Marine heatwaves in the Great Barrier Reef and Coral Sea, their mechanisms and impacts on shallow and mesophotic coral ecosystems

The Great Barrier Reef (GBR) World Heritage Area and adjacent Coral Sea Marine Park are under serious threat from global climate change. This study used sea surface temperature (SST) data to identify major marine heatwaves (MHWs) occurring in this region over the last three decades (1992–2022) and to map significant MHW events that have occurred between 2015 and2022. We investigated the mechanisms of the MHWs and identified potential coral refugia. MHWs in this region have increased in frequency, intensity and spatial extent. El Niño, especially when it is in phase with positive Indian Ocean Dipole, was the key remote driver leading to intense MHWs. However, the more recent strong MHWs (e.g., 2017 and 2022) occurred without these climatic events, signifying the impacts of long-term climate change and local drivers. We also found that reduced wind speed and shoaling mixed layer depth, often together with reduced cloudiness, were the main local drivers pre-conditioning these MHWs. Anomalous air-sea heat flux into the ocean, mainly controlled by shortwave solar radiation (cloudiness) and latent heat flux (wind), was the most constant contributor to the 2015–16 and 2019–20 MHW events. However, local oceanographic dynamics, especially horizontal advection and turbulent mixing, played important roles in MHW heat budgets. This study confirmed that shallow-water coral bleaching severity was positively related to the cumulative MHW intensity in these two MHWs. We identified the shallow reefs along the path of the North Queensland Current as potential coral refugia from bleaching because of the cooler waters upwelled from the ocean current. We also found that, except during abnormal weather events such as tropical cyclones, the mesophotic reefs in the Coral Sea Marine Park may be less susceptible to severe bleaching as the MHWs are more confined within the shallow mixed layer.
Presented at the 30th Conference of the Australian Meteorological and Oceanographic Society (AMOS) 2024

大堡礁（Great Barrier Reef，GBR）世界遗产地及邻近的珊瑚海海洋公园正面临全球气候变化带来的严峻威胁。本研究利用海表温度（sea surface temperature, SST）数据，识别了1992-2022年近三十年间该区域发生的主要海洋热浪（marine heatwaves, MHWs），并绘制了2015至2022年间发生的显著海洋热浪事件分布图。我们探究了海洋热浪的发生机制，并识别出潜在的珊瑚避难所。该区域的海洋热浪在发生频率、强度及空间范围上均呈上升趋势。厄尔尼诺现象，尤其是与正印度洋偶极子相位同步时，是引发强海洋热浪的关键远程驱动因子。不过，近年的强海洋热浪（如2017年与2022年事件）并未伴随此类气候事件，这凸显了长期气候变化与局地驱动因子的影响。我们还发现，风速降低、混合层变浅，往往还伴随云量减少，是为这些海洋热浪预先生成适宜条件的主要局地驱动因子。异常的海-气热通量进入海洋，主要受短波太阳辐射与潜热通量调控，前者由云量决定，后者受风速影响，是2015-2016年及2019-2020年海洋热浪事件最持续的贡献因子。不过，局地海洋动力学过程，尤其是水平平流与湍流混合，在海洋热浪的热量收支中发挥了重要作用。本研究证实，在这两次海洋热浪事件中，浅水珊瑚白化严重程度与累积海洋热浪强度呈正相关。我们识别出沿北昆士兰洋流路径分布的浅礁为潜在的珊瑚白化避难所，这得益于该洋流上涌带来的较冷水域。此外我们还发现，除热带气旋等异常天气事件期间外，珊瑚海海洋公园内的中深层礁体受严重白化影响的可能性更低，因为海洋热浪更多局限于浅层混合层内。本研究于2024年澳大利亚气象与海洋学会（Australian Meteorological and Oceanographic Society, AMOS）第30届学术会议上进行展示。