The Great Barrier Reef has seen better days, after a one-two punch of coral bleaching events struck it in 2016 and 2017. Warming sea temperatures have been pegged as the culprit, but that alone can't account for why the bleaching was so severe. Australian and Belgian researchers have now found that a "perfect thermal storm" of oceanographic conditions led to the 2016 mass bleaching.

Bleaching occurs when stressed coral expels the algae that lives inside it, turning the coral a bony white color and often killing it as a result. Warmer water is the general cause, but that itself is a symptom of wider conditions – most notably, climate change. A new study from James Cook University and the Catholic University of Louvain has examined just how the waters around the Great Barrier Reef heated up enough to cause the 2016 bleaching event – the worst in the reef's history.

The study examined satellite and oceanographic data, backed up by oceanography models to simulate the water currents in the area. Altogether, this information painted a more complicated picture of the conditions that affected over 90 percent of the Great Barrier Reef.

"What we presented is our best-informed attempt to reveal the mechanisms involved in causing the event, based on the available oceanographic data combined with the existing body of knowledge on the water circulation in and around the Torres Strait/Northern Great Barrier Reef region," says Professor Eric Wolanski, lead author of the study.

According to satellite data, the trouble started in the Gulf of Carpentaria, the body of water cupped within the northern coast of Australia. There, water temperature soared to 34° C (93° F) thanks to the El Nino effect, before flowing east into the Torres Strait reefs and eventually south to the Great Barrier Reef.

To make matters worse, the warm waters sat over the reefs for longer than usual and, combined with the influence of the strong Queensland sun, the coral had ample time to stew in the stressful conditions.

"Examining surface currents suggests that the North Queensland Coastal Current in the Coral Sea, which would normally flush and cool the Northern Great Barrier Reef, actually did the opposite," says Wolanski. "It reversed course and brought very warm water to the Northern Great Barrier Reef."

This specific combination of factors is what caused the 2016 event to be worse than usual. The researchers point out that 1998 was a particularly hot year, worsened by an El Nino event, and yet coral bleaching was relatively minor.

The study was published in the journal Estuarine, Coastal and Shelf Science.