Predictability of monsoons

Abstract

It is shown by numerical simulation that the variability of average pressure and rainfall for July due to short-period flow instabilities occurring in the absence of boundary anomalies can account for most of the observed variability at midlatitudes but not at low latitudes. On the basis of the available evidence it is suggested that a large part of the low-latitude variability is due to boundary anomalies in such quantities as sea-surface temperature, albedo and soil moisture, which, having longer time constants, are more predictable than the flow instabilities. Additional variability due to long-period natural fluctuations would likewise be more predictable. The degree of predictability of monsoons is a matter of considerable social and economic importance. Large agrarian populations exist in monsoon areas, and monsoon rains have a critical influence on food production and human welfare. The long-range prediction of average rainfall could be of immense value for water management and agricultural planning. In this chapter, evidence that mean flow conditions and precipitation patterns at low latitudes are in principle more predictable than those at high latitudes is presented. Among the low-latitude circulations are the African and Asian monsoons and perhaps also the southeasterly monsoon flow east of the North American Cordillera. In particular we wish to show that the natural flow instabilities on synoptic scales account for most of the interannual variability of monthly mean quantities at midlatitudes, but cannot explain the observed variability at low latitudes.