Dynamics of the Biennial Oscillation in the Equatorial Indian and Far Western Pacific Oceans

Many investigators have noted strong biennial atmospheric and oceanic variability, phase-locked to the calendar year, over the equatorial Indian and Pacific Oceans. A simple air–sea interaction theory suggests that biennial oscillations in the far western equatorial Pacific (120°–140°E) may originate from an air–sea interaction instability involving the mean seasonal wind cycle and evaporation. Over a wide range of realistic parameters the instability grows rapidly and is phase-locked to the calendar year in a similar way to the observations. Crucial to the mechanism are that the anomalous equatorial surface wind response is westerly under anomalous deep convection, that it lags the convection by at least 1 month, and that the zonal seasonal wind has a strong 12-month cycle. No such air–sea interaction instability is possible in the equatorial Indian Ocean because the water is either too cold for deep convection most of the year (western equatorial Indian Ocean, 40°–60°E) or the wind is westerly nearly all the year (central and eastern equatorial Indian Ocean 60°–100°E). Yet strong biennial oscillations occur in the central and eastern equatorial Indian Ocean. A second simple model suggests that the western equatorial Pacific biennial oscillation remotely drives these.