Field” Theory of Hydraulically Controlled Strait Flow

Abstract The free discharge of a layer of bottom water in a wide strait (e.g., the Denmark Strait) differs from the classical control problem because of the strong geostrophic turbulence. As a consequence, the cross-stream (x) variation of the time-averaged downstream velocity υ(x) is “underdetermined” and depends on more parameters than available conditions. To resolve this “degeneracy” the classical control condition is generalized; the result requires the discharge Q to be extremized with respect to the degeneracy parameters and with respect to the constraints. One of these constraints is that a branch point or a local stationary wave can be supported at some section of the long channel. By either maximizing Q or by requiring a stationary wave, useful approximations are obtained. Future work should consider the joint variational problem. Specific calculations are made for nonuniform potential vorticity in a rectangular channel and also for variable cross-stream bottom topography. In the latter case, th...

Topographic and boundary effects on steady and unsteady flow through straits

An overview of topographic and boundary effects in flows through straits is presented. The emphasis is on the various types of interaction of uniform and stratified flow with the topography of straits, especially the interaction with sills. The interaction of a single-layer flow in a channel with a sill (obstacle) depends on the Froude number of the flow and the relative height of the sill. Two classification schemes (one steady and one pseudo-steady) are given. For single-layer flow in straits where the effects of rotation are important a further parameter, the ratio of the channel width to a Rossby radius, is needed and a similar classification scheme as for the non-rotating flows can be obtained. Unsteady flows often result in bores (for single-layer flows) and internal bores (in stratified flows) and the behaviour of these is discussed. The flow of stratified flow past sills may generate lee waves, while the increased flow speeds at the sill may allow deep water to be drawn up and over the sill (aspiration). Features of the strait topography away from the sill, including slopes at the side of the channel and also downstream of the sill, channel curvature and channel widening have important effects on the character of the flow and on mixing.

Stratification and mixing in sea straits

Abstract Quantifying the transport through relatively narrow straits connecting basins with differing fluid properties is an important issue in both surface and abyssal flows. These flows range from large-scale flows in ocean straits connecting the major ocean basins down to small-scale regional flows, such as in estuaries and lagoons connected to the coastal ocean by relatively narrow straits or channels. The exchange rate through these straits is dependent on the topography, mixing and stratification, and for steady flows the interplay between these three effects can be quantified using the dimensionless parameter Gr T A 2 , where the Grashoff number Gr T and the aspect ratio A are evaluated within the strait. When Gr T A 2 is large, the exchange is determined by the well-known hydraulic solution. As Gr T A 2 becomes small the exchange rates progressively decrease below the hydraulic predictions toward values determined by diffusive control and characterized by strong vertical mixing. For systems with simple topographic controls, we show how the exchange rate in the strait may be linked to the external basin scale forcing characteristics. We review recent work on extending this picture to consider the effects of unsteadiness in two forms, time-dependent forcing in the basin due to seasonal effects and tidal forcing with an associated barotropic forcing in the strait itself.

On Stratified Shear Flow in Sea Straits of Arbitrary Cross Section

Equations and theorems governing the flow of an inviscid, incompressible, continuously‐stratified fluid in a gradually varying channel with an arbitrary cross section are developed. The stratification and longitudinal velocity are assumed to be uniform in the transverse direction, an assumption that is supported under the assumption of gradual topographic variations. Extended forms of Long's model and the Taylor–Goldstein equation are developed. Interestingly, the presence of topographic variation does not alter the necessary condition for instability (Richardson number ) nor the bounds on unstable eigenvalues (the semi‐circle theorem). The former can be proved using a new technique introduced herein. For the special case of homogeneous shear flow, generalized versions of the theorems of Rayleigh and Fjørtoft do depend on the form of the topography, though no general tendency toward stabilization or destabilization is apparent. Previous results on the bounds and enumeration of neutral modes are also extended. The results should be of use in the hydraulic interpretation of exchange flow in sea straits.

Instrument-induced linear flow resistance in Öresund

It is shown that the previously studied linear component of flow resistance in Öresund is an artefact due to underestimates of current velocities obtained from Aanderaa RCM7 and InterOcean S4 in strong current environments. The analysis uses two datasets. The first set contains current observations from RCM7 and S4 whereas the second set was taken with ADCP. Data were fitted to two different strait flow models. In the first model, the flow resistance is proportional to the flow rate squared. The second model has an additional term accounting for a possible linear relation between flow resistance and flow rate. From the models, it is shown that the linear term is significant in the first dataset but not in the second. It is then proved that the linear term is an artefact caused by non-linear current meter response that, unfortunately, was overlooked in some previous analyses.

Integral measurements of mass transport and heat content in the Strait of Gibraltar from acoustic transmissions

Three View the MathML sourceacoustic transceivers were deployed on either side of the eastern entrance of the Strait of Gibraltar during April–May 1996 to determine the feasibility of using acoustic methods to make routine, rapidly repeated, horizontally integrated measurements of flow and temperature in straits. Reciprocal transmissions between the transceivers were used to test the feasibility of using traditional ray differential travel times to monitor the component of flow along the acoustic paths. Transmissions directly across the Strait were used to test the feasibility of using horizontal arrival angle fluctuations and acoustic intensity scintillations to monitor the flow perpendicular to the acoustic path. The geometry was selected to provide ray paths that only sample the lower-layer Mediterranean water, so that the feasibility of monitoring the Mediterranean outflow using the various methods could be evaluated. The acoustic scintillation method did not yield useful current estimates, but the experimental parameters were not optimized for this approach. Since the low-frequency variability in log-amplitude was found to be highly correlated at receivers View the MathML source apart, it is possible that acoustic scintillation measurements using different receiver spacings and more rapid sampling might yield better results. The horizontal deflection method gave encouraging results at the time of neap tides, but less so during spring tides. For this approach, both theoretical estimates and measured phase differences between the horizontally separated receivers suggest that internal-wave-induced horizontal arrival angle fluctuations may fundamentally limit the precision with which arrival angles can be measured. Further work is needed to determine if a smaller horizontal spacing and higher signal-to-noise ratios would yield better results. Reciprocal travel time measurements diagonally across the Strait performed the best of the three methods, giving absolute flow estimates consistent with those derived from current-meter data. The fractional uncertainty variance for the lower layer tidal transport from a single tomographic path was estimated to be 0.017 (i.e. 98% of the a priori tidal transport variance was resolved). The spatial scales of the sub-tidal flow are thought to be significantly shorter than those of the tidal flow, however, which means that a more elaborate monitoring network is required to achieve the same performance for sub-tidal variability. Finally, sum travel times from the reciprocal transmissions were found to provide good measurements of the temperature and heatcontent in the lower layer.

Hydraulic Control of Three-Layer Exchange Flows: Application to the Bab al Mandab

Two-layer hydraulic models have proven to be extremely useful in understanding a number of exchange flows through straits, for example, the Strait of Gibraltar. There are, though, some strait flows that cannot be represented by a two-layer model. A very striking example is the Bab al Mandab. In the exchange between the Red Sea and the Gulf of Aden, three distinct water masses are observed in the strait during summer. The wintertime flow is, though, very different, with only two layers observed in the strait. In this paper a three-layer model of hydraulically controlled exchange flows is described The model can represent exchange flows that are controlled with respect to either the first or second (or both) internal modes. A number of different examples are discussed. The model is able to explain the qualitative features of the seasonal variation of the flow in the Bab al Mandab. In particular, the reversal of the direction of flow of the surface layer and the intrusion into the Red Sea of cold intermediate water from the Gulf of Aden during the summer months can be explained by upwelling in the Gulf of Aden forced by the southwest monsoon.

Resistance to Barotropic Tidal Flow in Straits by Baroclinic Wave Drag

Energy transfer from barotropic tides to baroclinic motions may take place at the ends of straits connecting stratified basins, implying generation of internal waves propagating into the basins. Different aspects of this have been described in the literature, including quantification of the energy transfer, studies of the resulting internal tides, and the relationship to diapycnal mixing in the basins. However, the accompanying resistance to oscillatory barotropic strait flows, by so-called baroclinic wave drag, has not been implemented earlier in models for strait flow. In the present paper a formulation for the instantaneous baroclinic wave drag force is suggested. This leads to a linear relationship between the strait flow and the synoptic sea level difference between the adjacent basins. The model is used to study tidal response in land-locked basins. The response is computed for three fjords where the resistance to the barotropic flow comes essentially from baroclinic wave drag. It is concluded that the strait-flow model handles baroclinic wave drag in a satisfactory way since the observed responses are well predicted by the model. Baroclinic wave drag should provide the major resistance to surface tides in the deep ocean, which indicates its importance in a wider context.

Topographic control of oceanic flows in deep passages and straits

Saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next, depending on the source of bottom water. Flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next. Examples in the Atlantic include flow in the Romanche Fracture Zone, the Vema Channel, the Ceara Abyssal Plain, the Anegada‐Jungfern passage, and the Discovery Gap, but there are many more. Theoretical predictions of volume flux using a method that requires only conductivity‐temperature‐depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and/or geostrophic estimates for seven cases. The ratio of prediction to volume flux estimate ranges from 1.0 to 2.7. Some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins. Theory of the influence of rotation on such critical flows is reviewed. Predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods.

Residence times in the upper Arctic Ocean

We use passive tracers in a one‐dimensional numerical model of the Arctic Ocean to determine the residence time in the mixed layer and the cold halocline. When run to a steady state, the model successfully reproduces most of the observed distribution of salinity and temperature in the Arctic above the Atlantic layer. Comparison of model‐calculated tritium concentrations with observational data also indicates that the transient properties are correct. An important component of the model is the implementation of a “shelf circulation” of about 0.8 Sv that simulates the observed production and interleaving of cold, highly saline shelf waters. We use the model to derive bulk residence times, which are about 25 years in the mixed layer and about 100 years in the halocline. These values are higher than those published in the literature. We explain how the model residence time is related to different tracer ages by generating age distributions with the model. It is shown that the weighted mean of these distributions corresponds well with published tracer age data. The model dynamics that correctly reproduce Arctic mixed‐layer and cold halocline vertical structure in salinity, temperature, density, and various tracers also appear to simulate the natural processes that filter out interannual fluctuations in the freshwater influx from runoff and Bering Strait flow. Under these conditions, the period of the variation must be over 30 years to get 50% of the signal through to Fram Strait. In accord with other investigations, this would suggest that short‐term fluctuations in Arctic river runoff are not the direct cause of freshwater anomalies in the northern North Atlantic.

On the origin of the Tsushima Warm Current

The origin of the Tsushima Warm Current (referred to as TWC) was investigated by surface drifter experiments and conductivity, temperature, and depth (CTD) surveys in the northeastern East China Sea (ECS) at three different times: July 1991, and April and November 1992. Trajectories of 10 satellite‐tracked drifters provide direct information for the first time on the surface flow patterns in each season, and CTD observations allow identification of warm and saline TWC waters. The results of the experiments argue against two historical concepts of TWC origins, i.e., (1) a northward flow transporting warm and saline water through the deep trough southwest of Kyushu toward the Korea Strait after separation from the Kuroshio and (2) a northeastward continuation of the Taiwan Current (TC) on the shelf of ECS after passing through the Taiwan Strait. A persistent northward current was found to exist both on the shelf west of the trough and on the western flank of the trough. The northward flow seems to bifurcate around the northwestern corner of the trough, splitting into a northward continuing flow on the shelf of 100–150 m and an eastward flow along the northern wall of the trough. The northward flow on the shelf, which might be the shore fringe of the Kuroshio, corresponds to the origin of TWC entering the Korea Strait. The eastward flow on the northern slope turned back to the south along the west coast of Kyushu and eventually joined the Kuroshio. This structure was accompanied by an anticyclonic eddy in the northern trough. The second concept, that of TWC originating from TC, contradicts the observed differences in physical properties between TWC and TC waters. The saline water in the Taiwan Strait flows out intermittently only during late winter‐early spring, and its salinity during other seasons is lower than that of the TWC water in the Korea Strait. Experiments also indicated a seasonal shift of drifter paths in the northeastern ECS and coexistence of cyclonic and anticyclonic eddies in the trough during the cold season.

Water masses and 200 m relative geostrophic circulation in the western Bransfield Strait region

Four hydrographic surveys to 200 db of 16–20 nm horizontal resolution carried out in the vicinity of western Bransfield Strait in the period November 1986 to March 1987 are used to provide a description of water masses and relative geostrophic circulation in the survey area. The water mass structure is characterized by two frontal structures with a complex vertical and horizontal interleaving of water from the Southern Drake Passage, Orkney Island shelf, Bellingshausen Sea and the shelf of Antarctic Peninsula. In the northwestern segment of the survey area, intrusions of Drake Passage water into the Bransfield are found beneath a frontal zone at a 100-m depth between Snow and Smith Islands. In the southeastern segment fresh, warm water from the Gerlache Straits flows over the cold salty water from the Antarctic Peninsula. A geostrophically balanced relative flow, termed the Bransfield Current, transports water along a second front between Bellingshausen Sea and Antarctic Peninsula from the Bellingshausen Sea and Gerlache Straits to the northeast. A cyclonic gyre exists around Low Island. In this current, the 200 db relative geostrophic mean current maximum is 8 cm s −1 and the geostrophic eddy kinetic energy is 18–55 cm 2 s −2, being strongest near the flow axis. Based on estimates of other mid-latitude ocean current areas with similar mean flows and eddy energies, the single particle diffusion coefficient is 1–3 × 10 7 cm 2 s −1. This circulation and diffusion could transport particles from the biologically rich Gerlache Strait to Livingston Island and the Drake Passage in 15–30 days, while particles can spread an average ± 50 km to either side of a track traced out by the mean flow.

Recent Progress in Strait Dynamics

A major change in the understanding of the dynamics of flow through straits has occurred over the last few years with the development of two layer hydraulic control models and their application to new observations in the Strait of Gibraltar. Observations of the flow through the Strait of Gibraltar suggest that there are at least two locations of critical flow where the Fronde number equals one, at the sill and at the narrowest section of the Strait, as required by hydraulic theory for maximal exchange through the Strait. Predictions of the inflow and outflow transports and the salinity difference between them from hydraulic theory are in agreement with the recent observations. The physical configuration of the Strait of Gibraltar does indeed appear to control the amount of exchange between the Mediterranean and Atlantic basins.New observational techniques utilized in the Gibraltar Experiment have dramatically portrayed the physical processes operating in the two layer exchange. Acoustic backscatter and Doppler current profiling have characterized hydraulic jumps and mixing, bore formation and propagation of internal solitary wave packets as no conventional measurements have done before. Time series current and, most importantly, salinity measurements on moorings have led to quantitative estimates of the inflow and outflow transports far superior to historical measurements. The new transports throught the Strait of Gibraltar are 30% smaller than historical estimates and, furthermore, tidal oscillations contribute nearly half of the total transport. Microstructure measurements have allowed regions of intense mixing in the Strait to be identified and the amount of mixing to be quantified. Finally the first conclusive evidence of the direct aspiration of deep water up and over a sill has been found in nearly all time series CTD stations at the Gibraltar sill.In this progress report and review, hydraulic control theory and Gibraltar observations are discussed. Then the effects of strait flows on the circulation in the adjoining seas and results from studies of other straits are described. Following a discussion on the generation and propagation of solitary waves in straits, a brief summary and list of outstanding problems are presented, with particular emphasis on the need to develop a fully time‐dependent hydraulic theory for the exchange through a strait.

Transports through Bering Strait: Annual and interannual variability

Reanalysis of the 1976–1977 mean monthly transport estimates for Bering Strait of Coachman and Aagaard (1981) shows a considerably stronger wind dependence than was calculated by Aagaard et al. (1985). We find (1) a long‐term mean transport of 0.8 Sv, (2) an annual transport cycle of amplitude 0.6 Sv, with the maximum in June, the minimum in February, and a secondary maximum in January probably associated with North Pacific blocking‐ridge activity, and (3) an interannual variability marked by a number of low‐flow years in the past two decades, including three of the four lowest‐transport winter periods during this century. In a new current time series from 1984 to 1985 we find anomalously strong and persistent northerly flow during a 2‐month period in which the current/wind correlation breaks down. This occurred during a prolonged period with southerly winds, and we believe the data point to an asymmetry in the dynamical response of the Bering Strait flow to major changes in wind direction.

Nonlinear integrable systems in the description of water flow in straits or channels

Nonlinear integrable systems in the description of water flow in straits or channels

Submersible Observations in the Straits of Florida: Geology and Bottom Currents

The submarine slopes that border the Straits of Florida off Miami and Bimini were traversed by the submersible Aluminaut, in August and September of 1967. The Bimini escarpment is characterized by a three-part zonation consisting of relatively strong northerly bottom currents in both deep and shallow zones, and an intermediate zone of low northerly bottom current velocities. From 538 m (the bottom of the traverse) to 222 m, there is a sloping, smooth, rock surface veneered with sand, ripple-marked by northward bottom currents of 50 cm/sec or more. The middle, low velocity zone, from 222 m to 76 m, exhibits a muddy slope of largely bank-derived material. The sediment surface exhibits tracks, burrows and mounds, which indicates that currents here are never as strong as in the rippled zones above and below. Observed current velocities in the middle zone were only 5 to 10 cm/sec. Above 76 m, a steep, vertical to overhanging cliff with large talus blocks at its base rises to a crest at 30 m. Currents in the upper zone are northward at 50 to 150 cm/sec. The inverted situation of higher energy bottom current conditions and associated sedimentary features and textures existing in the same area, but at a greater depth than low energy surface features and fine sediments, is of significance to the stratigraphic interpretation of ancient rocks. On the western side of the Straits, at the base of the Miami Terrace, is an elongate trough 825 m deep. The bottom here is characterized by ridges and mounds of muddy sand capped by thickets of living deep-water branching coral. The eastward-facing escarpment of the Miami Terrace exhibits ledge-like outcroppings of dark phosphatic limestone from depths of 719 m to the crest at 457 m where the traverse ended. An interesting finding of the dives in the Straits of Florida is the observation, based on both current measurements and sedimentary structures, that the bottom current on the western side of the Straits flows southward at observed velocities of 2 to 50 cm/sec. This southerly bottom flow is opposite to either the northerly Florida Current above or to the bottom current on the Bahama side of the Straits. The nature and orientation of the sedimentary structures, plus the combined observations of several Aluminaut dives in the same area, indicate that the southward bottom counterflow is persistent and not a temporary tidal reversal. An extensive sedimentary anticline in the west-central sector of the Straits may have been built by this bottom counter current bringing material from the north.

The Identification and Nomenclature of the Surface Water Masses in the Tasman Sea (Data to the End of 1954)

In this paper an examination of all available data on the hydrological characteristics of the Tasman Sea, prior to and including the year 1954, has permitted the identification and naming of eight surface water masses. Certain of their properties and general features of their season and region of occurrence and method of formation are summarized. Although little quantitative data are available some general features of the circulation of these water masses in the Tasman Sea are deduced from a study of their seasonal occurrence in relation to source regions. The Coral Sea water mass (chlorinity 19.60-19.70‰, temperature 20-26� C) flows from a source region in the north-west Coral Sea along the western side of the Tasman Sea and reaches maximum velocity off Sydney in October-December. The South Equatorial (chlorinity 19.50-19.60‰, temperature 24-26� C) also flows south along the western side of the Tasman Sea but reaches maximum velocity between February and March. These two water masses constitute the East Australian current. The Sub-Antarctic (chlorinity 19.15-19.30‰, temperature 10-14°C) is found at the surface in the south-eastern Tasman Sea between July and September. The Central Tasman (chlorinity 19.65-19.75‰, temperature 15-20‰C) flows to the west from its region of formation and generally flows north along the southern New South Wales coast in late winter. The South-west Tasman (chlorinity 19.50- 19.60‰, temperature 12-15°C) flows to the east in latitude 38�S. and curves south in a clockwise gyral off eastern Tasmania between October and December. The Xorth Bass Strait (chlorinity 19.66-19.75‰ temperature 12-17�C) flows from South Australia to the eastern approaches of Bass Strait. The East Central New Zealand (chlorinity 19.10-19.30‰, temperature 15-20°C) flows west through Cook Strait into the Tasman Sea in midsummer. The East and West Tasmanian (chlorinity 19.40- 19.50‰ temperature 10-14°C) form in midwinter in the southern part of Bass Strait and flow along the east and west coasts in the spring.