Yiing Jang Yang

Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea

A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.

Upper-ocean currents around Taiwan

Abstract Current velocity, measured by Shipboard Acoustic Doppler Current Profiler (Sb-ADCP) during 1991–2000, was used to study the upper-ocean ( The composite and moored currents revealed a branch of the Kuroshio that intruded steadily and persistently into the South China Sea. Part of the intruded Kuroshio flowed out of the South China Sea through the northern Luzon Strait and re-united with the main stream Kuroshio. The Kuroshio had two velocity maximum cores southeast of Taiwan, but gradually combined into one as the Kuroshio flowed north. The Kuroshio was deflected by the I-Lan Ridge east of Taiwan and the zonal-running shelf break northeast of Taiwan. At the shelf break, the Kuroshio split, with one branch intruding onto the shelf. West of the Luzon Strait, the Kuroshio intruded into the South China Sea. Some water flowed northward into the Taiwan Strait and re-joined the Kuroshio. Currents in the Taiwan Strait flowed primarily in a northward direction, except for the southward current near the coast of Mainland China. North of the Taiwan Strait, a branch of the northward flow followed the northern coast of Taiwan to join the Kuroshio. The composite current varied consistently from season to season. There was generally poor correlation between currents and local winds, especially in the deep-water regime. Remote forces were important in the currents around Taiwan.

The flow pattern north of Taiwan and the migration of the Kuroshio

Abstract The migration of the Kuroshio and mesoscale flow pattern north of Taiwan were studied using the historical ship-board acoustic Doppler current profiler (Sb-ADCP) current velocity and hydrographic measurements, moored current velocity, water temperature and salinity time series, snapshot current velocity observations and infrared images. The Kuroshio migrated both seasonally and intra-seasonally, with the former being more pronounced. The flow pattern north of Taiwan was significantly impacted by the seasonal migration of the Kuroshio. In summer the Kuroshio generally moved away from the shelf, colliding with the zonally running shelf breaks of the East China Sea (ECS) and splitting into an eastward mainstream and a northwestward branch current. Southwest of the branch current, a counterclockwise circulation was found along the edge of the shelf northeast of Taiwan, through which the subsurface Kuroshio water intruded forming a cold dome. In winter the Kuroshio moved close to and sometimes onto the northern shelf of Taiwan. The intrusion of the Kuroshio dominated the flow pattern in the region, causing the disappearance or obscuration of the counterclockwise circulation and cold dome. A sharp horizontal temperature front accompanying the horizontal velocity front was found on the northern shelf of Taiwan, through which the current flowed mainly northward against the winter monsoon. Intra-seasonal variations in flow patterns north of Taiwan and the intra-seasonal migration of the Kuroshio were also studied. In summer the counterclockwise circulation and cold dome migrated shoreward/seaward with the intra-seasonal migration of the Kuroshio. In winter the Kuroshio either intruded onto the shelf just off the northern tip of Taiwan or slightly further north. The resulting flow patterns north of Taiwan varied with the path of the intruding Kuroshio. Though the impact of the Taiwan Strait outflow on the flow pattern north of Taiwan was not negligible, it was not as great as the impact of the Kuroshio. Conversely, the intrusion of the Kuroshio strongly influenced the Taiwan Strait outflow. In summer counterclockwise circulation blocked the Taiwan Strait outflow and directly interacted with the Kuroshio. In winter the outflow joined with the on-shelf Kuroshio north of Taiwan.

Speed and Evolution of Nonlinear Internal Waves Transiting the South China Sea

Abstract In the South China Sea (SCS), 14 nonlinear internal waves are detected as they transit a synchronous array of 10 moorings spanning the waves’ generation site at Luzon Strait, through the deep basin, and onto the upper continental slope 560 km to the west. Their arrival time, speed, width, energy, amplitude, and number of trailing waves are monitored. Waves occur twice daily in a particular pattern where larger, narrower “A” waves alternate with wider, smaller “B” waves. Waves begin as broad internal tides close to Luzon Strait’s two ridges, steepening to O(3–10 km) wide in the deep basin and O(200–300 m) on the upper slope. Nearly all waves eventually develop wave trains, with larger–steeper waves developing them earlier and in greater numbers. The B waves in the deep basin begin at a mean speed of ≈5% greater than the linear mode-1 phase speed for semidiurnal internal waves (computed using climatological and in situ stratification). The A waves travel ≈5%–10% faster than B waves until they reach...

Kuroshio in the Luzon Strait

[1] Three acoustic Doppler current profilers (ADCPs) were deployed in the central Luzon Strait to monitor current velocity. The profilers were deployed from 1997 to 1999, with the duration of deployment varying by location. The observed current velocities indicated that the Kuroshio flowed consistently into the South China Sea. Further information provided by composite shipboard ADCP data showed that the Kuroshio intruded into the Luzon Strait through the deepest channels (∼20.5°N). Most of the intruded component made a loop and flowed out of the northern Luzon Strait, but a branch of the Kuroshio intruded into the South China Sea. While the current velocity obtained from mooring data did not show seasonal variation, significant intraseasonal variation ranging from several to 100 days was found. The seasonal reversal of monsoonal winds (i.e., northeasterly in winter and southwesterly in summer) did not cause noticeable variation in current velocity. The Miami Isopycnic Coordinate Ocean Model, together with monthly wind data, provided by the European Centre for Medium-Range Weather Forecasts was used to interpret the observed current velocity. Validation of the model results by the ADCP data showed that the model can explain the seasonal and interannual variations in the observed velocity fields. For example, the model reproduced the branch of the Kuroshio that consistently intruded into the South China Sea. The modeled velocity showed slight seasonal variation in the central Luzon Strait, although the large interannual signal could have hindered the seasonal timescale fluctuation. The interaction between the Kuroshio and South China Sea cyclonic flow caused variation in current velocities in both the Luzon Strait and the northern sector of the South China Sea. The model results also indicated that the Kuroshio axis bent clockwise within the Luzon Strait and flowed into and out of the South China Sea through the central and northern Luzon Strait, respectively. This pattern is similar with that shown by composite shipboard ADCP data. In summer, the Kuroshio looped west of the Luzon Strait, with a small intruding branch confined to the northwestern South China Sea. In winter, the intruding branch of the Kuroshio extended west and into the interior of the South China Sea. The net zonal transport of inflow and outflow across the Luzon Strait calculated from the model results was primarily westward, with the maximum varying from 4.8 to 6.5 Sv. Eastward transport was found occasionally in summer in some years, with a maximum from 0.3 to 2.2 Sv. While the Kuroshio intruded consistently into the South China Sea, transport out of the South China Sea was also observed. In summer, the current on the northern South China Sea shelf break contributed to the outflow. Variation in zonal transport was caused by variation in the sea surface height (SSH) west of northern Luzon caused by wind stress curl. Ekman transport, driven by monsoonal winds, and the strength of the Kuroshio off the Luzon Strait had little impact on the intrusion, which varied with fluctuation of the SSH west of northern Luzon.

Enhanced buoyancy and hence upwelling of subsurface Kuroshio waters after a typhoon in the southern East China Sea

Much has been documented worldwide on the implications of the passage of a tropical cyclone on a shelf ecosystem. In particular, wind mixing, resuspension and increased terrestrial runoff have thus far been pinpointed as the three major processes that bring about higher depth-integrated values of nutrients, chlorophyll a, primary and bacterial production, particulate organic carbon and nitrogen concentrations as well as biomass in the water column. Here, however, there is evidence to indicate that the cross-shelf upwelling of nutrient-rich subsurface Kuroshio water likely increased significantly after the passage of typhoon Herb in a normally downwelling region northwest of Taiwan. This phenomenon, most probably due to an enhanced buoyancy effect resulting from excessive rainfall, offers the best explanation for the lower temperatures yet higher salinity and larger amounts of nutrients that were observed in the deep and bottom coastal waters after the typhoon in July 1996. Further, there are indications that the episodic event might have pushed the Kuroshio towards the shelf-break, which then facilitated the onshore transport of subsurface Kuroshio waters. These new sources of nutrients along with nutrients brought in by the increased terrestrial runoff would eventually mix in or upwell to the euphotic zone on the shelf, thereby supporting new production.

Continental Slope Flow Northeast of Taiwan

Abstract Hydrographic observations and current measurements with a Shipboard Acoustic Doppler Current Profiler over the continental shelf–slope junction northeast of Taiwan during 10–17 August 1994 allow the construction of the mesoscale flow pattern generated by the collision of the Kuroshio and a stretch of the continental shelf that has turned to run nearly east–west. The pattern is made up of a deflected Kuroshio mainstream to the east, an intrusion of Kuroshio water onto the continental shelf region, a counterclockwise circulation over Mien-Hwa Canyon (MHC) immediately northeast of Taiwan, a deep southwestward countercurrent along the northern wall of MHC, and a seaward outflow of continental shelf water around the northern coast of Taiwan. The hydrography features a cold dome over the west side of MHC that consisted of subsurface Kuroshio water. A temperature–salinity plot of all the station data shows the incorporation in the neighborhood of Taiwan of continental shelf water into the Kuroshio.

Low frequency current variability on the shelf break northeast of Taiwan

A buoy-mounted Acoustic Doppler Current Profiler was deployed on the shelf break off the northeast coast of Taiwan to monitor current variations in the upper ocean. The acquired data show that the flow in the upper water column was initially southwest and then abruptly turned northwest. This abrupt change occurred in mid-October, starting from the surface layer and then gradually extending to the deeper layer. In contrast with this flow, the flow in the lower water column was southwest over the entire record, but its amplitude was reduced after the middle of October. The abrupt change of current from southwest to northwest is related to the intrusion of Kuroshio. Examination of two CTD casts showed the salinity of the upper ocean to have increased after the directional shift in mid-October, further indicating the Kuroshio intrusion. The sea level data at Keelung provided other evidence for the intrusion of Kuroshio. The sea level descended as the intrusion occurred and kept the low value until the end of the record. The northwest flow, which carried the water away from the northern coast of Taiwan, is responsible for this descent. Although the intrusion of Kuroshio was mainly confined to the upper ocean, it did have influence on the whole water column. Examination of the wind record at Pengchiayu showed that the time of Kuroshio intrusion was not coincident with the intensification of the northeasterly monsoon. The local wind and the current at 20 m were incoherent. Both the variation of Kuroshio current and the fluctuation of Kuroshio path may be responsible for the variation of the local current. Since the intrusion of Kuroshio has a weak relationship with local wind variation, it appears to be induced by non-local factors.

Observations of second baroclinic mode internal solitary waves on the continental slope of the northern South China Sea

[1] A temperature and current velocity mooring, located on the upper continental slope of the northern South China Sea, recorded a number of second baroclinic mode (mode 2) internal solitary waves (ISWs). These types of waves are seldom observed in nature. The mode 2 ISWs typically showed upward (downward) displacement of isotherms in the upper (lower) water column and three layers of eastward, westward, and eastward current from the uppermost to bottommost portions of a wave. In summer, westward-propagating mode 2 ISWs were observed only occasionally. These waves generally appeared after mode 1 ISWs, a feature that may relate to the diurnal tide with a period of approximately 24 hours. The displacement of isotherms induced by mode 2 ISWs was 20 ± 14 m at 75 m and ―22 ± 15 m at 240 m, and the characteristic time scale was approximately 8.0 ± 4.3 min. In winter, mode 2 ISWs were more active but mode 1 ISWs were rarely observed. Isotherm displacement by mode 2 ISWs in winter was 30 ± 18 m at 75 m and ―26 ± 16 m at 240 m, and the average characteristic time scale was 6.9 ± 4.6 min. The mode 2 ISWs thus had larger amplitudes and smaller time scales in winter than they did in summer. The observed vertical temperature profile also showed notable seasonal change. The thermocline was shallow in summer and deep in winter. In winter, vertical temperature profiles indicated that the main thermocline was located near middepth over the upper continental slope near the 350 m isobath. Mode 1 ISWs were more active in summer than in winter, reflecting the larger Ursell numbers for mode 1 ISWs in summer. Among mode 2 ISWs in summer, 90% appeared after mode 1 ISWs. These results suggest that mode 2 ISWs could be related to mode 1 ISWs. In contrast, mode 2 ISWs were more active in winter than in summer, with larger mode 2 Ursell numbers also found in winter. Among winter mode 2 ISWs, 72% appeared without mode 1 ISWs. Mode 2 ISWs in winter could be related to the main thermocline being located near middepth. These seasonal variations of mode 2 ISWs were correlated with the seasonal change of local stratification. Further study on the different generating mechanisms of mode 2 ISWs in summer and winter is needed.

Trapped Core Formation within a Shoaling Nonlinear Internal Wave

AbstractLarge-amplitude (100–200 m) nonlinear internal waves (NLIWs) were observed on the continental slope in the northern South China Sea nearly diurnally during the spring tide. The evolution of one NLIW as it propagated up the continental slope is described. The NLIW arrived at the slope as a nearly steady-state solitary depression wave. As it propagated up the slope, the wave propagation speed C decreased dramatically from 2 to 1.3 m s−1, while the maximum along-wave current speed Umax remained constant at 2 m s−1. As Umax exceeded C, the NLIW reached its breaking limit and formed a subsurface trapped core with closed streamlines in the coordinate frame of the propagating wave. The trapped core consisted of two counter-rotating vortices feeding a jet within the core. It was highly turbulent with 10–50-m density overturnings caused by the vortices acting on the background stratification, with an estimated turbulent kinetic energy dissipation rate of O(10−4) W kg−1 and an eddy diffusivity of O(10−1) m2...