Near-inertial Internal Waves Research Articles

Mooring observations of the near-inertial wave wake of Hurricane Ida (2009)

The near-inertial wave wake of Hurricane Ida is examined of the basis of horizontal velocity observations acquired from 7 moorings instrumented with acoustic Doppler current profilers deployed across the shelf break, slope, and at the abyssal plain of the Yucatan Peninsula, from 130m to ∼3300m. During the forced stage, background mean-flow consisted on a dominant cyclonic circulation of ∼100km of diameter intensified toward the Yucatan's shelf (topographic constraint) and bounded by anticyclonic vorticity northeastward (north 25°N). In the low frequency band, subinertial signals of ∈[5.5–7.5]day period propagating along the Yucatan shelf break. After the passage of Hurricane Ida, energetic near-inertial oscillations spread away from the storm's track over cyclonic vorticity. The wave's Eulerian frequency increases shoreward and toward the Yucatan's shelf. After Ida's passage, mooring data show a contrasting velocity response: semi-diurnal and diurnal tides are enhanced at the shelf break of the Yucatan Peninsula and near-inertial oscillations at the slope and abyssal plain. The near-inertial kinetic energy is largest to the right of the storm track because of the asymmetric wind-stress and amplified due to vorticity trapping near z=−500m, which is a proxy of the base of the mesoscale structure and where the mean-flow is nearly zero. The blue frequency shifted wave wake propagates downward at ∼57–70mday−1 and horizontally at 23–28kmday−1 leading a downward vertical energy flux of [1.3–1.6]×10−2Wm−2. This represents a 7–9% of the total wind power input to near-inertial oscillations that, ultimately, became available for interior ocean mixing. The results suggest that the most energetic wave packet propagated poleward and downward from a broad upwelling region located near the Hurricane's track. The vertical structure of the near-inertial kinetic energy is described as a sum of the first 12 standing vertical modes and as vertically propagating near-inertial internal waves. The amplification of near-inertial kinetic energy as the wave train propagates through the region of anticyclonic vorticity is consistent with the reduction of the Eulerian frequency (and mean-flow) at depth and the shrinking horizontal wavenumber in a critical layer. This work shows that energetic near-inertial oscillations of vertical wavelength of 850–1280m, penetrate well below the thermocline, and are concentrated to the right of the storm track in a region of anticyclonic vorticity.

Dynamics of the Changing Near-Inertial Internal Wave Field in the Arctic Ocean

ABSTRACTThe dynamics of the wind-generated near-inertial internal wave field in the Canada Basin of the Arctic Ocean are investigated using the drifting Ice-Tethered Profiler dataset for the years 2005 to 2014, during a decade when sea ice extent and thickness decreased dramatically. This time series, with nearly 10 years of measurements and broad spatial coverage, is used to quantify a seasonal cycle and interannual trend for internal waves in the Arctic, using estimates of the amplitude of near-inertial waves derived from isopycnal displacements. The internal wave field is found to be most energetic in summer when sea ice is at a minimum, with a second maximum in early winter during the period of maximum wind speed. Amplitude distributions for the near-inertial waves are quantifiably different during summer and winter, due primarily to seasonal changes in sea ice properties that affect how the ice responds to the wind, which can be expressed through the “wind factor”—the ratio of sea ice drift speed to wind speed. A small positive interannual trend in near-inertial wave energy is linked to pronounced sea ice decline during the last decade. Overall variability in the internal wave field increases significantly over the second half of the record, with an increased probability of larger-than-average waves in both summer and winter. This change is linked to an overall increase in variability in the wind factor and sea ice drift speeds, and reflects a shift in year-round sea ice characteristics in the Arctic, with potential implications for dissipation and mixing associated with internal waves.

Decay Mechanisms of Near-Inertial Mixed Layer Oscillations in the Bay of Bengal

Winds generate inertial and near-inertial currents in the upper ocean. These currents dominate the kinetic energy and contain most of the vertical shear in horizontal currents. Subsequent shear instabilities lead to mixing. In the Bay of Bengal, the annual mean wind energy input and near-inertial mixed layer energy is almost as large as in the mid-latitude storm tracks. Also, mixing associated with these waves is known to affect mixed layer heat content, sea surface temperature, and, thus, precipitation in coupled global models. Therefore, the mechanisms leading to the decay of these currents in the mixed layer and below are of considerable importance. Two such decay mechanisms are examined here. One mechanism is the downward propagation of near-inertial internal waves, which is aided by the mesoscale circulation and is observed with a rapidly profiling float. In a few days (faster than at mid-latitudes), the near-inertial wave group propagated from the base of the mixed layer to 250 m depth in the stratified interior. Another decay mechanism is enhanced shear generation at the mixed layer base from periodic alignment of rotating, near-inertial current shear and winds, which is observed with a mooring and analyzed with a simple two-layer model.

Sensitivity of near-inertial internal waves to spatial interpolations of wind stress in ocean generation circulation models

The oceanic near-inertial internal waves (NIWs) have been extensively studied using ocean general circulation models (OGCMs). Currently most OGCMs use the bilinear or bicubic interpolation to interpolate wind stress onto models’ surface grids. In this study, we examine the influences of bilinear and bicubic interpolations on the wavenumber and frequency spectra of wind stress and on the simulated NIWs in the ocean. It is demonstrated that both the bilinear and bicubic interpolations are equivalent to spatial low-pass filters with the former leading to more significant loss of wind stress variance at high wavenumbers. When coarse (e.g., 2°) wind stress is used to force OGCMs, the bilinear and bicubic interpolations significantly damp the wavenumber spectrum of wind stress at mesoscales, leading to decreased near-inertial wind stress variance. Using the bilinear (bicubic) interpolation could weaken the near-inertial wind work by ∼43% (22%) in the subtropical region (10°N–30°N) and by ∼16% (4%) at the midlatitudes (30°N–50°N).We propose a new interpolation method, i.e., the bi-sinc-function interpolation, which is able to retain all the wind stress variance within the Nyquist wavenumber. Compared to the bilinear and bicubic interpolations, the bi-sinc-function interpolation improves the simulations of NIWs and should be incorporated into OGCMs especially when coarse wind stress is used.

Role of Near-Inertial Internal Waves in Subthermocline Diapycnal Mixing in the Northern Gulf of Mexico

AbstractMoored ADCP data collected in the northern Gulf of Mexico are analyzed to examine near-inertial internal waves and their contribution to subthermocline diapycnal mixing based on a finescale parameterization of deep ocean mixing. The focus of the study is on the impact of near-inertial internal waves generated by an extreme weather event—that is, Hurricane Katrina—and by month-to-month variation in weather patterns on the diapycnal mixing. The inferred subthermocline diapycnal mixing exhibits pronounced elevation in the wake of Katrina. Both the increased near-inertial (0.8–1.8f, where f is the Coriolis frequency) and superinertial (>1.8f) shear variances contribute to the elevated diapycnal mixing, but the former plays a more dominant role. The intense wind work on near-inertial motions by the hurricane is largely responsible for the energetic near-inertial shear variance. Energy transfer from near-inertial to superinertial internal waves, however, appears to play an important role in elevating the superinertial shear variance. The inferred subthermocline diapycnal mixing in the region also exhibits significant month-to-month variation with the estimated diffusivity in January 2006 about 3 times the values in November and December 2005. The subseasonal change in the diapycnal mixing mainly results from the subseasonal variation of the near-inertial wind work that causes intensification of the near-inertial shear in January 2006.

Near-inertial internal waves observed near the tip of the northeastern coast of Noto Peninsula, Japan

Near-inertial internal waves observed near the tip of the northeastern coast of Noto Peninsula, Japan

Morphobathymetric analysis of the large fine-grained sediment waves over the Gulf of Valencia continental slope (NW Mediterranean)

Detailed analysis of recently acquired swath bathymetry, together with high-resolution seismic profiles and bottom sediment samples, revealed the presence of large-scale fine-grained sediment waves over the Gulf of Valencia continental slope. As many other deep-water sediment waves, these features were previously attributed to gravitational slope failure, related to creep-like deformation, and are here reinterpreted as sediment wave fields extending from 250m depth to the continental rise, at ~850m depth. Geometric parameters were computed from the high-resolution multibeam dataset. Sediment wave lengths range between 500 and 1000m, and maximum wave heights of up to 50m are found on the upper slope, decreasing downslope to minimum values of 2m high. Sediment waves on the lower part of the slope are quasi-stationary vertically accreting, whereas they show an upslope migrating pattern from the mid-slope to the upper part of the continental slope. High-resolution seismic profiles show continuous internal reflectors, with sediment waves merging down-section and sediment wave packages decreasing in thickness downslope. These sediment packages are thicker on the crest of each individual sediment wave and thinner on the downslope flank. 210Pb analyses conducted on sediment cores collected over the sediment wave fields also indicate slightly higher sediment accumulation rates on the wave crests. Sediment wave formation processes have been inferred from contemporary hydrodynamic observations, which reveal the presence of near-inertial internal waves interacting with the Gulf of Valencia continental slope. Internal wave activity is suggested to be the preferential mechanism for the transport and deposition of sediment, and the maintenance of the observed sediment wave fields.

Improve the Simulations of Near-Inertial Internal Waves in the Ocean General Circulation Models

AbstractThe near-inertial wind work and near-inertial internal waves (NIWs) in the ocean have been extensively studied using ocean general circulation models (OGCMs) forced by 6-hourly winds or wind stress obtained from atmospheric reanalysis data. However, the OGCMs interpolate the reanalysis winds or wind stress linearly onto each time step, which partially filters out the wind stress variance in the near-inertial band. In this study, the influence of the linear interpolation on the near-inertial wind work and NIWs is quantified using an eddy-resolving (°) primitive equation ocean model. In addition, a new interpolation method is proposed—the sinc-function interpolation—that overcomes the shortages of the linear interpolation.It is found that the linear interpolation of 6-hourly winds significantly underestimates the near-inertial wind work and NIWs at the midlatitudes. The underestimation of the near-inertial wind work and near-inertial kinetic energy is proportional to the loss of near-inertial wind stress variance due to the linear interpolation. This further weakens the diapycnal mixing in the ocean due to the reduced near-inertial shear variance. Compared to the linear interpolation, the sinc-function interpolation retains all the wind stress variance in the near-inertial band and yields correct magnitudes for the near-inertial wind work and NIWs at the midlatitudes.

Near-Inertial Internal Gravity Waves in the Ocean.

We review the physics of near-inertial waves (NIWs) in the ocean and the observations, theory, and models that have provided our present knowledge. NIWs appear nearly everywhere in the ocean as a spectral peak at and just above the local inertial period f, and the longest vertical wavelengths can propagate at least hundreds of kilometers toward the equator from their source regions; shorter vertical wavelengths do not travel as far and do not contain as much energy, but lead to turbulent mixing owing to their high shear. NIWs are generated by a variety of mechanisms, including the wind, nonlinear interactions with waves of other frequencies, lee waves over bottom topography, and geostrophic adjustment; the partition among these is not known, although the wind is likely the most important. NIWs likely interact strongly with mesoscale and submesoscale motions, in ways that are just beginning to be understood.

Observed near-inertial waves in the wake of Typhoon Hagupit in the northern South China Sea

Energetic near-inertial internal waves (NIWs) were observed on the continental slope of the northern South China Sea in September 2008. Characteristics of the observed near-inertial waves were examined based on current data recorded by a moored acoustic Doppler current profiler. Results of a simple slab model indicated that the NIWs were generated by the surface winds of Typhoon Hagupit. Following Hagupit’s passage, the wave field was dominated by baroclinic NIWs. The near-inertial currents were surface-intensified with a maximum of 0.52 m/s but still reached 0.1 m/s at the depth of 210 m. Moreover, the near-inertial currents were clockwise-polarized and slightly elliptical. A depth-leading phase of the near-inertial currents was evident, which indicated downward energy propagation. However, the rotary vertical wavenumber spectra suggested that upward energy propagation also existed, which was consistent previous theoretical study. The frequency of the NIWs, modified by the positive background vorticity, was 0.714 2 cycles per day, which was 0.02 f0 higher than the local inertial frequency (f0). The near-inertial kinetic energy evolved exponentially and had an e-folding timescale of about 3 days. The vertical phase and group velocity were estimated to be 10 and 2.1 m/h, respectively, corresponding to a vertical wavelength of 340 m. The NIWs were dominated by the second mode with a variance contribution of >50%, followed by the third mode, while the first mode was insignificant.

Quantifying High-Frequency Wind Energy Flux into Near-Inertial Motions in the Southeast Pacific

AbstractWind-forced internal waves close to the inertial frequency are ubiquitous throughout the world’s oceans, but observational constraints on their global energetics and impact on subsurface mixing remain scarce. This study reports on velocity measurements from three Electromagnetic Autonomous Profiling Explorers (EM-APEX) deployed in February 2009. These floats observed downward-propagating near-inertial internal waves near the Subantarctic and Polar Fronts of the Antarctic Circumpolar Current. These waves were episodic and enhanced at middepth between 500 and 1000 m. Depth-integrated kinetic energy varied between 1 and 7 kJ m−2 and averaged 1.6 kJ m−2 with typical group velocities of 40 m day−1, implying an average energy flux of 3 mW m−2 at the mixed layer base decreasing to approximately 25% of that value at 1500 m. Modeled currents forced by reanalysis winds along each float track agree with observed surface currents from EM-APEX, provided that mixed layer depth is restricted to the layer of weakest observable stratification (interpreted as the maximum depth that can remain mixed over an inertial period given the continual balance between mixing and restratification). This model estimates an average wind power of 3 mW m−2. Shipboard wind and current observations during a strong storm show an integrated wind work of 3.5 kJ m−2, comparable to the vertically integrated kinetic energy over the following month. Model wind work estimates are considerably less, likely because of the mixed layer depth used. A model with varying stratification in response to the wind provides a better match to the observations, emphasizing the importance of stratification within the mixed layer in amplifying wind energy input.

Observation of near-inertial internal waves in the abyssal Japan Sea

Observation of near-inertial internal waves in the abyssal Japan Sea

Intensified diapycnal mixing in the midlatitude western boundary currents.

The wind work on oceanic near-inertial motions is suggested to play an important role in furnishing the diapycnal mixing in the deep ocean which affects the uptake of heat and carbon by the ocean as well as climate changes. However, it remains a puzzle where and through which route the near-inertial energy penetrates into the deep ocean. Using the measurements collected in the Kuroshio extension region during January 2005, we demonstrate that the diapycnal mixing in the thermocline and deep ocean is tightly related to the shear variance of wind-generated near-inertial internal waves with the diapycnal diffusivity 6 × 10−5 m2s−1 almost an order stronger than that observed in the circulation gyre. It is estimated that 45%–62% of the local near-inertial wind work 4.5 × 10−3 Wm−2 radiates into the thermocline and deep ocean and accounts for 42%–58% of the energy required to furnish mixing there. The elevated mixing is suggested to be maintained by the energetic near-inertial wind work and strong eddy activities causing enhanced downward near-inertial energy flux than earlier findings. The western boundary current turns out to be a key region for the penetration of near-inertial energy into the deep ocean and a hotspot for the diapycnal mixing in winter.

Upscale Energy Transfer by the Vortical Mode and Internal Waves

AbstractDiapycnal mixing in the ocean is sporadic yet ubiquitous, leading to patches of mixing on a variety of scales. The adjustment of such mixed patches can lead to the formation of vortices and other small-scale geostrophic motions, which are thought to enhance lateral diffusivity. If vortices are densely populated, they can interact and merge, and upscale energy transfer can occur. Vortex interaction can also be modified by internal waves, thus impacting upscale transfer. Numerical experiments were used to study the effect of a large-scale near-inertial internal wave on a field of submesoscale vortices. While one might expect a vertical shear to limit the vertical scale of merging vortices, it was found that internal wave shear did not disrupt upscale energy transfer. Rather, under certain conditions, it enhanced upscale transfer by enhancing vortex–vortex interaction. If vortices were so densely populated that they interacted even in the absence of a wave, adding a forced large-scale wave enhanced the existing upscale transfer. Results further suggest that continuous forcing by the main driving mechanism (either vortices or internal waves) is necessary to maintain such upscale transfer. These findings could help to improve understanding of the direction of energy transfer in submesoscale oceanic processes.

Near-Inertial Internal Waves and Sea Ice in the Beaufort Sea*

Abstract The evolution of the near-inertial internal wavefield from ice-free summertime conditions to ice-covered wintertime conditions is examined using data from a yearlong deployment of six moorings on the Beaufort continental slope from August 2008 to August 2009. When ice is absent, from July to October, energy is efficiently transferred from the atmosphere to the ocean, generating near-inertial internal waves. When ice is present, from November to June, storms also cause near-inertial oscillations in the ice and mixed layer, but kinetic energy is weaker and oscillations are quickly damped. Damping is dependent on ice pack strength and morphology. Decay scales are longer in early winter (November–January) when the new ice pack is weaker and more mobile, decreasing in late winter (February–June) when the ice pack is stronger and more rigid. Efficiency is also reduced, as comparisons of atmospheric energy available for internal wave generation to mixed layer kinetic energies indicate that a smaller percentage of atmospheric energy is transferred to near-inertial motions when ice concentrations are >90%. However, large kinetic energies and shears are observed during an event on 16 December and spectral energy is elevated above Garrett–Munk levels, coinciding with the largest energy flux predicted during the deployment. A significant amount of near-inertial energy is episodically transferred to the internal wave band from the atmosphere even when the ocean is ice covered; however, damping by ice and less efficient energy transfer still leads to low Arctic internal wave energy in the near-inertial band. Increased kinetic energy below 300 m when ice is forming suggests some events may generate internal waves that radiate into the Arctic Ocean interior.

Near-inertial wave driven dissolved oxygen transfer through the thermocline of a large lake

The offshore regions of large temperate lakes are characterized by strong summer temperature stratification that limits vertical mass flux, and enables near-inertial internal wave motions. Here, we investigate the contribution of near-inertial baroclinic velocity shear on enhancing the vertical transport of dissolved oxygen (DO) through the thermocline of the central basin of Lake Erie. The lake is prone to severe annual hypoxia in the hypolimnion and also has strong near-inertial Poincaré wave activity. Using field measurements, analytical arguments and a numerical model under idealized conditions, we show that the near-inertial waves drive baroclinic shear instabilities that enhance the vertical turbulent diffusivity and reduce the rate of DO depletion in the hypolimnion by up to 12% over the entire basin. Results from modeling large-spatial variability in the enhanced thermocline flux to match the distribution of near-inertial wave energy density, indicate that the observed oxygen budgets will vary as a function of location of sampling.

Near-Inertial Internal Wave Field in the Canada Basin from Ice-Tethered Profilers

AbstractSalinity and temperature profiles from drifting ice-tethered profilers in the Beaufort gyre region of the Canada Basin are used to characterize and quantify the regional near-inertial internal wave field over one year. Vertical displacements of potential density surfaces from the surface to 750-m depth are tracked from fall 2006 to fall 2007. Because of the time resolution and irregular sampling of the ice-tethered profilers, near-inertial frequency signals are marginally resolved. Complex demodulation is used to determine variations with a time scale of several days in the amplitude and phase of waves at a specified near-inertial frequency. Characteristics and variability of the wave field over the course of the year are investigated quantitatively and related to changes in surface wind forcing and sea ice cover.

Tropical storm-induced turbulent mixing and chlorophyll-a enhancement in the continental shelf southeast of Hainan Island

Based on moored observations and remote sensing data in July and August 2005, energy sources for enhancing turbulent mixing and possible mechanisms of phytoplankton bloom in the continental shelf southeast of Hainan Island under the influence of Washi, a fast-moving and weak tropical storm, are analyzed in this paper. Observations show that strong near-inertial internal waves were generated by the rapidly changing wind stress and the near-inertial energy was dissipated quickly across the thermocline. The strong turbulent mixing associated with the near-inertial baroclinic shear instability occurred with maximum eddy diffusivity above 3.2×10−4m2s−1, and the surface chlorophyll-a (Chl-a) concentration after the storm increased by 22.2%. The Chl-a concentration augment was inferred to be an upper ocean biophysical response to the enhanced near-inertial turbulent mixing which could increase the upward nutrient flux into the surface low eutrophic zone during the passage of Washi.

Spatial structure of internal Poincaré waves in Lake Michigan

In this paper we examine the characteristics of near-inertial internal Poincare waves in Lake Michigan (USA) as discerned from field experiments and hydrodynamic simulations. The focus is on the determination of the lateral and vertical structure of the waves. Observations of near-inertial internal wave properties are presented from two field experiments in southern Lake Michigan conducted during the years 2009 and 2010 at Michigan City (IN, USA) and Muskegon (MI, USA), respectively. Spectra of thermocline displacements and baroclinic velocities show that kinetic and potential baroclinic energy is dominated by near-inertial internal Poincare waves. Vertical structure discerned from empirical orthogonal function analysis shows that this energy is predominantly vertical mode 1. Idealized hydrodynamic simulations using stratifications from early summer (June), mid-summer (July) and fall (September) identify the basin-scale internal Poincare wave structure as a combination of single- and two-basin cells, similar to those identified in Lake Erie by Schwab, with near-surface velocities largest in the center of the northern and southern basins. Near-inertial bottom kinetic energy is seen to have roughly constant magnitude over large swathes across the basin, with higher magnitude in the shallower areas like the Mid-lake Plateau, as compared with the deep northern and southern basins. The near-bottom near-inertial kinetic energy when mapped appears similar to the bottom topography map. The wave-induced vertical shear across thermocline is concentrated along the longitudinal axis of the lake basin, and both near-bottom velocities and thermocline shear are reasonably explained by a simple conceptual model of the expected transverse variability.

Observations of Near-Inertial Internal Gravity Waves Radiating from a Frontal Jet

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.