The Study On Distribution Of The Primary Productivity North And West Of The South China Sea

Using MODIS satellite data from July 2002-December 2014,we studied distribution of phytoplankton primary productivity,as well as other ocean conditions including the depth of thermocline top(DTT)in different seasons,and investigated possible responses of MODIS-derived Chl-a and primary productivity on DTT and wind speed during seasons of El-Nino years north and west of the South China Sea(SCS).Significant seasonal changes of Chl-a were observed in the western SCS where upwelling is generally prevailing in summer.In order to study further influence of El-Nino years on Chl-a in the summer upwelling in the western SCS(109-110°E,11-12°N),we chose 4 typical El-Nino/La-Nina years as representative samples,analyzed the upwelling velocities derived from QuikSCAT wind,SST and ENSO index in the west SCS,to investigate possible factors regluating variation of Chl-a in the west SCS.As the result indicated,primary productivity has obvious changes during the El Ni?o and La Ni?a compared with that during other years.It may suggest influence of the ENSO on phytoplankton Chl-a,also related closely with SST,wind and so on.During the winter monsoon of El Ni?o years,decreasing strength of the wind vectors in the northern SCS and increasing SST along the coastal area triggered decrease of productivity in the above regions;decreasing SST and strong wind in the northeastern SST induced the increasing productivity;there were roughly negative correlations between productivity and the multivariate ENSO index(MEI)for the whole SCS.And there are reverse patterns of SST,primary productivity and wind field as well as positive correlations between primary productivity and MEI index during La-Nina.Chlorophyll-a in the northern SCS is significantly related to wind vectors and the DTT.In the offshore open sea,the average wind speed is the strongest along with the lowest SST,strongest entrainment mixing and the deepest DTT in all four seasons,all which were most favorable to the effective transport of nutrients into the surface layer from the subsurface,arising the widespread increase of Chl-a.Because spring is the transition period between winter and summer with the weakest wind,and also is the season with the weakest mixing entrainment and the strongest stratification indicated by DTT,the Chl-a is the lowest in spring.Though the temperature in summer is higher,the summer monsoon is stronger than that in spring,which led to DTT deeper,higher Chlorophyll-a than that in spring.Under the influence of El Ni?o,there is a stronger signal ofChl-a variation in the upwelling region in the west SCS.There is also a higher correlation between Chl-a and the upwelling index,compared with other regions.During the El Ni?o events,along with the weakening southwest monsoon,the coastal wind stress became weaker,the upwelling tendency was suppressed,resulting in the surface water receiving less supplement of the deep nutrients,and lower primary productivity,and the decrease of the upwelling intensity would also lead to the increase in SST,enhancing stability of upper oceans,which also caused decrease in Chl-a concentration,in turn.