Internal Waves in the East Australian Current Alford, Matthew H.; Sloyan, Bernadette M.; Simmons, Harper L.
Geophysical research letters,
28 December 2017, Letnik:
44, Številka:
24
Journal Article
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Odprti dostop
Internal waves, which drive most ocean turbulence and add “noise” to lower‐frequency records, interact with low‐frequency current systems and topography in yet poorly known ways. Taking advantage of ...a heavily instrumented, 14 month mooring array, internal waves in the East Australian Current (EAC) are examined for the first time. Internal wave horizontal kinetic energy (HKE) is within a factor of 2 of the Garrett‐Munk (1976) spectrum. Continuum internal waves, near‐inertial waves, and internal tides together constitute a significant percentage of the total velocity variance. Mode‐1 internal tide energy fluxes are southward and much smaller than energy times group velocity, consistent with reflection at the continental slope of incident waves generated from near New Caledonia and the Solomon Islands. Internal tide HKE is highly phase variable, consistent with refraction by the variable EAC. Mode‐1 near‐inertial wave energy fluxes are of comparable magnitude and are equatorward and episodic, consistent with generation by storms farther poleward. These processes are considered together in the complex environment of the EAC.
Key Points
Internal waves are present in the EAC, with continuum levels twice the GM spectral level
Internal tides are reflective and nonstationary
Near‐inertial waves are equatorward propagating and quite energetic, consistent with distant radiation from storms
Abstract
Though unresolved by Argo floats, internal waves still impart an aliased signal onto their profile measurements. Recent studies have yielded nearly global characterization of several ...constituents of the stationary internal tides. Using this new information in conjunction with thousands of floats, we quantify the influence of the stationary, mode-1 M
2
and S
2
internal tides on Argo-observed temperature. We calculate the in situ temperature anomaly observed by Argo floats (usually on the order of 0.1°C) and compare it to the anomaly expected from the stationary internal tides derived from altimetry. Globally, there is a small, positive correlation between the expected and in situ signals. There is a stronger relationship in regions with more intense internal waves, as well as at depths near the nominal mode-1 maximum. However, we are unable to use this relationship to remove significant variance from the in situ observations. This is somewhat surprising, given that the magnitude of the altimetry-derived signal is often on a similar scale to the in situ signal, and points toward a greater importance of the nonstationary internal tides than previously assumed.
Abstract
The reflection of a low-mode internal tide on the Tasman continental slope is investigated using simulations of realistic and simplified topographies. The slope is supercritical to the ...internal tide, which should predict a large fraction of the energy reflected. However, the response to the slope is complicated by a number of factors: the incoming beam is confined laterally, it impacts the slope at an angle, there is a roughly cylindrical rise directly offshore of the slope, and a leaky slope-mode wave is excited. These effects are isolated in simulations that simplify the topography. To separate the incident from the reflected signal, a response without the reflector is subtracted from the total response to arrive at a reflected signal. The real slope reflects approximately 65% of the mode-1 internal tide as mode 1, less than two-dimensional linear calculations predict, because of the three-dimensional concavity of the topography. It is also less than recent glider estimates, likely as a result of along-slope inhomogeneity. The inhomogeneity of the response comes from the Tasman Rise that diffracts the incoming tidal beam into two beams: one focused along beam and one diffracted to the north. Along-slope inhomogeneity is enhanced by a partially trapped, superinertial slope wave that propagates along the continental slope, locally removing energy from the deep-water internal tide and reradiating it into the deep water farther north. This wave is present even in a simplified, straight slope topography; its character can be predicted from linear resonance theory, and it represents up to 30% of the local energy budget.
Abstract
Extending an earlier attempt to understand long-range propagation of the global internal-wave field, the energy E and horizontal energy flux F are computed for the two gravest baroclinic ...modes at 80 historical moorings around the globe. With bandpass filtering, the calculation is performed for the semidiurnal band (emphasizing M2 internal tides, generated by flow over sloping topography) and for the near-inertial band (emphasizing wind-generated waves near the Coriolis frequency). The time dependence of semidiurnal E and F is first examined at six locations north of the Hawaiian Ridge; E and F typically rise and fall together and can vary by over an order of magnitude at each site. This variability typically has a strong spring–neap component, in addition to longer time scales. The observed spring tides at sites northwest of the Hawaiian Ridge are coherent with barotropic forcing at the ridge, but lagged by times consistent with travel at the theoretical mode-1 group speed from the ridge. Phase computed from 14-day windows varies by approximately ±45° on monthly time scales, implying refraction by mesoscale currents and stratification. This refraction also causes the bulk of internal-tide energy flux to be undetectable by altimetry and other long-term harmonic-analysis techniques. As found previously, the mean flux in both frequency bands is O(1 kW m−1), sufficient to radiate a substantial fraction of energy far from each source. Tidal flux is generally away from regions of strong topography. Near-inertial flux is overwhelmingly equatorward, as required for waves generated at the inertial frequency on a β plane, and is winter-enhanced, consistent with storm generation. In a companion paper, the group velocity, ĉg ≡ FE−1, is examined for both frequency bands.
We present improvements in the modeling of the vertical wavenumber spectrum of the internal gravity wave continuum in high‐resolution regional ocean simulations. We focus on model sensitivities to ...mixing parameters and comparisons to McLane moored profiler observations in a Pacific region near the Hawaiian Ridge, which features strong semidiurnal tidal beams. In these simulations, the modeled continuum exhibits high sensitivity to the background mixing components of the K‐Profile Parameterization (KPP) vertical mixing scheme. Without the KPP background mixing, stronger vertical gradients in velocity are sustained in the simulations and the modeled kinetic energy and shear spectral slopes are significantly closer to the observations. The improved representation of internal wave dynamics in these simulations makes them suitable for improving ocean mixing estimates and for the interpretation of satellite missions such as the Surface Water and Ocean Topography mission.
Plain Language Summary
Internal waves (IWs) exist in the ocean interior due to differences in fluid densities. Breaking IWs cause mixing, which has important effects on ocean temperatures and nutrients. Interactions between internal tides generated by tidal flow over bathymetric features and near‐inertial waves generated by wind yield a spectrum of IWs at many frequencies. Here, we compare the IW spectrum in high‐resolution numerical simulations of a region in the North Pacific with observations from moored instruments. We study the effects of the “background” mixing components of the widely used K‐Profile Parameterization (KPP) vertical mixing scheme on the vertical structure of the IW field. The KPP background parameterizes the mixing action of IWs, which is not resolved in coarser‐resolution global ocean models. In our high‐resolution simulations, the IW field is highly active, and the KPP background components turn out to be mostly redundant in this setting. The modeled IW field lies closer to observations when we turn off the KPP background. Improved IW representation in ocean models can play an important role in the accurate representation of IW‐driven mixing in ocean simulations and interpretation of IW signatures from the upcoming Surface Water and Ocean Topography mission.
Key Points
Regional ocean simulations are ideal for examining sensitivity of internal gravity wave (IGW) spectra to model mixing parameters
Turning off the background components of K‐Profile Parameterization yields more realistic IGW vertical structure in high‐resolution regional models
IGW spectra are most correctly estimated in models away from tidal generation sites and lateral boundaries
The enegy flux from the wind to inertial mixed layer motions is computed for all oceans from 50 degreees S to 50 degrees N for the years 1996-99. Midlatitude storms produce the greatest fluxes, ...resulting in broad maxima near 40 degrees latitude during each hemisphere's winter, concentrated in the western portion of each basin.
Abstract
Temporal and spatial patterns of near-inertial kinetic energy (KEmoor) are investigated in a database of 2480 globally distributed, moored current-meter records (deployed on 690 separate ...moorings) and compared with the distribution of wind-forced mixed-layer energy flux FML. By computing KEmoor using short (30 day) multitaper spectral windows, the seasonal cycle is resolved. Clear winter enhancement by a factor of 4–5 is seen in the Northern Hemisphere for latitudes 25°–45° at all depths <4500 m, in close agreement with the magnitude, phase, and latitudinal dependence of the seasonal cycle of FML. In the Southern Hemisphere, data coverage is poorer, but a weaker seasonal cycle (a factor of 2) in both KEmoor and FML is still resolvable between 35° and 50°. When Wentzel–Kramers–Brillouin (WKB) scaled using climatological buoyancy-frequency profiles, summer KEmoor is approximately constant in depth while showing a clear decrease by a factor of 4–5 from 500 to 3500 m in winter. Spatial coverage is sufficient in the Northern Hemisphere to resolve broad KEmoor maxima in the western portion of each ocean basin in winter, generally collocated with FML maxima associated with storm forcing. The ratio of depth-integrated KEmoor to FML gives a replenishment time scale, which is about 10 days in midlatitudes, consistent with 1) previous estimates of the dissipation time scale of the internal wave continuum and 2) the presence of a seasonal cycle. Its increase to ≈70–80 days at lower latitudes is a possible signature of equatorward propagation of near-inertial waves. The seasonal modulation of the magnitude of KEmoor, its similarity to that in FML, and the depth decay and western intensification in winter but not in summer are consistent with a primarily wind-forced near-inertial field for latitudes poleward of ≈25°.
Abstract
The most comprehensive studies of the spatial and temporal scales of diffusivity rely on internal wave parameterizations that require knowledge of finescale shear and strain. Studies lacking ...either shear or strain measurements have to assume a constant ratio between shear and strain (
R
ω
). Data from 14 moorings collected during five field programs are examined to determine the spatial and temporal patterns in
R
ω
and the influence of these patterns on parameterized diffusivity. Time-mean
R
ω
ranges from 1 to 10, with changes of order 10 observed over a broad range of scales. Temporal variability in
R
ω
is observed at daily, weekly, and monthly scales. Observed changes in
R
ω
could produce a 2–3 times change in parameterized diffusivity. Vertical profiles of
R
ω
,
E
shear
, and
E
strain
(shear or strain variance relative to Garret–Munk) reveal that both local topographic properties and wind variability impact the internal wave field. Time series of
R
ω
from each mooring have strong correlations to either shear or strain, often only at a specific range of vertical wavenumbers. Sites fall into two categories, in which
R
ω
variability is dominated by either shear or strain. Linear fits to the dominant property (i.e., shear or strain) can be used to estimate a time series of
R
ω
that has an RMS error that is 30% less than the RMS error from assuming
R
ω
= 3. Shear and strain level vary in concert, as predicted by the Garret–Munk model, at high
E
shear
values. However, at
E
shear
< 5, strain variations are 3 times weaker than shear.
Abstract
The evidence for, baroclinic energetics of, and geographic distribution of parametric subharmonic instability (PSI) arising from both diurnal and semidiurnal tides in a global ocean general ...circulation model is investigated using 1/12.5° and 1/25° simulations that are forced by both atmospheric analysis fields and the astronomical tidal potential. The paper examines whether PSI occurs in the model, and whether it accounts for a significant fraction of the tidal baroclinic energy loss. Using energy transfer calculations and bispectral analyses, evidence is found for PSI around the critical latitudes of the tides. The intensity of both diurnal and semidiurnal PSI in the simulations is greatest in the upper ocean, consistent with previous results from idealized simulations, and quickly drops off about 5° from the critical latitudes. The sign of energy transfer depends on location; the transfer is positive (from the tides to subharmonic waves) in some locations and negative in others. The net globally integrated energy transfer is positive in all simulations and is 0.5%–10% of the amount of energy required to close the baroclinic energy budget in the model. The net amount of energy transfer is about an order of magnitude larger in the 1/25° semidiurnal simulation than the 1/12.5° one, implying the dependence of the rate of energy transfer on model resolution.
Abstract
Shipboard ADCP and towed CTD measurements are presented of a near-inertial internal gravity wave radiating away from a zonal jet associated with the Subtropical Front in the North Pacific. ...Three-dimensional spatial surveys indicate persistent alternating shear layers sloping downward and equatorward from the front. As a result, depth-integrated ageostrophic shear increases sharply equatorward of the front. The layers have a vertical wavelength of about 250 m and a slope consistent with a wave of frequency 1.01f. They extend at least 100 km south of the front. Time series confirm that the shear is associated with a downward-propagating near-inertial wave with frequency within 20% of f. A slab mixed layer model forced with shipboard and NCEP reanalysis winds suggests that wind forcing was too weak to generate the wave. Likewise, trapping of the near-inertial motions at the low-vorticity edge of the front can be ruled out because of the extension of the features well south of it. Instead, the authors suggest that the wave arises from an adjustment process of the frontal flow, which has a Rossby number about 0.2–0.3.
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