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Abstract The spatiotemporal evolution of marine heatwaves (MHWs) is explored using a tracking algorithm called Ocetrac that provides the objective characterization of MHW spatiotemporal evolution. Candidate MHW grid points are defined in detrended gridded sea temperature data using a seasonally varying temperature threshold. Identified MHW points are collected into spatially distinct objects using edge detection with weak sensitivity to edge detection and size percentile threshold criteria at each time step. Ocetrac then uses 3D connectivity to determine if these objects are part of the same event, but Ocetrac only defines the full MHW event after all time steps have been processed, limiting its use in predictability studies. Here, Ocetrac is applied to monthly satellite sea surface temperature data from September 1981 through January 2021. The resulting MHWs are characterized by their intensity, duration, and total area covered. The global analysis shows that MHWs in the Gulf of Maine and Mediterranean Sea are spatially isolated, while major MHWs in the Pacific and Indian Oceans are connected in space and time. The largest and most long-lasting MHW using this method lasts for 60 months from November 2013 to October 2018, encompassing previously identified MHW events including those in the northeast Pacific (2014–15), the Tasman Sea (2015–16, 2017–18), and the Great Barrier Reef (2016). Significance StatementThis study introduces Ocetrac, a method to track the spatiotemporal evolution of marine heatwaves (MHWs). It is applied to satellite sea surface temperature data from 1981 to 2021. The method objectively identifies and tracks MHWs in space and time while allowing for splitting and merging. The resulting MHWs are characterized by intensity, duration, and total area covered. Marine heatwaves can have significant ecological consequences, including biodiversity loss and mortality, geographical shifts, and range reductions in marine species and community structure changes when physiological thresholds are exceeded. This results in both ecological and economic impacts. Ocetrac provides a method of tracking the space and time evolution of MHWs that can provide a visualization that demonstrates the global impact of these events. 

Abstract The reappearance of a northeast Pacific marine heatwave (MHW) sounded alarms in late summer 2019 for a warming event on par with the 2013–2016 MHW known as The Blob. Despite these two events having similar magnitudes in surface warming, differences in seasonality and salinity distinguish their evolutions. We compare and contrast the ocean's role in the evolution and persistence of the 2013–2016 and 2019–2020 MHWs using mapped temperature and salinity data from Argo floats. An unusual near‐surface freshwater anomaly in the Gulf of Alaska during 2019 increased the stability of the water column, preventing the MHW from penetrating deep as strongly as the 2013–2016 event. This freshwater anomaly likely contributed to the intensification of the MHW by increasing the near‐surface buoyancy. The gradual buildup of subsurface heat content throughout 2020 in the region suggests the potential for persistent ecological impacts.