Numerical study of seasonal circulation and variability over the inner shelf of the northern South China Sea

Typhoon Vicente (2012) underwent an extreme rapid intensification (RI) over the northern South China Sea just before its landfall in south China. The extreme RI, the sudden track deflection, and the inner- and outer-core structures of Vicente were reasonably reproduced in an Advanced Research version of the Weather Research and Forecasting (WRF-ARW) Model simulation. The evolutions of the axisymmetric inner-core radar reflectivity and the primary circulation of the simulated Vicente before its landfall were verified against the Doppler radar observations. Two intensification stages were identified: 1) the asymmetric intensification stage (i.e., RI onset), represented by a relatively slow intensification rate accompanied by a distinct eyewall contraction; and 2) the axisymmetric RI stage with very slow eyewall contraction. Results from a storm-scale tangential wind tendency budget indicated that the primary spinup mechanism during the first stage was the radial eddy momentum transport, which was beneficial to accelerate primary circulation inside the radius of maximum wind (RMW) and thus conducive to eyewall contraction. In contrast, the principal spinup mechanism during the second stage was mainly ascribed to the forced secondary circulation in response to diabatic heating in the eyewall and boundary layer friction, which efficiently transported the absolute angular momentum radially inward and vertically upward to increase the primary circulation in the eyewall region throughout the troposphere. Further analysis revealed that the interaction between the monsoon circulation and storm-scale vorticity anomalies played an important role in erecting the tilted vortex and spinning up the midtropospheric TC circulation during the first stage.

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Spatial shifts in size structure, phylogenetic diversity, community composition and abundance of small eukaryotic plankton in a coastal upwelling area of the northern South China Sea

Journal of Plankton Research ◽

10.1093/plankt/fbaa046 ◽

2020 ◽

Author(s):

Fahui Gong ◽

Guihao Li ◽

Yaping Wang ◽

Qinyu Liu ◽

Fangjuan Huang ◽

...

Keyword(s):

South China Sea ◽

Spatial Patterns ◽

South China ◽

High Throughput Sequencing ◽

Size Structure ◽

Northern South China Sea ◽

Rrna Genes ◽

Inner Shelf ◽

Cell Abundance ◽

China Sea

Abstract Summer upwelling off Hainan Island (northern South China Sea) is influenced by fresher coastal water masses. To assess the ecological effects of this complicated oceanography, a cruise was conducted in August 2016. We investigated the spatial patterns of phytoplankton size structure, diversity and quantity of small eukaryotes (0.2–20 μm) across the upwelling system. In the inner-shelf waters, nanophytoplankton contributed a higher proportion to phytoplankton biomass than microphytoplankton and picophytoplankton. High-throughput sequencing of 18S rRNA genes and community analysis indicated that the horizontal variation in community structure of small eukaryotes was larger than the vertical. The relative sequence proportions of dinoflagellates, diatoms, ciliates and Micromonas in the surface layer gradually decreased from the inner shelf to the slope, whereas the Rhizaria, Bathycoccus and Ostreococcus exhibited an opposite trend. The spatial patterns of taxon-specific rDNA copy numbers were similar to the patterns based on the relative proportions. Redundancy analysis revealed that turbidity, salinity and ${\mathrm{NO}}_3^{-}$ were the most important environmental variables in structuring the community of small eukaryotes. The cell abundance of photosynthetic picoeukaryotes was significantly correlated with the rDNA abundance of Mamiellophyceae and the cell abundance of nanoeukaryotes was significantly correlated with the rDNA abundance of dinoflagellates plus diatoms.

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