Monsoon and Enso: Selectively Interactive Systems
Authors: Webster Peter J. , Yang Song 
Published: February 2007

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 time of year a forecast is initiated. Noting that most models used in interannual prediction emphasize the coupled physics of the Pacific Ocean basin, with the intent of encapsulating the essential structure of the El Niño‐Southern Oscillation (ENSO) system, lagged Southern Oscillation Index (SOI) correlations are compared with the model 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 by 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 be 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 mean summer vertical shear in the ‘South Asia’ 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 on 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.