海洋中尺度涡(百公里尺度)基本遍布各大洋。把同一时刻所有涡的面积加起来,接近全球水域面积的三分之一。这当然是算出来的。怎么形象地理解这回事呢?我们不妨玩一把数据可视化,把中尺度涡都挪去大西洋,看看能不能塞满它。为什么选大西洋?因为Wikipedia记载它覆盖了全球水域面积的29%,正好适合以涡填海。
资料记载
在挪涡之前,先看看哪些论文记载了中尺度涡覆盖面积在水域面积中的占比。
当提到涡覆盖了近三分之一水域面积这一数据时,大家多引用的是Alexis Chaigneau关于四个主要东边界上升流区内的中尺度涡的文章1:
The four major eastern boundary upwelling systems: “The total area covered by CE and AE represents 25–30% of the considered oceanic region (Table 1).”
Alexis还有一篇更早的08年的文章2,关注的是秘鲁沿岸的中尺度涡,也计算了涡在其研究海区的日常覆盖面积:
Mesoscale eddies off Peru: “The mesoscale structures cover an average total area of around 6.5105 km2, representing 25% of the study region.”
其它海域关于涡盖面积比例的文章就出现得比较晚。比如Haussman et al. (2017)3:
The Southern Ocean: “With a fraction of time and space of the SO covered by eddy cores of approximately 20%, this study suggests a small impact of eddies on deepening the climatological mean SO winter mixed layer. “
和Czeschel et al. (2018)4:
The eastern tropical South Pacific: “Both types of eddies have an average radius of about 70 km and on average 15 % of the ETSP are covered everyday with eddies (Fig. 1a)”
在我们南海,计算了涡覆盖比例的文献算是较多的。较早时,有修鹏研究员的文章5,统计水深1000m以上区域的南海中尺度涡平均覆盖了9.8%的南海面积。18年,有两篇文章重新提及了这个百分比。一篇是詹海刚研究员团队何庆友老师的文章6,从涡内Argo数量在总数量中等占比,推知南海涡覆盖面积占比大概在25%:
A total number of 970 and 924 Argo profiles are located within 1R of the compositecyclonic and anticyclonic eddies, respectively. They account for 27% and 23% of the total number of profiles that are associated with cyclonic and anticyclonic eddies, respectively. These proportions support that eddies occupy ∼25% of the oceanic surface in the SCS.
另一篇是董昌明教授团队的文章7,他们计算了涡覆盖占比的月变化(如下图):
Xiu et al. 5 suggested the mean area covered by eddies with depths greater than 1000 m was around 9.8% of the SCS area each year. Figure 3 illustrates the monthly variation of the percentage of eddy covered area occupying the whole size in the SCS. The mean area covered by eddies is 16.0–20.7% in the SCS, where the highest (lowest) percentage appears in March (January).
可能还有文献有相关记载,欢迎补充。
开始挪涡
所用数据
这套资料并未有中尺度涡边缘的位置数据,所以每个涡均以其等效半径所画的圆来表示。
挪涡方法
概括
随机生成一个大西洋里的经纬位置放被挪涡,找到能塞进这个位置的最大的涡,确保这个涡的圆形不与岸界,大西洋边界和其它涡圆形重叠。
具体步骤
- 先把大西洋的涡列个list,这些涡不需要挪。每挪一个涡进大西洋,就把这个涡的新位置依序放进这个list;
- 把要挪的不在大西洋的涡也建立一个list,这个list就是待挪涡的list。每挪走一个涡,这个list就短一点,直到list里所有涡都被挪走;
- 随机生成一个经纬度,作为可能的放涡的地方;诊断这个随机生成的经纬度是否在大西洋里;
- 如果在大西洋里,就计算该经纬度与大西洋边界、岸界和所有已在大西洋内的涡(包括本来就在的和此前挪进去的)的圆形边缘的距离;
- 上述距离的最小值代表了能塞进该位置的涡的最大半径。如果塞进一个半径大于该最小值的涡,则该涡可能与大西洋边界或岸界或已存在的涡发生重叠。
- 在待挪涡list里搜寻是否有涡可以满足半径小于上述距离最小值;
- 如果有,就把这个涡挪走;如果没有,就重新生成新经纬度,重复上述过程,直到所有涡都被挪进大西洋。
- 能否都挪完看人品。有可能会塞不下。
所需依赖
The M_Map toolbox 和它的 m_coasts.mat。
所用MATLAB版本
- R2015a
致谢
最终效果图
以1993年1月1日数据为例:
效果还是挺赞的,大西洋被塞得满满当当。比直接拿数据去算涡覆盖占比要费事多了,但是实现动画的过程和结果都很有趣,成功把科研升华为科普。
代码开源
https://github.com/chouj/MoveEddiesIntoAtlantic
参考文献
- Alexis Chaigneau, Gérard Eldin, Boris Dewitte (2009), Eddy activity in the four major upwelling systems from satellite altimetry (1992–2007), Progress in Oceanography, Volume 83, Issues 1–4, Pages 117-123, ISSN 0079-6611,
https://doi.org/10.1016/j.pocean.2009.07.012. - Alexis Chaigneau, Arnaud Gizolme, Carmen Grados (2008), Mesoscale eddies off Peru in altimeter records: Identification algorithms and eddy spatio-temporal patterns, Progress in Oceanography, Volume 79, Issues 2–4, Pages 106-119,
ISSN 0079-6611, https://doi.org/10.1016/j.pocean.2008.10.013. - Hausmann, U., McGillicuddy, D. J., and Marshall, J. (2017), Observed mesoscale eddy signatures in Southern Ocean surface mixed‐layer depth, J. Geophys. Res. Oceans, 122, 617– 635
- Czeschel, R., Schütte, F., Weller, R. A., and Stramma, L. (2018): Transport, properties, and life cycles of mesoscale eddies in the eastern tropical South Pacific, Ocean Sci., 14, 731-750, https://doi.org/10.5194/os-14-731-2018.
- Xiu, P.; Chai, F.; Shi, L.; Xue, H.; Chao, Y (2010). A census of eddy activities in the South China Sea during 1993–2007. J. Geophys. Res., 115.
- He, Qingyou & Zhan, Haigang & Cai, Shuqun & He, Yinghui & Huang, Gaolong & Zhan, Weikang. (2018). A New Assessment of Mesoscale Eddies in the South China Sea: Surface Features, Three-Dimensional Structures, and Thermohaline Transports. Journal of Geophysical Research: Oceans. 10.1029/2018JC014054.
- Sun, W.; Dong, C.; Tan, W.; Liu, Y.; He, Y. ; Wang, J. (2018), Vertical Structure Anomalies of Oceanic Eddies and Eddy-Induced Transports in the South China Sea. Remote Sens. 10, 795.