Sen Jan

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.

Transports and tidal current estimates in the Taiwan Strait from shipboard ADCP observations (1999–2001)

Tidal and mean flows in the Taiwan Strait are obtained from analysis of 2.5 years (1999–2001) of shipboard ADCP data using a spatial least-squares technique. The average tidal current amplitude is 0.46 ms � 1 , the maximum amplitude is 0.80 ms � 1 at the northeast and southeast entrances and the minimum amplitude is 0.20 ms � 1 in the middle of the Strait. The tidal current ellipses derived from the shipboard ADCP data compare well with the predictions of a high-resolution regional tidal model. For the mean currents, the average velocity is about 0.40 ms � 1 . The mean transport through the Strait is northward (into the East China Sea) at 1.8 Sv. The transport is related to the along Strait wind by a simple regression, transport (Sv)=2.42 + 0.12 � wind (ms � 1 ). Using this empirical formula, the maximum seasonal transport is in summer, about 2.7 Sv, the minimum transport is in winter, at 0.9 Sv, and the mean transport is 1.8 Sv. For comparison, this result indicates that the seasonal amplitude is almost identical to the classical estimate by Wyrtki (Physical oceanography of the southeast Asian waters, scientific results of marine investigations of the South China Sea and Gulf of Thailand, 1959–1961. Naga Report 2, Scripps Institute of Oceanography, 195 pp.) based on the mass balance in the South China Sea, while the mean is close to the recent estimate by Isobe [Continental Shelf Research 19 (1999) 195] based on the mass balance in the East China Sea. 2003 Elsevier Science B.V. All rights reserved.

Nutrient Fluxes through the Taiwan Strait in Spring and Summer 1999

Transports of water and nutrients (N and P) through the Taiwan Strait were calculated using chemical hydrography and currents observed in May and August 1999. The surveys were conducted along a transect across the strait in the middle section. The velocity fields were determined by phase-averaging currents measured using shipboard Acoustic Doppler Current Profiler (ADCP) on two repeats, which were separated by 1.5 cycles of the dominant M2 tide. Nutrient distributions were also derived from phase-averaged data. The volume transports determined from the two surveys were similar (2.0 Sv and 2.2 Sv, respectively). By contrast, the nutrient fluxes obtained in August (1.82 kmol N/s and 0.34 kmol P/s) were significantly higher than those in May (0.96 kmol N/s and 0.16 kmol P/s), apparently due to coastal upwelling under southwest monsoon in summer. The rather low N/P ratios (6.0 and 5.4 by atoms) of the nutrient fluxes were attributed to the widespread N-deficiency in the upper water column of the North Pacific. The nutrient fluxes were fed mainly through a meridional deep channel off southwest Taiwan. The nutrient contributions from the Taiwan Strait to the East China Sea in spring and summer are comparable to the total riverine contributions from the Changjiang (also know as the Yangtze River) and other smaller rivers for nitrogen, but 8–17 times larger than the latter for phosphate. Therefore, the Taiwan Strait inflow may serve as an important supplement for the P-limiting condition in the huge coastal plume in the East China Sea.

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.

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.

Coupled Ocean–Acoustic Prediction of Transmission Loss in a Continental Shelfbreak Region: Predictive Skill, Uncertainty Quantification, and Dynamical Sensitivities

In this paper, we quantify the dynamical causes and uncertainties of striking differences in acoustic transmission data collected on the shelf and shelfbreak in the northeastern Taiwan region within the context of the 2008 Quantifying, Predicting, and Exploiting Uncertainty (QPE 2008) pilot experiment. To do so, we employ our coupled oceanographic (4-D) and acoustic (Nx2-D) modeling systems with ocean data assimilation and a best-fit depth-dependent geoacoustic model. Predictions are compared to the measured acoustic data, showing skill. Using an ensemble approach, we study the sensitivity of our results to uncertainties in several factors, including geoacoustic parameters, bottom layer thickness, bathymetry, and ocean conditions. We find that the lack of signal received on the shelfbreak is due to a 20-dB increase in transmission loss (TL) caused by bottom trapping of sound energy during up-slope transmissions over the complex and deeper bathymetry. Sensitivity studies on sediment properties show larger but isotropic TL variations on the shelf and smaller but more anisotropic TL variations over the shelfbreak. Sediment sound-speed uncertainties affect the shape of the probability density functions of the TLs more than uncertainties in sediment densities and attenuations. Diverse thicknesses of sediments lead to only limited effects on the TL. The small bathymetric data uncertainty is modeled and also leads to small TL variations. We discover that the initial transport conditions in the Taiwan Strait can affect acoustic transmissions downstream more than 100 km away, especially above the shelfbreak. Simulations also reveal internal tides and we quantify their spatial and temporal effects on the ocean and acoustic fields. One type of predicted waves are semidiurnal shelfbreak internal tides propagating up-slope with wavelengths around 40-80 km, horizontal phase speeds of 0.5-1 m/s, and vertical peak-to-peak displacements of isotherms of 20-60 m. These waves lead to variations of broadband TL estimates over 5-6-km range that are more isotropic and on bearing average larger (up to 5-8-dB amplitudes) on the shelf than on the complex shelfbreak where the TL varies rapidly with bearing angles.

Nutrient supply in the Southern East China Sea after Typhoon Morakot

Author Posting.

Eddy‐Kuroshio interaction processes revealed by mooring observations off Taiwan and Luzon

The influence and fate of westward propagating eddies that impinge on the Kuroshio were observed with pressure sensor-equipped inverted echo sounders (PIESs) deployed east of Taiwan and northeast of Luzon. Zero lag correlations between PIES-measured acoustic travel times and satellite-measured sea surface height anomalies (SSHa), which are normally negative, have lower magnitude toward the west, suggesting the eddy-influence is weakened across the Kuroshio. The observational data reveal that impinging eddies lead to seesaw-like SSHa and pycnocline depth changes across the Kuroshio east of Taiwan, whereas analogous responses are not found in the Kuroshio northeast of Luzon. Anticyclones intensify sea surface and pycnocline slopes across the Kuroshio, while cyclones weaken these slopes, particularly east of Taiwan. During the 6 month period of overlap between the two PIES arrays, only one anticyclone affected the pycnocline depth first at the array northeast of Luzon and 21 days later in the downstream Kuroshio east of Taiwan.

Characteristics of short period secondary microseisms (SPSM) in Taiwan: The influence of shallow ocean strait on SPSM

Taking advantage of a unique opportunity provided by a dense array of coastal short-period seismic stations and the diverse bathymetry around Taiwan, we examine how the long-range coherent ambient noises are influenced by surrounding ocean settings using the cross-correlation functions (CCFs) between pairs of stations. The effective energy of the CCFs derived from three components of short-period seismometer data falls within the frequency range of the short period secondary microseism (SPSM). The spatial variations mapped from the amplitude asymmetry of CCFs and source migration images evidently demonstrate that the SPSM strengths are closely linked to the drastic changes in offshore ocean characteristics and result in much stronger SPSM in the shallow and narrow Taiwan Strait than in deep open seas of eastern Taiwan. The temporal variations of the CCF strengths exhibit very good correlations with the wind speeds and wave heights, explicitly indicating the observed SPSM is dominated by local sources generated from wind-driven ocean waves around offshore Taiwan.