Dynamics of warm-season marine heatwaves and their ecological impacts

Marine heatwaves in the summertime when temperatures may exceed organisms’ thermal thresholds (“warm-season MHWs”) have huge impacts on the health and function of ecosystems like kelp forests and coral reefs. While previous studies showed that MHWs have increased in frequency and severity over recent decades and are likely to become more frequent and severe under climate change, there has been less analysis of the thermal properties of warm-season MHWs. This dissertation aims to improve our understanding of the thermal properties of warm-season MHW in the past and future, and their impacts on marine organisms and ecosystems, which may help inform marine conservation and management under climate change. I first examined the trends in the thermal properties of warm-season MHWs at a global-scale from 1985 to 2019 using multiple metrics (e.g., duration, peak intensity, accumulated heat stress and heating rate), and found that the historical increase in accumulated heat stress was predominantly driven by the increased duration rather than the increased intensity. Next, I examined the ability of three Climate Model Intercomparison Project 6 models to simulate five key properties of warm-season MHWs, and found that the duration, accumulated heat stress and peak intensity are projected to increase. Meanwhile, the duration of pre-MHW exposure to sub-lethal heat stress and following recovery with no heat stress (i.e., “priming” conditions) and heating rate are projected to decrease, potentially reducing organisms’ ability to acclimate to heat stress. For some coral reef and kelp forest locations, the projected increases in the MHW duration and accumulated heat stress, however, are likely overestimated due to model limitations in simulating surface winds, deep convection and some other processes that influence MHW evolution. Lastly, employing comparative analyses and mixed-effects models with a global coral bleaching observation database and heat stress metrics, I demonstrated that priming conditions can mitigate some of the coral bleaching response to a subsequent MHW, provided the MHW is moderate. However, such protective effects of priming conditions may be decreasing due to climate change. The examination of priming frequency across coral reefs globally identified potential refugia where coral reefs have a higher likelihood of priming protection.Arts, Faculty ofGeography, Department ofGraduat