Diurnal Internal Tides Research Articles

Coastal‐trapped behavior of the diurnal internal tide at O'ahu, Hawai'i

AbstractThe influence of rotation on the structure and propagation of internal tides around O'ahu, Hawai'i is investigated using in situ observations and a tidally forced, primitive equation model with realistic bathymetry and stratification. Particular attention is given to the diurnal internal tide, which largely has been de‐emphasized in previous studies of the region because of the dominance of the semidiurnal internal tide but has been determined by recent studies to be a significant contributor to baroclinic variability. Though both diurnal and semidiurnal internal tides are generated primarily over Ka'ena Ridge to the northwest of the island, the diurnal internal tide propagates clockwise around the island as an imperfectly trapped wave, while the semidiurnal internal tide propagates away from the ridge, unaffected by rotation. The diurnal and semidiurnal internal tides fall into the superinertial frequency range; however, the diurnal frequency apparently is sufficiently close to inertial (∼ 1.4f) for rotation to affect internal tide propagation. The in situ observations support the model finding that diurnal trapping provides the primary source of baroclinic variability along the eastern coast of the island, a stretch of coastline otherwise sheltered from the internal tide energy generated over the Hawaiian Ridge. The findings in Hawai'i suggest that coastal trapping of superinertial internal tides may be a significant source of variability and mixing in other nearshore systems around the world.

Coherent and incoherent features, seasonal behaviors and spatial variations of internal tides in the northern South China Sea

Abstract Based on observations of the currents at six moorings from March 2010 to April 2011, the coherent and incoherent features, seasonal behaviors and spatial variations of internal tides (ITs) in the northern South China Sea (SCS) are investigated. Measurements of the currents indicate that both diurnal and semidiurnal ITs contain stronger coherent signals than incoherent ones at all moorings. In the measuring range, coherent internal tidal current variances explain ~ 70% of the semidiurnal motion at most moorings. However, the proportion of coherent signals in the diurnal motion shows a non-monotonically decreasing trend with the westward propagation of diurnal ITs. Coherent signals of diurnal and semidiurnal ITs exhibit different seasonal variability at the six moorings: Diurnal ITs are stronger in winter (December to February) and summer (June to August) than in spring (March to May) and autumn (September to November), whereas stronger semidiurnal ITs always appear in spring and autumn. Combining these results with the temporal variation of barotropic tidal currents at the Luzon Strait (LS), it can be concluded that the seasonal variability of ITs at the six moorings are determined by the barotropic tides at the LS. In addition, this study shows that there are asymmetric features of ITs to the east and west of the LS.

Internal Tide Convergence and Mixing in a Submarine Canyon

AbstractObservations from Eel Canyon, located on the north coast of California, show that elevated turbulence in the full water column arises from the convergence of remotely generated internal wave energy. The incoming semidiurnal and bottom-trapped diurnal internal tides generate complex interference patterns. The semidiurnal internal tide sets up a partly standing wave within the canyon due to reflection at the canyon head, dissipating all of its energy within the canyon. Dissipation in the near bottom is associated with the diurnal trapped tide, while midwater isopycnal shear and strain is associated with the semidiurnal tide. Dissipation is elevated up to 600 m off the bottom, in contrast to observations over the flat continental shelf where dissipation occurs closer to the topography. Slope canyons are sinks for internal wave energy and may have important influences on the global distribution of tidally driven mixing.

Three-dimensional numerical simulation of internal tides that radiated from the Luzon Strait into the Western Pacific

Recent satellite altimeter observations have indicated that internal tides (ITs) from the Luzon Strait (LS) propagate more than 2 500 km into the Western Pacific (WP). This study used a high-resolution three-dimensional numerical model to reproduce and examine the ITs radiation process. The propagation of diurnal and semidiurnal ITs showed different patterns and variations. Diurnal ITs with lower frequency were affected more by the earth’s rotation and they were bent more toward the equator than semidiurnal ITs. ITs phase speeds are functions of latitude and diurnal ITs showed greater distinctions of phase speeds during propagation. For M2 ITs, the wavelength remained nearly unchanged but the beam width increased significantly during propagation away from the LS. For diurnal ITs (K1 and O1), the wavelength decreased noticeably with latitude, while the beam width varied little during propagation because of blocking by land. Baroclinic energy was also examined as a complement to satellite results reported by Zhao (2014). The magnitude of the generated baroclinic energy flux reduced remarkably within 300 km from the generation site but it then decayed slowly when propagating into abyssal sea. Baroclinic energy of diurnal ITs was found to dissipate at a slower rate than semidiurnal ITs along the main propagation path in the WP.

Temporal variations in internal tide multimodal structure on the continental shelf, South China Sea

Temporal variations in multimodal structures of diurnal (D 1) and semidiurnal (D 2) internal tides were investigated on the continental slope of the Dongsha Plateau, based on 2-month moored acoustic Doppler current profiler observations. Harmonic analysis indicated that the D 1 components (K 1 and O 1) dominated the internal tide field. The vertical structure of the K 1 constituent presented a first-mode structure while the M 2 constituent seemed to exhibit a high-mode structure. Amplitude spectra analysis of the current data revealed differences in baroclinic current amplitudes between different water depths. Temporal variations in modal structures ware analyzed, based on the D 1 and D 2 baroclinic tides extracted from the baroclinic velocity field with band-pass filters. Analysis showed that the magnitude of the D 1 internal tide current was much larger than the D 2 current, and temporal variations in the modal structure of the D 1 internal tide occurred on an approximately fortnightly cycle. The EOF analyses revealed temporal transformation of multimodal structures for D 1 and D 2 internal tides. The enhancement of the D 1 internal tide was mainly due to the superposition of K 1 and O 1, according to the temporal variation of coherent kinetic energy.

Coherent and incoherent internal tides in the southern South China Sea

Coherent and incoherent internal tides (CITs and ICITs) in the southern South China Sea were investigated from two sets of 18-month mooring current records. The CITs were mainly composed of diurnal Q 1, O 1, P 1 and K 1 and semidiurnal M 2. The observed diurnal internal tides (ITs) were more coherent than the semidiurnal constituents. Coherent diurnal variance accounted for approximately 58% of the diurnal motion, whereas semidiurnal tides contained a much smaller fraction (35%) of coherent motion. The ICITs mainly consisted of motion at non-tidal harmonic frequencies around the tidal frequency, and showed clear intermittency. The modal decomposition of CITs and ICITs showed that CITs were dominated by mode- 1, whereas mode-1 and higher modes in ICITs signals showed comparable amplitudes. CITs and ICITs accounted for approximately 64% and 36% of the total kinetic energy of internal tides, respectively.

Seasonal characteristics of internal tides and their responses to background currents in the Luzon Strait

The spatial-temporal characteristics of internal tides (ITs) in the southwest Luzon Strait are examined, based on 9-month mooring current records from autumn 2008 to summer 2009. The results of spectral analysis show that the ITs in diurnal and semidiurnal frequencies are prominent at the mooring site, especially for the clockwise rotary component. The diurnal ITs are mostly dominated by the first mode except for that in spring when the second mode is relatively predominant. The semidiurnal ITs display a variable multimodal structure. Moreover, an apparent difference is detected in the kinetic energy of diurnal ITs. The energy is strongest in winter, and followed by that in summer, whereas the value is smallest in spring and autumn. It is suggested that the incoherent motions are responsible for the significant seasonal variations of diurnal ITs, reflecting interaction between diurnal ITs and the varying background conditions. However, the semidiurnal ITs are independent of seasonal change, whose energy is smaller and only one-third of the diurnal energy in winter. Nevertheless, the abnormal variations of semidiurnal ITs are also related to the variable background conditions. The incoherent semidiurnal constituent accounts for about 37% of the total semidiurnal tidal kinetic energy, but the diurnal tidal motions contain fewer incoherent component (22.2%).

Observed enhanced internal tides in winter near the Luzon Strait

AbstractSeasonal characteristics and nonlinear interaction of internal tides (ITs) near the Luzon Strait in the northeastern South China Sea are investigated using 285 day, in situ observation data. It is found that ITs, which are dominated by the first‐mode wave throughout the year, are the strongest in subsurface layer. Baroclinic incoherent diurnal (semidiurnal) variance accounts for about 85.7% (78.3%) of diurnal (semidiurnal) ITs. The amplitude and seasonal variation of the diurnal ITs are more prominent than those that are semidiurnal, e.g., the largest kinetic energy densities of diurnal and semidiurnal baroclinic tidal currents are 2.81 and 0.83 KJ/m2 in winter, respectively. It is considered that there are two reasons for the significantly enhanced ITs in winter: (1) it may be due to the Kuroshio intrusion, and (2) the enhanced diurnal ITs may be due to the enhanced diurnal barotropic tidal currents, while the enhanced semidiurnal ITs may be caused by the strong nonlinear interaction between diurnal IT constituents O1 and K1 due to their high vertical shears in subsurface layer. Thus, harmonic semidiurnal constituent D2 with a similar frequency of constituent M2 is induced; it subsequently enhances the semidiurnal ITs in this subsurface layer, and the associated energy is carried downward to enhance the semidiurnal ITs in the upper and lower layers.

Self-directed learning modules in spatial anatomy using CT scan images

Observed currents from ADCPs, deployed at 100 m and 150 m depths, on the continental shelf at five different locations along the west coast of India, were used to study the characteristics of both barotropic and baroclinic tidal currents. The observations extended over a 6-month period (March–August), which includes two seasons, pre-monsoon (March–April) and southwest (SW) monsoon (May–August) during 2008, 2009 and 2011. Tidal ellipses, constructed for M2 and K1 constituents, show that barotropic tidal currents propagate in along-isobath direction at the southern shelf (off Kollam at about 9°N) and they are oriented more or less in a cross-isobath direction at the northern shelf locations (off Jaigarh and Mumbai at about 17°N and 20°N, respectively). Maximum cross-isobath tidal current is found at northern shelf locations (for instance, about 32 cm s−1 off Mumbai) than those in south (about 10 cm s−1 off Bhatkal, at 13°N). This could be due to the amplification of semidiurnal tidal currents from south to north of the shelf due to an increase in shelf widths towards north. The rotary spectra of baroclinic currents showed large peaks at semidiurnal and diurnal bands with a dominant clockwise rotation showing the presence of strong internal tidal currents. Semidiurnal variability occurs mainly in M2 and S2 and diurnal variability occurs mainly in K1 and O1. An increase in the amplitude of semidiurnal and diurnal internal tide is apparent when the stratification on the shelf increases from pre-monsoon to SW monsoon period. The presence of strong internal tide during May to August is attributed to increased seasonal stratification on the shelf. EOF analysis showed that the first three modes are sufficient to describe most of the variability in both semidiurnal and diurnal internal tides on the shelf, as they represent about 70–90% of total variance. The small scale vertical shear in the velocity field, induced by diurnal internal tide, is found to be larger than that induced by semidiurnal internal tide.

Tomographic mapping of a coastal upwelling and the associated diurnal internal tides in Hiroshima Bay, Japan

AbstractTemperature variations caused by a typhoon were measured in the northern part of Hiroshima Bay by four coastal acoustic tomography (CAT) systems. The horizontal distributions of depth‐averaged temperature from 0 to 8 m were mapped at 10 min intervals between the 11 and 25 September 2013. The horizontal distributions of a coastal upwelling and the associated diurnal internal tides were reconstructed well by regularized inversion based on the grid segmented method, using one‐way travel time data along five successful sound transmission lines. Station‐to‐station ranges were corrected in such a way that sound speed (determined from one‐way travel time data) was equal to sound speed calculated from a couple of CTD (conductivity‐temperature‐depth) data sets on each transmission line. In addition, all station positions were adjusted to make focal points at the geographical positions of the transducers. The corrections increased the accuracy of temperature measurements to make temperature errors as small as 0.073–0.079°C. The high accuracy made it possible to map the temperature structure with a variation range of less than 0.5°C. An upwelling grew from 16 to 17 September, due to a typhoon‐derived northerly wind. The diurnal internal tide resonated with the semidiurnal external tide, which was pronounced after the upwelling decayed (18 September), around the time the spring tide occurred. The upwelling and mixing fractions were formulated. These fractions increased continuously as the upwelling grew. Complete mixing was observed during the upwelling's mature phase.

Structure and Variability of Internal Tides in Luzon Strait

AbstractThe Luzon Strait is the generation region for strong internal tides that radiate westward into the South China Sea and eastward into the western Pacific. Intrusions of the Kuroshio and strong mesoscale variability in the Luzon Strait can influence their generation and propagation. Here, the authors use eight moorings and two numerical models to investigate these relationships by quantifying the coherence of the diurnal and semidiurnal internal tides in the Luzon Strait. This study finds that the level of coherence of internal tide generation, energy, and energy flux is quite variable, depending on the specific location within the Luzon Strait. Large-scale spatial patterns in internal tide pressure and velocity exist across the region, shaped by the bathymetry, mean flow, and stratification. Internal tide coherence is lower (<30%) near large gradients in this pattern (predominantly along the eastern ridge), which are shifted by the variable Kuroshio and mesoscale fields. At other locations within the Luzon Strait, the internal tide is largely coherent (>80%), and simple calculations suggest that remote sources of internal tides could account for these small decreases in coherence. To the west of the Luzon Strait (away from the primary generation regions), the model suggests that diurnal internal tide energy is more coherent than semidiurnal.

Characteristics and seasonal variability of internal tides in the southern South China Sea

Two sets of current mooring records at the same location are used to investigate the generation and temporal variation of internal tides (ITs) near reefs in the southern South China Sea (SCS). The total records covered more than 900 days. ITs in this region are dominated by mode-1 diurnal motions, which are mainly composed of O1, P1 and K1 constituents. The major axes of both diurnal barotropic and baroclinic tidal ellipses are oriented in the northeast-southwest direction. The ITs over the entire observation period clearly show an approximately 14-day spring-neap cycle. Meanwhile, the diurnal ITs also displayed an evident seasonal change: their energy was higher in summer and winter than in spring and autumn. The modal decomposition of diurnal motion shows that their time-averaged kinetic energy in summer and winter was about 1.2kJ/m2, which was approximately 60% higher than the values in spring and autumn. The same seasonal variation also occurred at the diurnal barotropic tides with prominent O1, P1 and K1 constituents. Because stratification does not show a significant seasonal variation in the southern SCS, the enhanced diurnal ITs in winter and summer can be attributed to the barotropic forcing with the semi-annual cycle caused by the modulation of diurnal P1 and K1.

Internal tide radiation from the Luzon Strait

AbstractThe M2, K1, and O1 internal tides originating in the Luzon Strait are investigated using the sea surface height measurements by multiple satellites ERS‐2, Envisat, TOPEX/Poseidon, Jason‐1/2, and Geosat Follow‐On. A plane wave fit method is used to resolve multiple internal tides in arbitrary horizontal directions. The Luzon Strait is an energetic internal tide generation site, and radiates internal tides both westward into the South China Sea (SCS) and eastward into the western Pacific (WP). In the SCS, the K1 and O1 internal tides propagate over 1600 km, reaching the Vietnam coast; in the WP, they propagate over 2500 km and arrive to the Mariana Ridge and Guam. The K1 and O1 internal tides refract toward the Equator during propagation. The M2 internal tides in the SCS bifurcate into two beams. The northwestward beam is coincident with the frequent occurrence of internal solitary waves in this region, implying their causative relation. The phase speeds inferred from the altimetric along‐beam propagation agree with the theoretical values. Due to the influence of the Earth's rotation, the K1 and O1 phase speeds decrease remarkably from high to low latitudes. For the diurnal internal tides, the eastward radiation is about 50% greater than the westward radiation. For M2, the westward radiation is about two times the eastward radiation. The altimetric energy fluxes are about 50% of those in numerical model simulations.

Trapped diurnal internal tides, propagating semidiurnal internal tides, and mixing estimates in the California Current System from sustained glider observations, 2006–2012

From 2006–2012, along 3 repeated cross-shore transects (California Cooperative Oceanic Fisheries Investigations lines 66.7, 80, and 90) in the California Current System, 33609 shear and 39737 strain profiles from 66 glider missions are used to estimate mixing via finescale parameterizations from a dataset containing over 52000 profiles. Elevated diffusivity estimates and energetic diurnal (D1) and semidiurnal (D2) internal tides are found: (a) within 100km of the coast on lines 66.7 and 80 and (b) over the Santa Rosa-Cortes Ridge (SRCR) in the Southern California Bight (SCB) on line 90. While finding elevated mixing near topography and associated with internal tides is not novel, the combination of resolution and extent in this ongoing data collection is unmatched in the coastal ocean to our knowledge.Both D1 and D2 internal tides are energy sources for mixing. At these latitudes, the D1 internal tide is subinertial. On line 90, D1 and D2 tides are equally energetic over the SRCR, the main site of elevated mixing within the SCB. Numerous sources of internal tides at the rough topography in the SCB produce standing and/or partially-standing waves. On lines 66.7 and 80, the dominant energy source below about 100m for mixing is the D1 internal tide, which has an energy density of the D2 internal tide. On line 80, estimated diffusivity, estimated dissipation, and D1 energy density peak in summer. The D1 energy density shows an increasing trend from 2006 to 2012. Its amplitude and phase are mostly consistent with topographically-trapped D1 internal tides traveling with the topography on their right. The observed offshore decay of the diffusivity estimates is consistent with the exponential decay of a trapped wave with a mode-1 Rossby radius of 20–30km. Despite the variable mesoscale, it is remarkable that coherent internal tidal phase is found.

Seasonal variability and north–south asymmetry of internal tides in the deep basin west of the Luzon Strait

Temporal and spatial variability of internal tides (ITs) in the deep basin west of the Luzon Strait were examined, based on two 9-month mooring current observations from autumn 2008 to summer 2009. The baroclinic current measurements exhibited north–south asymmetry and temporal variation. At the southern site (19.5°N), the kinetic energy of diurnal IT was dominant over that of semidiurnal IT by a factor of 3 to 4, whereas at the northern site (~21°N), the diurnal energy was comparable to or a little larger than semidiurnal energy. At both sites, the motions of semidiurnal IT were recognized as seasonally invariant, while diurnal IT showed notable seasonal variation, namely, stronger in summer and winter but weaker in spring and autumn. The seasonal variation was mainly modulated by the astronomical tides in the Luzon Strait rather than by the seasonal thermocline. Both diurnal and semidiurnal ITs contained stronger coherent signals than incoherent counterparts at two sites, but the IT at the southern site was more coherent than that at the northern site. Diurnal IT is more coherent than the semidiurnal IT at each site. The spatial–temporal variations were probably due to IT interferences from various sources within the Luzon Strait and modulation by varying background conditions.

Tomography and the global predictability of mode‐1 internal tides

Twenty years ago, temporally coherent, mode-1 internal tides were detected in the central North Pacific using acoustic tomography. The spatial sampling of tomography is a natural filter for these internal waves. The signals were conjectured to have originated from the Hawaiian Ridge 2000 km to the south, and shortly thereafter Ray and Michum found radiation of internal tides from that undersea ridge using satellite altimetry. The two data types are complementary: tomography demonstrates astonishing temporal coherence, altimetry demonstrates astonishing spatial coherence. In the Atlantic, diurnal internal tides, resonantly trapped between Puerto Rico and their 30N turning latitude, were observed by AMODE tomography, while the semidiurnal variability consisted of a complicated interference pattern. While several conventional in situ observations indicated incoherent components to mode-1 internal-tide variability, both tomography (directly) and altimetry (indirectly) show the incoherent component is minimal. Resolution of mode-1 is challenging, but, without basis, oceanographers have been reluctant to accept the tomography measurements. A combined frequency and wavenumber tidal analysis of altimetry data has recently demonstrated that the amplitude and phase of mode‐1 internal tides are predictable, except, perhaps, in more variable regions. Attenuation appears to be negligible, and waves coherent in time and space extend across ocean basins.

Observed tidal currents on the continental shelf off the west coast of India

Observed currents from ADCPs, deployed at 100m and 150m depths, on the continental shelf at five different locations along the west coast of India, were used to study the characteristics of both barotropic and baroclinic tidal currents. The observations extended over a 6-month period (March–August), which includes two seasons, pre-monsoon (March–April) and southwest (SW) monsoon (May–August) during 2008, 2009 and 2011. Tidal ellipses, constructed for M2 and K1 constituents, show that barotropic tidal currents propagate in along-isobath direction at the southern shelf (off Kollam at about 9°N) and they are oriented more or less in a cross-isobath direction at the northern shelf locations (off Jaigarh and Mumbai at about 17°N and 20°N, respectively). Maximum cross-isobath tidal current is found at northern shelf locations (for instance, about 32cms−1 off Mumbai) than those in south (about 10cms−1 off Bhatkal, at 13°N). This could be due to the amplification of semidiurnal tidal currents from south to north of the shelf due to an increase in shelf widths towards north. The rotary spectra of baroclinic currents showed large peaks at semidiurnal and diurnal bands with a dominant clockwise rotation showing the presence of strong internal tidal currents. Semidiurnal variability occurs mainly in M2 and S2 and diurnal variability occurs mainly in K1 and O1. An increase in the amplitude of semidiurnal and diurnal internal tide is apparent when the stratification on the shelf increases from pre-monsoon to SW monsoon period. The presence of strong internal tide during May to August is attributed to increased seasonal stratification on the shelf. EOF analysis showed that the first three modes are sufficient to describe most of the variability in both semidiurnal and diurnal internal tides on the shelf, as they represent about 70–90% of total variance. The small scale vertical shear in the velocity field, induced by diurnal internal tide, is found to be larger than that induced by semidiurnal internal tide.

The variability of internal tides in the Northern South China Sea

An array of three bottom-mounted ADCP moorings was deployed on the prevailing propagation path of strong internal tides for nearly 1 year across the continental slope in the northern South China Sea. These velocity measurements are used to study the intra-annual variability of diurnal and semidiurnal internal tidal energy in the region. A numerical model, the Luzon Strait Ocean Nowcast/Forecast System developed at the U.S. Naval Research Laboratory that covers the northern South China Sea and the Kuroshio, is used to interpret the observed variation of internal tidal energy on the Dongsha slope. Internal tides are generated primarily at the two submarine ridges in the Luzon Strait. At the western ridge generation site, the westward energy flux of the diurnal internal tide is sensitive to the stratification and isopycnal slope associated with the Kuroshio. The horizontal shear at the Kuroshio front does not modify the propagation path of either diurnal or semidiurnal tides because the relative vorticity of the Kuroshio in Luzon Strait is not strong enough to increase the effective inertial frequency to the intrinsic frequency of the internal tides. The variation of internal tidal energy on the continental slope and Dongsha plateau can be attributed to the variation in tidal beam propagation in the northern South China Sea.

Variability of internal tides and near‐inertial waves on the continental slope of the northwestern South China Sea

AbstractStructure and variability of internal tides (IT) and near‐inertial waves (NIW) on the continental slope of the northwestern South China Sea were investigated, based on 9‐month moored current observations from autumn to early summer in 2008 and 2009. The diurnal IT kinetic energy, dominant over that of semidiurnal tides, is found to exhibit apparent seasonal variability—strongest in summer and weakest in winter—whereas the semidiurnal variance remained nearly uniform throughout the observation period. Moreover, the diurnal IT were more coherent (i.e., phase‐locked to the astronomical forcing) than the semidiurnal constituents. Coherent diurnal variance accounts for about 40% of diurnal motions, but semidiurnal tides contain a much smaller fraction (10%) of coherent motions. Further analysis demonstrates that the diurnal IT are dominated by the first mode, whereas the semidiurnal tides show a variable multimodal structure: the second mode is dominant in summer and comparable to the first mode in spring and autumn, but the first mode predominates in winter. Multimodal semidiurnal IT are more influenced by varying stratification structures and background currents and thus exhibit highly incoherent and intermittent behavior, which may wash out seasonal variability during their long propagation from the generation source. The observed NIW are seasonally independent and comparable to the semidiurnal motions. During the passage of Typhoon Hagupit, however, the NIW became the most energetic component of the inertia‐gravity waveband motions. NIW energy and shear were significantly enhanced and exceeded tidal counterparts by a factor of 2 to 3 in the upper layer.

Diurnal modulation of semidiurnal internal tides in Luzon Strait

A three-dimensional model is applied to the South China Sea to study the generation of internal tides in the Luzon Strait, a mixed diurnal dominant barotropic tidal environment. The computed internal tidal flux is in good agreement with limited observations and with estimates from the barotropic tidal inversion. The diurnal internal tides are simple harmonic oscillations. The semidiurnal internal tides, on the other hand, are highly nonlinear with striking daily amplitude variations. Comparison with an analytical mixed internal tide–lee wave model shows that the diurnal modulation is caused by interaction of barotropic diurnal tidal currents with the linear internal waves. The highly nonlinear semidiurnal internal tides likely is responsible for the observed diurnally modulated nonlinear internal waves in the South China Sea. The semidiurnal internal tides also appears to be the major source for the enormous vertical mixing in the Luzon Strait.