Weidong Zhai

Dynamics of the carbonate system in a large continental shelf system under the influence of both a river plume and coastal upwelling

National Basic Research Program of China (973 Program) [2009CB421201]; National Science Foundation of China (NSFC) [90711005, 40821063]; NSFC-RGC [40731160624, N_HKUST623/07]; SCOPE project

Runoff‐mediated seasonal oscillation in the dynamics of dissolved organic matter in different branches of a large bifurcated estuary—The Changjiang Estuary

National Natural Science Foundation of China [41276064, 41076044, 40776041]; Chinese Scholarship Council [2012-3022]

Cyclone-driven deep sea injection of freshwater and heat by hyperpycnal flow in the subtropics

China (973 Program) [2009CB421200]; Program of Introducing Talents of Discipline to Universities [B07034]; Academia Sinica Thematic Program AFOBi, Taiwan [NSC 98-2116-M-001-005]

A mechanistic semi‐analytical method for remotely sensing sea surface pCO2 in river‐dominated coastal oceans: A case study from the East China Sea

While satellite remote sensing has become a very useful tool contributing to assessments of sea surface partial pressure of carbon dioxide (pCO2) that subsequently allow quantification of air-sea CO2 flux, the application of empirical approaches in coastal oceans has proven challenging owing to the interaction of multiple controlling factors. We propose a “mechanistic semi-analytic algorithm” (MeSAA) to estimate sea surface pCO2 in river-dominated coastal oceans using satellite data. Observed pCO2 can be analytically expressed as the sum of individual components controlled by major factors such as thermodynamics (or temperature), mixing, and biology. With marine carbonate system calculations, temperature and mixing effects can be predicted using thermodynamic principles and by assuming conservative two end-member mixing of total dissolved inorganic carbon and total alkalinity (e.g., the Changjiang River and Kuroshio water in the East China Sea, ECS). Next, an integral expression for pCO2 drawdown due to biological effects can be parameterized using the chlorophyll a concentration (chla). We demonstrate the validity and applicability of the algorithm in the ECS during summertime. Sensitivity analysis shows that errors in empirical coefficients and three input satellite parameters (salinity, SST, chla) have limited influence on the algorithm, and satellite-derived pCO2 is consistent with underway data, even though no in situ pCO2 data from the ECS shelves was used to train the algorithm. Our algorithm has more physical and biogeochemical mechanistic meaning than empirical methods, and should be applicable to other similar systems.

Satellite views of the episodic terrestrial material transport to the southern Okinawa Trough driven by typhoon

Using satellite-derived water transparency (alias Secchi depth) images, we found clear signals of terrestrial material transport to the southern Okinawa Trough triggered by the Typhoon Morakot in August 2009. Three sources were identified: one is from the eastern coast of Taiwan, another is from the western coast of Taiwan, and the other is from the coast of mainland China. Carried by northward flows, typhoon-triggered terrestrial materials from both sides of Taiwans coasts were transported to the region northeast of Taiwan. Moreover, the terrestrial material from the coast of mainland China could cross the Taiwan Strait and be further transported to the region northeast of Taiwan. These typhoon-induced terrestrial materials off northeastern Taiwan could then be transported to the southern Okinawa Trough along the western edge of the Kuroshio. In addition to the particulate terrestrial material transported, nutrients might also be transported to the Kuroshio main stream. A significant phytoplankton bloom was observed along the Kuroshio path for about 300 km off northeast of Taiwan. Our results indicate that episodic cyclone-driven terrestrial material transport could be another source of mud in the southern Okinawa Trough.

Spring carbonate chemistry dynamics of surface waters in the northern East China Sea: Water mixing, biological uptake of CO2, and chemical buffering capacity

We investigated sea surface total alkalinity (TAlk), dissolved inorganic carbon (DIC), dissolved oxygen (DO), and satellite-derived chlorophyll-a in the connection between the Yellow Sea and the East China Sea (ECS) during April to early May 2007. In spring, Changjiang dilution water (CDW), ECS offshore water, and together with Yellow Sea water (YSW) occupied the northern ECS. Using 16 day composite satellite-derived chlorophyll-a images, several algal blooms were identified in the CDW and ECS offshore water. Correspondingly, biological DIC drawdown of 73 ± 20 μmol kg−1, oversaturated DO of 10–110 μmol O2 kg−1, and low fugacity of CO2 of 181–304 μatm were revealed in these two waters. YSW also showed CO2 uptake in spring, due to the very low temperature. However, its intrusion virtually counteracted CO2 uptake in the northern ECS. In the CDW and the ECS offshore water, Revelle factor was 9.3–11.7 and 8.9–10.6, respectively, while relatively high Revelle factor values of 11.4–13.0 were revealed in YSW. In the ECS offshore water, the observed relationship between DIC drawdown and oversaturated DO departed from the Redfield ratio, indicating an effect of chemical buffering capacity on the carbonate system during air-sea reequilibration. Given the fact that the chemical buffering capacity slows down the air-sea reequilibration of CO2, the early spring DIC drawdown may have durative effects on the sea surface carbonate system until early summer. Although our study is subject to limited temporal and spatial coverage of sampling, these insights are fundamental to understanding sea surface carbonate chemistry dynamics in this important ocean margin.

On the variations of sea surface pCO2 in the northern South China Sea: A remote sensing based neural network approach

[1] Using a neural networking (NN) approach, we developed an algorithm primarily based uponsea surface temperature (SST) and chlorophyll (Chla) to estimate thepartial pressure of carbon dioxide (pCO2) at the sea surface in the northern South China Sea (NSCS). Randomly selected in situ data collected from May 2001, February and July 2004 cruises were used to develop and test the predictive capabilities of the NN based algorithm with four inputs (SST, Chla, longitudes and latitudes). The comparison revealed a high correlation coefficient of 0.98 with a root mean square error (RMSE) of 6.9 matm. We subsequently applied our NN algorithm to satellite SST and Chla measurements, with associated longitudes and latitudes, to obtain surface water pCO2. The resulting monthly mean pCO2 map derived from the satellite measurements agreed reasonably well with the in situ observations showing a generally homogeneous distribution in the offshore regions. The pCO2 exerts a very dynamic feature in nearshore regions, especially in the coastal upwelling and estuarine plume regions. We identified three low pCO2 zones (<330 matm), two of which are influenced by coastal upwelling: off Hainan island in the western part of the NSCS; and off Guangdong province in the eastern part of the NSCS. The path of the Pearl River plume on the shelf was another zone with low pCO2. For the monthly mean pCO2 variations estimated based on the MODIS-SST and -Chla values, an RMSE of � 6 matm may be attributable to the measurement errors associated with MODIS measurements. As a first order estimation, we used the same sampling periods of remote sensing and in situ measurements, and were able to estimate pCO2 with an accuracy of 12.05 matm for onshore regions and 13.0 matm for offshore regions, but with combined uncertainties associated with the NN Testing algorithm and MODIS SST and Chla measurements.

Assessing the Dynamics of Dissolved Organic Matter in the Changjiang Estuary with Absorption and Fluorescence Spectroscopy

Changjiang Estuary is the largest estuary in China and is significantly influenced by anthropogenic perturbation and depositional processes. The south branch of this estuary is the dominant pathway of Changjiang river discharge, while the shallower north branch is dominated by salty water, and the water residence time is much longer, especially in the dry season. To examine the dynamics of dissolved organic matter (DOM) in such complex environments, two optical methods (absorption and fluorescence spectroscopy) combined with dissolved organic carbon (DOC) analysis were used to characterize the properties of DOM samples collected in different seasons (April, July, and October, 2011). The DOC concentration, absorption coefficient, and fluorescence intensities were highest in the Huangpu River (the last major tributary of south branch) that receives large amounts of sewage from Shanghai. Such input leads to not only an increase in the DOM level but also a notable change in the chemical composition (and hence reactivity) of DOM in the south branch. In the north branch, significant additions of DOM were observed in the dry season, which probably results from the release of water-soluble organic matter via intense resuspension. These results suggest that anthropogenic inputs from megacities and additions from resuspended sediments should be seriously considered in assessing the flux and chemical composition of DOM being discharged into the ocean.

A test of empirical and semi-analytical algorithms for euphotic zone depth with SeaWifs data off southeastern China

This study employs SeaWiFS data over the waters off the southeastern China to evaluate a semi-analytical algorithm for euphotic zone depth (Ze). This algorithm is based on waters inherent optical properties (IOPs), which can be near-analytically calculated from spectral remote-sensing reflectance, where remote-sensing reflectance can be derived from the normalized water-leaving radiance provided by SeaWiFS. In the Taiwan Strait, compared with in situ Ze (±3 hour within SeaWiFS collection), average error (ε) is 15.0 % and root mean square error (RMSE) is 0.074, with Ze in a range of 14-34 m from field measurements. In the South China Sea, compared with in situ Ze (±48 hour within SeaWiFS collection),ε is 5.1 % in summer and 22.6 in winter, while RMSE is 0.032 in summer and 0.129 in winter, with Ze in a range of 10-82 m from field measurements. For comparison, we also evaluate the performance of the empirical Ze algorithm that is based on chlorophyll concentration. It is found that the IOP-centered approach has higher accuracy compared to the chlorophyll-a centered approach (e.g. in the South China Sea in winter, ε is 55.3 % and RMSE is 0.219). The new algorithm is thus found not only worked well with waters of the Gulf of Mexico, Monterey Bay and the Arabian Sea, but also worked well with waters of the China Sea.