Mechanism of biennial oscillation in the tropical Indo-Pacific

Biennial oscillation is an important component of variability in the tropical Pacific and Indian oceans. The development of surface wind anomaly in the western equatorial Pacific is regarded instrumental for biennial oscillation. The mechanism of biennial oscillation has been investigated using combined cyclostationary EOF and regression analyses of various physical variables including sea surface temperature (SST), sea level pressure (SLP), and tropospheric wind and other derived quantities such as diabatic heating, velocity potential, and divergence of wind. The cyclostationary EOF analysis is suitable for examining physical mechanisms with strong temporal evolution characteristics such as biennial oscillation.

Change in deep convection in the central Pacific due to SST change and warm pool shift seems to be an important ingredient of biennial oscillation, as reflected in the diabatic heating anomaly field associated with the biennial oscillation. The present analysis shows, however, that the change in the mean Walker circulation and resulting change in the mean Hadley circulation may be instrumental for the biennial oscillation. Walker circulation anomaly is formed between the western and the eastern Pacific. Corresponding 700 mb divergence field also shows that the eastern Pacific plays a more active role than the central Pacific in alteration of the Walker circulation. Present analysis is consistent with the so-called "western Pacific oscillation" and supports it as a plausible negative feedback mechanism for the biennial oscillation.