Near-inertial Energy Research Articles

Controls on turbulent mixing on the West Antarctic Peninsula shelf

The ocean-to-atmosphere heat budget of the West Antarctic Peninsula is controlled in part by the upward flux of heat from the warm Circumpolar Deep Water (CDW) layer that resides below ~200m to the Antarctic Surface Water (AASW), a water mass which varies strongly on a seasonal basis. Upwelling and mixing of CDW influence the formation of sea ice in the region and affect biological productivity and functioning of the ecosystem through their delivery of nutrients. In this study, 2.5-year time series of both Acoustic Doppler Current Profiler (ADCP) and conductivity-temperature-depth (CTD) data are used to quantify both the diapycnal diffusivity κ and the vertical heat flux Q at the interface between CDW and AASW. Over the period of the study, a mean upward heat flux of ~1Wm−2 is estimated, with the largest heat fluxes occurring shortly after the loss of winter fast ice when the water column is first exposed to wind stress without being strongly stratified by salinity. Differences in mixing mechanisms between winter and summer seasons are investigated. Whilst tidally-driven mixing at the study site occurs year-round, but is likely to be relatively weak, a strong increase in counterclockwise-polarized near-inertial energy (and shear) is observed during the fast-ice-free season, suggesting that the direct impact of storms on the ocean surface is responsible for much of the observed mixing at the site. Given the rapid reduction in sea-ice duration in this region in the last 30 years, a shift towards an increasingly wind-dominated mixing regime may be taking place.

Overlooked Role of Mesoscale Winds in Powering Ocean Diapycnal Mixing.

Diapycnal mixing affects the uptake of heat and carbon by the ocean as well as plays an important role in global ocean circulations and climate. In the thermocline, winds provide an important energy source for furnishing diapycnal mixing primarily through the generation of near-inertial internal waves. However, this contribution is largely missing in the current generation of climate models. In this study, it is found that mesoscale winds at scales of a few hundred kilometers account for more than 65% of near-inertial energy flux into the North Pacific basin and 55% of turbulent kinetic dissipation rate in the thermocline, suggesting their dominance in powering diapycnal mixing in the thermocline. Furthermore, a new parameterization of wind-driven diapycnal mixing in the ocean interior for climate models is proposed, which, for the first time, successfully captures both temporal and spatial variations of wind-driven diapycnal mixing in the thermocline. It is suggested that as mesoscale winds are not resolved by the climate models participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5) due to insufficient resolutions, the diapycnal mixing is likely poorly represented, raising concerns about the accuracy and robustness of climate change simulations and projections.

A three-component model for the coupled evolution of near-inertial waves, quasi-geostrophic flow and the near-inertial second harmonic

We derive an asymptotic model that describes the nonlinear coupled evolution of (i) near-inertial waves (NIWs), (ii) balanced quasi-geostrophic flow and (iii) near-inertial second harmonic waves with frequency near $2f_{0}$, where $f_{0}$ is the local inertial frequency. This ‘three-component’ model extends the two-component model derived by Xie & Vanneste (J. Fluid Mech., vol. 774, 2015, pp. 143–169) to include interactions between near-inertial and $2f_{0}$ waves. Both models possess two conservation laws which together imply that oceanic NIWs forced by winds, tides or flow over bathymetry can extract energy from quasi-geostrophic flows. A second and separate implication of the three-component model is that quasi-geostrophic flow catalyses a loss of NIW energy to freely propagating waves with near-$2f_{0}$ frequency that propagate rapidly to depth and transfer energy back to the NIW field at very small vertical scales. The upshot of near-$2f_{0}$ generation is a two-step mechanism whereby quasi-geostrophic flow catalyses a nonlinear transfer of near-inertial energy to the small scales of wave breaking and diapycnal mixing. A comparison of numerical solutions with both Boussinesq and three-component models for a two-dimensional initial value problem reveals strengths and weaknesses of the model while demonstrating the extraction of quasi-geostrophic energy and production of small vertical scales.

Near-inertial motions in the DeSoto Canyon during Hurricane Georges

Hurricane Georges passed directly over an array of 13 moorings deployed in the DeSoto Canyon in the northern Gulf of Mexico on 27–28 September 1998. Current velocity data from the mooring array were analyzed together with a primitive-equation model simulation with realistic hurricane forcing, to characterize the generation and propagation of the hurricane-generated near-inertial waves. The model successfully reproduces the observed mean (sub-inertial) and near-inertial motions. The upper ocean response is strongly impacted by the canyon ‘wall’: a strong jet is formed along the slope, and the near-inertial motions on the shelf are rapidly suppressed. The model results moreover suggest that strong near-inertial waves in the mixed layer are mostly trapped in an energy flux recirculating gyre around the canyon. This gyre retains the near-inertial energy in the canyon region and enhances the transfer of near-inertial energy below the mixed layer. Additional simulations with idealized topographies show that the presence of a steep slope rather than the canyon is fundamental for the generation of this recirculating gyre. The near-inertial wave energy budget shows that during the study period the wind generated an input of 6.79×10–2Wm–2 of which about 1/3, or 2.43×10–2Wm–2, was transferred below the mixed layer. The horizontal energy flux into and out of the canyon region, in contrast, was relatively weak.

Three-Dimensional Distribution of Turbulent Mixing in the South China Sea

AbstractA three-dimensional distribution of turbulent mixing in the South China Sea (SCS) is obtained for the first time, using the Gregg–Henyey–Polzin parameterization and hydrographic observations from 2005 to 2012. Results indicate that turbulent mixing generally increases with depth in the SCS, reaching the order of 10−2 m2 s−1 at depth. In the horizontal direction, turbulence is more active in the northern SCS than in the south and is more active in the east than the west. Two mixing “hotspots” are identified in the bottom water of the Luzon Strait and Zhongsha Island Chain area, where diapycnal diffusivity values are around 3 × 10−2 m2 s−1. Potential mechanisms responsible for these spatial patterns are discussed, which include internal tide, bottom bathymetry, and near-inertial energy.

The diapycnal mixing in the upper Pacific estimated from GTSPP observations

Diapycnal mixing (DM) in the upper 600 m of the Pacific Ocean was estimated based on the huge amount of the observations from Global Temperature-Salinity Profile Programme (GTSPP), using the strain version of the finescale parameterization. It is found that DM in each season exhibits similar distribution pattern, but differs in details. The intensification of DM is related to bottom roughness, surface near-inertial energy, and proximity to the equator. The intensified DM caused by rough topography shows in the profiles near the Mendocino fracture zone in the northeast Pacific, and the heightened DM caused by wind-generated near-inertial energy appears in the westerly region of the Southern Ocean. As compared to previous estimates, the DM estimate in this work has better spatial coverage and finer resolution, and more importantly it contains the seasonal variability. Furthermore, the resulting DM dataset is gridded, rendering it suitable for modeling applications.

Latitudinal Dependence of Wind-Induced Near-Inertial Energy

AbstractMidlatitude storms, accounting for the majority of wind energy input to near-inertial motions in the ocean, are known to shift their track significantly from one year to another. The consequence of such storm track shifts on wind-induced near-inertial energy (NIE) is yet unknown. Here, the latitudinal dependence of wind-induced NIE is first analyzed in the framework of the slab model and then tested using two numerical ocean models. It is found that the NIE input by pure inertial wind stress forcing, which dominates the wind energy input to near-inertial motions, is independent of latitude. As a consequence, the NIE generated by white-noise wind stress forcing is also latitudinally independent. In contrast, the NIE generated by red-noise wind stress forcing shows strong dependence on latitude owing to longer inertial periods at lower latitudes capable of sampling greater inertial wind stress forcing. Given that the observed surface wind stress spectra are red, results from this study suggest that an equatorward shift of the storm track is likely to result in an increase in wind-induced NIE in the ocean, while the opposite is true for a poleward shift.

Effect of surface mesoscale eddies on deep-sea currents and mixing in the northeastern South China Sea

Recent studies suggest that deep-reaching surface-generated eddies result in anomalous current velocities in the deep sea, and ultimately lead to energy transfer from mesoscale to small-scale motions. Here we examine the influence of mesoscale eddies on deep-sea subinertial and near-inertial currents, and on possible enhanced oceanic mixing in the deep South China Sea (SCS). We analyzed current velocity data for nearly a full water column. Data were obtained using acoustic Doppler current profilers and recording current meters on a deep-sea mooring system at a depth of 2100m in the northeastern SCS from October 2012 to May 2013. A highly nonlinear southwestward-propagating anticyclonic eddy was detected via a resolved sea-surface-level anomaly. This eddy induced pronounced subinertial currents with a characteristic time scale of 1–2 months and a maximum velocity of up to 0.2ms–1 at the subsurface and 0.1ms–1 at great depth. Near-inertial energy co-occurring with subinertial flows showed a distinctive vertical propagation trend during strong subinertial oscillations in the deep sea. During periods of strong subinertial and near-inertial kinetic energy, estimates of diapycnal diffusivity in the deep ocean showed approximately 10-fold enhancement, with a mean value of 1.2×10–3m2s–1 compared to the background value of 1.4×10–4m2s–1. The results provide observational evidence of the effect of surface-observed mesoscale motions on benthic currents and ocean mixing in the deep SCS.

Features of near-inertial motions observed on the northern South China Sea shelf during the passage of two typhoons

Features of near-inertial motions on the shelf (60 m deep) of the northern South China Sea were observed under the passage of two typhoons during the summer of 2009. There are two peaks in spectra at both sub-inertial and super-inertial frequencies. The super-inertial energy maximizes near the surface, while the sub-inertial energy maximizes at a deeper layer of 15 m. The sub-inertial shift of frequency is induced by the negative background vorticity. The super-inertial shift is probably attributed to the near-inertial wave propagating from higher latitudes. The near-inertial currents exhibit a two-layer pattern being separated at mid-depth (25–30 m), with the phase in the upper layer being nearly opposite to that in the lower layer. The vertical propagation of phase implies that the near-inertial energy is not dominantly downward. The upward flux of the near-inertial energy is more evident at the surface layer (<17 m). There exist two boundaries at 17 and 40 m, where the near-inertial energy is reflected upward and downward. The near-inertial motion is intermittent and can reach a peak of as much as 30 cm/s. The passage of Typhoon Nangka generates an intensive near-inertial event, but Typhoon Linfa does not. This difference is attributed to the relative mooring locations, which is on the right hand side of Nangka’s path (leading to a wind pattern rotating clockwise with time) and is on the left hand side of Linfa’s path (leading to a wind pattern rotating anti-clockwise with time).

The spatiotemporal variation of the wind-induced near-inertial energy flux in the mixed layer of the South China Sea

On the basis of the QSCAT/NCEP blended wind data and simple ocean data assimilation (SODA), the wind-induced near-inertial energy flux (NIEF) in the mixed layer of the South China Sea (SCS) is estimated by a slab model, and the model results are verified by observational data near the Xisha Islands in the SCS. Then, the spatial and temporal variations of the NIEF in the SCS are analyzed. It is found that, the monthly mean NIEF exhibits obvious spatial and temporal variabilities, i.e., it is large west of Luzon Island all the year, east of the Indo-China Peninsula all the year except in spring, and in the northern SCS from May to September. The large monthly mean NIEF in the first two zones may be affected by the large local wind stress curl whilst that in the last zone is probably due to the shallow mixed layer depth. Moreover, the monthly mean NIEF is relatively large in summer and autumn due to the passage of typhoons. The spatial mean NIEF in the mixed layer of the SCS is estimated to be about 1.25 mW/m2 and the total wind energy input from wind is approximately 4.4 GW. Furthermore, the interannual variability of the spatial monthly mean NIEF and the Nino 3.4 index are negatively correlated.

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.

Near-Inertial Mixing in the Central Arctic Ocean

Abstract Observations were made in April 2007 of horizontal currents, hydrography, and shear microstructure in the upper 500 m from a drifting ice camp in the central Arctic Ocean. An approximately 4-day-long time series, collected about 10 days after a storm event, shows enhanced near-inertial oscillations in the first half of the measurement period with comparable upward- and downward-propagating energy. Rough estimates of wind work and near-inertial flux imply that the waves were likely generated by the previous storm. The near-inertial frequency band is associated with dominant clockwise rotation in time of the horizontal currents and enhanced dissipation rates of turbulent kinetic energy. The vertical profile of dissipation rate shows elevated values in the pycnocline between the relatively turbulent underice boundary layer and the deeper quiescent water column. Dissipation averaged in the pycnocline is near-inertially modulated, and its magnitude decays approximately at a rate implied by the reduction of energy over time. Observations suggest that near-inertial energy and internal wave–induced mixing play a significant role in vertical mixing in the Arctic Ocean.

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 &amp;gt;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.

Tropical storm-forced near-inertial energy dissipation in the southeast continental shelf region of Hainan Island

Near-inertial motion is an important dynamic process in the upper ocean and plays a significant role in mass, heat, and energy transport across the thermocline. In this study, the dissipation of wind-induced near-inertial energy in the thermocline is investigated by using observation data collected in July and August 2005 during the tropical storm Washi by a moored system at (19A degrees 35'N, 112A degrees E) in the continental shelf region off Hainan Island. In the observation period, the near-inertial part dominated the observed ocean kinetic energy and about 80% of the near-inertial energy dissipated in the upper layer. Extremely strong turbulent mixing induced by near-inertial wave was observed in the thermocline, where the turbulent energy dissipation rate increased by two orders of magnitude above the background level. It is found that the energy loss of near-inertial waves in the thermocline is mainly in the large-scales. This is different from the previous hypothesis based on "Kolmogorov cascade" turbulence theory that the kinetic energy is dissipated mainly by small-scale motions.

On the spatial and temporal distribution of near‐inertial energy in the Southern Ocean

We use an eddying realistic primitive‐equation model of the Southern Ocean to examine the spatial and temporal distribution of near‐inertial wind‐power input (WPI) and near‐inertial energy (NIE) in the Southern Ocean. We find that the modeled near‐inertial WPI is almost proportional to inertial wind‐stress variance (IWSV), while the modeled NIE is modulated by the inverse of the mixed‐layer depth. We go on to assess recent decadal trends of near‐inertial WPI from trends of IWSV based on reanalysis wind stress. Averaged over the Southern Ocean, annual‐mean IWSV is found to have increased by 16% over the years 1979–2011. Part of the increase of IWSV is found to be related to the positive trend of the Southern‐Annular Mode over the same period. Finally, we show that there are horizontal local maxima of NIE at depth that are almost exclusively associated with anticyclonic eddies.

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.

Simulation of wind-induced near-inertial oscillations in a mixed layer near the east coast of Korea in the East/Japan Sea

Using a simple damped slab model, it was possible to show that a local wind induced 88% (15 of 17) of the near-inertial oscillations (NIO) observed in the mixed layer near the east coast of Korea from 1999 to 2004. The model, however, overestimated the energy level in about two-thirds of the simulated cases, because the slab model was forced with winds whose characteristic period was shorter than the damping time scale of the model at 1.5 d. At the observation site, due to typhoons and orographic effects, high-frequency wind forcing is quite common, as is the overestimation of the energy level in the slab model results. In short, a simple slab model with a damping time-scale of about 1.5 d would be enough to show that the local wind was the main energy source of the near-inertial energy in this area, but the model could not be used to accurately estimate the amount of the work done by the wind to the mixed layer.

Observations of near‐inertial energy in Lake Superior

Directly measured velocity data collected in Lake Superior between 2008 and 2011 show that currents in the open waters of the lake are dominated by near‐inertial energy. The near‐inertial signal is composed almost entirely of clockwise rotation, with vertical structure dominated by the first baroclinic mode, where waters above and below the thermocline are roughly 180° out of phase with each other. The strength of the oscillations is strongly related to the strength of the stratification; in periods of the year when the water column is well‐mixed (typically late autumn and late spring) the near‐inertial signal is very weak; when stratification exists, near‐inertial oscillations can occur. Combining the velocity amplitudes with an estimate of the thermocline displacement allows estimation of the dominant direction and horizontal wavelength of the near‐inertial field, showing that horizontal wavelengths are on the order of 50–100 km, and the direction of the waves veers counter‐clockwise over the course of the season with a period of ∼ 1 month. Observations of backscatter suggest that inertial oscillations may be responsible for re‐suspension of bottom sediments, which could have significant ecological consequences.

A year of internal Poincaré waves in southern Lake Michigan

A unique set of full year, deep water observations from the middle of Lake Michigan's southern basin are analyzed to quantify the seasonal variability of the dominant near‐inertial internal Poincaré wave. At this mid‐lake location, the Poincaré wave is seen to describe more than 80% of the observed surface current variability for much of the year, with characteristic near‐inertial frequency and clockwise‐rotating velocities. The dominance of the near‐inertial seiche on the flow decreases with depth. The wave persists during the “stratified period,” roughly May through late December, and is supported by as few as 1–2 degrees of thermal stratification over 150 m; only after complete water column mixing does the wave go dormant for January through April. The strongest Poincaré wave activity is seen to correspond to the period of strongest summer thermal stratification (August), in spite of the relatively weak winds at this time. A simple inertial slab model optimized with linear friction is shown to capture the seasonal variability of the near‐inertial energy at this location reasonably well. The vertical structure of the wave shows good agreement with that calculated with a standard normal modes formulation, which is in turn used to characterize the potential shear and mixing caused by the wave. Late‐spring and summer events of elevated Poincaré wave activity are shown to generate sufficiently strong shear with persistent periods of sub‐1 Richardson numbers within the thermocline, suggesting that the near‐inertial seiche is likely generating thermocline instabilities in the lake's interior.

Enhanced vertical mixing within mesoscale eddies due to high frequency winds in the South China Sea

The South China Sea is a marginal basin with a complex circulation influenced by the East Asian Monsoon, river discharge and intricate bathymetry. As a result, both the mesoscale eddy field and the near-inertial energy distribution display large spatial variability and they strongly influence the oceanic transport and mixing.With an ensemble of numerical integrations using a regional ocean model, this work investigates how the temporal resolution of the atmospheric forcing fields modifies the horizontal and vertical velocity patterns and impacts the transport properties in the basin. The response of the mesoscale circulation in the South China Sea is investigated under three different forcing conditions: monthly, daily and 6-hourly momentum and heat fluxes.While the horizontal circulation does not display significant differences, the representation of the vertical velocity field displays high sensitivity to the frequency of the wind forcing. If the wind field contains energy at the inertial frequency or higher (daily and 6-hourly cases), then submesoscale fronts, vortex Rossby waves and near inertial waves are excited as ageostrophic expression of the vigorous eddy field. Those quasi- and near-inertial waves dominate the vertical velocity field in the mixed layer (vortex Rossby waves) and below the first hundred meters (near inertial waves) and they are responsible for the differences in the vertical transport properties under the various forcing fields as quantified by frequency spectra, vertical velocity profiles and vertical dispersion of Lagrangian tracers.