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Researchers at the Scripps Institution of Oceanography at UC San Diego and Yale University report that accelerated warming in the tropical Indian Ocean can influence rainfall, ocean salinity, and circulation in the Atlantic Ocean.

Shineng Hu, a postdoctoral researcher at Scripps Oceanography and Alexey Fedorov, a professor of oceanic and atmospheric sciences at Yale University, said the Indian Ocean warming might have the effect of bolstering the Atlantic meridional overturning circulation (AMOC), a flow of water in the Atlantic Ocean that moderates Europe’s climate to a large extent. This effect appears to be compensating for a slowing of the AMOC that has been trending in recent decades.

Their computer model analysis, funded in part by the National Science Foundation, provides more evidence of the complexity of the relationships among climate drivers through what scientists call global teleconnections. In this case, the ocean phenomena are 5,000 miles apart and separated by land masses.

“Indian Ocean warming is one of the most robust features of anthropogenic warming,” said Hu. “Its sea-surface temperature is now 1 degree C (1.8 degrees F) warmer than it was in the mid-20th century, more pronounced than other tropical ocean basins.”

Hu and Fedorov ran models to simulate the effects of this warming beyond the Indian Ocean and were surprised to find the AMOC flow grew stronger in lockstep with the warming.

“Our results demonstrate that very different components of the climate system can be intrinsically connected,” said Fedorov. “We have shown that the warming of the tropical Indian Ocean relative to the other two tropical ocean basins can effectively control the strength of the Atlantic meridional overturning circulation . The greater the relative warming is, the stronger the AMOC becomes. Consequently, should the Indian Ocean warming slow down in the future, the AMOC might decline faster.”

The fate of the AMOC has been a central plot point in global warming scenarios and even Hollywood movies. The circulation cycles warm water from the tropics north toward Greenland. There, colder, saltier water sinks and returns southward closer to the seafloor. Scientists have surmised that if that flow were to slow down or stop, catastrophic results ranging from dramatic global sea-level change to extreme cooling in Europe would ensue. The most likely causes of such a slowdown are local sources such as the large-scale melt of glaciers in Greenland and of sea ice in the Arctic Ocean. The thought among scientists is that the flood of freshwater on the surface ocean in the north Atlantic would alter the physics of the AMOC, stifling it if not bringing it to a full halt.

What is the relationship between Indian Ocean warming and the AMOC? The two researchers suggest that the warming sets in motion a chain reaction. The warm water evaporates into the atmosphere, creating more precipitation around the Indian Ocean. The heat thus dissipates in that region, but in doing so, influences other prevailing air flows in the tropical Atlantic Ocean and elsewhere. The effect of that is to stifle rainfall over the tropical Atlantic Ocean, which means more saline water at the surface flows toward high latitudes, supplying the salt that the circulation needs to keep moving. 

The authors of the Indian Ocean study argue that their finding suggests a need for scientists to take the global influence of the Indian Ocean into greater consideration.

“This work shows that tropical ocean temperature patterns, by modulating rainfall over the Atlantic and surrounding continents, can drive AMOC change,” said Scripps climate modeler Shang-Ping Xie, who works with Hu but was not involved with the study. “This is an interesting idea worth further evaluation with observations and model experiments.”

The study, “Indian Ocean warming can strengthen the Atlantic meridional overturning circulation,” appears in the journal Nature Climate Change.

Hu’s work was supported by the Scripps Institutional Postdoctoral Fellowship.