Cho-Teng Liu

The Kuroshio East of Taiwan: Moored Transport Observations from the WOCE PCM-1 Array

Observations from the WOCE PCM-1 moored current meter array east of Taiwan for the period September 1994 to May 1996 are used to derive estimates of the Kuroshio transport at the entrance to the East China Sea. Three different methods of calculating the Kuroshio transport are employed and compared. These methods include 1) a “direct” method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure, 2) a “dynamic height” method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles, and 3) an “adjusted geostrophic” method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements. The first two methods are largely independent and are shown to produce very similar transport results. The latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates. These methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations. The mean transport of the Kuroshio over the 20-month duration of the experiment ranges from 20.7 to 22.1 Sv (1 Sv ≡ 106 m3 s−1) for the three methods, or within 1.3 Sv of each other. The overall mean transport for the Kuroshio is estimated to be 21.5 Sv with an uncertainty of 2.5 Sv. All methods show a similar range of variability of ±10 Sv with dominant timescales of several months. Fluctuations in the transport are shown to have a robust vertical structure, with over 90% of the transport variance explained by a single vertical mode. The moored transports are used to determine the relationship between Kuroshio transport and sea-level difference between Taiwan and the southern Ryukyu Islands, allowing for long-term monitoring of the Kuroshio inflow to the East China Sea.

Winter upwelling off Luzon in the northeastern South China Sea

A major upwelling region centered about 100 km offshore between 16° and 19°N off northwest Luzon in the South China Sea is revealed in the distributions of temperature, salinity, and dissolved oxygen concentration during a cruise in December 1990. Upward movement of water low in dissolved oxygen can be identified to a depth of 300 m. Climatological distributions of temperature and upper ocean heat content and tracer distribution obtained from a recent numerical experiment lend support to the existence of this upwelling region. Results from a general circulation model of the South China Sea further show that upwelling off Luzon occurs between October and January, in agreement with the climatological data. Current structures obtained from a simple two-layer model and the general circulation model both suggest that this upwelling is not driven by local winds but contains a large, remotely forced component arising from the basin circulation. The upwelling is sustained by offshore Ekman drift from above and a converging northward undercurrent from below. The would-be downwind coastal jet in the surface layer is largely offset by currents associated with the winter circulation gyre. Upwelled water with high nutrient content spreads southwestward in January and could be an important nutrient source for the surface water in the interior South China Sea in spring.

Exchange of water masses between the East China Sea and the Kuroshio off northeastern Taiwan

Abstract At least six water masses take part in the mixing processes between the East China Sea and the Kuroshio off northeastern Taiwan: the Kuroshio Surface Water (SW), Kuroshio Tropical Water (TW), Kuroshio Intermediate Water (IW), East China Sea Water (ECSW), Coastal Water (CW) and the Taiwan Strait Water (TSW). SW is depleted in nutrients and normalized alkalinity but has the highest temperature and pH of all these waters. TW has relatively high temperature, and the highest salinity of all waters. The salinity maximum in the Kuroshio is usually between 100 and 300 m deep, with large interannual and seasonal variability. IW is characterized by a salinity minimum, high nutrient content and alkalinity, but low pH and oxygen. ECSW is low in salinity, temperature and nutrients, but high in oxygen and normalized calcium and alkalinity. CW has low salinity and nutrient content but is high in normalized alkalinity. TSW is generally depleted in nutrients. The characteristics of the above mentioned waters are discussed. The mixing percentages of SW, TW, IW, and the composite Shelf Surface Water (composed of ECSW, CW and TSW) off the northeast corner of Taiwan in September 1988 and December 1989 are calculated.

Prototypical solitons in the South China Sea

[1]xa0Surface signatures associated with non-linear internal waves are often seen in satellite images of the western South China Sea (SCS) slope and shelf. Observation in the deep sea, to the east, are rare. Here we report on the evolution of an energetic packet as it propagated through the deep central basin of the SCS, toward the western slope and shelf. The waves have amplitudes estimated at 170 m, half widths of 3 km, and phase speeds of 2.9 ± 0.1 m/s, faster than the mode-1 linear phase speed of 2.6 m/s. The shape and observed phase speed were consistent with the Korteweg-deVries (KdV) model over the 65-km path that they were tracked. The intrinsic velocity shear of the waves is small compared to pre-existing shears, and the waves exhibit weak turbulence. The KdV fit and a satellite-derived estimate of horizontal wave extent imply a westward energy flux of 4.5 GW for each crest.

Taiwan Current(Kuroshio)and Impinging Eddies

Considerable westward or nothwestward propagating eddies were found east of Taiwan that cross-explains the anomalies in the repeated hydrography, trajectory of drifting buoys and altimetric analyses. The sea level differences (SLD) across the Taiwan Current (Kuroshio) in the East Taiwan Channel (ETC) are utilized in order to examine the possible implication of eddies in the Taiwan Current transport. It is concluded that Taiwan is impinged by both cyclonic and anticyclonic mesoscale eddies at an interval of about 100 days. An approaching anticyclonic eddy will result in a higher SLD across the ETC and a larger mass transport of Taiwan Current, and, vice versa, a reduction of both SLD and the mass transport in the ETC as a cyclonic eddy arrives. The SLD-inferred northward transport in the ETC is highly coherent at the 100-day band with westward propagating eddies that originated in the interior ocean. The generation mechanism of these eddies are, however, still unclear. Leakage of the Kuroshio water to the east of the Ryukyu Islands is suggested due to the presence of cyclonic eddies. This 100-day rate of eddy-impingement invalidates any observation of 4 months or less, whether with direct or indirect measurements, because any conclusions depend on the presence or absence of eddies. To minimize the contamination from eddies, either long-term observations or eddy-removal procedures are required.

Variability of the chemical hydrography at the frontal region between the East China Sea and the Kuroshio north-east of Taiwan

Abstract The hydrography across the frontal region between the East China Sea and the Okinawa Trough north-east of Taiwan, observed during the summer of 1985 and 1988 and the early spring of 1987, was governed mainly by mixing across the front and the topographically induced upwelling of the modified Kuroshio water in the Okinawa Trough during the periodic shelfward migration of the Kuroshio. The location of the front relative to the shelf break seemed to be temporally variable. Topographically induced upwelling was evident during the summer of 1988 and the early spring of 1987 when the front was located close to the shelf break. It might not have occurred in the summer of 1985 when the front was further offshore. The end-member composition of the upwelling water was similar in both seasons. It originated from about 300 m with a temperature and salinity of 13 °C and 34·4 psu. It was rich in nutrients and poor in oxygen with concentrations of nitrate, phosphate, silicate and oxygen of 16, 1, 18 and 160 μM respectively. This upwelling water is potentially a major source of nutrients to the East China Sea. The deep water in the Okinawa Trough at temperatures below 15 C did not participate in cross-shelf mixing. Its chemical characteristics did not change significantly from year to year.

Mean transport and seasonal cycle of the Kuroshio east of Taiwan with comparison to the Florida Current

Moored observations of Kuroshio current structure and transport variability were made across the channel between northeast Taiwan and the Ryukyu Islands at 24 degreesN from September 19, 1994, to May 27, 1996. This was a cooperative, effort between the United States and Taiwan. The moored array was designated PCM-1, for the World Ocean Circulation Experiment (WOCE) transport resolving array. The dominant current and transport variability occurred on 100-day timescales and is shown by Zhang et al. [2001] to be caused by warm mesoscale eddys merging with the Kuroshio south of the array causing offshore meandering and flow splitting around the Ryukyu Islands. An annual transport cycle could not be resolved from our 20-month moored record because of abasing from the 100-day period events. Sea level difference data were used to extend the transport time series to 7 years giving a variation in the range of the annual transport cycle of 4-10 Sv, with a mean range closer to 4 Sv. The seasonal maximum of 24 Sv occurred in the summer and the seasonal minimum of 20 Sv occurred in the fall. A weaker secondary maximum also occurred in the winter. The cycle of Kuroshio transport appears to result from a combination of local along-channel wind forcing and Sverdrup forcing over the Philippine Sea. Our estimate of the mean transport of the Kuroshio at the entrance to the East China Sea from the moored array is 21.5 +/- 2.5 Sv. The mean transpacific balance of meridional flows forced by winds and thermohaline processes at this latitude requires an additional mean northward flow of 12 Sv with an annual cycle of +/-8 Sv along the eastern boundary of the Ryukyu Islands. The mean transport and annual cycle of the Kuroshio were found to be in reasonable agreement with basin-scale wind-forced models. Remarkable similarities are shown to exist between the mean western boundary currents and their seasonal cycles in the Atlantic (Florida Current and Antilles Current) and Pacific (Kuroshio and boundary current east of Ryukyu Island chain) at the same latitude. However, detailed comparison shows that the mean Kuroshio is weaker and more surface intensified than the mean Florida Current, while the Kuroshio-transport variability is significantly larger.

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.

Nonlinear internal waves from the Luzon Strait

In the northeastern South China Sea, fast westward moving nonlinear internal waves (NLIWs) emanate nearly daily from the Luzon Strait during spring tide. Their propagation speed of about 2.9 meters per second is faster than NLIWs previously observed in the worlds oceans. The amplitudes of these waves reach 140 meters or more, and they are the largest free propagating NLIWs observed to date in the interior ocean. These NLIWs energize the top 1500 meters of the water column, moving water up and down at timescales as short as 20 minutes. While their associated energy density and energy flux are the largest observed to date, the exact source of these giant waves has yet to be determined.

Sea surface slope as an estimator of the Kuroshio volume transport east of Taiwan

The Kuroshio volume transport (KVT) at 24.5°N near Taiwan was directly measured for the first time during the period from September 1994 to May 1996 through the channel between Taiwan and Iriomote, Japan. Adjusted sea level difference anomaly (SLDA) across the Kuroshio is examined as a proxy for long-term monitoring of KVT. Regression analyses were performed on the directly measured KVT and SLDA between Ishigaki and sea level stations on the Taiwan side (Keelung, Kenfang and Suao). The best estimators of KVT are: (1) KVT1=0.32×SLDA1 (Ishigaki-Suao) + 20.85 Sυ±2.3 Sυ for daily KVT and (2) KVT10=0.37 × SLDA10 (Ishigaki-Suao) + 20.87 Sυ±1.4 Sυ for 10-day low-passed KVT.