Combined Role of the MJO and ENSO in Shaping Extreme Warming Patterns and Coral Bleaching Risk in the Great Barrier Reef

Local meteorology over the Great Barrier Reef (GBR) can significantly influence ocean temperatures, which in turn impacts coral ecosystems. While El Ni & ntilde;o-Southern Oscillation (ENSO) provides insight into the expected synoptic states, it lacks details of anticipated sub-seasonal weather variability at local scales. This study explores the influence of the Madden-Julian oscillation (MJO) on Australian tropical climate, both independently and in combination with ENSO, focusing on GBR impacts. We find that during El Ni & ntilde;o periods, including the summer of 2009/10, faster propagating MJO patterns can disrupt background warm, dry conditions, and potentially provide cooling relief via increased cloud cover and stronger winds. In La Ni & ntilde;a periods, such as the summer of 2021/22, the MJO tends to be prevented from passing the Maritime continent, forcing it to remain in a standing pattern in the Indian Ocean. This leads to decreased cloud cover and weaker winds over the GBR, generating warm ocean anomalies. Bleaching is likely when tropical corals are exposed to ocean temperatures above a threshold for a prolonged period. In austral summer, tropical weather over the Great Barrier Reef (GBR) can vary from hot and sunny to stormy with rain and strong winds. During El Ni & ntilde;o, summer weather over the GBR is typically warm, still, and dry, increasing the likelihood of coral bleaching due to increased exposure to solar radiation and decreased mixing. During La Ni & ntilde;a, tropical storms, with cooling effects through increased rainfall and cloud cover, are more typical. The Madden-Julian oscillation (MJO) is an eastward moving storm pattern near the equator that can also influence the background climate over the GBR. We find that the MJO can significantly influence the weather variability over the GBR, altering the expected states of El Ni & ntilde;o and La Ni & ntilde;a periods. Composite maps show how the Madden-Julian oscillation (MJO) can change meteorological patterns on the Great Barrier Reef Cluster analysis is used to show the types of MJO propagation patterns likely to occur during El Ni & ntilde;o and La Ni & ntilde;a periods Ocean temperature variability is discussed with El Ni & ntilde;o/La Ni & ntilde;a phases as the background states and the MJO as a sub-seasonal modulator