During the southern hemisphere (boreal) summer, a rise of cold water to the surface is observed east of the ocean basin.

Twenty-eight dead, a billion animals killed, 80,000 square kilometers of charred land, and more than two thousand homes destroyed. The fires that have ravaged Australia for several months have reached a record level of intensity, in a country yet used to facing the fires of the southern summer every year.

Behind this igneous devastation is a combination of factors, including an atmospheric circulation anomaly identified only thirty years ago: the Indian Ocean Dipole (IOD).

Usually, in the Indian Ocean, surface water temperatures at the equator are, on average, relatively warmer in the east, on the Indonesian coast, and colder in the west, along Africa. Because of this difference, the winds generally blow from west to east. Seasonally, during the boreal summer, a rise of cold water to the surface is observed east of the ocean basin, with monsoon winds. However, this situation, which constitutes the “neutral phase” of IOD, may experience major upheavals.

In certain years, from September to November, the phenomenon of interaction between the ocean and the atmosphere is accentuated. We are then seeing a greater rise in cold surface water towards the eastern Indian Ocean, around the Indonesian islands, which is causing a drop in precipitation.

Conversely, along the African coasts, a sudden rise in ocean temperatures is noted. This is called the “positive phase” of the dipole, which recalls a phenomenon well known in the Pacific for the lot of deadly disasters it generates: El Niño. “It is a comparable phenomenon, but its duration and its mechanisms differ,” summarizes Eric Guilyardi, climatologist at CNRS.

This “positive phase” was precisely at work at the end of 2019, “in a particularly strong proportion, with very marked temperature increases in the west,” said Etienne Kapikian, a forecaster at Météo France. According to the Australian Bureau of Meteorology (BOM), this positive dipole is even the most powerful ever recorded by its services since the beginning of its study, thirty-five years ago.

The consequences of these cold water displacements? They “force atmospheric movements”, summarizes Etienne Kapikian. On the African coast, this unusual mass of warm water favors rising thermal surpluses, and at the same time causes a strong rainy activity, even cyclonic. In early December 2019, the entire Horn of Africa to the north of the island of Madagascar was affected by rainfall more than 200% above normal in this season. At least 120 people have been killed in Kenya in the floods.