Jae-Hun Park

The formation and fate of internal waves in the South China Sea

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

Internal wave observations in the South China Sea: The role of rotation and non‐linearity

Abstract Observations of internal waves travelling across the deep basin of the South China Sea provide an opportunity for exploring the effects of rotation and non‐linearity on their evolution. Time series measurements using inverted echo‐sounders at three locations illustrate the progressive steepening of the internal tide generated in Luzon Strait and the subsequent development of short non‐linear internal wave trains. Potential mechanisms for internal tide generation are discussed in terms of tidal beam interaction with near‐surface stratification and mode 1 response to flow over a ridge. For transformation of an internal tide under the influence of non‐linearity and rotation, we apply Boyds (2005) criterion for wave stability in a rotating flow to separate waves dominated by non‐linearity, which can be expected to steepen and break, from waves that are inhibited from breaking due to rotational dispersion of energy into internal inertial gravity waves. Wave breaking in this context refers to the point at which the wave becomes steep enough for non‐hydrostatic effects to come into play, with the subsequent generation of a short period non‐linear internal wave train. For a monotonic M2 internal tide generated over the eastern ridge, breaking is predicted close to the location at which remotely sensed images first indicate the presence of short non‐linear internal waves; steepening of a K1 internal tide, on the other hand, is predicted to be dispersed by rotational effects before steepening. Time series observations acquired just west of the eastern ridge provide initial conditions for applying the stability criterion to mixed internal tides over a spring‐neap cycle and for comparison with time series measurements at two additional sites further west. When the stability criterion is applied to wave slopes, initial conditions predicted to be unstable generally result in the formation of high frequency non‐linear internal wave trains. In the case of initial conditions for which rapid steepening is not predicted we see evidence of the interaction between rotation and non‐linearity, specifically the formation of ‘corner waves’ with the characteristic parabolic shape first predicted by Ostrovsky (1978).

Internal Tides in the Southwestern Japan/East Sea

Abstract This paper investigates the internal tidal energy distribution in the southwestern Japan/East Sea using vertical round-trip travel time (τ) data from 23 pressure-sensor-equipped inverted echo sounders (PIES). The τ records are analyzed by bandpass filtering to separate time-dependent variability of the semidiurnal and diurnal bands. The semidiurnal internal tides exhibit a horizontal beam pattern of high energy, propagating into the open basin. They originate from a restricted portion of the shelf break where the Korea Strait enters the Ulleung Basin. The generation appears to occur at ∼200-m water depth near 35.5°–35.7°N and 130°–131°E, where the slope of bottom topography matches that of the wave characteristics, coinciding with the location where the semidiurnal barotropic cross-slope tidal currents are strongest. Maximum vertical displacement of the thermocline interpreted as a long-wave first baroclinic mode from the measured τ is about 25 m near the generation region. Annual and monthly var...

Three-Dimensional Double-Ridge Internal Tide Resonance in Luzon Strait

AbstractThe three-dimensional (3D) double-ridge internal tide interference in the Luzon Strait in the South China Sea is examined by comparing 3D and two-dimensional (2D) realistic simulations. Both the 3D simulations and observations indicate the presence of 3D first-mode (semi)diurnal standing waves in the 3.6-km-deep trench in the strait. As in an earlier 2D study, barotropic-to-baroclinic energy conversion, flux divergence, and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides. The resonance in the 3D simulation is several times stronger than in the 2D simulations for the central strait. Idealized experiments indicate that, in addition to ridge height, the resonance is only a function of separation distance and not of the along-ridge length; that is, the enhanced resonance in 3D is not caused by 3D standing waves or basin modes. Instead, the difference in resonance between the 2D and 3D simulations is attributed to the topographic blocking ...

Program Studies the Kuroshio Extension

The Kuroshio Extension system links to North Pacific climate through its role in subtropical-subpolar exchange, the formation and distribution of mode waters, and the intensification of the extratropical storm track across the North Pacific. The Kuroshio Extension System Study (KESS) offers a window into these processes through integrated measurements of the ocean and atmosphere and through modeling efforts (Figure 1). The northward flowing waters of the Kuroshio western boundary current leave the Japanese coast to flow eastward as a free jet—the Kuroshio Extension. The Extension forms a vigorously meandering boundary between the warm subtropical and cold northern waters.

A Multi-Index GEM Technique and Its Application to the Southwestern Japan/East Sea

Abstract This paper demonstrates a new gravest empirical mode (GEM) technique that constructs multi-index lookup tables of temperature (T) and specific volume anomalies (δ) using historical hydrocasts as a function of three indices: round-trip travel time (τ) from sea floor to the surface, sea surface temperature, and pressure. Moreover, the historical hydrocasts are separated into non-mixed-layer (NML) and mixed-layer (ML) groups, and a single GEM field is constructed for each group. This is called the MI-GEM technique. The appropriate dates for MI-GEM fields are determined by the monthly distribution of the number of NML and ML profiles in the historical hydrocasts, which are also well correlated with the strength of the winds during the 2 yr of observations. The T and δ profiles that are determined by this MI-GEM technique capture 92% and 88% of the T and δ variances in the depth range of 0–200 db. These values reduce by about one-third of the unexplained error variance of the residual GEM, which was r...

Second-Mode Internal Tides in the East China Sea Deduced from Historical Hydrocasts and a Model

during the two-year deployment period. To obtain temperature (T) and specific volume anomaly (d) profiles from t and sea surface temperature (SST), observed respectively by PIES and satellite, we constructed multi-index gravest empirical mode (MI-GEM) fields [Park et al., 2005] using historical hydrocasts. However, comparison between the MI-GEM fields and the historical hydrocasts reveals that the MI-GEM fields fail to capture 44% and 52% of the T and d variance, respectively, in the p = 100–200-dbar layer. This may be caused by 2nd- or higher-mode variations of vertical structure, since t is insensitive to all but the lowest baroclinic mode [Watts and Rossby, 1977]. [4] In this study, we demonstrate that 2nd-mode ITs in the ECS are responsible for failure of the MI-GEM fields to captureadequatelyTand dvariationsinthep=100–200-dbar layer as seen in historical hydrographic measurements. We verify our observations with a numerical simulation of ITs.

Comment on ''Current system east of the Ryukyu Islands'' by A. Nagano et al.

[1] In a recent paper, Nagano et al. [2007] (hereafter N2007) discussed the variation of current structure and volume transport of the Ryukyu Current System (RCS) east of the Ryukyu Islands using an inverse technique with hydrographic-section data collected from three cruises. Their paper appears to be important for understanding this western boundary current, since it is based on repeated hydrographic surveys in the region east of the Ryukyu Islands. But acquiring meaningful results from inverse calculations requires great care. Fiadeiro and Veronis [1982, p. 160] say ‘‘Because inverse analysis always supplies a solution, it appeared that the assumed reference level matters less in inverse theory than in the hydrographer’s [level-of-no-motion] approach. Actual computations have shown that is not true. A bad assumption leads to bad results. The ‘advantage’ of inverse theory is that one sees how bad the results are.’’ The inverse technique minimizes the velocity at the reference level, and that point is indeed raised in the final paragraph of N2007, which used the reference level of 100 dbar. To determine whether this shallow reference level is an appropriate condition or not, one should investigate the effect of choosing other reference levels, in particular deeper levels. In this note, we perform two inverse calculations to calculate the current east of the Ryukyu Islands from the same data as one of the N2007 cruises but using a reference level of 2000 dbar. The resulting RCS circulation field is, we believe, more realistic than that of N2007. [2] Three hydrographic surveys were carried out during May–June, September and October 2002 along three lines (AE, E, OS) east of the Ryukyu Islands (Figure 1). May– June and September cruises used XCTD/XBT casts along the E line, but the October cruise collected CTD casts along all three lines. Surprisingly, absolute geostrophic velocity sections, determined by N2007 with a 100-dbar reference level, revealed large values up to 30–40 cm s 1 at 1000 dbar, the deepest level shown (Figures 2, 3b and 4c in N2007). We have calculated full-depth absolute geostrophic velocity sections using the inverse results of N2007 and plotted them to 3000 dbar (Figures 2a, 2b, 3a, 3b, 4a and 4b). Figures 2b, 3b and 4b show strong deep currents with maximum speeds of 20–50 cm s 1 on each line. In each case, note the contour lines of velocity are nearly vertical and strong currents extend to the bottom. For example, a current velocity of 57 cm s 1 touches the bottom near 3100 dbar in the AE section. Is the computed strong deepcurrent field real, or is it an artifact of the shallow reference level? We address this question by carrying out the inverse calculation using a reference level of 2000 dbar instead of 100 dbar, and compare results using the two reference levels. We present results for the October cruise data only, because the XBT/XCTD casts reached only 1000 dbar; also the CTD measurements provide more accuracy in the inverse calculations.

Evidence of Bottom-Trapped Currents in the Kuroshio Extension Region

As partof theKuroshioExtensionSystemStudy, observations fromfivecurrentmetermoorings revealthat the abyssal currents are weakly bottom intensified. In the framework of linear quasigeostrophic flow, the best fittedverticaltrappingdepthsrangefrom8to15 kmintheabsenceofsteeptopography,butonemooringnear an isolated seamount exhibited vertical trapping that was more pronounced and energetic with a vertical trapping depth of 5 km. The ratios of current speeds and geostrophic pressure streamfunctions at the sea surface compared to the bottom are 88% in the absence of steep topography, 63% near an isolated seamount, and overall on average 83% of their value at a reference depth of 5300 m. It is hypothesized that weakly depth-dependent eddies impinging upon topographic features introduce to the flow the horizontal length scales of the topography, and these smaller lateral scales are subject to bottom intensification.

Observations of flow variability through the Kerama Gap between the East China Sea and the Northwestern Pacific

The Kerama Gap, near the middle of the Ryukyu Island chain, is the deepest channel with a sill depth of 1050 m connecting the East China Sea (ECS) to the Northwestern Pacific. We measured the flow through the Kerama Gap from June 2009 to June 2011. The 2 year mean transport, 2.0 ± 0.7 Sv, is into the ECS from the Northwestern Pacific; it contributes about 11% of the mean Kuroshio transport in the ECS at the PN line. Subtidal standard deviation of the transport through the Kerama Gap is 3.2 Sv, comparable to that of the PN-line Kuroshio transport (4.0 Sv), suggesting a significant effect of Kerama Gap transport on temporal variability of the Kuroshio transport in the ECS. Comparison with time series of satellite-measured sea surface height maps reveals that temporal variability of the Kerama Gap transport is related to the arrival of mesoscale eddies from the east: high (low) transport into the ECS is associated with the presence of a cyclonic (anticyclonic) eddy south of the Kerama Gap.