A year-long deployment of a mooring in the South Java Current (SJC) of Indo
nesia provides a fascinating insight into this poorly understood, semi-annu
ally reversing boundary current. A striking three-week period of southeastw
ard flow begins in mid-May 1997. An analytical model directly account for c
hanges in velocity at this time due to the passage of a westerly wind-force
d, downwelling Kelvin wave from the equatorial western Indian Ocean. The en
tire water column is warmed, with a fresh cap overlying salty water, consis
tent with the Indian Ocean source. Following the wave passage, the SJC is n
orth-westward, and the prevailing southeasterly monsoon winds lead to upwel
ling of cold, salty water. In early August, the SJC abruptly returns to sou
th-eastward flow, and remains so until November 1997 in the face of steady
south-easterly local winds. The anomalous flow direction and cooler water a
re related to an upwelling Kelvin wave, forced by unseasonal prolonged east
erly 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 eva
poration, and a lack of freshwater advection due to the regionally reduced
precipitation during the 1997-98 El Nino.