Near-inertial Waves Research Articles

Restratification of the Surface Mixed Layer with Submesoscale Lateral Density Gradients: Diagnosing the Importance of the Horizontal Dimension

Abstract A depth-cycling towed conductivity–temperature–depth (CTD) and vessel-mounted acoustic Doppler current profiler (ADCP) were used to obtain four-dimensional measurements of the restratification of the surface mixed layer (SML) at a submesoscale lateral density gradient near the subtropical front. With the objective of studying the role of horizontal processes in restratification, the thermohaline and velocity fields were monitored for 33 h by 16 small-scale (≤15 km2) surveys centered on a drogued float. Daytime warming by insolation caused a unidirectional displacement of the initially vertical isopycnals toward increasing density. Across the entire SML (50-m vertical scale), solar insolation accounted for 60% of observed restratification, but over 10-m scales, the percentage decreased with depth from 80% at 25–35 m to ≤25% at 55–65 m. Below 35 m, stratification was enhanced by the vertically sheared horizontal advection of the lateral density gradient due to a near-inertial wave of ∼100-m vertical wavelength that rotated anticyclonically at the inertial frequency. The phase and similar period (25.4 h) of the local inertial period to the diurnal cycle ensured constructive interference with isopycnal displacements due to insolation. Restratification by sheared advection matched that predicted due to vertically sheared inertial oscillations generated during the geostrophic adjustment of a density front, but direct wind forcing may also have generated the wave that was subsequently modified by interaction with mesoscale vorticity associated with a nearby large-scale front. By further including the effects of lateral uncompensated thermohaline inhomogeneity, the authors can account for 100% ± 20% of the observed N 2 during daytime restratification. No detectable restratification due to the slumping of horizontal density gradients under gravity alone was found.

Mesoscale Eddy–Internal Wave Coupling. Part I: Symmetry, Wave Capture, and Results from the Mid-Ocean Dynamics Experiment

Abstract Vertical profiles of horizontal velocity obtained during the Mid-Ocean Dynamics Experiment (MODE) provided the first published estimates of the high vertical wavenumber structure of horizontal velocity. The data were interpreted as being representative of the background internal wave field, and thus, despite some evidence of excess downward energy propagation associated with coherent near-inertial features that was interpreted in terms of atmospheric generation, these data provided the basis for a revision to the Garrett and Munk spectral model. These data are reinterpreted through the lens of 30 years of research. Rather than representing the background wave field, atmospheric generation, or even near-inertial wave trapping, the coherent high wavenumber features are characteristic of internal wave capture in a mesoscale strain field. Wave capture represents a generalization of critical layer events for flows lacking the spatial symmetry inherent in a parallel shear flow or isolated vortex.

Inertial oscillations as deep ocean response to hurricanes

We discuss the deep ocean response to passing hurricanes (aka typhoons), which are considered as generators of near-inertial, internal waves. The analysis of data collected in the northwestern parts of the Pacific and Atlantic oceans in the hurricane season permit us to assess the deep ocean response to such a strong atmospheric forcing. A large number of moorings (more than 100) in the northwestern Pacific have allowed us to characterize the spatial features of the oceanic response to typhoons and the variable downward velocity of near-inertial wave propagation. The velocity of their downward propagation varies in the range 1–10 m/hour. It is higher in the regions of low stratification and high anticyclonic vorticity. The inertial oscillations generated by a hurricane last for 10–12 days. The mean anticyclonic vorticity in the region increases the effective frequency of inertial oscillations by 0.001–0.004 cyc/hour.

Boundary Mixing Associated with Tidal and Near-Inertial Internal Waves

Abstract Moorings deployed on the south (August–November 2002) and north (November 2002–June 2003) flanks of the Kaena Ridge, Hawaii, are used to document the flow variability associated with mixing within 200 m from the boundary, deep along the ridge, as part of the Hawaii Ocean Mixing Experiment (HOME). At both sites, strong temperature inversions are detected with vertical scales of ∼100 m. A Thorpe-scale analysis of the overturns yields a time-averaged dissipation near the bottom at the south site (1.2 × 10−8 W kg−1) that is 10 times higher than the north site (1.9 × 10−9 W kg−1), with both higher than the dissipation at similar depths 30 km from the ridge. On the south flank, observed horizontal currents and vertical displacements are dominated by the semidiurnal internal tide. On the north flank, the semidiurnal tide is less energetic than on the south, with a different vertical structure as tidal amplitudes decrease toward the boundary. These differences are attributed to greater separation from the bottom of downward-propagating internal tides at the north site compared to the south site, resulting in higher mixing at the south site. Near the boundary, near-inertial to diurnal oscillations are more energetic at the north than the south site. This asymmetry is attributed to near-inertial internal waves that are generated north of the ridge by winter storms; the ridge shadows the equatorward-propagating near-inertial internal waves leading to negligible amplitudes on the southern lee side. The near-inertial waves combine with internal tidal motions to create high strain conditions that lead to mixing at the north site.

The limiting form of inertial instability in geophysical flows

The instability of a rotating, stratified flow with arbitrary horizontal cross-stream shear is studied, in the context of linear normal modes with along-stream wavenumberkand vertical wavenumberm. A class of solutions are developed which are highly localized in the horizontal cross-stream direction around a particular streamline. A Rayleigh–Schrödinger perturbation analysis is performed, yielding asymptotic series for the frequency and structure of these solutions in terms ofkandm. The accuracy of the approximation improves as the vertical wavenumber increases, and typically also as the along-stream wavenumber decreases. This is shown to correspond to a near-inertial limit, in which the solutions are localized around the global minimum offQ, wherefis the Coriolis parameter andQis the vertical component of the absolute vorticity. The limiting solutions are near-inertial waves or inertial instabilities, according to whether the minimum value offQis positive or negative.We focus on the latter case, and investigate how the growth rate and structure of the solutions changes withmandk. Moving away from the inertial limit, we show that the growth rate always decreases, as the inertial balance is broken by a stabilizing cross-stream pressure gradient. We argue that these solutions should be described as non-symmetric inertial instabilities, even though their spatial structure is quite different to that of the symmetric inertial instabilities obtained whenkis equal to zero.We use the analytical results to predict the growth rates and phase speeds for the inertial instability of some simple shear flows. By comparing with results obtained numerically, it is shown that accurate predictions are obtained by using the first two or three terms of the perturbation expansion, even for relatively small values of the vertical wavenumber. Limiting expressions for the growth rate and phase speed are given explicitly for non-zerok, for both a hyperbolic-tangent velocity profile on anf-plane, and a uniform shear flow on an equatorial β-plane.

Nutrient flux into an intense deep chlorophyll layer in a mode-water eddy

Abstract An intense deep chlorophyll layer in the Sargasso Sea was reported near the center of an anticyclonic mode-water eddy by McGillicuddy et al. [2007. Eddy–wind interactions stimulate extraordinary mid-ocean plankton blooms, Science, accepted]. The high chlorophyll was associated with anomalously high concentrations of diatoms and with a maximum in the vertical profile of 14C primary productivity. Here we report tracer measurements of the vertical advection and turbulent diffusion of deep-water nutrients into this chlorophyll layer. Tracer released in the chlorophyll layer revealed upward motion relative to isopycnal surfaces of about 0.4 m/d, due to solar heating and mixing. The density surfaces themselves shoaled by about 0.1 m/d. The upward flux of dissolved inorganic nitrogen, averaged over 36 days, was approximately 0.6 mmol/m2/d due to both upwelling and mixing. This flux is about 40% of the basin wide, annually averaged, nitrogen flux required to drive the annual new production in the Sargasso Sea, estimated from the oxygen cycle in the euphotic zone, the oxygen demand below the euphotic zone, and from the 3He excess in the mixed layer. The observed upwelling of the fluid was consistent with theoretical models [Dewar, W.K., Flierl, G.R., 1987. Some effects of wind on rings. Journal of Physical Oceanography 17, 1653–1667; Martin, A.P., Richards, K.J., 2001. Mechanisms for vertical nutrient transport within a North Atlantic mesoscale eddy. Deep-Sea Research II 48, 757–773] in which eddy surface currents cause spatial variations in surface stress. The diapycnal diffusivity at the base of the euphotic zone was 3.5±0.5×10−5 m2/s. Diapycnal mixing was probably enhanced over more typical values by the series of storms passing over the eddy during the experiment and may have been enhanced further by the trapping of near-inertial waves generated within the eddy.

Numerical experiment on Kyucho around the Tango Peninsula induced by Typhoon 0406

A numerical experiment using a three dimensional level model was performed to clarify the mechanism generating a strong coastal current, Kyucho, induced by the passage of Typhoon 0406 around the tip of the Tango Peninsula, Japan in June 2004. Wind stress accompanied by Typhoon 0406 was applied to the model ocean with realistic bottom topography and stratification condition. The model well reproduced the characteristics of Kyucho observed by Kumaki et al. (2005), i.e., the strong alongshore current with maximum velocity of 53 cm s−1 and its propagation along the peninsula with propagation speed of about 0.6 m s−1 one half-day after the typhoon’s passage. Coastal-trapped waves (CTW) accompanied by downwelling were induced along the northwest coast of the peninsula by the alongshore wind stress. The energy density flux due to the CTW flowed eastward along the coast, and indicated scattering of the CTW around the eastern coast of the peninsula. In addition, significant near-inertial internal gravity waves were also caused in the offshore region from the west of the Noto Peninsula to the north of the Tango Peninsula by the typhoon’s passage. The energy flux density of the near-inertial fluctuations flowed southward off the Fukui coast, and part of the energy flux was trapped on the tip of the Tango Peninsula, flowing with the coast on its right. It was found that the strong current, Kyucho, at the northeastern tip of the Tango Peninsula was generated by superposition of the near-inertial internal gravity waves and subinertial CTW.

Turbulence of near-inertial waves in the continuously stratified fluid

Abstract By using the normal form of continuously stratified “primitive” equations of geophysical fluid dynamics with density (in the ocean), or potential temperature (in the atmosphere) playing the role of the vertical coordinate, we decouple vortex and wave motions in the system, introduce normal variables, and derive the effective Hamiltonian for waves with frequencies close to the inertial frequency (near-inertial waves, NIW). We then apply the weak turbulence approach to the random-phase ensembles of these waves. We show how the anisotropic scale-invariance of NIW may be exploited in order to obtain the stationary power-law spectra. The non-decay anisotropic scale-invariant dispersion laws of the NIW-type were not studied previously in the weak-turbulence literature.

Seasonal and Spatial Variability of Near-Inertial Kinetic Energy from Historical Moored Velocity Records

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°.

High-Frequency Internal Waves on the Oregon Continental Shelf

Abstract Measurements of vertical velocity by isopycnal-following, neutrally buoyant floats deployed on the Oregon shelf during the summers of 2000 and 2001 were used to characterize internal gravity waves on the shelf using measurements of vertical velocity. The average spectrum of Wentzel–Kramers–Brillouin (WKB)-scaled vertical kinetic energy has the level predicted by the Garrett–Munk model (GM79), plus a narrow M2 tidal peak and a broad high-frequency peak extending from about 0.1N to N and rising a decade above GM79. The high-frequency peak varies in energy coherently with time across its entire bandwidth. Its energy is independent of the tidal energy. The energy in the “continuum” region between the peaks is weakly correlated with the level of the high-frequency peak energy and is independent of the tidal peak energy. The vertical velocity is not Gaussian but is highly intermittent, with a calculated kurtosis of 19. The vertical kinetic energy varies geographically. Low energy is found offshore and nearshore. The highest energy is found near a small seamount. High energy is found over the rough topography of Heceta Bank and near the shelf break. The highest energy occurs as packets of high-frequency waves, often occurring on the sharp downward phase of the M2 internal tide and called “tidal solibores.” A few isolated waves with high energy are also found. Of the 1-h periods with the highest vertical kinetic energy, 31% are tidal solibores, 8% are isolated waves, and the remainder of the periods appear unorganized. The two most energetic tidal solibores were examined in detail. As compared with the steady, propagating, two-dimensional, inviscid, internal-wave solutions to the equations of motion with no background shear [i.e., the Dubreil–Jacotin–Long (DJL) equation], all but the most energetic observed waveforms are too narrow for their height to be solitary waves. Despite the large near-N peak in vertical kinetic energy, the M2 internal tide contributes over 80% of the energy, ignoring near-inertial waves. The tidal solibores make a very small contribution, 0.5%, to the overall internal-wave energy.

Global Patterns of Low-Mode Internal-Wave Propagation. Part II: Group Velocity

Abstract Using a set of 80 globally distributed time series of near-inertial and semidiurnal energy E and energy flux F computed from historical moorings, the group velocity ĉg ≡ FE−1 is estimated. For a single free wave, observed group speed |ĉg| should equal that expected from linear wave theory. For comparison, the latitude dependence of perceived group speed for perfectly standing waves is also derived. The latitudinal dependence of observed semidiurnal |ĉg| closely follows that expected for free waves at all latitudes, implying that 1) low-mode internal tides obey linear theory and 2) standing internal-tidal waves are rare in the deep ocean for latitudes equatorward of about 35°. At higher latitudes, standing waves cannot be easily distinguished from free waves using this method because their expected group speeds are similar. Near-inertial waves exhibit scattered |ĉg| values consistent with the passage of events generated at various latitudes, with implied frequencies ω ≈ 1.05–1.25 × f, as typically observed in frequency spectra.

Dynamics in the Deep Canada Basin, Arctic Ocean, Inferred by Thermistor Chain Time Series

Abstract A 50-day time series of high-resolution temperature in the deepest layers of the Canada Basin in the Arctic Ocean indicates that the deep Canada Basin is a dynamically active environment, not the quiet, stable basin often assumed. Vertical motions at the near-inertial (tidal) frequency have amplitudes of 10– 20 m. These vertical displacements are surprisingly large considering the downward near-inertial internal wave energy flux typically observed in the Canada Basin. In addition to motion in the internal-wave frequency band, the measurements indicate distinctive subinertial temperature fluctuations, possibly due to intrusions of new water masses.

Spatial and Temporal Patterns of Small-Scale Mixing in Drake Passage

Abstract Temperature and salinity profiles obtained with expendable CTD probes throughout Drake Passage between February 2002 and July 2005 are analyzed to estimate turbulent diapycnal eddy diffusivities to a depth of 1000 m. Diffusivity values are inferred from density/temperature inversions and internal wave vertical strain. Both methods reveal the same pattern of spatial variability across Drake Passage; diffusivity estimates from inversions exceed those from vertical strain by a factor of 3 over most of Drake Passage. The Polar Front (PF) separates two dynamically different regions. Strong thermohaline intrusions characterize profiles obtained north of the PF. South of the PF, stratification is determined largely by salinity, and temperature is typically unstably stratified between 100- and 600-m depth. In the upper 400 m, turbulent diapycnal diffusivities are O(10−3 m2 s−1) north of the PF but decrease to O(10−4 m2 s−1) or smaller south of the PF. Below 400 m diffusivities typically exceed 10−4 m2 s−1. Diffusivities decay weakly with depth north of the PF, whereas diffusivities increase with depth and peak near the local temperature maximum south of the PF. The meridional pattern in near-surface mixing corresponds to local maxima and minima of both wind stress and wind stress variance. Near-surface diffusivities are also found to be larger during winter months north of the PF. Wind-driven near-inertial waves, strong mesoscale eddy activity, and double-diffusive convection are suggested as possible factors contributing to observed mixing patterns.

The high-resolution vertical structure of internal tides and near-inertial waves measured with an ADCP over the continental slope in the Bay of Biscay

ADCP measurements of the velocity structure in the permanent thermocline at two locations over the continental slope in the Bay of Biscay are presented. The vertical variation of the contribution of the inertia-gravity waveband to the kinetic energy, vertical motion, and current shear are analysed. The semi-diurnal tides together with near-inertial waves appear to provide over 70% of the high-frequency kinetic energy (>1/3 cpd). Over the vertical range of the ADCP observations the phase of the harmonic M 2 tide changes up to 155°, while the kinetic energy varies in the vertical by a factor of 3.8, showing the importance of the contribution of internal waves to the observed tidal motion. Both semi-diurnal internal tidal waves and near-inertial waves have a vertically restricted distribution of the variance of the horizontal and vertical velocity, as in internal wave beams. The short-term 14-day averaged amplitude and phase lag of the M 2 tide shows large temporal changes, with a characteristic 40–45 day time scale. These changes are probably related to variations in generation sites and propagation paths of the internal tide, because of changes in the temperature and salinity stratification due to the presence of meso-scale eddies. The relatively large shear in the inertia-gravity wave band, mainly at near-inertial frequencies, supports low-gradient Richardson numbers that are well below 1 for nearly half of the time. This implies that the large shear may support turbulent mixing for a large part of the time.

Sediment transport processes from the topset to the foreset of a crenulated clinoform (Adriatic Sea)

Crenulated clinoforms of complex and uncertain origin characterize large portions of the Late-Holocene prograding mud wedge in the western Adriatic continental shelf. Sediment failure was originally postulated as the most plausible mechanism for the formation of the crenulations. Subsequent work has shown that, although the origin of the crenulations may have been related to deformation processes, their maintenance through time seems to be better explained by different sediment accumulation rates in the flat and steep flanks. In order to establish relationships between active sediment dynamics, across-shelf transport and sediment accumulation in these crenulated clinoforms, two tripods and a mooring were deployed off the Pescara River during autumn and winter 2002–2003 as part of the EuroSTRATAFORM program, and in combination with the Po and Apennine Sediment Transport and Accumulation (PASTA) study. The tripods were placed on the shallow topset region and close to the clinoform roll-over point (i.e., offlap break), at 12 and 20-m water depth, respectively, and the mooring was located at 50-m depth, in the crenulated foreset region. Several sediment-resuspension events were recorded, mainly related to Bora and Sirocco storms, during which wave–orbital and current velocities increased considerably. Sediment transport in the topset region was predominantly towards the SE, following the direction of the coastal current and the bathymetry, but showing a significant offshore component at the roll-over point that was intensified during storm events. Currents at the foreset region were also directed to the SE. In mid-waters they were clearly aligned with the local bathymetry, whereas near the bottom they had an important and persistent offshore component. This current behavior seems to be associated with an intense bottom Ekman transport that causes the near-bottom current to be deflected to the left (i.e., offshore) with respect to the direction of the surface current. This mechanism enhances the suspended-sediment transport from the topset down the foreset region along the Adriatic prograding mud wedge, contributing to the basinward clinoform progradation and controlling the depth of the clinoform roll-over point. In addition, activity of near-inertial internal waves was also recorded by the near-bottom instrument deployed in the foreset region. During periods characterized by a strong near-inertial signal, increases of the water turbidity clearly coincided with an intensified offshore velocity component, which suggest that this mechanism also contributes to the transport of suspended sediment across the clinoform. Both the bottom Ekman transport and the internal waves are mechanisms that could be responsible for the formation/maintenance of the Adriatic seafloor crenulations until present-day, although several arguments suggest that the latter likely plays the major role.

Latitudinal and seasonal variability of gravity-wave energy in the South-West Indian Ocean

Abstract. Vertical temperature profiles obtained by radiosonde and Raman lidar measurements are used to investigate a climatology of total energy density of gravity waves (GW) in the Upper Troposphere (UT) and the Lower Stratosphere (LS) from 1992 to 2004 above Mahé (4° S, 55° E), Tromelin (15° S, 54° E) and La Réunion (21° S, 55° E) located in the tropical South-West Indian Ocean. The commonly used spectral index value (p≈5/3) of the intrinsic frequency spectrum is used for calculating estimated total energy density in the UT and LS. Estimated total energy density provides good estimation of total energy density in the LS but underestimates total energy density by one half in the UT above Mahé and Tromelin probably due to the activity of near-inertial frequency waves. Estimated total energy density reveals a strong seasonal variability as a function of latitude and convection as an evident active source of GW activity in the LS in austral summer. Above La Réunion, a semi-annual GW activity is observed in the LS with the signature of the subtropical barrier in the UT. Moreover, radiosondes and Raman lidar provide consistent GW surveys in the UT/LS at heights<23 km above La Réunion.

Numerical study of enhanced energy dissipation near a seamount

Using a two-dimensional primitive equation model, we examine nonlinear responses of a semidiurnal tidal flow impinging on a seamount with a background Garrett-Munk-like (GM-like) internal wavefield. It is found that horizontally elongated pancake-like structures of high vertical wavenumber near-inertial current shear are created both in the near-field (the region over the slope of the seamount) and far-field (the region over the flat bottom of the ocean). An important distinction is that the high vertical wavenumber near-inertial current shear is amplified only at mid-latitudes in the far-field (owing to a parametric subharmonic instability (PSI)), whereas it is amplified both at mid-and high-latitudes (above the latitude where PSI can occur) in the near-field. In order to clarify the generating mechanism for the strong shear in the near-field, additional numerical experiments are carried out with the GM-like background internal waves removed. The experiments show that the strong shear is also created, indicating that it is not caused by the interaction between the background GM-like internal waves and the semidiurnal internal tides. One possible explanation is proposed for the amplification of high vertical wavenumber near-inertial current shear in the near-field where tide residual flow resulting from tide-topography interaction plays an important role in transferring energy from high-mode internal tides to near-inertial internal waves.

Near-Inertial Waves on the New England Shelf: The Role of Evolving Stratification, Turbulent Dissipation, and Bottom Drag

Abstract Energetic variable near-inertial internal waves were observed on the springtime New England shelf as part of the Coastal Mixing and Optics (CMO) project. Surface warming and freshwater advection tripled the average stratification during a 3-week observational period in April/May 1997. The wave field was dominated by near-inertial internal waves generated by passing storms. Wave evolution was controlled by a balance among wind stress, bottom drag, and turbulent dissipation. As the stratification evolved, the vertical structure of these near-inertial waves switched from mode 1 to mode 2 with associated changes in the magnitude and location of wave shear. The growth of mode-2 waves was attributable to a combination of changing wind stress forcing and a nonlinear coupling between the first and second vertical modes through quadratic bottom stress. To explore both forcing mechanisms, an open-ocean mixed layer model is adapted to the continental shelf. In this model, surface wind stress and bottom stress are distributed over the surface and bottom mixed layers and then projected onto orthogonal vertical modes. The model replicates the correct magnitude and evolving modal distribution of the internal waves and confirms that bottom stress can act to transfer energy between internal wave modes.

Temporal Variability of High Vertical Wavenumber Shear over the Izu-Ogasawara Ridge

Recent numerical studies (Hibiya et al., 1996, 1998, 2002) showed that the energy cascade across the internal wave spectrum down to small dissipation scales was un- der strong control of parametric subharmonic instabilities (PSI) which transfer en- ergy from low vertical mode double-inertial frequency internal waves to high vertical mode near-inertial internal waves. To see whether or not the numerically-predicted energy cascade process is actually dominant in the real deep ocean, we examine the temporal variability of vertical profiles of horizontal velocity observed by deploying a number of expendable current profilers (XCPs) at one location near the Izu- Ogasawara Ridge. By calculating EOFs, we find the observed velocity profiles are dominated by low mode semidiurnal (~double-inertial frequency) internal tides and high mode near-inertial internal waves. Furthermore, we find that the WKB-stretched vertical scales of the near-inertial current shear are about 250 sm and 100 sm. The observed features are reasonably explained if the energy cascade down to small dissi- pation scales is dominated by PSI.

Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf

Abstract Integrated observations are presented of water property evolution and turbulent microstructure during the spring restratification period of April and May 1997 on the New England continental shelf. Turbulence is shown to be related to surface mixed layer entrainment and shear from low-mode near-inertial internal waves. The largest turbulent diapycnal diffusivity and associated buoyancy fluxes were found at the bottom of an actively entraining and highly variable wind-driven surface mixed layer. Away from surface and bottom boundary layers, turbulence was systematically correlated with internal wave shear, though the nature of that relationship underwent a regime shift as the stratification strengthened. During the first week, while stratification was weak, the largest turbulent dissipation away from boundaries was coincident with shear from mode-1 near-inertial waves generated by passing storms. Wave-induced Richardson numbers well below 0.25 and density overturning scales of several meters were observed. Turbulent dissipation rates in the region of peak shear were consistent in magnitude with several dimensional scalings. The associated average diapycnal diffusivity exceeded 10−3 m2 s−1. As stratification tripled, Richardson numbers from low-mode internal waves were no longer critical, though turbulence was still consistently elevated in patches of wave shear. Kinematically, dissipation during this period was consistent with the turbulence parameterization proposed by MacKinnon and Gregg, based on a reinterpretation of wave–wave interaction theory. The observed growth of temperature gradients was, in turn, consistent with a simple one-dimensional model that vertically distributed surface heat fluxes commensurate with calculated turbulent diffusivities.