Role of the Asian monsoon on the interannual variability of the global climate system

Abstract

The role of the Asian summer monsoon on the interannual variability of the global climate system particularly relevant to the ENSO time scales is discussed, by examining the statistical and dynamical links between the Asian summer monsoon, the atmosphere/ocean system in the tropics and the westerly flow régimes in the extratropics. The Asian monsoon, the ocean and the atmosphere in the tropical Pacific are tightly linked together as one climate system, named here as the MAOS (Monsoon and the Coupled Atmosphere/Ocean System). The MAOS prominently shows the biennial oscillatory nature which tends to have anomalous states starting in the northern summer monsoon season and persisting for about one year (Yasunari, 1990a: 1991). The anomalous state of the MAOS produces the anomalous atmospheric circulation over the subtropics and the extratropics of the north Pacific during summer through the early winter, through the modulation of the subtropical high and the stationary Rossby wave propagation mechanism. In the mid winter, this anomalous circulation over the north Pacific is evolved to the hemispheric winter anomalous circulation with wavenumber-one and/or-two structure. The anomalous circulation over Eurasia associated with this hemispheric anomalous flow regime seems to provide a favorable condition for the extensive (or diminished) snow cover area over central Asia, which in turn is responsible for the reversed anomalous state of the next Asian summer monsoon and the MAOS. That is, the biennial nature of the climate system in the northern hemisphere may be due, at least partly, to this two-way interactions between the tropics and the extratropics. In these processes, the Asian monsoon plays a key role as a transmitter of climate signals between the tropics and the extratropics through the land/atmosphere/ocean interaction in the seasonal cycle. In addition, it is strongly suggested that the North Atlantic Oscillation (NAO), in reality, plays a crucial role in the timing of the occurrence of the ENSO event, by stochastically amplifying or damping the biennial oscillation of this coupled climate system. That is, the more or less irregular ENSO cycle may result from this interaction between the MAOS and the NAO, where the former seems to have the nature of an almost-intransitive climate system, while the latter seems to represent the more chaotic nature of the westerly flow régime.