Abstract

On interannual and decadal time scales, ocean circulation in the southeast Indian Ocean, the Indonesian Throughflow and the Leeuwin Current system, is strongly modulated by climate variability in the tropical Pacific. Stronger trade winds and subtropical cells in the Pacific correspond to a stronger Indonesian Throughflow and a stronger Leeuwin Current, due to dynamic connections of the Pacific and the southeast Indian Ocean through equatorial and coastal waveguides. Historical sea level records reveal that a strengthening of the Pacific subtropical cells since the early‐1990’s has reversed a multi‐decadal weakening tendency from the 1960’s. Multi-decadal weakening trend of the Pacific trade winds during 1960’s to early 1990’s has induced cooling anomalies in the thermocline depth of the equatorial western Pacific, whereas the rebound of the trades after 1994 has almost reversed the thermocline anomalies. The reversal of multi-decadal signals has transmitted into the southeastern Indian Ocean, causing a decline in the southward transports of the Indonesian Throughflow and the Leeuwin Current by 20-30% before 1994 followed by a rebound of the transport up to the present, according to high-resolution ocean models. An ensemble of coupled climate models for the Intergovernmental Panel on Climate Change forced by observed anthropogenic green-house gases, however, produces a much weaker decline trend in the current transports during the second half of the 20th century and no reversal. The discrepancy suggests that the observed multi-decadal trends of the internal climate system are larger than the climate-change-induced secular trend during the past few decades, so that long instrumental records are necessary for the detection of human induced global change signals in the two circulation systems.