The Asian monsoon and predictability of the tropical ocean-atmosphere system

The influence of the Asian monsoon on the predictability of the tropical ocean-atmosphere is studied using long-term records of monsoon rainfall, sea-level pressure, sea surface temperature (SST) and surface wind data. Results indicate that the Asian monsoon plays an important role in mediating interannual variations of SST, surface pressure and surface wind in a quasi-biennial oscillation of the tropical ocean-atmosphere as found in previous studies. During the boreal spring, monsoon-related atmospheric and oceanic anomalies change most rapidly, while the east-west gradients in sea-level pressure and SST across the equatorial Pacific are at a minimum. The so-called 'spring predictability barrier' (SPB) is associated with the phase-locking of interannual anomalies to the annual cycle variation of the tropical ocean-atmosphere. Results suggest that the Asian monsoon defines two climatic states in the tropical ocean-atmosphere system, corresponding to the warm and cold phases of the tropical biennial oscillation. A weak Asian monsoon is associated with enhanced memory for predictions starting from late boreal spring through to summer and with a well-defined SPB for those starting from boreal fall to winter. On the other hand, a strong monsoon is associated with a recovery of memory at long lags (> 4-6 months) from the boreal fall through to the following spring, indicating a partial breakdown or 'tunnelling' effect for the SPB. The above results may be exploited to improve seasonal-to-interannual predictions of the monsoon-ocean-atmosphere system.