Using the 1.5-layer long Rossby wave model forced by the seasonal European Remote Sensing (ERS) satellite scatterometer wind, we demonstrate that the seasonal variability of the South Equatorial Countercurrent (SECC) is due to the interplay of two types of forced Rossby waves: the resonantly forced Rossby waves north of 10°S and the locally forced Rossby waves south of 10°S. The resonantly forced Rossby waves north of 10°S are due to the annually varying tropical Pacific wind, and the locally forced Rossby waves south of 10°S in the western basin are associated with the Western Pacific Monsoon. Both types of waves are intensified westward in the SECC region but have different phases across the SECC; this latitudinal phase jump results in the seasonal variability of the SECC, which reaches its maximum in March and its minimum in August. Favorable agreement between the seasonal SECC signals from the model and from the satellite sea surface height measurements confirms this new insight about the seasonal modulation of the SECC.