Near-inertial Oscillations Research Articles

Currents observed north of Oahu during the first five years of HOT

Shipboard ADCP sections from 30 Hawaii Ocean Time-series cruises over a 5-year period show a mean flow 15 degrees north of west, reaching a maximum of 17 cm s − 1 about 50 km north of Oahu. This North Hawaiian Ridge Current is highly variable from cruise to cruise and from year to year. It was generally strong in the sections 1988–1991 and weak or absent in 1992 and 1993. There is no prominent annual cycle. Unresolved short-period oscillations are so energetic as to obscure the mean current in individual sections. The 5-year mean is highly significant, however, at five times the estimated standard error. Currents also were measured with a shipboard ADCP during the nominally 3-day occupations of the HOT Station ALOHA. The mean over 32 cruises was near 6 cm s −1 in magnitude at all depths from 20 to 200 m, but the median was near zero. Hence, the northern boundary of the North Hawaiian Ridge Current was usually south of ALOHA. The inter-cruise standard deviation of velocity varied from 22 cm s −1 at 20 m to 11 cm s −1 at 200 m for the zonal component, and 13 cm s −1 to 8 cm s − 1 for the meridional component. Intra-cruise variability during the 17 cruises with the best sampling in depth and time was dominated by a linear trend, near-inertial oscillations, and diurnal and semidiurnal tides. Each of these four elements had an rms value near 6 cm s −1 at 20 m, decreasing to 2–4 cm s −1 at 250 m. The vertical shear field was dominated by the diurnal tide and near-inertial oscillations; the semidiurnal tide and the trend had larger vertical scales of variability and consequently made smaller contributions to the shear.

Warm Pool Physics in a Coupled GCM

The physics of the Indo-Pacific warm pool are investigated using a coupled ocean atmosphere general circulation model. The model, developed at the Max-Planck-Institut fur Meteorologie, Hamburg, does not employ a flux correction and is used with atmospheres at T42 and T21 resolution. The simulations are compared with observations, and the model's mean and seasonal heat budgets and physics in the Indo-Pacific warm pool region are explored for the T42 resolution run. Despite the simulation of a split intertropical convergence zone, and of a cold tongue that extends too far to the west, simulated warm pool temperatures are consistent with observations at T42 resolution, while the nl resolution yields a cold bias of 1 K. At T42 resolution the seasonal migration of the warm pool is reproduced reasonably well, as are the surface heat fluxes, winds, and clouds. However, simulated precipitation is too small compared to observations, implying that the surface density flux is dominated by fluxes of heat. In the Pacific portion of the warm pool, the average net heat gain of the ocean amounts to 30-40 W m(-2). In the northern branch, this heat gain is balanced by vertical advection, while in the southern branch, zonal, meridional, and vertical advection cool the ocean at approximately equal rates. At the equator, the surface heat flux is balanced by zonal and vertical advection and vertical mixing. The Indonesian and Indian Ocean portions of the warm pool receive from the atmosphere 30 and 50 W m(-2), respectively, and this flux is balanced by vertical advection. The cooling due to vertical advection stems from numerical diffusion associated with the upstream scheme, the coarse vertical resolution of the ocean model, and near-inertial oscillations forced by high-frequency atmospheric variability. The seasonal migration of the warm pool is largely a result of the seasonal variability of the net surface heat flux; horizontal and vertical advections are of secondary importance and increase the seasonal range of surface temperature slightly everywhere in the warm pool, with the exception of its southern branch. There, advection reduces the effect of the surface flux. The seasonal variability of the surface heat flux in turn is mainly determined by the shortwave radiation, but evaporation modifies the signal significantly. The annual cycles of reduction of solar radiation due to clouds and SST evolve independently from each other in the Pacific portion of the warm pool; that is, clouds have little impact on SST. In the Indian Ocean, however, clouds limit the maximum SST attained during the annual cycle. In the western Pacific and Indonesian portion of the warm pool, penetrative shortwave radiation leads to convective mixing by heating deeper levels at a greater rate than the surface, which experiences heat losses due to turbulent and longwave heat fluxes. In the deeper levels, there is no mechanism to balance the heating due to penetrative radiation, except convection and its attendant mixing. In the Indian Ocean, however, the resulting vertical heating profile due to the surface fluxes decreases monotonically with depth and does not support convective mixing. Concurrently, the warm pool is shallower in the Indian Ocean compared with the western Pacific, indicating that convective mixing due to penetrative radiation is important in maintaining the vertical structure of the Pacific portion of the warm pool.

Near-Inertial Wave Propagation into the Pycnocline during Ocean Storms: Observations and Model Comparison

Abstract Observations of near-inertial oscillations collected during the Ocean Storms Experiment in the northeast Pacific Ocean are compared with results from a linear, numerical model on a β plane, developed by Zervakis and Levine. A slab mixed layer model, forced by the observed wind time series, is used to identify three isolated events of local generation in October, January, and March for detailed analysis. Synoptic storm track maps are used to estimate the initial horizontal wavenumber of the mixed layer currents that are used as initial conditions to the modal. A comparison of the modal with the observed currents reveals some differences and similarities. Overall the January and March events are better represented by the model than the October event. The timescale of the initiation of vertical propagation of energy from the mixed layer occurs almost immediately in October rather than after 8 days in January and March-this difference cannot be explained by the model. The observed vertical and tempor...

A case study of Doppler-shifted inertial oscillations in the Norwegian Coastal Current

A case study with emphasis on near-inertial oscillations with frequency below the inertial frequency ƒ has been performed based on current measurements from 40 m depth in the Norwegian Coastal Current. The measurements show evidence of transient features with periods of 15 and 130 h while the local half-pendulum day is 13.9 h. The currents related to the 15-h period show an elliptic pattern in clockwise rotation, while the currents corresponding to the 130-h period show east-west oscillations. By a modal decomposition of the velocity field it has been shown that energetically, the barotropic and first baroclinic mode dominate the field. Based on data analysis and application of a reduced gravity model with an associated uniform geostrophic current, the oscillations are considered with regard to the Doppler shift wave-current interaction mechanism. Perturbations on the basic current as plane waves propagating in the current direction, have been considered. The dispersion relationships obtained show two near-inertial and one low frequency wavemode of the Rossby type related to the sloping interface. The Doppler shift affects the dispersion properties for upstream and downstream propagating waves in different ways. For the downstream propagating waves the dispersion relationships are slightly modified compared with propagation in stationary mediums. For the upstream propagating waves the Doppler shift distorts the dispersion properties, resulting in a dispersion relation with both positive and negative group velocities and thus zero group speed for a certain wavelength with corresponding frequencies below ƒ for that waveband. In accordance with observations, the model shows zero group speed for a wavelength of 45 km with a corresponding frequency of 0.9 ƒ. The velocity components for near-inertial wavemodes show velocities in anti-cyclonic and cyclonic rotations. The pronounced observed oscillations with frequency 0.9 ƒ are explained as a result of the different dispersion properties for upstream and downstream propagating waves, where upstream propagating waves in the waveband of zero group speed are amplified due to accumulation of energy. Also the long periodic velocity field corresponds reasonably well with observations where the model shows eas-twest oscillations for symmetric disturbances.

Direct Observations of the Ekman Balance at 10°N in the Pacific

Abstract The wind-driven transport and velocity structure are estimated from direct current measurements and geostrophic shears along a transpacific section at 10°N. The total velocity field is dominated by the North Equatorial Current and its eddies, while the shear features near-inertial oscillations and a wind-driven current spiral. The section-averaged cross-track ageostrophic shear can be approximated as a slab layer 30–40 m thick with a velocity of 0.05 m s−1 overlying a sheared layer in which the velocity goes to zero below 80 m. The resulting zonally integrated ageostrophic mass transport is 62 (±10) × 109 kg s−1 northward, similar to the estimate of 52 (±10) × 109 kg s−1 predicted by the Ekman balance using winds measured from the ship. Climatological winds yield similar transport. The zonally averaged velocity relative to the top of the thermocline forms a clockwise spiral, decaying with depth. As has been found at higher latitudes, this mean wind-driven spiral results from the diurnal cycling o...

Near-inertial current oscillations off the mid-east coast of Korea

Near-inertial current oscillations were observed at depths of 620 and 790 m at a current meter mooring station in water 840 m deep off the mid-east coast of Korea. The oscillatory inertial motions with an amplitude of up to 8 cm/s existed during 73 days between August 27 and November 7 of 1986 with a large temporal variation in speed. Spectral analysis of the data was conducted and the result showed that the most dominant signal in the records was clockwise rotation whose frequency was close to the inertial frequency. The peak frequency of the near-inertial oscillations was slightly greater than the local inertial frequency ( f = 0.0511 cph) by 2–5%. These clockwise oscillations had the mean amplitude of about 2.5 cm/s, reaching an instantaneous maximum of 7.5 cm/s. The mean ratio of the semi-major axis to the semi-minor one was found to be about 1.2 and the mean direction of the major axis was nearly parallel to the isobaths around the mooring station. High near-inertial energy parallel continuously during the first 15 days and then appeared in the form of group wave packets lasting several days during the remaining period. Three prominent events of wave packets were more energetic at the deeper depth. The phase difference between the two horizontal near-inertial current vectors changed largely with time, but it remained relatively constant during each of the three energetic events, changing its sign from one event to another. The horizontal near-inertial current vector rotated clockwise with increasing depth during one event, but counterclockwise during the other two events. The corresponding phase propagation was upward with a phase speed of 2.5 cm/s for the clockwise polarization with depth, but downward with 1.8 and 1.1 cm/s for the counterclockwise rotating events.

Vorticity measurement in a region of coastal ocean eddies by observation of near‐inertial oscillations

Continuously repeated hourly profiles of horizontal velocity have been collected with the Cartesian diver profiler on each side of a front. The location was roughly 100 km west of Point Buchon, California, in a region of the California current dominated by offshore coastal jets and strong eddies. Concurrent temperature records from the quasi‐Lagrangian diver indicate that one set of 21 profiles was taken on the southerly side of the front with cold surface water, while an immediately following set of 16 was collected on the other side. For the 3 day collection period, average advection of the diver was .2 m/s to the WNW. The horizontal kinetic energy levels of the profiles are the same for either side of the front, and are 4.2 J/m³, roughly twice the Garrett‐Munk model level, but these also include subinertial baroclinic velocities. Cross spectral analysis of the repeated profiles is used to determine the dominant intrinsic rotation frequencies of horizontal velocity for vertical wavenumbers between .00625 and .019 cpm. In the absence of local vorticity, this would be expected to be the local inertial frequency f. On the cold (southerly) side of the front, most rotation frequencies were above f, while most were below f on the warm side. The mean frequencies were 1.18f and 0.78f. The difference between these dominant intrinsic frequencies and f can be attributed to local relative vorticity. These vorticity estimates are consistent with the westward advection of the diver being due to positions first south of then north of the central region of an offshore jet.

Observations of Internal and Near-Inertial Oscillations at Drake Passage

The kinetic energy levels Of horizontal oscillations near and above the inertial frequency are described based on 84 current records made during 5 years at Drake Passage. Instrument depths ranged from 280 to 3600 m. Moorings spanned the passage, though the best depth and time coverage is for a central location. For each variance spectrum of horizontal current, the frequency band above the local inertial and semidiurnal tidal peaks was fitted to the Garrett and Munk internal wave spectrum in the form of The fit is determined by the log-log decay (p) of energy density (NE0) with frequency (ω), where E0 is the energy density at 1 cph (ω0) normalized by stability frequency N. Although these parameters were generally near the “universal” values, there was considerable spatial variability—more than the year-to-year variability. Values of p were found to increase with depth; values of E0 were larger in the northern passage and at the continental slopes suggesting source regions for wave energy. The fitted relation was integrated between local inertial and the appropriate stability frequency to examine the total kinetic energy of horizontal motions in the internal wave field. This internal wave energy was normalized by stability frequency and plotted versus depth for three distinct bathymetric regimes. Both energy levels and vertical distributions differ, with greater energy and more depth variation over the rugged bathymetry of the northern passage. The prominent inertial peak was examined on the basis of its frequency, peak height and peak width at 50% of peak values. Values of these parameters fell within the range of values obtained by Fu for the North Atlantic. The time and space variations of inertial oscillations was described based mainly on band-passed records centered at local inertial frequencies. The phase coherence for inertial oscillations decreases with increasing vertical instrument separation with a vertical coherence scale somewhere in the order of 700–1500 m. For horizontal separations of less than 20 km the phase coherence is significant in all cases, and for separations greater than 60 km the inertial waves are not coherent. The partitioning of kinetic energy of horizontal oscillations between 2 hours and 2 days is examined. We earlier estimated that 49% of this energy is found in narrow pass-bands containing the dominant short period tides. Here we find that additionally 10 and 27% of that fluctuation kinetic energy is accounted for by inertial oscillations and internal waves, respectively.

Over-reflection of internal-inertial waves from the mixed layer

Near-inertial oscillations associated with downward energy propagation are commonly observed in the upper ocean. Stern (1977) has suggested that these observations may be internal-inertial waves over-reflected from the shear zone at the base of the mixed layer. In this paper we develop a criterion for over-reflection as a function of wavenumber and frequency for a class of shear flows in the mixed layer. By examining the vertical profile of the vertical wave action flux we demonstrate that the source of the over-reflection is the shear at the base of the mixed layer, which is maintained by the wind-induced turbulent Reynolds stress, here parametrized as a body force. The relationship between over-reflection and the wave-induced Lagrangian-mean flow is determined. We also determine a criterion for unstable waves, and show that these are contiguous in wavenumber-frequency space with points of resonant over-reflection. However, the growth rates of these unstable waves are quite small, and in practice unstable waves will be indistinguishable from waves generated by over-reflection.

Transient coastal currents and inertio-gravity waves

Solutions are found for the flow field generated near a coast by the sudden application of an alongshore wind stress. Alongshore geostrophy is not assumed, and so the near-inertial oscillations are retained. The model is alongshore independent, hydrostatic, continuously stratified, has a well-mixed surface layer, and admits vertical diffusion in the anterior. The asymptotic form of the solution for large time and fixed position shows that the inertial oscillations decay like t −1/2, leaving behind an accelerating coastally trapped baroclinic jet and a steady upwelling circulation. Calculations for up to 5 days show coastal inhibition of the inertial oscillations within the Rossby radius, outside which they increase in large parts of the ocean interior. The increase is due in part to the downward leakage of coastal inertial energy from the mixed layer and in part to the offshore leakage near the bottom. Several weeks may be needed to obtain significant decay everywhere in the flow field. Estimates of certain properties of the inertial oscillations in the model are in fair agreement with observations.

An experimental study of critical layers

Laboratory experiments have been made to investigate the development of internal gravity waves as they approach a critical layer where their phase speed is equal to that of the mean flow. The waves are produced in the accelerating flow of a stratified fluid in a long tilted tube in which the lower boundary has sinusoidal corrugations. As found in earlier experiments, the waves are not observed to propagate beyond the critical layer. Near the layer their amplitude increases, with the development of regions in which the fluid is gravitationally unstable. Kelvin-Helmholtz instability is not observed, perhaps because of viscous effects.A model is devised which describes the weakly nonlinear development of the waves. This is solved numerically. The results compare favourably with the experiments until gravitational instability is imminent. The numerical model is used to estimate both the second order Eulerian ‘jet’, which develops below the critical layer, and the Stokes drift. In the cases examined, the maximum drift is stronger than the jet and opposite in direction. The numerical model predicts the regions of wave breaking quite well.Internal gravity waves in the ocean may be modified by transient critical layers, for example those caused by vertically-propagating near-inertial oscillations.

Near-Inertial Internal Waves Observed on the Outer Shelf in the Middle Atlantic Bight in the Wake of Hurricane Belle

On 10 August 1976 Hurricane Belle passed rapidly over the highly stratified shelf of the New York Bight. Records from Aanderaa current-meter moorings show that the response to the hurricane depended strongly on bathymetry. At deeper stations (∼70 m depth), intense, first-mode, internal near-inertial oscillations were generated at frequencies ∼1% less than the local inertial frequency. At shallower stations (∼50 m depth), only weak, heavily damped second-mode oscillations were observed in the current records, with no corresponding inertial signals in temperature. In the Hudson Shelf Valley, inertial motion occurred only near the surface. This was probably due to topographic effects. The divergence and curl of the wind stress contributed equally to the forcing. The response at the deeper stations is consistent with Geisler's (1970) theory for the open ocean in which a hurricane leaves a wake of internal-inertial oscillations if it travels faster than the internal phase speed and if its horizontal scale is comparable to the internal Rossby radius. The observed frequency shifts (subinertial motion) and observed relative vorticity are consistent with Mooer's (1975a) theory that relative and planetary vorticities combine to give an effective inertial frequency. Here it is suggested that lack of strong inertial motion at the shallower stations is due to a lack of resonance and the likelihood that frictional effects are more important in shallower water, resulting in a more heavily damped response.

The Coastal Boundary Layer in Lake Ontario: Part II. The Summer-Fall Regime

Field observations on coastal currents near Oshawa on the north shore of Lake Ontario during summer and fall are described. The technique of observations (flag-station chain) has been given in Part I, together with a description of the dynamic regime during the spring period. Observations during summer and fall show that the kinetic energy level of water movements increases considerably from spring to summer and again from summer to fall. Much as in the spring, a nearshore band (some 10 km wide) becomes a unique kind of “boundary layer” in which mid-lake motions adjust to the presence of the shares. In this shore zone, currents are shore-parallel and relatively persistent. During summer and fall, mid-lake motions are wave-like, consisting mainly of near-inertial oscillations. Within the shore zone, the current-like motions are associated with thermocline displacements, upward if the current flows to the east, downward if it flows to the west, so that the pressure gradients caused by the non-uniform density distribution are at least partly balanced by the Coriolis force. The lakewide flow pattern is mainly determined by a few periods of relatively strong winds, which can reverse an opposing current and lead to a complete man exchange between the shore zone and mid-lake as a consequence of large thermocline movements. Advection of momentum from the outer zone into the coastal zone also plays a significant role in maintaining coastal currents.

Outfall simulation experiment in Lake Ontario

Existing data on the diffusion of pollutants in the Great Lakes originate from experiments carried out with near-surface release of tracers. Sewage outfalls (and some other discharge pipes) are on the other hand usually located at lake bottom some distance off-shore, and the question arises how effluents discharged at depth diffuse, when their density is equal to lake water density. Of particular interest is the case when a thermocline is interposed between outfall and free surface. Evidence on current structure in the shore zone indicates that bottom currents under such conditions are very slow (mostly below 5 cm s −1) and are mostly due to near-inertial oscillations, so that the direction rotates regularly. In order to obtain some experimental evidence on diffusion under such conditions, rhodamine B dye was released in Lake Ontario, near Oshawa, in water 20 m deep, 3 m from the bottom, from a 10 m long diffuser pipe arranged horizontally. The intention was to carry out surveys of the resulting plume for prolonged periods (order 10 days). The experimental difficulties of maintaining steady dye flow so far below the surface proved, however, unexpectedly great, so that only a few surveys could be carried out in 1970. The observed dye concentration profiles show some striking peculiarities. Vertical sections across the dye plume were regular (somewhat similar to Gaussian profiles). Horizontal distributions were by contrast many-peaked, mostly very much wider than comparable dye plumes near the surface, and on some occasions consisted only of discrete patches of dye, hundreds of metres distant. These features of diffusion may be attributed to the wave-like character of flow at low residual velocity.