Abstract
New estimates of mode-1 M2 internal tide energy flux are computed from an extended Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter dataset that includes both the original and ...tandem tracks, improving spatial resolution over previous estimates from O(500 km) to O(250 km). Additionally, a new technique is developed that allows separate resolution of northward and southward components. Half-wavelength features previously seen in unseparated estimates are shown to be due to the superposition of northward and southward wave trains. The new technique and higher spatial resolution afford a new view of mode-1 M2 internal tides in the central North Pacific Ocean. As with all altimetric estimates, only the coherent or phase-locked signals are detectable owing to the long repeat period of the tracks. Emanating from specific generation sites consistent with predictions from numerical models, internal tidal beams 1) are as narrow as 200 km and 2) propagate a longer distance than previously observed. Two northward internal tidal beams radiating from the Hawaiian Ridge, previously obscured by coarse resolution and the southward Aleutian beam, are now seen to propagate more than 3500 km across the North Pacific Ocean to reach the Alaskan shelf. The internal tidal beams are much better resolved than in previous studies, resulting in better agreement with moored flux estimates.
Abstract
Towed shipboard and moored observations show internal gravity waves over a tall, supercritical submarine ridge that reaches to 1000 m below the ocean surface in the tropical western Pacific ...north of Palau. The lee-wave or topographic Froude number,
Nh
0
/
U
0
(where
N
is the buoyancy frequency,
h
0
the ridge height, and
U
0
the farfield velocity), ranged between 25 and 140. The waves were generated by a superposition of tidal and low-frequency flows and thus had two distinct energy sources with combined amplitudes of up to 0.2 m s
−1
. Local breaking of the waves led to enhanced rates of dissipation of turbulent kinetic energy reaching above 10
−6
W kg
−1
in the lee of the ridge near topography. Turbulence observations showed a stark contrast between conditions at spring and neap tide. During spring tide, when the tidal flow dominated, turbulence was approximately equally distributed around both sides of the ridge. During neap tide, when the mean flow dominated over tidal oscillations, turbulence was mostly observed on the downstream side of the ridge relative to the mean flow. The drag exerted by the ridge on the flow, estimated to
for individual ridge crossings, and the associated power loss, thus provide an energy sink both for the low-frequency ocean circulation and the tidal flow.
Abstract
In the South China Sea (SCS), 14 nonlinear internal waves are detected as they transit a synchronous array of 10 moorings spanning the waves’ generation site at Luzon Strait, through the ...deep basin, and onto the upper continental slope 560 km to the west. Their arrival time, speed, width, energy, amplitude, and number of trailing waves are monitored. Waves occur twice daily in a particular pattern where larger, narrower “A” waves alternate with wider, smaller “B” waves. Waves begin as broad internal tides close to Luzon Strait’s two ridges, steepening to O(3–10 km) wide in the deep basin and O(200–300 m) on the upper slope. Nearly all waves eventually develop wave trains, with larger–steeper waves developing them earlier and in greater numbers. The B waves in the deep basin begin at a mean speed of ≈5% greater than the linear mode-1 phase speed for semidiurnal internal waves (computed using climatological and in situ stratification). The A waves travel ≈5%–10% faster than B waves until they reach the continental slope, presumably because of their greater amplitude. On the upper continental slope, all waves speed up relative to linear values, but B waves now travel 8%–12% faster than A waves, in spite of being smaller. Solutions of the Taylor–Goldstein equation with observed currents demonstrate that the B waves’ faster speed is a result of modulation of the background currents by an energetic diurnal internal tide on the upper slope. Attempts to ascertain the phase of the barotropic tide at which the waves were generated yielded inconsistent results, possibly partly because of contamination at the easternmost mooring by eastward signals generated at Luzon Strait’s western ridge. These results present a coherent picture of the transbasin evolution of the waves but underscore the need to better understand their generation, the nature of their nonlinearity, and propagation through a time-variable background flow, which includes the internal tides.
Near-inertial waves (NIWs) are a special class of internal gravity waves with periods set by planetary rotation and latitude (e.g., at 30° latitude, one cycle per 24 hours). They are notable because ...they contain most of the observed shear in the ocean and around half the kinetic energy. As such, they have been demonstrated to mix the upper ocean and to have the potential to mix the deep ocean enough to be important for climate simulations. NIWs are principally generated as a result of a resonant coupling between upper-ocean currents and mid-latitude atmospheric cyclones. Here, we report on simulated NIWs in an eddy-resolving general circulation model that is forced by a realistic atmosphere, and we make comparisons to NIWs observed from moored and shipboard measurements of currents. The picture that emerges is that as much as 16% of NIW energy (which is season dependent) radiates out of the mixed layer and equatorward in the form of low-mode, long-lived internal gravity waves; they transmit energy thousands of kilometers from their regions of generation. The large amount of energy in near-inertial motions at a given site is a combination of a local response to wind forcing and waves that have traveled far from where they were generated.
Energy flux is a fundamental quantity for understanding internal wave generation, propagation, and dissipation. In this paper, the estimation of internal wave energy fluxes < u'p' > from ocean ...observations that may be sparse in either time or depth are considered. Sampling must be sufficient in depth to allow for the estimation of the internal wave-induced pressure anomaly p' using the hydrostatic balance, and sufficient in time to allow for phase averaging. Data limitations that are considered include profile time series with coarse temporal or vertical sampling, profiles missing near-surface or near-bottom information, moorings with sparse vertical sampling, and horizontal surveys with no coherent resampling in time. Methodologies, interpretation, and errors are described. For the specific case of the semidiurnal energy flux radiating from the Hawaiian ridge, errors of 610% are typical for estimates from six full-depth profiles spanning 15 h.
Abstract
A strong internal tide is generated in the Luzon Strait that radiates westward to impact the continental shelf of the South China Sea. Mooring data in 1500-m depth on the continental slope ...show a fortnightly averaged incoming tidal flux of 12 kW m−1, and a mooring on a broad plateau on the slope finds a similar flux as an upper bound. Of this, 5.5 kW m−1 is in the diurnal tide and 3.5 kW m−1 is in the semidiurnal tide, with the remainder in higher-frequency motions. Turbulence dissipation may be as high as 3 kW m−1. Local generation is estimated from a linear model to be less than 1 kW m−1. The continental slope is supercritical with respect to the diurnal tide, implying that there may be significant back reflection into the basin. Comparing the low-mode energy of a horizontal standing wave at the mooring to the energy flux indicates that perhaps one-third of the incoming diurnal tidal energy is reflected. Conversely, the slope is subcritical with respect to the semidiurnal tide, and the observed reflection is very weak. A surprising observation is that, despite significant diurnal vertical-mode-2 incident energy flux, this energy did not reflect; most of the reflection was in mode 1.
The observations are consistent with a linear scattering model for supercritical topography. Large fractions of incoming energy can reflect depending on both the geometry of the shelfbreak and the phase between the modal components of the incoming flux. If the incident mode-1 and mode-2 waves are in phase at the shelf break, there is substantial transmission onto the shelf; if they are out of phase, there is almost 100% reflection. The observations of the diurnal tide at the site are consistent with the first case: weak reflection, with most of it in mode 1 and almost no reflection in mode 2. The sensitivity of the reflection on the phase between incident components significantly complicates the prediction of reflections from continental shelves.
Finally, a somewhat incidental observation is that the shape of the continental slope has large regions that are near critical to the dominant diurnal tide. This implicates the internal tide in shaping of the continental slope.
Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and ...the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
The global distribution and 54‐year time dependence of the energy‐flux from the wind to near‐inertial motions is computed by driving a slab mixed‐layer model with NCEP/NCAR Reanalysis winds, ...improving upon previous estimates Alford, 2001; Watanabe and Hibiya, 2002. The slab model is solved spectrally with frequency‐dependent damping. The resulting solutions are more physically sensible than the previous, and more skillful at high latitudes, where the inertial frequency approaches the 4×‐daily sampling of the Reanalysis winds. This enables Alford's calculation, whose domain was limited to ±50°, to be extended to the poles. The high‐latitude reliability is demonstrated by direct comparison with a high‐resolution regional model (REMO) in the NE Atlantic. The total power input, 0.47 TW, has increased by 25% since 1948, paralleling observed increases in extratropical cyclone frequency and intensity. If believable, the trend may have important consequences for modulation of the meridional overturning circulation.
Abstract
In the Beaufort Sea in September of 2015, concurrent mooring and microstructure observations were used to assess dissipation rates in the vicinity of 72°35′N, 145°1′W. Microstructure ...measurements from a free-falling profiler survey showed very low
(10
−
10
) W kg
−1
turbulent kinetic energy dissipation rates
ε
. A finescale parameterization based on both shear and strain measurements was applied to estimate the ratio of shear to strain
R
ω
and
ε
at the mooring location, and a strain-based parameterization was applied to the microstructure survey (which occurred approximately 100 km away from the mooring site) for direct comparison with microstructure results. The finescale parameterization worked well, with discrepancies ranging from a factor of 1–2.5 depending on depth. The largest discrepancies occurred at depths with high shear. Mean
R
ω
was 17, and
R
ω
showed high variability with values ranging from 3 to 50 over 8 days. Observed
ε
was slightly elevated (factor of 2–3 compared with a later survey of 11 profiles taken over 3 h) from 25 to 125 m following a wind event which occurred at the beginning of the mooring deployment, reaching a maximum of
ε
= 6 × 10
−10
W kg
−1
at 30-m depth. Velocity signals associated with near-inertial waves (NIWs) were observed at depths greater than 200 m, where the Atlantic Water mass represents a reservoir of oceanic heat. However, no evidence of elevated
ε
or heat fluxes was observed in association with NIWs at these depths in either the microstructure survey or the finescale parameterization estimates.
Turbulence-enhanced mixing of upper ocean heat allows interaction between the tropical atmosphere and cold water masses that impact climate at higher latitudes thereby regulating air-sea coupling and ...poleward heat transport. Tropical cyclones (TCs) can drastically enhance upper ocean mixing and generate powerful near-inertial internal waves (NIWs) that propagate down into the deep ocean. Globally, downward mixing of heat during TC passage causes warming in the seasonal thermocline and pumps 0.15 to 0.6 PW of heat into the unventilated ocean. The final distribution of excess heat contributed by TCs is needed to understand subsequent consequences for climate; however, it is not well constrained by current observations. Notably, whether or not excess heat supplied by TCs penetrates deep enough to be kept in the ocean beyond the winter season is a matter of debate. Here, we show that NIWs generated by TCs drive thermocline mixing weeks after TC passage and thus greatly deepen the extent of downward heat transfer induced by TCs. Microstructure measurements of the turbulent diffusivity (Formula: see text) and turbulent heat flux (
Formula: see text) in the Western Pacific before and after the passage of three TCs indicate that mean thermocline values of Formula: see text and
Formula: see text increased by factors of 2 to 7 and 2 to 4 (95% confidence level), respectively, after TC passage. Excess mixing is shown to be associated with the vertical shear of NIWs, demonstrating that studies of TC-climate interactions ought to represent NIWs and their mixing to accurately capture TC effects on background ocean stratification and climate.
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