Ching-Sheng Chern

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.

On the Kuroshio branch in the Taiwan strait during wintertime

Abstract Past investigations reported that warm and saline waters originating from the Kuroshio region occupied the northern South China Sea (NSCS) during wintertime. These waters were called a branch of the Kuroshio, and an offshoot of them was long believed to flow persistently along the west coast of Taiwan, contrary to the winter NE monsson. However, these descriptions are not completely consistent with recent evidence. Field measurements in the eastern Taiwan Strait now available display a quasi-stationary oceanic front, whose alignment is roughly normal to the transverse of the strait, which separates the warmer, more saline water to the south, from the colder, fresher China coastal water in the north during most of the winter. This feature implies the further northward movement of southeasterly Kuroshio water having been stagnated by cold waters to the north of the front. Therefore, the intruded Kuroshio water driven by the NE monsoon tends to accumulate in the NSCS, and eventually establishes an anti-cyclonic warm core eddy there. The first event of the intrusion of warm waters from the eddy into, or even through, the Taiwan Strait usually appears when the NE wind weakens from a strong cold-air outbreak in winter or early spring. We believe the inception of the Kuroshio branch in the strait starts from such an intrusion event.

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.

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.

Numerical Study of the Upper-Layer Circulation in the South China Sea

Upper-layer circulation in the South China Sea has been investigated using a three-dimensional primitive equation eddy-resolving model. The model domain covers the region from 99° to 122°E and from 3° to 23°N. The model is forced by the monthly averaged European Centre for Medium-Range Weather Forecasts (ECMWF) model winds and the climatological monthly sea surface temperature data from National Oceanographic Data Center (NODC). Inflow and outflow through the Taiwan Strait and the Sunda shelf are prescribed monthly from the Wyrtki estimates. Inflow of the Kuroshio branch current in the Luzon Strait is assumed to have a constant volume transport of 12 Sv (1 Sv = 106 m3/s), and the outflow from the open boundary to the east of Taiwan is adjusted to ensure the net volume transport through all open boundaries is zero at any instant. The model reveals that a cyclonic circulation exists all year round in the northern South China Sea. During the winter time this cyclonic eddy is located off the northwest of Luzon, coinciding with the region of positive wind stress curl in this season. This cyclonic eddy moves northward in spring due to the weakening of the northeast winds. The cyclonic circulation becomes weak and stays in the continental slope region in the northern South China Sea in the summer period. The southwest wind can raise the water level along the west coast of Luzon, but there is no anticyclonic circulation in the northern South China Sea. After the onset of the northeast monsoon winds in fall, the cyclonic eddy moves back to the region off the west coast of Luzon. In the southern South China Sea and off the Vietnam coast, the model predicts a similar flow structure as in the previous related studies.

A numerical study of the summertime flow around the Luzon Strait

Luzon Strait, a wide channel between Taiwan and Luzon islands, connects the northern South China Sea and the Philippine Sea. The Kuroshio, South China Sea gyre, monsoon and local topography influence circulation in the Luzon Strait area. In addition, the fact that the South China Sea is a fairly isolated basin accounts for why its water property differs markedly from the Kuroshio water east of Luzon. This work applies a numerical model to examine the influence of the difference in the vertical stratification between the South China Sea and Kuroshio waters on the loop current of Kuroshio in the Luzon Strait during summer. According to model results, the loop current’s strength in the strait reduces as the strongly stratified South China Sea water is driven northward by the southwest winds. Numerical results also indicate that Kuroshio is separated by a nearly meridional ridge east of Luzon Strait. The two velocity core structures of Kuroshio can also be observed in eastern Taiwan. Moreover, the water flowing from the South China Sea contributes primarily to the near shore core of Kuroshio.

Influence of the Seasonal Thermocline on the Intrusion of Kuroshio across the Continental Shelf Northeast of Taiwan

The hydrographic surveys in an area immediately northeast of Taiwan showed that the Kuroshio surface water intruded onto the shelf in the spring and there was a thick mixed layer and weak vertical stratification in the Kuroshio at the time. During the summer season, a strong thermocline was developed in the Kuroshio and the flow shifted offshore from Taiwan in front of the continental shelf break of the East China Sea. A numerical model is used to examine the effect of this seasonal thermocline on the flow pattern of the survey area. We find that the surface strength of the disturbance above the Su-Ao ridge is closely related to the occurrence of the on-shelf intrusion of Kuroshio. The presence of a seasonal thermocline in the Kuroshio can greatly diminish this disturbance in the surface level.

Observations of a freshwater pulse induced by Typhoon Morakot off the northern coast of Taiwan in August 2009

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