Using the ensemble empirical mode decomposition method, this study systematically investigates the multiple timescales of the southern annular mode (SAM) and identifies their relative contributions to the persistence of the SAM. Analyses show that the persistence of SAM mainly depends on the contribution of longer-timescale variabilities, especially the cross-seasonal variability of SAM. When subtracting the cross-seasonal variability from the SAM, the long-term positive covariance between the eddy forcing and zonal flow disappears. Composite analysis shows that the meridional shift of zonal wind, eddy momentum forcing and baroclinicity anomalies can be maintained for more than 40 days only under the circumstance of strong cross-seasonal variability, indicating the dominant role played by the cross-seasonal variability for the low-frequency persistence of the SAM. Analysis further shows that the cross-seasonal variability of the SAM, in addition to the internal dynamics, is associated with the extratropical air–sea interaction. About half of the strong cross-seasonal SAM events are accompanied by evident extratropical dipolar SST anomalies, which mostly occur in austral summer. The cross-seasonal dependence of the low-frequency change in SAM suggests that the contribution of longer-timescale variabilities, especially the cross-seasonal variability, cannot be neglected in subseasonal prediction of the SAM.

本研究采用集合经验模态分解方法，系统地研究了南环模态（SAM）的多个时间尺度，并确定了它们对SAM持续性的相对贡献。分析表明，SAM的持久性主要取决于较长时间尺度变化的贡献，尤其是SAM的跨季节变化。当从SAM中减去跨季节变化时，涡流强迫和纬向流动之间的长期正协方差消失了。综合分析表明，纬向风的经向偏移、涡动强迫和斜压异常只有在跨季节变化较强的情况下才能维持40天以上，表明跨季节变化对SAM低频持续存在的主导作用。分析进一步表明，除了内部动力学外，SAM的跨季节变化与温带海气相互作用有关。大约一半的强跨季节SAM事件伴有明显的温带偶极SST异常，主要发生在南方夏季。SAM 低频变化的跨季节依赖性表明，在 SAM 的次季节预测中，不能忽视较长时间尺度变化的贡献，特别是跨季节变化。大约一半的强跨季节SAM事件伴有明显的温带偶极SST异常，主要发生在南方夏季。SAM 低频变化的跨季节依赖性表明，在 SAM 的次季节预测中，不能忽视较长时间尺度变化的贡献，特别是跨季节变化。大约一半的强跨季节SAM事件伴有明显的温带偶极SST异常，主要发生在南方夏季。SAM 低频变化的跨季节依赖性表明，在 SAM 的次季节预测中，不能忽视较长时间尺度变化的贡献，特别是跨季节变化。