A new Indonesian coral-based record of surface ocean salinity shows that the location of the most significant hydroclimatic feature in the Southern Hemisphere, the South Pacific Convergence Zone (SPCZ), a band of high clouds and precipitation, influences a major current in the far western Pacific Ocean.
The current, the Indonesian Throughflow, transports warm, fresh water from the North Pacific Ocean south across the equator and into the Indian Ocean, transferring the heat and water that build up in the western Pacific into the Indian Ocean, and then on to the Atlantic. The record is part of a new study published in Geophysical Research Letters, and suggests that over geologic time changes, the mean position of the SPCZ could be a key regulator of the Indonesian Throughflow and the transfer of ocean heat out of the Pacific Ocean.
To create the coral salinity record, Lamont-Doherty Earth Observatory and Center for Climate and Life paleoceanographer Brad Linsley, the study’s lead author, analyzed samples taken from a set of corals living in the tropical waters of Indonesia’s central Makassar Strait, which runs between the islands of Borneo and Sulawesi.
The chemistry of coral skeletons is extremely sensitive to changes in their environment, including variations in temperature and salinity. And like trees, coral skeletons produce annual rings that provide a wealth of information about past climate and ocean conditions. Scientists extract core samples from living corals by scuba diving and collecting a sample from the coral skeleton; geochemical analyses of these samples reveals how ocean temperature, circulation, and salinity change over time.
Linsley’s analysis resulted in the first multi-century record of sea surface salinity for the Makassar Strait. The previous instrumental record dated back to 1979. The new record spans more than 250 years, from 1742 to 2004, at near-monthly resolution.
While assembling the new record, Linsley noticed something strange in the years 1998, 1992, and 1983: the salinity of seawater in the Makassar Strait was two full salinity units higher — a huge amount to physical oceanographers — than in most years. Normally, each spring, the addition of fresher and more buoyant water into the Makassar Strait acts as a brake or plug on the Indonesian Throughflow. But in these three specific years this low-salinity brake was missing or greatly reduced.
A 2009 study by Vincent et al. found that the SPCZ, which is more than 4,000 km east of the Makassar Strait, sometimes moves from its usual position. The SPCZ typically stretches northwest to southeast from the Solomon Islands, across Fiji and Samoa, to Tonga, but, during some El Niño events, it rotates about 15 degrees counter clockwise up onto the equator. The years when this shift, or “zonal event,” occurs correspond with the three years where Linsley noted high-salinity conditions in the Makassar Strait.
“When these odd SPCZ positions occur, all this salty water runs through the Makassar Strait, preventing the fresh water brake from effecting the Indonesian Throughflow,” Linsley said. “It shows that there’s a strong teleconnection between this key interocean current and areas of the equatorial and South Pacific Ocean far to the east.”
Further analysis of the coral salinity record revealed that this influx of higher-salinity seawater from the western Pacific into the Makassar Strait is a regular event, which has occurred every 10 to 20 years since the mid-1700s.
The authors suggest that the SPCZ can act like a switch, allowing saltier currents flow through the Strait — and that the mean position of the SPCZ over a range of time-scales could influence the Indonesian Throughflow and perhaps global climate. Their discovery highlights the interconnectedness of the climate system, and suggests feedbacks on other key aspects of tropical climate like El Niño.
“For me, these are exciting and completely unexpected results,” Linsley said. “I think they identify the position of the SPCZ as an important control on the salinity, temperature, and maybe velocity of the Indonesian Throughflow.”
Linsley says the findings also emphasize the value of corals to climate science.
“If you look very closely, these corals are recording exactly what’s going on,” he said. “The details they record are remarkable and they allow us to extend the instrumental record and gain a longer-term perspective.”
The research was funded by the National Science Foundation, with support for data analyses from World Surf League PURE. The other study authors are Arnold L. Gordon of Lamont-Doherty; Henry C. Wu of the Laboratoire d’Océanographie et du Climat, IRD France-Nord and the Université Paris-Saclay; Tim Rixen of the Leibniz Center for Tropical Marine Ecology and the University of Hamburg; Christopher D. Charles of Scripps Institution of Oceanography; and Michael D. Moore of Redmond, WA.