Daji Huang

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.

Real-time monitoring of nutrients in the Changjiang Estuary reveals short-term nutrient-algal bloom dynamics

The Changjiang Estuary is a large-river estuary ecosystem in the East China Sea, and its plume, the Changjiang Diluted Water (CDW), transports a large mass of nutrients ( NO3– +  NO2–, PO43–, SiO32–) to the shelf sea, leading to substantial eutrophication; the CDW also supports high primary production. However, relationships between nutrient delivery and phytoplankton responses have been difficult to establish, as many nutrient delivery events and algal blooms are episodic, and the CDW may expand or become detached with changing winds. To study the relationship between nutrient delivery events, algal blooms and estuarine metabolism dynamics, a buoy system was deployed in the CDW from 9 September to 10 October 2013, with measurements of chlorophyll a and dissolved nutrients. Day-to-day nutrient increases covaried with salinity decreases, regulated by both the spring-neap tidal cycle and wind events. Several specific nutrient injection periods were detected, each followed by nutrient drawdown and chlorophyll a accumulation (algal blooms). Each algal bloom had its own unique pattern of nutrient uptake based on change in nutrient ratios (ΔN:ΔP; ΔN:ΔSi) and appeared to be dominated by different algal groups. These events occurred under weak wind and stable hydrodynamic conditions. Ecosystem metabolism based on net community production (NCP) showed that the upper estuarine ecosystem was autotrophic when chlorophyll a accumulated, but heterotrophic when wind-induced mixing strengthened, and upwelling brought organic-rich water to the near surface. In spite of several short-lived algal blooms, the average NCPdaily was negative during the observation period, indicating a net source of CO2 to the atmosphere.

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.