Study On The Spatio-temporal Variations And The Causes Of The Upwelling In Zhoushan Sea Area

Upwelling is low temperature water brought many nutrients,which has a profound impact on fishery resources,ocean circulation and climate characteristics.The Zhoushan Archipelago is composed of 9 small islands,which has coastal upwelling.It is a famous Zhoushan fishing ground in our country and has attracted the attention of a lot of researchers.This paper combines the chlorophyll a(Chl-a)concentration data,the SST(Sea Surface Temperature)data,the SSW(Sea Surface Wind)data and the Ni(?)o3.4Index(INi(?)o3.4)to discuss the definition of Zhoushan upwelling area,study the spatio-temporal variations of the upwelling,and analyze the influence of El Ni(?)o on Zhoushan upwelling intensity,range and other characteristics,and study the mechanism of the upwelling caused by wind field.Using the chlorophyll a concentration data from July to August in 2002-2011,analyze the variations of chlorophyll a concentration in the Zhoushan upwelling area.In addition,using the difference in SST between the upwelling area and the open sea and the corresponding chlorophyll a concentration distribution range,the upwelling boundary is defined.The research results show that the summer SST in Zhoushan sea area is much lower than the outer sea area,ranging from 29°N-31°N,122°E-123°E,and the average temperature difference in the center is 1.5?.The average chlorophyll a concentration in July reached 6.6 mg/m~3,and the chlorophyll a concentration difference between the upwelling area and the non-upwelling area was large,indicating that the upwelling was in a strong period.The average concentration of chlorophyll a in August reached6.4mg/m~3,but the difference between the years was large,with obvious inter-annual variation characteristics.Combining the distribution characteristics of chlorophyll a concentration and the SST difference data,it can be defined that the Zhoushan upwelling boundary temperature threshold in July is 0.75°C,and the threshold in August is 0.5°C.Use the monthly average SST in June,July and August from 2002 to 2018 as the anomaly field to conduct EOF analysis(Empirical Orthogonal Function analysis).At the same time,by analyzing the monthly average SST distribution map for many years,the range of Zhoushan upwelling and the change of the location of the low temperature center are discussed.The research results show that:(1)It has upwelling in Zhoushan in summer in June,and the upwelling intensity is strongest in July,and the upwelling intensity is weakened in August.There are changes in the low temperature center of upwelling in the Zhoushan waters.In the same month,the temperature difference between different years is quite different,and it can be seen that there are interannual changes in the strength of upwelling.(2)SST anomaly field EOF analysis shows that the first two variance contribution rates in June were 53.7%and 26.5%,the first two variance contribution rates in July were 50.8%and 26.5%,and the first two variance contributions in August The rates are 38.2%and 34.5%.Zhoushan upwelling presents a 2-3 year cyclical change.(3)In June,the Zhoushan upwelling was distributed like a long snake,covering almost the entire Zhoushan Archipelago,and the center of the upwelling was roughly between Zhoushan Island and Meishan Island.In July,the upwelling mainly concentrated near the Zhoushan Archipelago,extending from Meishan Island to the Ma'an Islands in a ring shape.The upwelling centers were mainly on the four islands of Ma'an Islands,Shengsi Islands,Qushan Island and Chuanhu Islands.The upwelling in August is mainly concentrated near the Zhoushan Archipelago,extending from Meishan Island to Ma'an Islands.It is distributed in a ring shape.There are two areas in the center of the upwelling.Using SST data and SSW data from 1995 to 2016,calculate the SSTA(Sea Surface Temperature Anomaly)and climate summer average distribution SST in 1998 and 2016,and analyze the abnormal changes in SST and SSW of the Zhoushan sea area in 1998 and2016 after two super El Ni(?)o events.The research results show:(1)In the summer of1998 and 2016 after the El Ni(?)o event,the sea surface temperature in the Zhoushan upwelling area showed warm anomalies.Compared with the climatic summer average upwelling center,the temperature increase was 0.5-0.9?and 0.75-0.8?,respectively.After the El Ni(?)o event,the overall area of the upwelling area decreased in 1998 and 2016,and the Zhoushan upwelling weakened,indicating that the El Ni(?)o had an enhanced effect on the upwelling.(2)In the summer of 1998,southwest wind is in vogue in the southern part of Zhoushan sea area,and the maximum wind speed reached 4.5m/s,while the northern part was dominated by southerly wind,and the wind speed gradually weakened.In the summer of 2016,the Zhoushan sea area was dominated by the southeast wind,and the wind speed declined by degrees from south to north.Under the influence of the southwest wind in 1998,the range of upwelling was significantly larger than in 2016.Using the monthly average sea surface wind field distribution in Zhoushan sea area from 2002 to 2018,this paper discusses the distribution of summer wind field in Zhoushan sea area.At the same time,combined with the monthly average sea surface temperature distribution from June to August,different wind directions are analyzed.The impact on Zhoushan upwelling.The EOF analysis of the SSW anomaly field is carried out,and the temporal and spatial changes of SSW are discussed.The results of the study showed that:(1)Southerly wind prevailed in the southeastern part of the Zhoushan Archipelago in June,while southeast wind prevailed in Zhoushan Islands itself and its northwest.In July,the southerly wind prevails in the southeastern part of the Zhoushan Archipelago,the southeast wind prevails in the northwest of the Zhoushan Archipelago,and the southerly wind prevails in the Zhoushan Archipelago itself.In August,the southeast wind prevailed throughout the Zhoushan waters.The complex distribution of wind fields in the Zhoushan sea area in summer is the main reason for the change of Zhoushan upwelling intensity and range in summer.(2)In July,southerly and southwester winds is in vogue around Zhoushan Archipelago.Compared with June and August,the overall range of upwelling and the center of upwelling are wider.The southwesterly wind and the southerly wind are conducive to the development of upwelling,while the southeast wind will relatively restrain the development of upwelling.(3)SSW anomaly field EOF analysis shows that the first two variance contribution rates in June were 43.96%and 24.68%,the first two variance contribution rates in July were 52.9%and 27.1%,and the first two variance contribution in August rates were 45.6%and 28.7%.The sea surface wind speed has a periodic variation of 4-5 years.