Revealing The Multiple-scale Spatiotemporal Variations Of The Changjiang River Plume By The Self-Organizing Map(SOM) Method

The mixing and stratification of the Changjiang River plume and ambient seawater have significant impacts on the current circulations,water mass compositions,sedimentations,ecological processes,and marine productivities in the Changjiang River Estuary and its adjacent waters.Numerous researches have done in past years,while most of them were analyses of the steady state and instantaneous surveyed field.A few other studies used the realistic simulation and analyzed the results with empirical orthogonal function(EOF)method.These could hardly reflect the expansion variations and the underlying mechanisms in a simple and objective manner.In this paper,the self-organizing map(SOM)is used to analyze the data from the realistic simulation with a well-validated numerical model.SOM can extract essential features and recognize multimodal patterns from a lot of complicated data.The results showed that SOM could reproduce the salinity and temperature variations in the Changjiang River Estuary and its adjacent region on seasonal and synoptic scales.Focusing on the penetrating fronts in the Zhe-Min Coastal Water,SOM also revealed the complicated spatiotemporal variabilities of Changjiang River plume in a succinct way.The major conclusions of this study are as follows:The variations of sea surface temperature and sea surface salinity in Changjiang River Estuary and its adjacent can be analyzed into 9 patterns with SOM respectively.These patterns not only show the seasonal characteristics of Changjiang River Plume,but also reveal its characteristics in spring-neap tide scale and synoptic scale.The water in the plume area and in the west coast of the Yellow Sea was cold.The low-salinity and low-temperature water tongue in west coast of the Yellow Sea intruded southeastward into the relatively warm areas in the Taiwan warm current region,forming a strong temperature gradient.In summer,affected by the huge Changjiang River runoff and the southerly wind,the coverage of the Changjiang diluted water increased and it extended northeastward to Jeju Island.Cold water patch in Subei coastal water decreased and the southeastward cold water tongue was also weakened.There was a low-temperature zone around Zhoushan Islands,which appeared in spring and summer.In spring and autumn,the Changjiang River discharge was moderate and the wind field was transitional,so did the Changjiang River plume.On the short-term scale,the distribution of Changjiang River plume was highly related to the tide.The coverage of the Changjiang River plume was larger and the surface temperature was higher during neap tide than during spring tide.These were due to the fact that vertical mixing of water in neap tide was relatively weak,making the Changjiang River plume to expand in the surface.In summer,the warm water in the surface layer was mixed with the cold water beneath,which was vigorous in spring tide and weak in neap tide.Using the SOM method,the spatiotemporal characteristics of penetrating fronts in Zhe-Min Coast Water was identified.It was also able to classify and identify the rapid changing surface current fields in that area.It was shown that penetrating fronts usually occurred in October and November,and during this period penetrating fronts pointed to the east or southeast.Small scale penetrating was also observed occasionally in summer and early autumn,which pointed to the northeast.The penetrating fronts appeared mainly between 28°N and 30°N,especially between 28.5°N and 29.5°N.The direction of the current field during the penetrating front period were alternating,and there were often surface coastal currents and cross-shelf transport and vortices in the offshore areas.Finally,we investigated the mechanism of the penetrating fronts.The Kuroshio subsurface water invaded to the bottom of Zhe-Min coastal water,and Taiwan Warm Currents entered the upper-middle region of Zhe-Min coastal water,along with the Changjiang River plume,were all in favor of amplifying the vertical stratification.The region between 28°N and 30°N was the tidal wave divergence region where the horizontal turbulent diffusion coefficient of the water was small,and the horizontal Ekman number and the vertical mixed coefficient were significantly lower than that in adjacent areas,causing fluctuation of pycnocline and penetrating front.In October and November,barotropic pressure caused by the sea level height was also favorable for the generation of penetrating front.The wind field was also a notable factor driving the penetrating front.After a long period of strong northerly wind and north-easterly wind,penetrating fronts tended to occur.Low speed northerly wind and southeast wind were favorable for the maintenance of penetrating fronts.This might be due to the fact that under the influence of the northerly wind,the southward expansion of the Changjiang River plume increased under the geostrophic balance.After the wind or when the wind was weak,water body converged and moved offshore,triggering the penetrating fronts.