제주도 주변해역의 양자강 저염수 분석과 예측

Abstract

Since the massacre of shellfish due to abnormally low salinity in Jeju 1996, the Changjiang Diluted Water(CDW) has become one of the critical issue in Jeju in summer. The reason why CDW may damage Jeju is because Jeju locates in the middle of the CDW pathway. This makes Jeju devoting to coping with CDW approximately one month from late July. The first thing to do this is to figure out expected time and pathway of the CDW influx. However, since we do not have any observation between Changjiang river mouth and Jeju and salinity can not be observed from satellite, we can find CDW only after its arrival at Jeju coasts. In this circumstances, forecasting CDW with numerical model has been one of the most important topics in Jeju. The massacre of shellfish in Jeju 1996 has made CDW a famaous issue in the world as well as in Jeju, resulting in lots of research works. Now, we know many things such as structure, extension, seasonal, non-seasonal migration, etc..However, most results so far focus on the initial movement of CDW near the China coasts qualitatively, and these are not so helpful to predict the pathway and time of its influx to Jeju quantitatively. The core problem is to precisely calculate a long migration of CDW more than a month without any observation. Before developing the model, we analyze the data in the adjacent seas of Korea in August during 1990-2013 observed by NFRDI(National Fishery and Resear Development Institute). Compared with the reduction of Changjiang river discharge in summer after the construction of three George dam, the salinity around Korea rather decreased a little. This is due to the change of CDW pathway by winds, which means that the north pacific high air mass expands northwestward according to climate change. The water analysis with mixing ratio reveals that CDW flows eventually to the East Sea roughly along the isobath of 100m in the South Sea after flowing in the southwest sea of Jeju. During the migration in the South Sea, approximately 70% of CDW is mixed with ambient sea water and only 20% of CDW remains in the East Korean Warm Currnet. The model development was started by the reproduction of CDW movement of 1996 with ROMS and the model results made it possible to analyze CDW. The analysis tells us that the cause of patch formation in 1996 was not wave like salinity front, baroclinic instability, tide induced vertical mixing etc. suggested so far, but wind induced vertical mixing. In addition, comparing the maximum freshwater volumes passing through Datong in Changjiang river and the Korea Strait, we found that it takes about 2 months from Datong to the Korea Strait and also that about 90% of Changjiang river discharge flows into the East Sea in two months. The model was improved by reproducing the CDW distributions observed in the East China Sea by KIOST from 2006 to 2009. By the short observation period, the data exhibited the CDW distributions relatively precisely and played a key role in improving the model. The improved model was first applied to forecasting the CDW migration in 2010. The accuracy of first forecast was confirmed by observation and thereafter we could figure out the movement of CDW in detail by the coorperation of model calculation and observation. After flowing into the west sea of Jeju in late July, CDW approached 20km away from the west coast of Jeju in early August, moving to the Jeju Strait along the outer edge of the Jeju current. In this state, easterly winds generated by a typhoon pushed the CDW northward and made the salinity in Chuja island drastically dropped by 3 psu in 12 hours. This shows that CDW is strongly influential on the South Sea. After the typhoon passed, continuous southerly winds pushed the CDW toward Jeju and eventually to the coasts of Jeju crossing over the Jeju current. Throughout thess processes, we found that the southerly wind is needed for CDW to cross over the Jeju current. Model prediction on the migration of CDW was also confirmed by observation consecutively in 2011, resulting in getting credibility. In 2011, CDW moved directly toward Jeju and threatened Jeju eastward. The usual pattern of CDW movement toward Jeju in late July is a bypass to expand southeastward first in early July and then northward. The threatening of CDW was disappeared after a typhoon passing on August 10th accompanied by the upwelling of Yellow Sea Bottom Cold Water(YSBCW). Sea surface temperature in 2011 was remarkably low in the entire Yellow Sea, the South Sea, and also north of the East China Sea as well as the west sea of Jeju. YSBCW was developed in early 2011 by the arctic oscillation and expanded southeastward along the Yellow Sea trough to the west sea of Jeju in summer. The typhoon passed northward along the middle of the Yellow Sea, driving YSBCW upwelled and mixed with surface water, which resulted in abnormally low surface temperature. The surface salinity increased during this process. As the accuracy of the model forecasting has been verified by observation consecutively in 4 years from 2010 and 2013, the action plan of Jeju to CDW has been based on model forecasting from observation. In 2012, abnormally low salinity was observed in west sea. Two typhoons passed through the East China Sea and generated strong easterly wind which pushed CDW northward. After the typhoon, southerly wind drove CDW eastward and reached the west sea. Since the action of Jeju to CDW was conducted based on the model forecasts, there was few observation on CDW and nothing to be analyzed in 2013. The forecasting model has been established through the above processes and the forecast on CDW is open to the public via the homepage of Jeju KMA. This was possible because the observation and modeling institution mutually fed back. Having done with this research, our interpretation of CDW was broaden.