热带印度洋海温异常单、偶极模态及其相互作用

Two Modes of Sea Surface Temperature Anomalies in the Tropical Indian Ocean: Monopole,Dipole and Their Interactions

利用最近50多年的GISST和NCEP的OISST海表温度资料研究了印度洋海温变化的空间分布型和多重时间尺度及其相互作用。结果表明,热带印度洋海温主要存在两种空间分布型,即全海盆符号一致的单极和东、西部符号相反的偶极。单极既存在长期增暖趋势,也存在年际振荡;偶极则以年际变化为主。在去掉由EOF重建的单极后,热带印度洋东、西部海温表现为显著的反相关关系;对17次典型偶极子个例的分析表明,对偶极子本身而言,偶极子的演变更像是一种翘翘板似的局地振荡。单极在长期趋势和年际时间尺度上对偶极的影响是不同的。长期趋势缩短了偶极子的生命期,在冷期,印度洋海温经历了由负单极到正偶极再到负单极的演变,偶极子的异常信号最早出现在热带西印度洋;在暖期,印度洋海温经历了由正单极到正偶极再到正单极的演变,偶极子的异常信号最早出现在热带东印度洋。对年际时间尺度的变化而言,印度洋海温异常由负单极向正偶极再到正单极转换,偶极子位于一种单极向另一种单极的转换过程之中,在此过程中,印度洋海温表现为明显自西向东的传播特征。

Based on the last 5 decadal sea surface temperature （SST） data, the spatial distributions and multi-time scale of the SST anomalies （SSTA） in the tropical Indian Ocean （TIO） and their interactions are investigated. The results show that the spatial distributions of SSTA are characterized by monopole with the same anomalous signs in basin-scale and by dipole with the reverse signs in the western and eastern TIO. The monopoles are characterized not only by warming tendency but by interannual variations, while the dipoles are just dominated by the interannual variations. After subtracting the monopole reconstructed by the empirical orthogonal function （EOF） results, the western and eastern TIO SSTA turn out to be significantly anticorrelated, and the EOF analyses of the 17 typical dipole events show that the areas of the positive and negative SSTA appear to be immovable. The composite analysis of the TIO SSTA shows that the two counterparts evolve out-of-phase absolutely. These results demonstrate that the evolution of the dipole itself is rather a local seesaw-style oscillation. The influence of monopole on dipole is different from long-range tendency to interannual timescale. The long-range tendency shortens the lifecycle of the dipole. In the cold episode, the tropical Indian Ocean SSTA shift from negative monopole to positive dipole, finally to negative monopole, and the dipole anomalous signals appear in the western Indian Ocean first. While in the warm period, the SSTA alter from positive monopole to positive dipole, eventually to positive monopole, and the abnormal signals of dipole exist in the eastern Indian Ocean first. At the interannual timescale, the SSTA experience from negative monopole to positive dipole to positive monopole. The dipole occurs in the course Of transition from one kind of monopole to another. During this shift the TIO SSTA appear to migrate from west to east obviously. The dipole is also influenced by the basin-wide long-range warming tendency significantly. After subtracting the monopole reconstructed by EOF, the SSTA of western and eastern TIO show distinct negative correlation, but they show distinct positive correlation when the monopole is taken into account. It turns out that the basin-wide warming conceals some dipole events, for example the strong negative dipole in 1996 and 1998 respectively. Except some strong dipole events （e. g. , 1997）, the SSTA with the same sign in the warm period show the magnitude differences of the east and the west in TIO, i.e., an SSTA zonal gradient exists across the basin, which directly results in the doubts of the existence of dipole. Actually, the dipole and the warm tendency are completely independent in the temporal and spatial distributions.