Barotropic and Baroclinic Tides in the Central North Pacific Ocean Determined from Long-Range Reciprocal Acoustic Transmissions
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Travel times of reciprocal 1000-km range acoustic transmissions, determined from the 1987 Reciprocal Tomography Experiment, are used to study barotropic tidal currents and a large-scale, coherent baroclinic tide in the central North Pacific Ocean. The difference in reciprocal travel times determines the tidal currents, while the sum of reciprocal travel times determines the baroclinic tide displacement of isotachs (or equivalently, isotherms). The barotropic tidal current accounts for 90% of the observed differential travel time variance. The measured harmonic constants of the eight major tidal constituents of the barotropic tide and the constants determined from current meter measurements agree well with the empirical–numerical tidal models of Schwiderski and Cartwright et al. The amplitudes and phases of the first-mode baroclinic tide determined from sum travel times agree with those determined from moored thermistors and current meters. The baroclinic tidal signals are consistent with a large-scale, phase-locked internal tide, which apparently has propagated northward over 2000 km from the Hawaiian Ridge. The amplitude, phase, and polarization of the first-mode M2 baroclinic tidal displacement and current are consistent with a northward propagating internal tide. The ratio of baroclinic energy to barotropic energy determined using the range-averaging acoustic transmissions is about 8%, while a ratio of 26% was determined from the point measurements. The large-scale, internal tide energy flux, presumed northward, is estimated to be about 180 W m−1.Keywords:
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Barotropic fluid
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Abstract A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M2) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01°–horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model’s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M2 barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and south of Niihau and Kauai. The majority of the baroclinic energy (1.7 GW) is radiated out of the model domain, while 0.45 GW is dissipated close to the generation regions. The modeled baroclinic dissipation within the 1000-m isobath for the Kaena Ridge agrees to within a factor of 2 with the area-weighted dissipation from 313 microstructure profiles. Topographic resolution is important, with the present 0.01° resolution model resulting in 20% more barotropic-to-baroclinic conversion compared to when the same analysis is performed on a 4-km resolution simulation. A simple extrapolation of these results to the entire Hawaiian Ridge is in qualitative agreement with recent estimates based on satellite altimetry data.
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This thesis deals with the internal tide in the deep ocean, which is generated by the barotropic tide flowing over the bottom topography. The energy flux from the barotropic tide to the internal-wa ...
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Abstract The Regional Oceanic Modeling System (ROMS) is applied in a nested configuration with realistic forcing to the Southern California Bight (SCB) to analyze the variability in semidiurnal internal wave generation and propagation. The SCB has a complex topography with supercritical slopes that generate linear internal waves at the forcing frequency. The model predicts the observed barotropic and baroclinic tides reasonably well, although the observed baroclinic tides feature slightly larger amplitudes. The strongest semidiurnal barotropic to baroclinic energy conversion occurs on a steep sill slope of the 1900-m-deep Santa Cruz Basin. This causes a forced, near-resonant, semidiurnal Poincaré wave that rotates clockwise in the basin and is of the first mode along the radial, azimuthal, and vertical directions. The associated tidal-mean, depth-integrated energy fluxes and isotherm oscillation amplitudes in the basin reach maximum values of about 5 kW m−1 and 100 m and are strongly modulated by the spring–neap cycle. Most energy is locally dissipated, and only 10% escapes the basin. The baroclinic energy in the remaining basins is orders of magnitudes smaller. High-resolution coastal models are important in locating overlooked mixing hotspots such as the Santa Cruz Basin. These mixing hotspots may be important for ocean mixing and the overturning circulation.
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Sill
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Barotropic and baroclinic tides were simulated for the Indonesia Seas using a primitive equation, terrain‐following coordinate model, the Regional Ocean Model System (ROMS) with four tidal constituents (M 2 , S 2 , K 1 , and O 1 ). The region's intricate topography as well as interactions between the Pacific and Indian Ocean tides within the Indonesian Seas resulted in complex barotropic and baroclinic tidal fields. The semidiurnal tides entered from both the Pacific and Indian oceans converging in Makassar Strait and the Ceram Sea with an amphidromic point forming in the Timor Sea. Diurnal tides were dominated by the Pacific Ocean tide. The model successfully replicated the observed tidal elevation fields as determined from TOPEX/POSEIDON crossovers with better performance for the semidiurnal constituents, RMS differences of 4–6 cm, than the diurnal constituents, RMS differences of 7–10 cm. A baroclinic response was apparent in the elevations, and the locations of the observed baroclinic elevation response in TOPEX/POSEIDON data agreed with that of the model. Velocities were baroclinic for all constituents with high spatial variability, particularly near sills and in straits. Extensive interactions occurred in the internal tidal fields: between a beam and its own reflections, between internal tides generated at different locations (i.e., different sides of a channel, or beams generated nearby), and between the barotropic and baroclinic tidal beams. Owing to propagation, even regions >100 km from sills showed significant vertical and horizontal variability resulting from internal tides. This resulted in extremely complex internal tidal fields with high variability, both spatially and temporally during a tidal cycle.
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Internal M2 tides near Hawaii are investigated with a two-dimensional, two-layer numerical model. It is seen that along the Hawaiian Ridge barotropic tidal energy is transformed into baroclinic internal tides that propagate in both northeast and southwest directions, as previously hypothesized. The internal tide for a certain beam is seen to propagate well over 1000 km. with an approximate decay scale of 1000 km. An asymmetric pattern in the baroclinic energy flux is observed to the north and south of the Hawaiian Ridge due to the spatially inhomogeneous baroclinic energy sources. The surface manifestation of the M2 internal tide in the model is compared with analysis results from TOPEX/Poseidon satellite altimetry. The baroclinic short-wave variation of a few centimeters amplitude, superposed on the barotropic surface amplitude, agrees well with the altimeter analyses. This, together with snapshots of the interfacial disturbance, allows the authors to sketch the propagation pattern of internal waves emanating northward and southward from the Hawaiian Ridge. Tidal current ellipses in the upper layer are dominated by the baroclinic internal tide with large spatial variability in their magnitude compared to the barotropic tidal ellipses. The M2 baroclinic energy flux is over 10 kW m−1 for the strongest energy beam propagating toward the northeast. Along the western Hawaiian Ridge about 3.8 GW of tidal power is converted from barotropic to baroclinic motion. The average northward or southward flux density for the first baroclinic mode is about 1.35 kW m−1 in the western Hawaiian Ridge. Also, if 2.7 kW m−1 (1.35 kW m−1 to each direction) is assumed for the whole 2000-km-long Hawaiian Ridge, a total of 5.4 GW is obtained. This value indicates that there is still a large uncertainty in the rate of barotropic to radiating baroclinic energy conversion along the Hawaiian Ridge.
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Tidal power
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The time-depth structure of the baroclinic diurnal tide has been examined with the aid of current and temperature profiles on the West Florida Continental Shelf. Of interest is the fact that the diurnal frequencies (e.g., the K1 and O1 tides) are near the “critical frequency” corresponding to the bottom slope and density stratification at the experimental location. The baroclinic semidiurnal tide was rather weak and most of the semidiurnal tidal energy was contained in the barotropic currents. This large ratio of barotropic-to-baroclinic, semidiurnal tidal energy is in agreement with the results obtained by Koblinsky (1979) from previous (current meter) measurements in the same area. In contrast, the baroclinic diurnal tide is quite strong and exhibits appreciable structural variations with time. The diurnal oscillations are predominantly of low vertical modal order, and there is no evidence of the concentrated “beams” of internal tidal energy which have sometimes been observed in other areas (e.g., Torgrimson and Hickey, 1979). However, the diurnal structure is modulated in a fashion which seems to be more complicated than can be accounted for by a simple “beating” effect between the K1 and O1 constituents. This relatively rapid modulation in amplitude and vertical structure indicates that there was present a significant transient component in either the generation or propagation of the internal diurnal tide. It is shown that variations in the vertical shear of low-frequency currents which occurred were in the correct sense and were potentially of sufficient amplitude to produce a subcritical bottom slope for the diurnal constituents during one period of the experiment. In this same period, there is clear evidence of near-bottom intensification of the diurnal oscillations. The data also show that the internal diurnal oscillations are propagating up-slope, away from the shelf break.
Barotropic fluid
Internal tide
Stratification (seeds)
Diurnal cycle
Current meter
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