2013年南沙群岛海域温跃层的季节变化及形成机理

摘要

With the survey and remote sensing data,we revealed the seasonal characteristics of the thermocline in the sea area around Nansha Islands sea area in 2013.Specifically,the mean value of the thermocline upper-bounds depth was almost the same in spring,summer and winter,which was around 45 to 47 m,and it was maximum in autumn which was as deep as 60 m.For the thermocline thickness,in the seasons of summer,autumn and winter it demonstrated almost the same mean value ranging from 85 to 87 m,while a slightly smaller value of 78 m was found in spring .The mean value of the thermocline intensity was almost the same throughout the year from spring to winter,which ranged from 0.13 to 0.15℃/m.The formation mechanism of such seasonal characteristics of ther-mocline upper-bounds depth was also revealed.In spring,it was deep in the west whereas shallow in the east.This is because that the western area was controlled by relatively low net heat fluxes,high wind speeds and the negative wind stress curl,and it is also influenced simultaneously by the mesoscale warm eddy and the anticyclonic circula-tion from the coastal area off the eastern Indo-China Peninsula;contrarily,the eastern area was controlled by rela-tively high net heat fluxes,low wind speed as well as the Ekman pumping effect induced by the wind stress curl.In summer,the depth was almost uniform from west to east.It was because that,in the north of 10°N,the wind-in-duced vortex mixing,though being strong,was undermined by the Ekman pumping,whereas in the south of 10°N the wind-induced vortex mixing,which although was weak,was affected by the negative wind stress curl.In au-tumn,the mean depth was 15 m deeper than those in the other three seasons.The reason was that the sea area of Nansha Islands was controlled by a warm eddy which induced the anticyclonic circulation that subsequently deep-ened the thermocline upper-bounds depth through the convergence of upper layer seawater.When it came to win-ter,the Ekman pumping induced by the positive wind stress curl and the cyclconic circulation caused by the cold eddy worked together,which lifted up the mean value of the thermocline upper-bounds depth by 15 m compared with that in autumn.%利用调查数据及遥感数据揭示了2013年南沙群岛海域温跃层的季节变化特征,温跃层上界深度平均值春、夏、冬季基本一致,介于45~47 m之间,秋季最大,达60 m;温跃层厚度平均值夏、秋、冬季基本一致,介于85~87 m之间,春季相对较小,为78 m.温跃层强度平均值春、夏、秋、冬季几乎一致,介于0.13~0.15℃/m之间.调查海域温跃层上界深度季节变化的形成机理为:春季西深东浅的原因是西部受净热通量较小、大风速、负的风应力旋度以及中南半岛东部外海的中尺度暖涡和反气旋环流共同作用,东部近岸海域净热通量高值、风速相对较小及风应力旋度引起的Ekman抽吸效应共同控制;夏季深度分布较均匀的原因是10°N以北风致涡动混合强但受Ekman抽吸影响,10°N以南风致涡动混合弱但风应力旋度为负值;秋季深度较其他季节平均加深15 m的原因是南沙群岛海域被暖涡占据,暖涡引起的反气旋式环流使得温跃层上界深度被海水辐聚下压;冬季正的风应力旋度产生的Ekman抽吸和冷涡引起的气旋式环流共同作用,使得温跃层上界深度较秋季平均抬升15 m.