Near-inertial Kinetic Energy Research Articles

Near-Inertial Response of a Salinity-Stratified Ocean

Abstract We study the near-inertial response of the salinity-stratified north Bay of Bengal to monsoonal wind forcing using 6 years of hourly observations from four moorings. The mean annual energy input from surface winds to near-inertial mixed layer currents is 10–20 kJ m−2, occurring mainly in distinct synoptic “events” from April–September. A total of fifteen events are analyzed: Seven when the ocean is capped by a thin layer of low-salinity river water (fresh) and eight when it is not (salty). The average near-inertial energy input from winds is 40% higher in the fresh cases than in the salty cases. During the fresh events, 1) mixed layer near-inertial motions decay about two times faster and 2) near-inertial kinetic energy below the mixed layer is reduced by at least a factor of three relative to the salty cases. The near-inertial horizontal wavelength was measured for one fresh and one salty event; the fresh was about three times shorter initially. A linear model of near-inertial wave propagation tuned to these data reproduces 2); the thin (10 m) mixed layers during the fresh events excite high modes, which propagate more slowly than the low modes excited by the thicker (40 m) mixed layers in the salty events. The model does not reproduce 1); the rapid decay of the mixed layer inertial motions in the fresh events is not explained by the linear wave propagation at the resolved scales; a different and currently unknown set of processes is likely responsible.

Near- and Superinertial Internal Wave Responses and the Associated Energy Transfer after the Passage of Tropical Cyclone Fitow at a Midlatitude Shelf Slope

Abstract Observations from a mooring station at the East China Sea (ECS) shelf slope revealed both near- and superinertial dynamic responses to tropical cyclone (TC) Fitow. Different from the typical near-inertial response, near-inertial internal waves (NIWs) after TC Fitow showed a complicated phase pattern due to its superposition with parametric subharmonic instability (PSI)-generated M1 subharmonic waves. The wind-injected near-inertial kinetic energy (NIKE) was largely restrained to the upper 250 m. Wave packet analysis revealed the co-occurrence of enhanced NIKE, circularly polarized near-inertial currents, veering NIW propagation direction, and shrinking NIW vertical wavenumber at the base of the Kuroshio (∼180 m). This indicated the trapping and stalling of the TC-generated NIWs. Intense high-frequency internal waves (HFIWs) appeared immediately after TC Fitow which had an average period of ∼24 min and lasted ∼8 h. HFIWs also existed before the arrival of TC Fitow with a regular semidiurnal cycle. However, the HFIW after TC did not follow the semidiurnal cycle and had much larger amplitudes and longer-lasting periods. Local generation of supercritical flow over a slope or evolution from propagating internal tide as modified by TC may have induced these HFIWs. Along with the occurrence of intense HFIWs after TC Fitow, intense energy transfers from NIWs to HFIWs were identified. Due to the limited vertical propagation of TC-induced NIWs, it was the PSI-generated M1 subharmonic wave rather than the wind-induced NIW that contributed most to the energy transfer. Significance Statement Winds blowing over the ocean generate not only surface waves but also another type, so-called internal waves, which can travel down into the ocean. These internal waves carry a large amount of wind-injected energy into the ocean interior and are thought to play an important role in sustaining ocean turbulent mixing and circulation. We provided evidence that the passage of a tropical cyclone (TC) can induce different types of internal wave responses: near-inertial and high-frequency internal waves. Interactions and energy transfer between different internal waves occurred. With the TC-induced near-inertial internal waves trapped in the upper ocean, the background M1 subharmonic waves contributed most to the interactions with high-frequency internal waves. This fact is crucial in understanding the propagation and dissipation of wind-injected energy in the ocean, which plays an important role in controlling the ocean interior mixing and large-scale circulation.

Spontaneous near-inertial wave generation from mesoscale eddy: Energy transformation

The energy transformation between inertial oscillations (IOs), near-inertial waves (NIWs), and mesoscale eddies during spontaneous NIW generation is analyzed by the kinetic energy equations of the multiple temporal–spatial scale interaction between slow and fast motions and then quantitatively estimated by numerical simulations. The evolution of perturbed quasi-geostrophic mesoscale eddies is accompanied by IOs. The nonlinear interaction of IOs and mesoscale eddies results in spontaneous energy transfer from mesoscale eddies to NIWs. However, IOs act as catalysts in the spontaneous NIW energy transformation process, because there is no energy transfer between NIWs and IOs for longer than one inertial period and NIW energy is entirely transferred from mesoscale eddies. The energy conversion rate (ECR) from a propagating eddy to NIWs is significantly enhanced by the resonance of NIWs and the nonlinear coupling of IOs and mesoscale eddies during the spontaneous NIW generation. The time-averaged NIW ECR from a propagating an anti-cyclonic mesoscale eddy (AE) is approximately 5.725 × 10−6 Wm−2 and nearly 16 times higher than those from the standing AE. The magnitude of global near-inertial kinetic energy generated spontaneously from mesoscale eddies is approximately on the order of 2 GW. Therefore, the spontaneous adjustment of mesoscale eddies is a non-negligible NIW generation mechanism and plays a crucial role in the process of supplying energy from mesoscale eddies to the diapycnal mixing in the ocean interior.

A mechanism of enhanced subsurface near-inertial kinetic energy in the East China Sea associated with successive typhoons

Near-inertial waves (NIWs) play an important role in diapycnal processes and energy dissipation. A mooring observation deployed on the continental shelf in the East China Sea captured anomalously intensified subsurface near-inertial kinetic energy (NIKE) during the passage of Typhoon Danas (2013). An early study has investigated the role of Parametric Subharmonic Instability (PSI) induced by internal tides in the intensification of the subsurface intensified near-inertial velocity. However, results based on regional numerical simulations reveal that strong subsurface near-inertial velocity persists even in the absence of tidal effects, implying the existence of additional sources of NIWs. Our analyses showed that after excluding the effect of PSI, approximately 30% of the remaining subsurface NIKE can be attributed to another Typhoon Fitow (2013), which occurred a week prior to Typhoon Danas. Constrained by the Kuroshio current and the continental shelf, the NIKE generated by Typhoon Fitow propagates northward and reaches the mooring location, leading to the intensified subsurface NIW signal. Our simulation, together with the observations, suggests complicated NIW dynamics in continental shelf regions, involving interactions between successive typhoons, topography and background current, and differing from the open ocean. These interactions will further influence vertical mixing on the continental shelf along the pathway of NIW.

Using NIW Observations to Assess Mixed Layer Parameterizations: A Case Study in the Tropical Atlantic

AbstractTropical sea surface temperature (SST) biases can cause atmospheric biases on global scales, hence SST needs to be represented well in climate models. A major source of uncertainties is the representation of turbulent mixing in the oceanic boundary layer, or mixed layer (ML). In the present study we focus on near‐inertial wave (NIW) induced mixing. The performance of two mixing schemes, Turbulent Kinetic Energy and K‐profile parameterization (KPP), is assessed at two sites (11.5°N, 23°W and 15°N, 38°W) in the tropical Atlantic. At 11.5°N, turbulence observations (eddy diffusivities, shear and stratification) are available for comparison. We find that the schemes differ in their representation of NIWs, but both under‐represent the observed enhanced diffusivities below the observed ML. However, we find that the models do mix below the ML at 15°N when a storm passes nearby. The near‐inertial oscillations remain below the ML for the following 10 days. Near‐inertial kinetic energy (NIKE) biases in the models are not directly correlated with the wind speed, the MLD biases, or the stratification at the ML base. Instead, NIKE biases are sensitive to the vertical mixing scheme parameterization. NIKE biases are lowest when the KPP scheme is used.

Joint Effects of Winds and Tides on Near-Inertial Internal Waves in the Northern South China Sea: A Three-Dimensional Numerical Study

Abstract Joint effects of winds and tides on near-inertial internal waves (NIWs) are numerically investigated via a series of three-dimensional quasi-realistic simulations in the northern South China Sea (NSCS). Model results demonstrate that in the presence of wind-induced NIWs, more tidal energy is transferred to NIWs, while in the presence of tide-induced NIWs, the extreme wind (cyclone) would inject less near-inertial kinetic energy (NIKE). The interaction between wind-induced and tide-induced NIWs produces total NIKE more (or less) than a linear superposition of that generated by wind and tide forcing alone at different sites in the NSCS. Specifically, near the Luzon Strait, both tides and winds make positive contributions to the local near-inertial energy input, resulting in more than 30% enhancement of total NIKE (>0.5 kJ m−2). However, in some deep-water regions along the cyclone paths, energy is transferred from cyclones to NIWs and also from NIWs to internal tides. Due to this “energy pipeline” effect, tide- and wind-induced NIWs contribute to weakening of total NIKE (∼0.3 kJ m−2 or 30%). Additionally, sensitivity experiments with varying initial tidal phases indicate that the interaction between wind-induced NIKE and tide-induced NIKE is robust in most of the model domain (over 80%) under different phase alignments between wind- and tide-induced NIWs. From the perspective of cyclones, tide-induced NIKE is comparable to wind-induced NIKE in the Luzon Strait before the arrival of cyclones, while tide-induced NIKE is two orders of magnitude smaller than wind-induced NIKE in most of the NSCS after the arrival of cyclones. Overall, our results highlight the joint effects of wind and tide forcing on the local NIW dynamics in the NSCS. Significance Statement Near-inertial internal waves (NIWs) are ubiquitous phenomena in stratified water, which can significantly influence the ocean mixing, especially across the thermocline. NIWs are generated mainly by wind forcing near the sea surface, but also by internal tide breaking in the ocean interior. Hence, a question that arises is whether winds or tides play dominant roles in generating NIWs. In fact, due to the interaction between wind- and tide-induced NIWs, the total near-inertial kinetic energy (NIKE) is not merely a linear superposition of that generated by winds and tides forcing alone. By carrying out numerical experiments, we find that extreme winds can help to transfer more energy from tides to NIWs, while tides would suppress energy transferring from winds to NIWs. As a result, this fact is crucial in accurately reproducing NIW dynamics in a targeted region, thereby determining redistribution of local ocean mixing intensity.

Observations of Near-Inertial Internal Waves over the Continental Slope in the Northeastern Black Sea

The article presents observations of near-inertial internal waves (NIWs) in the slope waters of the Black Sea in winter and summer. Rotary spectral analysis of a time series of sea current velocity measurements revealed the prevailing anticyclonic component of the motions near the local inertial frequency f. The clockwise rotation of the velocity vector with depth implies that the NIWs propagate downwards. The amplitude of NIWs usually was 0.1–0.2 m s−1. NIWs were observed in the layer of the permanent pycnocline and the seasonal pycnocline, which attenuate below depths of 160 m and 80 m in winter and summer, respectively. The amplitude of the near-inertial kinetic energy (NIKE) showed a close relationship with vertical stratification. During winter, NIKE exhibited maximum values in the layer of the permanent pycnocline, whereas, in summer, it was primarily observed in the seasonal pycnocline layer. The near-inertial oscillations were generally more energetic in winter.

Observations of Parametric Subharmonic Instability of Diurnal Internal Tides in the Northwest Pacific

Abstract Based on yearlong observations from three moorings at 12°, 14°, and 16°N in the northwest Pacific, this study presents observational evidence for the occurrence and behavior of parametric subharmonic instability (PSI) of diurnal internal tides (ITs) both in the upper and abyssal ocean around the critical latitudes (O1 IT: 13.44°N; K1 IT: 14.52°N), which is relatively less explored in comparison with PSI of M2 ITs. At 14°N, near-inertial waves (NIWs) feature a “checkerboard” pattern with comparable upward- and downward-propagating components, while the diurnal ITs mainly feature a low-mode structure. The near-inertial kinetic energy at 14°N, correlated fairly well with the diurnal KE, is the largest among three moorings. The bicoherence analysis, and a causality analysis method newly introduced here, both show statistically significant phase locking between PSI triads at 14°N, while no significant signals emerge at 12° and 16°N. The estimated PSI energy transfer rate shows a net energy transfer from diurnal ITs to NIWs with an annual-mean value of 1.5 × 10−10 W kg−1. The highly sheared NIWs generated by PSI result in a 2–6 times larger probability of shear instability events at 14°N than 12° and 16°N. Through swinging the local effective inertial frequency close to either O1 or K1 subharmonic frequencies, the passages of anticyclonic and cyclonic eddies both result in elevated NIWs and shear instability events by enhancing PSI efficiency. Particularly, different from the general understanding that cyclonic eddies usually expel NIWs, enhanced NIWs and instability are observed within cyclonic eddies whose relative vorticity can modify PSI efficiency. Significance Statement Parametric subharmonic instability (PSI) effectively transfers energy from low-mode internal tides (ITs) to high-mode near-inertial waves (NIWs), triggering elevated mixing around critical latitudes. This study provides observational evidence for the occurrence of PSI of diurnal ITs in the northwest Pacific and its role in enhancing shear instability. Generally, anticyclonic eddies act to trap NIWs while cyclonic eddies tend to expel NIWs. Here we document elevated NIWs and shear instability within both anticyclonic and cyclonic eddies, which shift the local effective inertial frequency close to either O1 or K1 subharmonic frequencies, thereby enhancing PSI efficiency. Processes associated with PSI and the modulation of PSI efficiency by mesoscale eddies have significant implications for improving mixing parameterizations in ocean circulation and climate models.

Observation of Near‐Inertial Waves Induced by Typhoon Lan in the Northwestern Pacific: Characteristics, Energy Fluxes and Impact on Diapycnal Mixing

AbstractBased on mooring observations from October 10 to 5 November 2017 at four stations in the Northwestern Pacific, the characteristics of four strong near‐inertial wave (NIW) packets generated by the typhoon Lan were examined. The wave‐packet analysis revealed that for the NIWs with larger horizontal wavelengths, as their interactions with the background currents were weakened, the observed frequency was close to the intrinsic frequency. The near‐inertial kinetic energy (NIKE) between two cyclonic eddies penetrated deeper (∼620 m) than that in a negative vorticity region. A ray‐tracing model suggested that it was the strong positive vorticity to the north that caused the northward propagating NIWs to be reflected, and then the reflected NIWs were accelerated to propagate downward at ∼70 m depth where the stratification was strongest. In these two cases, furthermore, the efficiency of the downward propagation of NIKE was at a comparable level of 21%–25%. Energy budget analysis indicated that about 5%–25% of the near‐inertial wind work was injected into the upper 50–200 m, approximately 6%–25% of which could be further radiated to the deeper ocean. On average, after the passage of the typhoon Lan, the dissipation rate increased by 3–8 times, and for the enhanced diapycnal mixing, ∼42% of the energy was provided by the typhoon‐induced strong NIWs.

Observed near-inertial waves generated by tropical and extratropical cyclones in the East China Sea

Both tropical and extratropical cyclones can affect the East China Sea (ECS) shelf in summer. Based on a current mooring deployed in 2014, near-inertial waves (NIWs) triggered by both extratropical and tropical cyclones were examined. During the observation period, three tropical cyclones (i.e., NEOGURI, MATMO, and NAKRI) passed by the mooring site. MATMO and NAKRI led to obvious near-inertial responses. The weakest tropical cyclone (MATMO) caused the strongest NIWs due to its large translation speed and favorable position relative to the mooring site. Mode 1 dominates the vertical structure of the NIWs caused by tropical cyclones. Due to the modulation of the background vorticity, the NIWs show a significant blueshift. However, the NIWs caused by extratropical cyclones are mainly concentrated near the surface layer with a higher proportion of mode-2 NIWs. The highest value of the near-inertial kinetic energy generated by extratropical cyclones is about half of that generated by tropical cyclones. In addition, both two kinds of cyclones can lead to an enhancement of sub-inertial current in the ECS shelf. The nonlinear interactions between NIWs and semidiurnal internal tides (D2) are also evidenced, with the NIWs being a potential trigger for D2-f waves. This study highlights the important role of NIW on the nonlinear interactions and energy transfer within the internal wave spectral continuum in the continental shelf region.

Observed oceanic response to Tropical Cyclone Amphan (2020) from a subsurface mooring in the Bay of Bengal

Upper ocean responses to the super Tropical Cyclone (TC) Amphan and modulation of energy dissipation by internal tides were investigated using in situ observations from a subsurface mooring in Bay of Bengal (BoB). The observations showed that the entire observed water column (150–800 m) cooled significantly, up to a maximum of 2 ℃, and salinity increased significantly at 150–200 m within one week after the TC, mainly due to the TC-induced upwelling. Dynamic response was characterized by strong near-inertial oscillation above 120 m. The near inertial waves (NIWs) had a maximum velocity and kinetic energy at 112.6 m, about 0.34 m/s and 59.5 J/m3, respectively. The energy spectrum and bicoherence analysis indicated that, the nonlinear wave-wave interaction between NIWs (f) and semi-diurnal tide (D2), as well as the self-interaction of NIWs were enhanced after the passage of TC. The nonlinear wave-wave interaction processes were able to transfer energy to higher mode internal waves, thus accelerating energy attenuation. Hence, the observed e-folding time for near-inertial kinetic energy (6.2 days) was only 41 % of the theoretical e-folding time. This study provided a new evidence for the mechanism of energy dissipation of NIWs and semi-diurnal tide during a TC condition in the southern BoB.

Near-Inertial Waves Reaching the Deep Basin in the South China Sea after Typhoon Mangkhut (2018)

Abstract Typhoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm s−1. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ∼3.9 and ∼2.5 m s−1 north and south of the typhoon’s track, respectively, while those of mode 3 were ∼2.1 and ∼1.7 m s−1, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode-2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ∼300 m and lengthened the duration of energetic NIWs observed in the north. Significance Statement Near-inertial waves (NIWs), generally caused by wind (e.g., typhoons and monsoons) in the upper ocean, are one of the two types of energetic internal waves widely observed in the ocean. After their generation near the surface, energetic NIWs propagate downward and equatorward, thereby significantly contributing to turbulent mixing in the upper and deep ocean and acting as a mechanism of energy transfer from the surface to the deep ocean. The unprecedented NIW observations in the South China Sea describe the generation, propagation, and vertical normal modes of typhoon-induced NIWs in the upper and deep oceans, and contribute to knowledge regarding the dynamic responses of abyssal processes to typhoons.

Numerical simulation of upper ocean responses to Typhoon Maria (2018) in the Northwest Pacific: Behavior of sub-mesoscale processes

A deeper understanding of the upper ocean response to typhoons is beneficial to the further improvement of typhoon forecasting. However, because of model horizontal resolution limitations, we still do not understand how sub-mesoscale processes in the ocean may modulate the response of the upper ocean to typhoons. In this study, we set up one-way online nested parent (mesoscale resolving: Exp_9 km) and child (sub-mesoscale permitting: Exp_3 km) models to investigate how the 15–45-km sub-mesoscale processes in the ocean affected the thermal and dynamical response processes of the upper ocean to Super Typhoon Maria (2018). It was found that the sub-mesoscale processes and their induced vertical heat transport (VHT) were mainly concentrated at the eddy, and most of the sub-mesoscale processes and their accompanying VHT disappeared after the typhoon, indicating that typhoons may be destroyers of sub-mesoscale processes. The differences in the sea temperature response between Exp_3 km and Exp_9 km can reached 1.5 °C in the eddy-rich sea surface region and 3 °C at the thermocline. This thermal response difference could be attributed to the difference in vertical diffusion and the difference in total advection processes simulated by the two models. Furthermore, the sub-mesoscale VHT played an important role in causing smaller sea surface cooling (SSC) and larger subsurface warming to the right side of the typhoon, and considering more sub-mesoscale processes improved the simulation accuracy of the SSC on the right side of the typhoon. The difference in near-inertial kinetic energy (NIKE) and the difference in fD1 (a peak frequency approximately equal to the sum of f and D1) kinetic energy between the two experiments were mainly found in the eddy-rich region to the right side of the typhoon, accounting for, at most, 15% of Exp_3 km's NIKE and 50% of Exp_3 km's fD1 kinetic energy, respectively, and resolving more sub-mesoscale processes could limit the lateral propagation of NIKE, especially in warm eddy. The kinetic energy and vertical shear of the near-inertial currents (NICs) and fD1 currents of Exp_3 km were larger than those of Exp_9 km, suggesting that sub-mesoscale processes may enhance ocean vertical mixing. Moreover, sub-mesoscale processes promoted the downward propagation of NIKE, which may be of great significance for promoting NICs-induced ocean mixing into the deep ocean and accelerating the decay of NICs. This work is of great importance for improving the understanding of typhoon-induced thermal and dynamical responses, and thus for improving the typhoon prediction.

Modal content of typhoon-induced near-inertial waves around the East China Sea

Recent studies show that typhoon-induced near-inertial waves (NIWs) exhibit different modal content in the shallow and deep regions, whereas the underlying mechanism has not been fully understood. Based on a three-dimensional numerical model, we simulate the NIWs induced by two typhoons (Songda and Meari in 2011) in the East China Sea and its adjacent deep waters in the northwest Pacific Ocean. Although the near-inertial kinetic energy induced by the two typhoons has different intensity and spatial distribution, the modal content of NIWs induced by them shares a common characteristic: In the shallow regions (water depth smaller than 200 m), the NIWs are mainly dominated by mode-1; whereas in the deep regions (water depth larger than 200 m), the NIWs are dominated by higher modes (mode-2 to mode-5). Different mechanisms are found to account for the different modal content in shallow and deep regions. The dominance of higher-mode wind forcing is the cause for the dominance of higher-mode NIWs in the deep regions. The two-layer-structure stratification is the cause for the dominance of mode-1 NIWs in the shallow regions.

Amplification of typhoon-generated near-inertial internal waves observed near the Tsushima oceanic front in the Sea of Japan

It is not fully understood how near-inertial kinetic energy (NIKE) is spatially distributed near Tsushima oceanic front (TOF) as a typhoon travels across the region. Underneath TOF, a year-round mooring covering a major part of water column was implemented in 2019. During summer, three massive typhoons (Krosa, Tapah, and Mitag) consecutively traversed the frontal area and delivered a substantial amount of NIKE into surface mixed layer. According to a mixed-layer slab model, NIKE was widely distributed near the cyclone’s track. The mooring observation exhibited the vertical distribution and pathways of surface-generated NIKE in response to the successive typhoon events. According to the modal decomposition, first three modes mostly explain the NIKE’s elevations following the typhoon events. According to ray-tracing experiments based on the internal-wave theory, large-scale near-inertial waves (NIWs) rapidly descend to a depth greater than 1000 m, while mesoscale NIWs slowly descend and rarely reached beyond the main pycnocline. Following the passage of Tapah, a profound energy mass was found nearly stationary at shallow depths coincident with vertical shear of geostrophic current. We infer that the descending rate of NIWs fell and then they were amplified through the energy conservation when the waves came from the north side of TOF.

Spatial and seasonal variations of near-inertial kinetic energy in the upper Ross Sea and the controlling factors

The spatial distribution and seasonal variation of near-inertial kinetic energy (NIKE) in the upper Ross Sea (RS) are examined using the 1/4° NEMO3.6-LIM3 sea ice physical–biological coupled model. The annual-mean surface and mixed layer-integrated NIKE have large values at the shelf break around Iselin Bank and the northeast area outside the continental shelf. The spatial distribution of the surface and mixed layer-integrated NIKE in the austral summer RS is mainly controlled by the near-inertial wind stress magnitude (NIWSM), which acts as the energy source. The northeast high NIKE agrees with the large NIWSM there. Semidiurnal tides only contribute to the NIKE peak around the Iselin Bank and shelf break. The change of mixed layer depth (MLD) can induce spatial discrepancies between surface and mixed layer-integrated NIKE. The shallower MLD in the western RS limits the near-inertial energy in a thin layer, which has induced the large surface NIKE there. The NIKE in the upper RS has a pronounced seasonal variation with the largest surface and mixed layer-integrated NIKE both observed during austral summer. The seasonal variation of NIWSM and sea ice concentration play an important role in inducing the seasonal variation of NIKE. While the NIWSM acts as the main energy source of NIKE, the presence of sea ice can prevent the wind from directly acting on the surface ocean, inducing the low NIKE during austral winter.

Observation of near-inertial waves in the wake of four typhoons in the northern South China Sea

Based on the velocity and temperature data recorded by two acoustic Doppler current profilers (ADCPs) at a mooring system deployed in the northern South China Sea (SCS), this study investigates the characteristics of near-inertial waves (NIWs) induced by typhoons Bebinca, Barijat, Mangkhut and Yutu in 2018. For the dynamical response, besides the motion of near inertial frequency induced by typhoons, the motion of 2 f ([1.80–2.20] f, f is the local inertial frequency) and f D1 (a harmonic wave with a frequency equal to the sum of frequencies of NIWs and diurnal tides) frequency will also increase. For near-inertial motions, the maximum near-inertial kinetic energy (NIKE) is confined to depths above 150 m. For stronger (weaker) wind forcing, the longer (shorter) the response time of the ocean to the atmospheric forcing is, and the shorter (longer) the response time is required in relaxation stage. There are upward and downward propagating energies after the passage of typhoons, and the upward propagating energy mainly occur in the stage of the geostrophic balance adjustment. The current structure suggests that the NIWs in the vertical direction are two antisymmetric rotary vortices in a near-inertial period, which is similar to the structure of the Langmuir circulation. Besides, the horizontal near-inertial currents (NICs) are much stronger than the vertical NICs, and the stronger the NIWs are, the stronger the horizontal NICs relative to the vertical NICs are. For the temperature response, the temperature variation reflects a clear stratified vertical structure. In the forcing stage, the upper layer becomes colder, the lower layer becomes warmer, and the thickness and intensity of the thermocline decrease. In the relaxation stage, the upper layer warms and the lower layer cools, and the thickness and intensity of thermocline increase.

Observation of Near-Inertial Internal Gravity Waves in the Southern South China Sea

Two sets of more than 850 days of mooring records and satellite altimeter data are used to explore the features and spatiotemporal evolution of near-inertial waves (NIWs) near Nansha Island in the southern South China Sea (SCS). The observed NIWs are dominated by clockwise (downward energy propagation) motions and show a clear blue shift with a distinct peak frequency of 1.09 f during two large NIW events. The near-inertial kinetic energy (NIKE) is primarily concentrated in the upper layer and radiated downward. The largest value of depth-integrated NIKE reaches 3.5 KJ/m2. Besides, the NIWs are dominated by the first three modes, which account for 80% of the total NIKE. Moreover, the depth-integrated NIKE exhibits an apparent seasonal variation, with the largest NIKE in winter, which is almost three times larger than that in other seasons. Every large NIKE event is attributed to the passage of storms and is dominated by mode-2 NIWs. The dominance of the mode-2 NIWs is likely caused by the interaction between NIWs and mesoscale eddies.

Parametric Subharmonic Instability of Diurnal Internal Tides in the Abyssal South China Sea

Abstract Internal waves close to the seafloor of abyssal oceans are the key energy suppliers driving near-bottom mixing and the upwelling branches of meridional overturning circulation, but their spatiotemporal variability and intrinsic mechanisms remain largely unclear. In this study, measurements from 10 long-term moorings were used to investigate the internal wave activities in the abyssal South China Sea, which is an important upwelling zone. Strong near-inertial internal waves (NIWs) with current velocity pulses exceeding 5 cm s−1 were observed to dominate the near-bottom internal wave field at approximately 14°N. These abyssal NIWs were phase-coupled with diurnal internal tides (D1), and both displayed common seasonal variations that were larger in winter and summer, providing evidence of diurnal parametric subharmonic instability (PSI) near its critical latitudes (CLs). Emitted from the bottom, near-inertial kinetic energy rapidly decreased by one order of magnitude from depths of ∼120 to ∼620 m above the bottom. Near rough topographies, the abyssal PSI was shifted poleward to approximately 14.8°N by negative relative vorticities of passing anticyclonic eddies or topographic Rossby waves. Compared with flat topography, PSI near rough topography was significantly promoted by topographic-localized strong D1 with high-mode structures, creating abyssal NIW bursts. Bottom-reaching shipboard conductivity–temperature–depth profiles revealed that the bottom mixed layers became much thicker when approaching CLs, suggesting that abyssal PSI potentially accelerates the ventilation and upwelling of bottom water. The observational results presented here illustrate notable spatiotemporal variations in abyssal NIWs regulated by PSI and call for consideration of PSI to better understand near-bottom mixing and upwelling.

A numerical study of near-inertial motions in the Mid-Atlantic Bight area induced by Hurricane Irene (2011)

Abstract. Hurricane Irene generated strong near-inertial currents (NICs) in ocean waters when passing over the Mid-Atlantic Bight (MAB) of the US East Coast in late August 2011. It is demonstrated that a combination of valuable field data and detailed model results can be taken advantage of to study the development and decay mechanism of this event. Numerical results obtained with the Regional Oceanic Modeling System (ROMS) are shown to agree well with the field data. Both computed and observed results show that the NICs were significant in most areas of the MAB region except in the nearshore area where the stratification was totally destroyed by the hurricane-induced strong mixing. Based on the energy budget, it is clarified that the near-inertial kinetic energy (NIKE) was mainly gained from the wind power during the hurricane event. In the deepwater region, NIKE was basically balanced by the vertical turbulence diffusion (40 %) and downward divergence (33 %), while in the continental shelf region, NIKE was mainly dissipated by the vertical turbulence diffusion (67 %) and partially by the bottom friction (24 %). Local dissipation of NIKE due to turbulence diffusion is much more closely related to the rate of the vertical shear rather than the intensity of turbulence. The strong vertical shear at the offshore side of the continental shelf led to a rapid dissipation of NIKE in this region.