Jiliang Xuan

Cross‐shelf exchange in the shelf of the East China Sea

A high-resolution, 3-dimensional primitive equation model is used to investigate the cross-shelf exchange in the East China Sea (ECS). Favorable comparisons between field data and model simulations from both climatological run and hindcast run for 2006 indicate that the model has essential skills in capturing the key physics of the ECS. Temporal and spatial variations of the cross-shelf exchanges are further analyzed. It was demonstrated from both observations and simulations that in 2006 high saline water could be delivered to the north of the Changjiang River mouth (near 32 degrees N) as a result of stronger than typical cross-shelf exchanges at the shelf break and flows through the Taiwan Strait with an annual mean rate of 2.59 and 1.83 Sv, respectively. A few new places at the shelf break were also identified where persistent and vigorous onshore or offshore exchanges occur throughout the year. Cross-shelf exchange is largely determined by the along-shelf geostrophic balance with weak seasonality, which is modulated in upper layers by northeasterly monsoon from early-fall to late-spring and at seabed by bottom friction during December-January, May, and August-September. Nonlinear effect, with strong spatial variations and intraseasonal variability, is a secondary but persistent contributor to the net seaward transport, except for northeast of Taiwan where the nonlinear effect becomes significant but more varied.

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.

Cross‐shelf transport of terrestrial Al enhanced by the transition of northeasterly to southwesterly monsoon wind over the East China Sea

Aluminum (Al) is one of the key parameters of GEOTRACES. In spring 2011, we examined the distribution of dissolved Al in the East China Sea (ECS) to assess the potential passage of coastal water across the shelf to the western Pacific. Measurement of dissolved Al in coastal, shelf, and shelf break waters indicated decreasing concentrations with distance from the coast. However, the northward increase of dissolved Al in the upper 100 m along the shelf break transect, corresponding to the northward flow of Kuroshio Water along the Okinawa Trough from the north of Taiwan to the Tokara strait, indicates contributions of terrestrial material. The presence of an Al plume indicated cross-shelf transport at the subsurface at a potential density of 23.3–24.0 kg/m3. This plume originates from the near-bottom layer along the coast off Zhejiang and Fujian Provinces, and first moves southeastward (to the north of Taiwan) and then northeastward (to shelf break with export at 29°N, 127°E). We calculated the cross-shelf fluxes of water and dissolved Al based on simulation results of the 3-D Massachusetts Institute of Technology general circulation model (MITgcm). The calculated cross-shelf fluxes of Al at the subsurface layer indicate that the ECS is highly efficient in pumping Al-rich coastal waters northward to the Japan Sea/East Sea and/or eastward into the western Pacific. We also identified an important role of the monsoon, with a change from northeasterly wind to southwesterly wind in spring, on the cross-shelf transport of Al by use of numerical tracer experiments.

Tidal and residual currents in the Qiongzhou Strait estimated from shipboard ADCP data using a modified tidal harmonic analysis method

In spring 2013, 33 repeat shipboard Acoustic Doppler Current Profile (ADCP) surveys were conducted to measure the tidal current in the Qiongzhou Strait (QS). The major tidal currents and the residual current along a section across the QS were estimated using a modified tidal harmonic analysis method based on the inverse technique. A simple simulation and comparisons with previous observations demonstrated that the tidal currents estimated using the modified tidal harmonic analysis method are reasonable, and this method was able to control the magnitude and deviation of the estimation error. The direction of the major axis of tidal current ellipses is generally along the strait. Diurnal tidal constituents are dominant among the five tidal current constituents (K1, O1, M2, S2, and MSf). The ratio of the amplitudes of O1, K1, M2, S2, and MSf, averaged along the section across the QS is 1:0.79:0.42:0.27:0.29. The residual current along the entire section is all westward; the averaged velocity over the section is 6.0±2.1 cm s−1; the associated volume transport through the section is −0.065 ± 0.046 Sv (Sv = 106 × m3 s−1), in which the second value denotes the uncertainty of first value. Dynamic analysis indicates that tidal current activity is more dominant than mean current and eddy activity, and tidal rectification and sea level difference between two entrances of the QS are important in maintaining the residual current through the strait.

Cross‐shelf water exchange in the East China Sea as estimated by satellite altimetry and in situ hydrographic measurement

Combining satellite altimetry and in situ hydrographic measurement, we estimated the cross-shelf transport (CST) and its spatial and temporal variations across 200 m isobath in the East China Sea (ECS) from 1993 to 2014. The vertically-integrated CST can be dynamically divided into three parts: surface Ekman transport, geostrophic transport, and bottom Ekman transport. The results show that the 22-year-mean, sectionally-integrated CST to be 1.7±2.0 Sv (positive in the on-shelf direction), comprised of bottom and surface Ekman transports of 2.7±1.0 Sv and 0.6±0.6 Sv, respectively, that are partially offset by a geostrophic transport of -1.5±1.7 Sv. The sectionally-integrated CST shows significantly high power at roughly annual period from 1999-2001, with lower power at intra-annual period. The time-averaged, vertically-integrated CST to the northeast of Taiwan is the main source of sectionally-integrated CST. The vertically-integrated CST also shows significant variations in the 6–15 month period band to the northeast of Taiwan as well. The temporal variations of the sectionally-integrated and vertically-integrated CST are both controlled primarily by geostrophic transport and modulated by bottom Ekman transport. In the upper 50 m, the geostrophic current to the northeast of Taiwan exhibits large mean and significant variability. The empirical orthogonal function analysis of vertical structure of geostrophic current shows two significant modes with strong annual signal. The first mode is associated with the migration of Kuroshio axis near Taiwan, while the second mode is associated with the variation of the meander of the Kuroshio to the northeast of Taiwan. This article is protected by copyright. All rights reserved.