MONSOON AND ENSO - SELECTIVELY INTERACTIVE SYSTEMS
MONSOON AND ENSO - SELECTIVELY INTERACTIVE SYSTEMS
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DOI：
10.1002/qj.49711850705
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发表时间：
1992-07-01
影响因子：
8.9
通讯作者：
YANG, S
中科院分区：
文献类型：
--
作者：
WEBSTER, PJ;YANG, S
我们试图构建一个逻辑框架，以破译决定耦合气候系统年际变化的物理过程。特别令人感兴趣的是，当观测预测相关性迅速下降时，北方春季出现“可预测性障碍”的原因。这一障碍是许多模型的特性，与预测开始时的年份无关。注意到年际预测中使用的大多数模式强调太平洋海盆的耦合物理，目的是概括厄尔尼诺-南方涛动（ENSO）系统的基本结构，因此将滞后的南方涛动指数（SOI）相关性与模式结果进行了比较。滞后的SOI相关性在春季也迅速下降。从这个意义上说。耦合的海洋-大气模式的行为方式与实际系统非常相似，至少像SOI所定义的那样。我们提出：(i)春季是一个在耦合的海洋-大气预报模式中误差可能增长最快的时期，或（ii）存在其他未包括在简单耦合模式公式中的对系统的影响。这两个命题都基于观察。通过对相关性减弱周期的考察，发现赤道气压梯度在相关性减弱时趋于最小。这表明海洋-大气系统在春季可能是最不稳定的，因此会受到误差增长的影响。与此同时，南亚夏季风正在迅速发展。由于季风环流每年在相位和振幅上都有很大的变化，因此每年春季海洋-大气系统可能会受到变化和脉冲强迫的影响。根据南亚地区夏季海洋垂直切变的大小，定义了大尺度季风的季风强度指数。“强”和“弱”季风季节由该指数决定，并显示与独立的大尺度外发长波辐射场一致。与异常季风相关的是全球尺度的、连贯的夏季环流模式。特别重要的是，强于（弱）平均夏季信风与强（弱）季风期有关。因此，在太平洋上空的低空风场中检测到可变季风的信号，该信号将通过表面应力传递给太平洋。寻找夏季风环流场异常的较长时期背景。以夏季风指数为基础，合成了季风季节强弱年份的年周期。早在前一个冬春季，包括南亚和热带印度洋在内的全球大部分地区的环流场就存在明显的大尺度相干差异。虽然有限的数据期使得结论的绝对性难以证实，但结果表明，变化的季风（以及太平洋贸易体制的信号）沉浸在一个更大尺度和缓慢演变的环流系统中。根据观测，季风和沃克环流呈正交关系。有人提出，这两种循环是选择性地相互作用的。在春天。快速增长的季风支配着近赤道的沃克环流。在秋冬季节，季风最弱，对流相对靠近赤道，这时Walker环流最强，可能主导冬季季风。在夏季，季风可能占主导地位。提出了数值实验来验证这两个命题。
We attempt to construct a logical framework for the deciphering of the physical processes that determine the interannual variability of the coupled climate system. Of particular interest are the causes of the 'predictability barrier' in the boreal spring when observation prediction correlations rapidly decline. The barrier is a property of many models and occurs irrespective of what tinic of year a forecast is initiated. Noting that most models used in interannual prediction emphasize the coupled physics ot the Pacific Ocean basin, with the intent of encapsulating the essential structure of the El Nino-Southern OSCillation (ENSO) system, lagged Southern Oscillation Index (SOI) correlations are compared with the mode results. The lagged SOI correlations also decrease rapidly in springtime. In that sense. the coupled ocean atmosphere models are behaving in a manner very similar to the real system, at least as it is defined hy the SOI.We propose that (i) the springtime is a period where errors may grow most rapidly in a coupled ocean-atmosphere forecast model or (ii) there are other influences on the system that are not included in the simple coupled-model formulations. Both propositions are based on observations. By examining the period of correlation decrease, it is noticed that the equatorial pressure gradients tend to he a minimum at the time of the correlation decrease. suggesting that the ocean-atmosphere system may be least robust during the spring and, thus, subject to error growth. At the same time the south Asian summer monsoon is growing very rapidly. As the monsoon circulation is highly variable in both phase and amplitude from year to year, the ocean-atmosphere system may be subject to variable and impulsive forcing each spring.A monsoon intensity index, based on the magnitude of the ocean summer vertical shear in the 'South Asian' region, was defined for the broad-scale monsoon. 'Strong' and 'weak' monsoon seasons were determined by the index and were shown to be consistent with the independent broad-scale outgoing long-wave-radiation fields. Associated with the anomalous monsoons were global scale, coherent summer circulation patterns. Of particular importance was that stronger (weaker) than average summer trade winds were associated with strong (weak) monsoon periods. Thus, a signal of the variable monsoon was detected in the low-level wind fields over the Pacific Ocean that would be communicated to the Pacific Ocean through surface stresses.A longer-period context for the anomalous summer monsoon circulation fields was sought. Based on the summer monsoon index, annual cycles for the years in which there were strong and weak monsoon seasons were composited. Large-scale coherent differences were apparent in the circulation fields over most of the globe including south Asia and the tropical Indian Ocean as far as the previous winter and spring. Although the limited data period renders the absoluteness of the conclusions difficult to confirm, the results indicate that the variable monsoon (and hence the signal in the Pacific Ocean trade regime) are immersed in a larger scale and slowly evolving circulation system. Based in the observation that the monsoon and the Walker circulation appear to be in quadrature. it is proposed that these two circulations are selectively interactive. During the springtime. the rapidly growing monsoon dominates the near-equatorial Walker circulation. During autumn and winter, the monsoon is weakest with convection fairly close to the equator, the Walker circulation is then strongest and may dominate the winter monsoon. During the summer the monsoon may dominate. Numerical experiments are proposed to test both propositions.