用TOPEX/POSEIDON高度计数据分析热带大西洋海面高度距平的低频变化

Low frequency variability of sea surface level in the tropical Atlantic Ocean derived from TOPEX/POSEIDON altimeter data

利用1992年10月~2002年7月的TOPEX/POSEIDON(T/P)卫星高度计月平均格点数据分析了热带大西洋(15°S^25°N,5°~50°W)海面高度距平的低频变化。由热带大西洋大约10 a海面高度距平变化的标准差分析得到:在赤道附近海区(2°~5°N,25°~45°W)、非洲沿岸海区(11°~16°N,16°~18°W)海面高度波动剧烈。对海面高度距平进行经验正交函数(EOF)分析,得到EOF的3个模态分别占有方差比例为51.5%,13.2%和7.9%。第一模态揭示的是热带辐合带(ITCZ)的季节性迁移导致海面高度距平沿着ITCZ平均位置经向倾斜的1 a周期变化,第一模态还显示了太阳辐射的季节差异引起南北两个海盆海面高度的整体升降。第二模态描述了中心分别位于(3°N,40°W)和(7°N,45°W)附近两个涡漩的变化。第三模态表征的是几内亚海湾上升流和赤道北部下降流在6~7月强度达到最大。对EOF时间系数曲线的经验模态分解(EMD),结果表明热带大西洋低频变化包含的成分主要有:0.5,1,2,4和6 a。其中1 a周期是热带大西洋海面高度变化最主要的周期成分,0.5 a周期和2 a周期也是热带大西洋海面高度变化的重要形式;而4 a和6 a周期所占的比例较小。另外EMD方法还分解出1997~1998年太平洋El Nino事件对热带大西洋海面高度的影响。

The sea surface variability of the tropical Atlantic （TA, 15°S-25°N, 5°-50°W） is studied by using TOPEX/POSEIDON （T/P） altimeter data from October 1992 to July 2002. Patterns of variance of the sea surface anomaly （SLA） deviations from the ten-year （1992-2002） mean show that there are two highvariation areas： the first is at the basin near the equator （2°-5°N,25°-45°W）, the second is along the coast of Africa （11°- 16°N, 16°-18°W）. Empirical orthogonal function （EOF） analysis is used to study the spatial and temporal variabilities of sea level. The first three EOFs account for 51.5 %, 13.2 %, 7.9 % of the total va- riance, respectively. The first one not only describes the meridional tilting of the TA along the mean location of the Intertropical Convergence Zone （ITCZ） with an annual period due to the location of the ITCZ moving seasonally, but also presents variability over the annual time scale of sea level due to the variability of solar radiation. The second EOF describes two gyres centered at 3~N,40~W and 7°N,40°W, respectively with an annual time scale. The third EOF shows Guinea Gulf upwelling and north equatorial downwelling peak in June-July. Using empirical mode decomposition（EMD） method is to decompose time coefficients of the first three EOFs. The result shows that the annual cycle is the dominant component of the sea level variabilities in the TA, while half-year cycle and two-year cycle are also important components. The EMD analysis also reveals that the variation of sea surface height （SSH） in the TA has a response to the response to the EI Nifio events occurred in the Pacific in 1997-1998.