Could a forecast model predict the 2020 Great Barrier Reef marine heatwave?

Details

The 2020 marine heatwave in the Great Barrier Reef and Coral Sea led to mass coral bleaching. Sea surface temperature anomalies reached +1.7 °C across the whole Great Barrier Reef and Coral Sea and exceeded +2 °C across broad regions, relative to 1990–2012. The heatwave reached Category 2 (Strong), and warm anomalies peaked between mid-February and mid-March 2020. Its peak intensity aligned with regions of reduced cloud cover and weak winds.

Researchers used a marine heatwave framework to assess whether an operational coupled ocean-atmosphere prediction system, the Australian Community Climate and Earth System Simulator Seasonal version 1, could capture the heatwave’s severity, duration, and spatial extent. For initial-week predictions, predicted severity generally agreed with the magnitude and spatial extent of observed severity for that week. The model ensemble mean did not capture the heatwave’s development phase at lead times beyond the first week. The model underestimated its spatial extent: at least Moderate severity reached up to 95% of the study area, and at least Strong severity reached up to 43%. Most forecast ensemble members, however, correctly predicted the period of Strong severity in the first week of the model forecast. The model correctly predicted heatwave conditions to persist from mid-February to mid-March, but did not capture the heatwave’s end.

The failure to predict the end and other periods of less skilful prediction were related to subseasonal variability due to weather systems, including the passage of tropical cyclones not simulated in the model. The researchers concluded that evaluating daily to weekly forecasts of ocean temperature extremes on subseasonal timescales is an important step toward methods for developing operational forecast products for extremes in early warning systems.