Xiao-Hua Zhu

Coastal acoustic tomography system and its field application

The coastal acoustic tomography system (CATS), composed of five moored acoustic stations, has been constructed to measure current fields. The system is developed with special considerations in mind, including the use of Global Positioning System clock signals in the synchronization of the system clock timing among the multiple acoustic stations, and the use of the differently coded Gold sequences to identify the acoustic signals corresponding to individual stations from a received signal. The CATS was successfully applied to map the structure of strongly nonlinear tidal currents in the coastal sea. In spite of the limited spatial resolution caused by inadequate sound transmission data, the two-dimensional tidal vortices features of growth, translation, and decay processes are reconstructed through an inverse analysis of the acoustic travel time obtained among the station pairs. It is evident that the CATS is a powerful tool for measuring variable current fields generated in the coastal seas.

The Kuroshio nutrient stream and its temporal variation in the East China Sea

[1]xa0Using in situ data from 88 cruises from 1987 to 2009 in the East China Sea, downstream nutrient flux (the product of velocity and nutrient concentration) and nutrient transport (integration of flux over a section) by Kuroshio were examined. The presence of a maximum nutrient flux core in the middle layer was confirmed. Seasonal variation in the nutrient flux was not significant and was much smaller than interannual variations. The change in the Kuroshio speed and current structure were major causes for interannual variations in the nutrient flux. The downstream nitrate transport by the Kuroshio in the East China Sea had a mean value of 170.8 kmol s−1 and a standard deviation of 41.6 kmol s−1. The mean seasonal nitrate transport ranged between about 161 and 177 kmol s−1 and the absolute interannual variation from about 100 to 280 kmol s−1. The phosphate flux and phosphate transport can be approximately estimated by the ratio (13.64) of nitrate concentration to phosphate concentration. The nitrate concentration in the middle and bottom layers across the Kuroshio in the East China Sea was found to increase significantly over the 23 year period, and especially after 2004 but not at ratios with oxygen that suggest increased remineralization of organic matter. The nutrient transport, however, did not increase significantly because increases in the surface layer were offset by decreases in the middle and bottom layers caused by reduction in velocity in the density ranges of 26.0 to 27.2 σθ below the Kuroshio.

The Northeastward current southeast of Okinawa Island observed during November 2000 to August 2001

[1]xa0In order to estimate the variability of current structure and mean geostrophic volume transport in the southeast of Okinawa Island, the round-trip acoustic travel times between sea bottom and surface at nine sites were measured by Inverted Echo Sounder with pressure gauge (PIES) from November 2000 to August 2001. Vertical sections of geostrophic velocity are calculated by using vertical profiles of specific volume anomaly derived by the Gravest Empirical Mode method from the PIES data. In the upper 500 m layer over a slope shallower than 1000 m depth, the northeastward current reached up to 60 cm s−1 during the occupation of an anticyclonic eddy, and decreased to −15 cm s−1 during the occupation of a cyclonic eddy, around the mean of 20 cm s−1. The temporal mean geostrophic volume transport relative to the 2000 dbar level was estimated to be 6.1 Sv (1 Sv ≡ 106 m3 s−1) northeastward in the west of 128.85°E.

The Kuroshio East of Taiwan and in the East China Sea and the Currents East of Ryukyu Islands during Early Summer of 1996

Using hydrographic data and moored current meter records and the ADCP observed current data during May–June 1996, a modified inverse method is applied to calculate the Kuroshio east of Taiwan and in the East China Sea and the currents east of Ryukyu Islands. There are three branches of the Kuroshio east of Taiwan. The Kuroshio in the East China Sea comes from the main (first) and second branches of the Kuroshio east of Taiwan. The easternmost (third) branch of the Kuroshio flows northeastward to the region east of Ryukyu Islands. The net northward volume transports of the Kuroshio through Section K2 southeast of Taiwan and Section PN in the East China Sea are 44.4×106 and 27.2×106 m3s−1, respectively. The western boundary current east of Ryukyu Islands comes from the easternmost branch of the Kuroshio east of Taiwan and an anticyclonic recirculating gyre more east, making volume transports of 10 to 15×106 m3s−1. At about 21°N, 127°E southeast of Taiwan, there is a cold eddy which causes branching of the Kuroshio there.

Manifestation of the Pacific Decadal Oscillation in the Kuroshio

[1]xa0Pacific Decadal Oscillation (PDO) index is strongly correlated with vertically integrated transport carried by the Kuroshio through the East China Sea (ECS). Transport was determined from satellite altimetry calibrated with in situ data and its correlation with PDO index (0.76) is highest at zero lag. Total PDO-correlated transport variation carried by the ECS-Kuroshio and Ryukyu Current is about 4 Sv. In addition, PDO index is strongly negatively correlated, at zero lag, with NCEP wind-stress-curl over the central North Pacific at ECS latitudes. Sverdrup transport, calculated from wind-stress-curl anomalies, is consistent with the observed transport variations. Finally, PDO index and ECS-Kuroshio transport are each negatively correlated with Kuroshio Position Index in the Tokara Strait; this can be explained by a model in which Kuroshio path is steered by topography when transport is low and is inertially controlled when transport is high.

The role of wind on the detachment of low salinity water in the Changjiang Estuary in summer

in July 2006, and the role of wind on detaching the LSW in particular, are explored with a three-dimensional numerical model. The real-case simulation and the sensitivity experiments results show that wind plays a crucial role in the detachment events and is highlighted in three aspects. First, wind is the most important dynamic factor in the two detachment processes of the LSW. Wind mixing, wind-driven northward current and wind-induced upwelling are three driving forces on detaching the LSW, which increase the salinity in the upper layer in the detachment region along the 30 m isobath and separate the offshore LSW from the nearshore main body of LSW. The diagnostic analysis further indicates that the increase of salinity in the detachment region is mainly due to northward current which transports high salinity water from the south. Second, a critical wind speed, namely a southeasterly wind above 8.0 m/s, is found to be related to the timing of the detachment events. A sensitivity experiment further confirms this critical wind speed and no detachment occurs when the wind speed is below 8.0 m/s. Third, the southwesterly wind plays a key role in the magnitude of the spatial size of the detached LSW. Before the detachment occurs, a persistent southwesterly wind induces northeastward expansion of the LSW and consequently forms larger LSW offshore after detachment, which is verified by another sensitivity experiment with modified wind direction.

Interannual to decadal variability of the Kuroshio Current in the East China Sea from 1955 to 2010 as indicated by in-situ hydrographic data

The temporal and spatial variability of the Kuroshio Current was analyzed. Current data were estimated from hydrographic data collected from areas within the central East China Sea (PN section) from 1955 to 2010 and the Tokara Strait (TK section) from 1987 to 2010. To reduce the bias caused by cruise-dependent spatial resolution among the data, grid-consistent temperature and salinity fields were reconstructed by use of a regression relationship to account for anomalies between observed stations and grid points. The mass imbalance problem between the PN and TK sections, which appears stochastically when viewed by use of the dynamic method, was solved by use of the inverse method. The estimated Kuroshio volume transport (KVT) was found to be closely consistent with that of current observations and had an uncertainty of 2.4xa0Sv. The KVT seemed to have neither a regime shift in approximately 1976 nor a sharply increasing trend. The KVT was dominated by 2–5xa0year modulating interannual variability with an amplitude of 2.8xa0Sv, followed by weak 20-year decadal variability with an amplitude of 0.33xa0Sv. Empirical orthogonal function analysis of the currents suggested that the temporal and spatial variability of the Kuroshio Current in the PN section was dominated by a transport mode, manifested by the high variability of current on the seaward side of current core with expansion or shrinkage of the core. In contrast, the temporal and spatial variability of the Kuroshio Current in the TK section was dominated by a meandering mode, as indicated by the migration of the Kuroshio axis in the south gap of the Tokara Strait.

Kuroshio Stream path variation and its associated velocity structures south of Shikoku, Japan

During 1993-1995, twelve repeat observations of the Kuroshio south of Shikoku, Japan were carried out for obtaining a vertical velocity section of the upper 300m, using a towed-type acoustic Doppler current profiler (ADCP). Among the twelve observations, the Kuroshio took the onshore path 9 times and the offshore path 3 times. The cross-stream velocity distribution around the Kuroshio stream axis is asymmetric (steeper on the onshore side than on the offshore side) for the onshore path while it is symmetric for the offshore one. The core with a maximum velocity is submerged to 100-200m depths for the onshore case and located at the near surface for the offshore case. In the combined analysis with the Rapid Bulletin of Ocean Conditions, the Kuroshio mainly took an onshore path with stream-axis positions less than 80km from Cape Ashizuri, but the distance was increased over 150km at the frequency once a year.

Mapping Tidal Current Structures in Zhitouyang Bay, China, Using Coastal Acoustic Tomography

The first Chinese coastal acoustic tomography (CAT) experiment for mapping the tidal currents in Zhitouyang Bay near Zhoushan Island was successfully performed with seven acoustic stations from July 12 to 13, 2009. Using CAT, the horizontal distributions of the tidal currents in the tomography domain were calculated by the inverse analysis, in which the travel time differences for sound traveling reciprocally between the station pairs are used as data. The specified tidal current structures, such as the strong east-west oscillation of the tidal current, the branched current, and the tidal vortices, were reconstructed as snapshots at the successive tidal phases. The relative vorticity calculated from the inverted current fields served to specify the current structures, such as tidal vortices. The inversion-estimated uncertainty of (0.02-0.08) m s-1 narrowed the root-mean-square difference (RMSD) of (0.00-0.11) m s-1 between the 3-min interval original data and the hourly mean data for all the sampled data, which may be regarded as a measure of error. Throughout the tidal phases, the divergence from the inverted current showed a positive (negative) distribution in the shallow (deep) region as an overall view. However, the divergence for the entire tomography domain was nearly equal to zero, corresponding to no net transport. This result implies that the observational errors are quite small for the present experiment. This experiment reaffirms that coastal acoustic tomography is an accurate and efficient observational method for continuously mapping tidal current structures in coastal regions that are characterized by heavy shipping traffic and active fishing.

Variation in the Kuroshio intrusion: Modeling and interpretation of observations collected around the Luzon Strait from July 2009 to March 2011

This study analyzes the observed subtidal currents, 1/12° global HYCOM model results, and the observed time series to interpret seasonal and interannual patterns in the behavior of the Kuroshio intrusion around the Luzon Strait (LS). The observations include current measurements conducted at mooring station N2 (20°40.441′N, 120°38.324′E) from 7 July 2009 to 31 March 2011, surface geostrophic currents derived from the merged absolute dynamic topography, and the trajectory of an Argo float during the winter of 2010–2011. Results from mooring station N2 confirmed the seasonal changes in the Kuroshio intrusion and the variation of the Kuroshio intrusion during El Nino event from July 2009 to April 2010 and La Nina even from June 2010 to March 2011. The strongest Kuroshio intrusion occurs in the winter, with successively weaker currents in spring, autumn, and summer. Comparison of relative differences (Δmax (z)) in the maximum absolute value of monthly average zonal velocity components |Umax (z)| showed that the Kuroshio intrusion was stronger during the 2009–2010 winter (El Nino) than the 2010–2011 winter (La Nina). Furthermore, the relative differences (Δmax (z)) in deeper layers exceed those of the surface layer. Circulation patterns in surface geostrophic currents and the Argo float trajectory confirmed the results of mooring station N2. The Kuroshio intrusion velocity variation modeled using the 1/12° global HYCOM model resembled the observation on both seasonal to interannual scales. Modeled variation in the zonal mean velocity anomaly was also consistent with Nino3, Nino4, and North Equatorial Current (NEC) bifurcation latitude indices, indicating concurrent impacts of the ENSO influence. Monsoon winds strongly affect the seasonal variation while the weak upstream Kuroshio transport induced by El Nino, strongly affects the interannual variation, such as 2009–2010 winter. In 2010–2011 winter, the impact of winter monsoon forcing still exists in the LS. However, the stronger upstream Kuroshio transport during this period did not allow the Kuroshio to penetrate into the LS deeply. This explains why the 2009–2010 winter Kuroshio intrusion (El Nino event) was stronger than that of the 2010–2011 winter (La Nina event).