Joe Wang

Seasonal variation of the circulation in the Taiwan Strait

Abstract The Taiwan Strait is an essentially meridional channel connecting the East and South China Seas. There is often a northward current on the east side and a southward current on the west side. The source water feeding the eastern boundary current is South China Sea Water in summer and Kuroshio Branch Water in other seasons. The current on the west side carries colder and fresher China Coastal Water southward. Both currents are modulated by the annual cycle of monsoon wind forcing, which reinforces the northward current in summer but southward current in other seasons. Further, both currents are partially impeded by a bottom ridge (Changyun Rise) in the middle reaches of the strait. The combination of monsoon and topography forcing leads to the winter blocking of northward current, spring renewal of northward intrusion, minimal blocking of northward intrusion in summer, and fall emergence of China Coastal Current. A recent hydrographic data set, satellite images and a numerical model lend support to these findings.

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.

Whistler instability: Electron anisotropy upper bound

If the electron distribution function is approximately bi-Maxwellian with T⊥/T‖ > 1, where the subscript symbols denote directions perpendicular and parallel to the background magnetic field, and if this temperature anisotropy is sufficiently large, the whistler anisotropy instability is excited. This mode is studied using two-dimensional particle-in-cell simulations in a spatially homogeneous plasma model. Theory predicts a threshold electron anisotropy for this instability which depends inversely on the electron parallel β. The simulations show that wave-particle scattering by enhanced whistler fluctuations maintain the initially bi-Maxwellian character of the electron distribution, and that this scattering imposes an upper bound on the electron T⊥/T‖ commensurate with that predicted by linear theory.

The intrusion of the Kuroshio across the continental shelf northeast of Taiwan

Hydrographic observations in an area immediately northeast of Taiwan in April 1989 indicate an on-shelf intrusion of Kuroshio water across a sharply curved continental shelf break. It appears that a part of the Kuroshio on the cyclonic side overran the shelf break and penetrated northward as a shallow surface current. The remainder of the Kuroshio, presumably affected by the shoaling topography, largely turned and ran along the shelf break to the northeast. Between the two, the flow was weak and disorganized. Conservation of potential vorticity and constancy of the Bernoulli function in an analytical, reduced-gravity model of the incidence of a baroclinic current upon a step shelf lead to an on-shelf flow field that is compatible to the observed hydrographic distributions. In particular, the integrated balance of along-step momentum yields an expression of the angle of intrusion in terms of the incidence angle and of the ratio of step depth to the depth of the upper layer of the ocean. In addition, the transport of the intrusion is shown to be equal to the product of the depth ratio squared and the incident transport. Calculations for the condition of the April survey yield results in agreement with the observation.

Blocking of the Kuroshio by the continental shelf northeast of Taiwan

In an area immediately northeast of Taiwan where in April 1989 an on-shelf intrusion of the Kuroshio was observed, hydrographic observations from August 1991 indicate a blocking of the Kuroshio at the continental shelf break. It appears that the Kuroshio approaches the shelf break as a near-bottom current and is deflected. The deflection apparently gives rise to a countercurrent along the shelf break which was observed by a buoy-mounted acoustic Doppler current profiler deployed within a month of the hydrographic survey. The on-shelf hydrographic distribution accompanying the deflection exhibits a field of near-bottom eddies dominated by a particularly well organized large pool downstream and to the right of the approaching Kuroshio but well separated from the Kuroshio. The hydrographic properties of the pool are similar to those of the uplifted Kuroshio subsurface water observed in April 1989, suggesting that it may be the remnant of the Kuroshio subsurface water that intruded during the previous winter. A theory of the blocking is put forth in terms of the approach of a bottom current toward a step rise of the bottom in a two-layered, reduced-gravity ocean model in which the thickness of the current exceeds the rise in topography. The theory predicts a complete blocking that features a flow deflection in front of the step and the presence on the step of a preexisting region of high baroclinic pressure, similar to the observation. The theory also predicts approximately correctly the track of the isotherm that bounds the large pool on the side facing the on-rushing Kuroshio.

Wind relaxation as a possible cause of the South China Sea Warm Current

Winter appearance of a northeastward warm current off the southern coast of China against gale force winds is well documented but lacks a plausible explanation. Relaxation of northeasterly winds is envisaged here as a possible cause of the South China Sea Warm Current in winter. A three-dimensional circulation model for the South China Sea is first driven to equilibrium by climatological forcings. Thereafter, wind forcing is relaxed from the 15th day of each month for 9 days. In winterlike months from December to April, the wind relaxation invariably triggers a northeastward current of which the location and alongshore span are comparable to that of the observed warm current. This current is driven by the pressure gradient along the northwestern boundary of the South China Sea, sea level being high to the southwest and low to the northeast. The sea level gradient is built up by the monsoon-driven southwestward coastal current along the northwestern boundary and, after wind relaxes, triggers a return current and a sea level drop that expand southwestward from the southern coast of China to the east coast of Vietnam. The current is initially barotropic, becoming increasingly baroclinic in time as warm waters from the south are advected northeastward. The model also suggests that the sea level gradient is present in most of the months of the year, but is not as dramatic as in winter to trigger fundamental changes in the circulation of the South China Sea.

Coprostanol distribution in marine sediments off southwestern Taiwan.

One of the major industries in southern Taiwan is pigfarming along the Kaoping River; some two million animals are being raised along the river banks. Excretions from pigs, treated and untreated, are discharged directly into the river and eventually carried to the sea. Twenty-four surface sediments and one sediment core off southwestern Taiwan were analyzed to determine quantitatively the extent of coprostanol addition and its distribution and to obtain the input of coprostanol over the past. Geographically, the percent coprostanol is highest around the river mouth, and decreases to about 1% at the shelf break and about 0% at a distance of approximately 40 nautical miles (74 km) from the river mouth. The progressive seaward decline of percent coprostanol from the river mouth can be attributed to [1] dilution of coprostanol by uncontaminated sediment and/or sediment containing relatively lower levels of coprostanol, [2] dilution of coprostanol by biogenic sterols, and [3] probably degradation of coprostanol. Further, the Kaoping Canyon sediments contain relatively high percent coprostanol; this can be attributed to [1] a more direct input of the river sediments because the canyon is well aligned with the river, [2] currents in the canyon being alternate upcanyon and downcanyon which tend to keep sediments in the canyon, and [3] the possible blocking effect of a topographic high in the canyon. A sediment core exhibits comparatively higher percent coprostanol in the top 15 cm, indicating an increased input of coprostanol over the past 20 years.

Large variability of the Kuroshio at 23.75°N east of Taiwan

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.

A Composite View of Surface Signatures and Interior Properties of Nonlinear Internal Waves: Observations and Applications

Abstract Surface signatures and interior properties of large-amplitude nonlinear internal waves (NLIWs) in the South China Sea (SCS) were measured during a period of weak northeast wind (∼2 m s−1) using shipboard marine radar, an acoustic Doppler current profiler (ADCP), a conductivity–temperature–depth (CTD) profiler, and an echo sounder. In the northern SCS, large-amplitude NLIWs propagating principally westward appear at the tidal periodicity, and their magnitudes are modulated at the spring–neap tidal cycle. The surface scattering strength measured by the marine radar is positively correlated with the local wind speed when NLIWs are absent. When NLIWs approach, the surface scattering strength within the convergence zone is enhanced. The sea surface scattering induced by NLIWs is equivalent to that of a ∼6 m s−1 surface wind speed (i.e., 3 times greater than the actual surface wind speed). The horizontal spatial structure of the enhanced sea surface scattering strength predicts the horizontal spatial s...

Transition of Tidal Waves from the East to South China Seas over the Taiwan Strait: Influence of the Abrupt Step in the Topography

Observations of tidal waves between the East and South China Seas (ECS and SCS) over the Taiwan Strait (TS) suggest that the diurnal tides simply appear as one southward-propagating wave from the ECS to the SCS through the TS. The semidiurnal tides, however, behave differently in that they appear as a southward-propagating Kelvin wave in the western TS and a nearly standing wave in the eastern TS, and then diminish rapidly over the shallow shoal in the southern TS. A smaller-domain model, with sea-level boundary conditions derived from a larger-domain tidal model, was first used to simulate tides in the TS to an overall percentage of accuracy of about 90%. Subsequent numerical experiments and theoretical analysis revealed that the southward-propagating semidiurnal tides to be impeded and then reflected as they arrive at an abrupt, deepened step in the topography of the southern TS. This reflection enhances the amplitudes of the incident semidiurnal tides and contributes to the formation of a nearly standing wave in the eastern TS. The southward-propagating diurnal tides in the TS are connected by the diurnal tides in the northern SCS when the amplitudes of the two tide systems are comparable and their phases nearly equal at the step.