In late 2015, about 8000 hectares of the forested tidal wetland along a 1000 km stretch of the southern coastline of Australia's Gulf of Carpentaria experienced extensive dieback of the mangrove trees. Mangroves are an important part of the local ecosystem, providing habitat for many marine and coastal species. They also protect coastlines from extreme weather and erosion, filtering out sediment from river run-off to protect seagrass, as well as absorbing considerable amounts of greenhouse gases. Earlier studies reported that a combination of ocean-atmospheric conditions, including the El Niño of 2015-16, were likely important in causing this environmental catastrophe. However, the exact cause of the dieback event remains unclear. Mangroves routinely cope with the seasonally low of sea level in winter and typically experience ~10% canopy loss during El Niño, even during strong El Niño of 1997-98. This leads to the question: what was unusual about 2015-16 El Niño that resulted in up to 100% dieback at some locations?

We address this critical question by examining a new record of the 'greenness' of the mangroves derived from satellite data. Covariance between local sea-level height and mangrove 'greenness' indicates that El Niño events have resulted in systematic dieback with about a ~6-month lag between minimum sea-level height and lowest greenness. However, compared to other El Niño events, a record-low sea level occurred during winter 2015 due to an early onset of the El Niño. Various features of the 2015-16 El Niño and its impact on sea level and surface climate in the Gulf are also discussed and a 'mangrove stress index' is proposed based on the large-scale variables. Understanding the drivers of such dieback events will allow for improved capability in the monitoring of such ecological disasters and promises great value in the optimization of risk management for policymakers in future.