Joe Wang

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.

Synoptic features of the Kuroshio at 23.75°N were quantified using nine ship‐based surveys between September 2012 and September 2014. The new ship‐based data set provides an unprecedented view of the Kuroshio east of Taiwan and suggest tremendous variability in its velocity, hydrography, volume, heat, and salt transports, and water masses. The Kuroshio maximum velocity varied in 0.7–1.4 m s⁻¹; the core current width, delineated with the limit v ≥ 0.2 m s⁻¹, ranged from 85 to 135 km, and the thickness varied from 400 to 600 m. A dual velocity maximum in the Kuroshio core current, though unexpected, was observed in three of nine cruises. The Kuroshio core transport, integrated from the directly measured velocity, varied between 10.46 and 22.92 Sv (1 Sv=10⁶ m³ s⁻¹). The corresponding heat transport referenced to 0°C was 0.838–1.793 × 10¹⁵ W, and the salt transport was 345.0–775.9 × 10⁶ kg s⁻¹. The geostrophic transport estimated using the thermal wind relation with the observed hydrographic data and reference velocity at 900 m is comparable to the directly measured Kuroshio transport during most of the surveys, suggesting the directly measured velocity is mostly in geostrophic balance.

The reliability of two-and-a-half-year surface velocity data collected from the two High-Frequency radar systems (CODAR) operated at Suao and Han-Ben northeast of Taiwan is evaluated via comparison with real-time drifter trajectory. The result suggests apparent seasonal dissimilarity which might due to strongly stratified water characteristic at observing area, and the localized large differences of speed measurements between 15-meter-deep drifter and surface radar system could be the result of actually comparing two separate layers of water whose flow pattern is weakly related to each other.

The CODAR is a High-Frequency (HF) radar system. Recently, CODAR becomes widely used for monitoring ocean surface currents remotely in nearly real time. However, some environmental effects often hampers or weakens the strength of CODAR system, which would deteriorate the data quality of CODAR and result in missing data in the designated observation region. In this study, we analyzed complete CODAR datasets by using both modal decomposition methods of real-vector Empirical Orthogonal Function (EOF) and the Karhunen-Loève Expansion (KLE), respectively. More than 96% of total variances of currents in the whole observation region can be interpreted by the first 20 modes of both methods, thus the first 20 modes of both methods were further used for data reconstruction and data filling. In the data filling experiment, the incomplete dataset was generated by depleting artificially assigned grid points in the CODAR original complete dataset, then both the EOF and the KLE methods were applied, in accompany with the least square and the iteration procedures to estimate the amplitude of each mode. Results show that the real-vector EOF method in accompany with the least square procedure is the best among the four methodologies, when the percentage of occurrence of the missing data is less than 57% of the whole dataset. However, all these four methods were not adequate for filling incomplete dataset, if the percentage of occurrence of the missing data exceeds 71%.

Using a three‐dimensional model with idealized settings, this study evaluated the effect of the Kuroshio on the generation and propagation of semidiurnal M₂ and diurnal K₁ baroclinic tides (BCTs) in the Luzon Strait (LS). The influences of the varied Kuroshio transport, the location of the Kuroshio, and the separation distance of the two meridional ridges were investigated. The absence of the Kuroshio in the model leads to 30% decreases in the westward baroclinic energy flux (F_bc^x) emanating from the west slope of the east ridge compared to the F_bc^x with the presence of the Kuroshio. Compared to the F_bc^x without the Kuroshio's influence, the westward F_bc^x is increased by 20–40% when the Kuroshio is located in either the middle or the west portions of the LS, whereas both the westward and eastward F_bc^x are 25–40% decreased as the Kuroshio is moved to the east of the east ridge. The wavelength of the mode 1 M₂ BCT is similar to the separation distance of the two ridges, favoring a resonance generation for M₂ rather than K₁ BCTs. The increase of the Kuroshio transport from 20 to 30 Sv (1 Sv = 10⁶ m³ s⁻¹) enhances the density front but weakens the westward F_bc^x owing to the effects of energy advection and lateral friction from the Kuroshio. The influence of the Kuroshio on the westward F_bc^x of K₁ is opposite of that on M₂. The characteristic slope of K₁ is closer to the isopycnal slope across the Kuroshio than M₂, thereby causing the discrepancy.