Semiannual variation in the western tropical Pacific Ocean

Analysis of altimeter data combined with results from a high‐resolution general circulation model has revealed the existence of a semiannual variation in the western tropical Pacific. The semiannual variation cannot be explained by local Ekman pumping alone, but is also related to the westward propagation of Rossby waves originating in the central tropical Pacific, corresponding in the northern hemisphere with the Inter‐Tropical Convergence Zone and in the southern hemisphere with the South Pacific Convergence Zone. This signal intensifies toward the western boundary and appears to have a notable impact on the circulation there. In the Mindanao Current, the semiannual variation is of comparable strength with the annual variation. This finding is supported by a moored acoustic Doppler current profile measurement during the period from October 1999 to July 2002.