Extreme Marine Warming Across Tropical Australia During Austral Summer 2015-2016

During austral summer 2015-2016, prolonged extreme ocean warming events, known as marine heatwaves (MHWs), occurred in the waters around tropical Australia. MHWs arose first in the southeast tropical Indian Ocean in November 2015, emerging progressively east until March 2016, when all waters from the North West Shelf to the Coral Sea were affected. The MHW maximum intensity tended to occur in March, coinciding with the timing of the maximum sea surface temperature (SST). Large areas were in a MHW state for 3-4 months continuously with maximum intensities over 2 degrees C. In 2016, the Indonesian-Australian Basin and areas including the Timor Sea and Kimberley shelf experienced the longest and most intense MHW from remotely sensed SST dating back to 1982. In situ temperature data from temperature loggers at coastal sites revealed a consistent picture, with MHWs appearing from west to east and peaking in March 2016. Temperature data from moorings, an Argo float, and Slocum gliders showed the extent of warming with depth. The events occurred during a strong El Nino and weakened monsoon activity, enhanced by the extended suppressed phase of the Madden-Julian Oscillation. Reduced cloud cover in January and February 2016 led to positive air-sea heat flux anomalies into the ocean, predominantly due to the shortwave radiation contribution with a smaller additional contribution from the latent heat flux anomalies. A data-assimilating ocean model showed regional changes in the upper ocean circulation and a change in summer surface mixed layer depths and barrier layer thicknesses consistent with past El Nino events. Plain Language Summary From the southeast tropical Indian Ocean to the Coral Sea, waters across tropical Australia experienced extreme warming during austral summer 2015-2016. This widespread warming was mainly caused by changes in the atmospheric conditions related to the strong El Nino. These regions had reduced cloud cover and weakened winds, leading to enhanced oceanic heating by the atmosphere. In the Indonesian-Australian Basin and surroundings areas, the shallower mixed layer depth further intensified the near-surface warming. We used satellite sea surface temperature and a range of instruments to document the warming events near the coast and in the deeper ocean. The warmer than average conditions persisted through austral winter and weakened in spring.