A year-long deployment of a mooring in the South Java Current (SJC) of Indonesia provides a fascinating insight into this poorly understood, semi-annually reversing boundary current. A striking three-week period of southeastward flow begins in mid-May 1997. An analytical model directly account for changes in velocity at this time due to the passage of a westerly wind-forced, downwelling Kelvin wave from the equatorial western Indian Ocean. The entire water column is warmed, with a fresh cap overlying salty water, consistent with the Indian Ocean source. Following the wave passage, the SJC is north-westward, and the prevailing south-easterly monsoon winds lead to upwelling of cold, salty water. In early August, the SJC abruptly returns to south-eastward flow, and remains so until November 1997 in the face of steady south-easterly local winds. The anomalous flow direction and cooler water are related to an upwelling Kelvin wave, forced by unseasonal prolonged easterly wind anomalies in the equatorial Indian Ocean. After a small reversal of flow in November 1997, the SJC is south-eastward, as expected during the north-east monsoon. A trend toward increasing salinities in the record is attributed to the increased input of salty Indian Ocean water, enhanced evaporation, and a lack of freshwater advection due to the regionally reduced precipitation during the 1997-98 El Niño.