| Coastal upwelling brings cold seawater with rich nutrient from the subsurface to surface, which plays a significant role in the blooms of phytoplankton and formation the fishing ground. Zhoushan coastal area is one of the most important upwelling zone and fishing ground in China. In this paper, the spatial and temporal characteristics of summer upwelling along Zhoushan coast(121°E-124°E, 29°N-30°N), and dynamic mechanism related to wind is investigated with monthly mean remote sensing SST( Sea Surface Temperature) and wind data from 2002 to 2013, which is useful to the research of formation of fishing ground and variation fishery resources Study.1. Zhoushan Coastal UpwellingBased on years of satellite remote sensing average SST data,the low temperature can be found in some small region in June. In July, it expanded significantly and formed a dramaticlly cold water center, and the center temperature was lower than 25℃(temperature in adjacent areas>26℃),The geographical range was 29°30′N-31°N,122°E-123°E. In August, this cold water mass continued to expand with center temperature 26.5℃(temperature in adjacent areas >27.5℃), The geographical range was 29°15′N-31°15′N,122°E-123°E. In September, the cold water area began to disappear. In July and August, the temperature in center of cold water mass was 2 ℃lower than the average water temperature. Zhoushan coastal area existed different intensities upwelling corresponding to the cold area between June and September, particularly evident in July and August.2. Spatial and temporal characteristics of upwelling in Zhoushan costal areaFor further analyzing summer upwelling in Zhoushan coastal area, we decomposed monthly average SST and SSTA by EOF method in July and August, the result showed that modal variance contribution rates were 71.66% and 16.55% in July, 87.03% and 7.3% in August. All of them pass the North significance testing, and the first model in July had a positive correlation. SSTA in this sea area had the good consistency. The runoff in the Yangtze River and Qiantang River increase in July made the flow rate in zhoushan islands became relatively slow, SST variable rate had relatively smaller value in zhoushan islands and the temperature variation was relatively stable. Also, there was a strong upwelling, which maintained inter-annual variation from 2 to 3 years. The variation trend of SSTA in the second model showed in the northwestern area was positive, but it had an opposite trend in southeastern area, indicating that temperature in these two regions had the opposite trend. In summer, north area of East-sea was prevailed by southeast winds, and south area of East-sea was prevailed by southwest winds. The sea surface temperature variability was larger, and there was a consistent feature between space distribution of wind field and SST anomaly. In the inter-annual change, changing period of upwelling was more obvious during 2 to 3 years. However, the spatial and temporal distribution of upwelling kept the same in July and August, the value in August was slightly smaller than that in July, due to changes of wind field.3.The wind-driven upwelling mechanism in Zhoushan costal areaThe wind speed had a significant negative correlation to SST(negative correlation) in July and August. In the period from 2002 to 2013, wind speed and SST in July and August exhibited periodic change as a 2-3 year periodic change in July while no obvious change in August. The intensity of south wind and west wind had negative relationship with SST, which meaned southwest wind played an important role in low-temperature SST. Furthermore, The meridional wind and SST had significant inter-annual variations, in July, the changing trend of wind-driven upwelling maintained the same with the meridional wind. The speed of mean meridional wind in July was higher(2m/s) than that in August, the scale of wind-driven upwelling in July was twice as much as that in August. In July, it took 19 days to bring sea water from 50 m depth to sea surface, while low temperature in sea surface in August had a significant contribution. In addition, prevailing south winds may be happen earlier causing lower temperature. |
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