Using an ocean general circulation model that incorporates data assimilation, we show that the northward flowing portion of the subtropical gyre in the South Pacific undergoes a well defined decadal variation. The maximum variability of ocean temperature is located at around 10 ¿S and is largest in the western side of the basin with a vertical extension of up to 1000 m. The anomalous temperature shows a rapid warm-to-cold transition in the late 1970s with a peak-to-peak value of more than 1 ¿C. Coupled with this change is a strengthened circulation in the northern portion of the subtropical gyre. Further analysis indicates that the decadal change in the south tropical pacific is linked to increased Ekman suction induced by enhanced cyclonic wind-stress curl in the region. ¿ 2001 American Geophysical Union