The Annual Cycle, Intraseasonal Oscillations, and Roadblock to Seasonal Predictability of the Asian Summer Monsoon
The Annual Cycle, Intraseasonal Oscillations, and Roadblock to Seasonal Predictability of the Asian Summer Monsoon
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DOI：
10.1175/jcli3901.1
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发表时间：
2006-10
影响因子：
4.9
通讯作者：
B. Goswami;Guoxiong Wu;T. Yasunari
中科院分区：
文献类型：
--
作者：
B. Goswami;Guoxiong Wu;T. Yasunari
在可预测性概念框架内研究了亚洲夏季风（ASM）可预测性有限的因素。季节平均值的可预测性取决于季风年周期 (MAC) 的年际变率 (IAV)，并且由 IAV 的可预测“外部”分量与不可预测的“内部”IAV 相比的相对贡献来确定。回顾了缓慢过程的贡献，例如与厄尔尼诺-南方涛动（ENSO）相关的海气相互作用或局部温暖的海洋-大气相互作用对产生 MAC 的 IAV 的贡献。提供了这些海气相互作用调节 MAC 的经验证据。内部 IAV 的估计是根据观测和大气模型模拟得出的。与夏季气候可预测的大部分热带地区相比，内部变率远小于外部变率，亚洲季风的可预测性有限似乎是由于外部IAV对该地区的贡献相对较弱，与内部IAV的贡献相当。研究了 ASM 区域内部 IAV 较大的原因，并提出 MAC 内部 IAV 主要是由于 MAC 与夏季季节内振荡 (ISO) 之间的相互作用造成的。 ISO 导致 MAC 内部 IAV 的两种机制已被阐明。如果 ISO 的空间结构对季节平均值的空间结构有显着的投影，并且正相位和负相位的出现频率不相等，则 ISO 异常的季节偏差会影响季节平均值。提出了支持这一点的证据。此外，研究还表明，由与“缓慢的年周期”相关的每年变化的强迫调节的混乱的夏季 ISO 可以导致季节性平均的 IAV。还提供了 ISO 活动的 IAV 与季节平均值或 MAC 的 IAV 相关的经验证据。因此，亚洲季风仍然是一个难以预测的系统。然而，为了利用可预测信号，必须改善模型的系统偏差并准确模拟夏季 ISO 的时空结构。
Factors responsible for limited predictability of the Asian summer monsoon (ASM) are investigated within a conceptual framework for predictability. Predictability of the seasonal mean depends on the interannual variability (IAV) of the monsoon annual cycle (MAC) and is determined by relative contribution of the predictable "external" component of IAV compared to the unpredictable "internal" IAV. Contributions of slow processes such as those involving air-sea interactions associated with the El Nino-Southern Oscillation (ENSO) or local warm ocean-atmosphere interactions in generating IAV of the MAC are reviewed. Empirical evidence that these air-sea interactions modulate the MAC is presented. Estimates of internal IAV have been made from observations as well as atmospheric model simulations. In contrast to a large part of the Tropics where the summer climate is predictable, with the internal variability being much smaller than the external one, the limited predictability of the Asian monsoon appears to be due to the fact that the contribution from the external IAV over the region is relatively weak and comparable to that from internal IAV. Cause for large internal IAV over the ASM region is investigated, and it is proposed that the internal IAV of the MAC is primarily due to interaction between the MAC and the summer intraseasonal oscillations (ISOs). Two mechanisms through which ISOs lead to internal IAV of the MAC are unraveled. The seasonal bias of the ISO anomalies can influence the seasonal mean if the spatial structure of the ISO has significant projection on that of the seasonal mean and if frequency of occurrence of positive and negative phases is unequal. Evidence supporting this is presented. In addition, it is demonstrated that the chaotic summer ISOs modulated by the annually varying forcing associated with the "slow annual cycle" can lead to IAV of the seasonal mean. Empirical evidence that IAV of ISO activity is related to IAV of the seasonal mean or MAC is also presented. Thus, the Asian monsoon would remain a difficult system to predict. To exploit the predictable signal, however, it is imperative that systematic bias of the models is improved and the space-time structure of the summer ISOs is simulated accurately.