Conference Agenda

Satellite and Argo observations of dissolved oxygen responses to “Wind Pump” in the Bay of Bengal

Huabing Xu^{1, 3}, Danling Tang^{1, 3*}, Jinyu Sheng², Yupeng Liu^{1, 3}, Yi Sui²

Effects of tropical cyclones (TCs) “Wind pump” on dissolved oxygen (DO) in subsurface waters (20-200 m) over the Oxygen Minimum Zones (OMZs) in the Bay of Bengal (BoB) are examined based on Argo and satellite data. Five TCs (Hudhud, Five, Vardah, Maarutha and Mora) sweeping the central BoB during 2013-2018 are considered. Our analyses reveal three types of DO temporal variability caused by the storm-induced mixing and upwelling. The first type occurred during TC Hudhud features temporal increases of DO in subsurface waters (37-70 m) caused mainly by intense vertical mixing and downwelling. The second type features DO reductions in subsurface waters respectively after four TCs (Hudhud, Five, Maarutha and Mora) attributed to storm-induced upwelling. The third type occurring during TC Vardah features temporal increases of DO at depths between 40-79 m and decreases at depths between 80-150 m due to the combined effect of strong vertical mixing and upwelling. These three types of DO responses can occur in different areas, depending on TC intensity, translational speed and Ekman pumping. The temporal DO variability is also influenced by the shallow oxycline (58.3±16.7 m), mesoscale eddies and biochemical processes. Due to TC intensification, a pre-existing oceanic cyclonic eddy produced a large upwelling and induced a long time of DO decrease in the subsurface layer. This study suggests three different types of DO responses along TC track in the OMZ, which would help us to evaluate the influence of TC on OMZ.

In studies of upwelling usually data from infrared and optical sensors are used which provide information on the sea surface temperature (SST) and the chlorophyll-a (Chl-a) concentration. In this paper, we show that also synthetic aperture radars (SAR) images can also give valuable contribution to such studies. Upwelling regions become detectable by SAR because they are associated with a reduction of the radar backscatter due to 1) a change of the stability of the air-sea interface or/and 2) the presence of biogenic slicks. While the change of the stability of the air-sea interface due to the presence of cold surface water in the upwelling region causes only a small reduction of the radar backscatter, biogenic cause a very strong reduction, usually of more than 10 dB. In areas of strong upwelling, the biological productivity is high due to increased nutrient supply from lower water levels. The biota living in this area secrete surface active material that ascends to the sea surface and forms there biogenic slicks, which often cover large areas in the form as monomolecular layers. The biogenic slicks damp the short-scale surface waves, which are responsible for the radar backscattering, as strongly as mineral oil films, and thus areas covered with the surface films become visible on SAR images as areas of strongly reduced radar backscatter (dark areas). Biogenic slicks often are entrained in the surface current field of eddies and thus render the structure of eddies visible on SAR images.

This paper focuses on upwelling events off the coast of Namibia (caused by northward directed coastal winds), off the coast of South Africa (caused by intensification of the meandering Agulhas Return Current), off the southern coast of Sicily (caused by strong southeastward directed coastal winds, in particular by the Mistral), and off the north coast of Taiwan (caused by the interaction of the Kuroshio Current with shallow bottom topography). These upwelling events are studied using Sentinel-1 and GF-3 SAR images, Modis SST and Chl-a maps and model data of geostrophic surface currents. We show that this synergism yields new insights into upwelling mechanism. In particular, we show that upwelling events are often associated with the generation of filaments, internal waves, and small-scale eddies, which are detectable by SAR.

Rain is one common phenomenon usually observed on SAR images. Its signatures on C-band images are often composed of very bright patches and adjacent dark patches. It is caused by the radar backscattering or attenuation from rain-induced structures on the sea surface (ring waves, splash products, and turbulence) and the hydrometeors in the atmosphere (liquid water or ice). Although some models have been developed and laboratory experiments have been conducted to explain rain impact at C-band, the vertical non-uniform distribution of raindrops and the presence of ice aloft are often ignored. In this study, we co-analyze Sentinel-1 C-band SAR data together with high temporal and spatial resolution weather radar NEXRAD. NEXRAD provide different precipitation products, including rain rate (1 hour/0.25 km), basic reflectivity (less than 5 min /0.5 km) and hydrometeor classification (less than 5 min/0.25 km). More than 747 SAR images in both co- and cross-polarization have been collocated to provide statistics of NRCS under rain, at 1 km resolution and less than 5 min between SAR and NEXRAD. Our results evidence that NRCS at both VV and VH increase with rain rate, for low to moderate wind regimes. In addition, the very bright patches obtained at both polarizations are found to be in relation with melting ice particles in the atmosphere, by examining the NEXRAD hydrometeor classification. Our analysis also reveals the possible importance of rain impact after the rainfall, with a decrease of the backscatter observed after intense rainfall. This indicates the need to analyze the rain event history to take into account rain effect persistency after the rain event. Finally, examples in the specific case of hurricane are discussed.

.

A semi-automated pocket FerryBox monitoring system was installed on a cargo ship in October 2015 to continuously record in situ chemical oceanographic data along a transect between two cities across the Bohai Strait, China for one year. In 2016, 2 cruises by research vessels that had an automated FerryBox system installed covered the Bohai Strait in the summer and winter. This report summarizes the outcome of this one-year campaign that used FerryBox as a water quality monitoring tool in the Bohai Strait. Temperature, salinity, dissolved oxygen, turbidity and saturation were the 5 parameters selected to describe the seasonal patterns in the Bohai Strait during the one-year period. Evidence of a short-term spring bloom event was found between the FerryBox transects, and the hydrodynamic factors that are the main controlling forces of the water quality in the strait are discussed. The importance of using FerryBox as a valuable and robust tool in future coastal operational observation networks is emphasized

Sand excavation can transport sediments into surrounding waters and thus raise the suspended sediment concentration. However, assessment of the area that is influenced by sand excavation and the strength of this influence is not easy due to the temporal and spatial variability of the background suspended sediment concentrations. Remote sensing can provide data before and after sand excavation activities and thus provides a possibility to estimate the variation in suspended sediment resulting from sand excavation. Sand excavation generally occurs in rivers or estuaries. The medium resolution of ocean color satellite data makes it difficult to obtain appropriate information in estuaries or rivers because of the spatial resolution of the sensors, the narrow spans of the rivers and the effects from the adjacent land. Sand excavation in the Pearl River Estuary has become frequent in recent years due to the development of urbanization in China. The Hong Kong-Zhuhai-Macau Bridge crosses the Pearl River Estuary and is the largest bridge and tunnel project in the world. The suspended sediment generated by upstream sand excavation was doubted to have a significant impact on the suspended sediment in the tunnel region. In this paper, we assessed the impact of upstream sand excavation on the suspended sediment in the Hong Kong-Zhuhai-Macau Bridge construction area using Landsat OLI, ETM+ and TM data. Regional suspended sediment algorithms were developed for Landsat using a symbolic regression method based on data from 25 cruises in the study area from 2003 to 2014. A band shift was conducted on the remote sensing reflectance data from Landsat ETM+ and OLI to produce a time series of suspended sediment that was internally consistent with that of Landsat TM data. The suspended sediment distribution was extracted and used to compare two different conditions, with and without sand excavation. The correlation of suspended sediment in different regions in the surrounding waters, including the correlation between the construction regions and the sand excavation regions, was calculated. Our results indicate that the sand excavation in the upstream region of the Pearl River Estuary has limited impact on the surface suspended sediment concentrations in the Hong Kong-Zhuhai-Macau Bridge tunnel area.