Credit: DAVID M. LAWRENCE

Between the lush islands of Indonesia lies one of the bottlenecks of the global ocean circulation — the Indonesian throughflow, which connects the Pacific and Indian oceans. Over 80% of the water flowing from the Pacific to the Indian Ocean in the low latitudes passes through a tiny 45-km-wide channel in the Makassar Strait, which separates the islands of Borneo and Sulawesi. Through this narrow passage, comparatively warm and fresh Pacific waters are delivered into the Indian Ocean and transported further westward, towards Africa.

In 1976–1977, the characteristics of the tropical Pacific background climate changed distinctly, first noticed because the large-scale climatic seesaw of the El Niño/Southern Oscillation became biased towards more El Niño-like conditions. The equatorial Pacific sea surface temperatures rose sharply, beyond expectations from global warming. At the same time, the easterly trade winds weakened.

Lana Wainwright at the University of Tasmania, Australia, and colleagues suggest that these changes in the Pacific Ocean affected the Indian Ocean. According to their data analysis, the strength of the Indonesian throughflow was about 23% weaker after the 1976–1977 climate shift (Geophys. Res. Lett. 35, L03604; 2008). As a result, the thermocline became shallower and cooler on the Indian Ocean side of the passage.

The origin of the Pacific climate shift in 1976–1977 is still a matter of debate, with natural variability and human-induced global warming both being considered as possible contributors. It is clear, however, that the change in the Pacific Ocean had an effect on the waters around it.