Physical drivers of chlorophyll variability in the open South China Sea

Abstract

The variability of chlorophyll a concentration (Chl a) in the open South China Sea (SCS) was examined using observations from two Bio‐Argo floats. During the period of September 2014 to August 2015, there was a permanent subsurface Chl a maximum (SCM) in the depth range of 48 to 96 m in the central basin of the SCS. In the northern basin, the SCM disappeared in winter, replaced by enhanced surface layer phytoplankton with high Chl a. The values of the SCM were influenced by the vertical displacement of isotherms. Strong wind forcing and surface cooling were the main physical drivers of high surface Chl a in winter. In the north, stronger wind than in the center, lower sea surface temperature (SST) than in the center, and Kuroshio water intrusion were more favorable for the upward transport of nutrient‐rich deep water. A large amount of nitrate could be advected from the Taiwan Strait and shallow continental shelf to the northern basin in winter. A combination of strong wind mixing, surface cooling, Kuroshio water intrusion, and horizontal advection caused the winter surface phytoplankton bloom in the north.

Temporal and Vertical Variations of Particulate and Dissolved Optical Properties in the South China Sea

“…The continuous observations of the vertical variations provided by profiling floats allow us to build a 2‐D (depth‐time) view of upper‐ocean [Chla], b bp , FDOM, oxygen, as well as physical properties during the winter of 2014–2015. Float 0347 recorded concurrent MLD deepening from early October 2014 and an enhancement of surface [Chla] ([Chla] ML > 0.1 mg/m 3 , Figure c), consistent with studies based on remotely sensed satellite data (also see Zhang et al, ). This “surface bloom” attained its peak (maximal [Chla] ML reached 1.75 mg/m 3 , a 15‐fold increase in magnitude) in December, was sustained for about 2.5 months and was followed by a fast decay in February, with [Chla] ML attaining low values (< 0.1 mg/m 3 ) at the end of May.…”

“…Generally, in the northern SCS, the DCM appeared in spring after the MLD shoaling, deepened in summer with increasing solar illumination (PAR0), shoaled in the autumn, and disappeared in winter due to strong mixing (Zhang et al, ). This evolution is typical of oligotrophic regions such as the North Pacific subtropical gyre and Mediterranean Sea (Letelier et al, ; Mignot et al, ), with, however, shallower MLD in the SCS.…”

“…As there were no nitrate sensors on the floats, two depth values are derived as the proxy for the nitracline: (i) the 22 °C isotherm ( z T22 ) which was used by Zhang et al () as the proxy for nutricline and could be expected to covary with the depth where [NO] 3 reaches 0–5 μmol/kg in SCS based on a regional empirical relationship (Chen et al, ); and (ii) the depth which the neural‐network model CANYON finds [NO 3 ] = 2 μmol/kg ( z NO3 ; Sauzède et al, ). CANYON estimates [NO 3 ] based on the date, location, temperature, salinity, and dissolved oxygen.…”

“…As there were no nitrate sensors on the floats, two depth values are derived as the proxy for the nitracline: (i) the 22°C isotherm (z T22 ) which was used by Zhang et al (2016) as the proxy for nutricline and could be expected to covary with the depth where [NO] 3 reaches 0-5 μmol/kg in SCS based on a regional empirical dFDOM was calculated as the difference between FDOM and a FDOM model based on a multiple linear relationship (MLR) with both temperature and salinity (Carlson et al, 2010), formulated as…”

Achieving Highly Efficient Atmospheric CO2 Uptake by Artificial Upwelling

“…Due to the strength of winter monsoons and the cooling of surface temperature, the water column in the SCS and ECS was well-mixed, with the mixed-layer depth (MLD) deepening from summer to winter [40][41][42]. Liu et al (2002) observed a deeper subsurface Chl a maximum (SCM) at 75 m in September 1998 and a shallower SCM at 40~50 m at the same site in January 1999 in the Northern SCS [43].…”