Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Benthuysen, J. A., Oliver, E. C. J., Chen, K., & Wernberg, T. Editorial: advances in understanding marine heatwaves and their impacts. Frontiers in Marine Science, 7, (2020): 147, doi:10.3389/fmars.2020.00147.

Description: Editorial on the Research Topic Advances in Understanding Marine Heatwaves and Their Impacts In recent years, prolonged, extremely warm water events, known as marine heatwaves, have featured prominently around the globe with their disruptive consequences for marine ecosystems. Over the past decade, marine heatwaves have occurred from the open ocean to marginal seas and coastal regions, including the unprecedented 2011 Western Australia marine heatwave (Ningaloo Niño) in the eastern Indian Ocean (e.g., Pearce et al., 2011), the 2012 northwest Atlantic marine heatwave (Chen et al., 2014), the 2012 and 2015 Mediterranean Sea marine heatwaves (Darmaraki et al., 2019), the 2013/14 western South Atlantic (Rodrigues et al., 2019) and 2017 southwestern Atlantic marine heatwave (Manta et al., 2018), the persistent 2014–2016 “Blob” in the North Pacific (Bond et al., 2015; Di Lorenzo and Mantua, 2016), the 2015/16 marine heatwave spanning the southeastern tropical Indian Ocean to the Coral Sea (Benthuysen et al., 2018), and the Tasman Sea marine heatwaves in 2015/16 (Oliver et al., 2017) and 2017/18 (Salinger et al., 2019). These events have set new records for marine heatwave intensity, the temperature anomaly exceeding a climatology, and duration, the sustained period of extreme temperatures. We have witnessed the profound consequences of these thermal disturbances from acute changes to marine life to enduring impacts on species, populations, and communities (Smale et al., 2019). These marine heatwaves have spurred a diversity of research spanning the methodology of identifying and quantifying the events (e.g., Hobday et al., 2016) and their historical trends (Oliver et al., 2018), understanding their physical mechanisms and relationships with climate modes (e.g., Holbrook et al., 2019), climate projections (Frölicher et al., 2018), and understanding the biological impacts for organisms and ecosystem function and services (e.g., Smale et al., 2019). By using sea surface temperature percentiles, temperature anomalies can be quantified based on their local variability and account for the broad range of temperature regimes in different marine environments. For temperatures exceeding a 90th-percentile threshold beyond a period of 5-days, marine heatwaves can be classified into categories based on their intensity (Hobday et al., 2018). While these recent advances have provided the framework for understanding key aspects of marine heatwaves, a challenge lies ahead for effective integration of physical and biological knowledge for prediction of marine heatwaves and their ecological impacts. This Research Topic is motivated by the need to understand the mechanisms for how marine heatwaves develop and the biological responses to thermal stress events. This Research Topic is a collection of 18 research articles and three review articles aimed at advancing our knowledge of marine heatwaves within four themes. These themes include methods for detecting marine heatwaves, understanding their physical mechanisms, seasonal forecasting and climate projections, and ecological impacts.

Description: We thank the contributing authors, reviewers, and the editorial staff at Frontiers in Marine Science for their support in producing this issue. We thank the Marine Heatwaves Working Group (http://www.marineheatwaves.org/) for inspiration and discussions. This special issue stemmed from the session on Advances in Understanding Marine Heat Waves and Their Impacts at the 2018 Ocean Sciences meeting (Portland, USA).

Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Sen Gupta, A., Thomsen, M., Benthuysen, J. A., Hobday, A. J., Oliver, E., Alexander, L. V., Burrows, M. T., Donat, M. G., Feng, M., Holbrook, N. J., Perkins-Kirkpatrick, S., Moore, P. J., Rodrigues, R. R., Scannell, H. A., Taschetto, A. S., Ummenhofer, C. C., Wernberg, T., & Smale, D. A. Drivers and impacts of the most extreme marine heatwaves events. Scientific Reports, 10(1), (2020): 19359. doi:10.1038/s41598-020-75445-3.

Description: Prolonged high-temperature extreme events in the ocean, marine heatwaves, can have severe and long-lasting impacts on marine ecosystems, fisheries and associated services. This study applies a marine heatwave framework to analyse a global sea surface temperature product and identify the most extreme events, based on their intensity, duration and spatial extent. Many of these events have yet to be described in terms of their physical attributes, generation mechanisms, or ecological impacts. Our synthesis identifies commonalities between marine heatwave characteristics and seasonality, links to the El Niño-Southern Oscillation, triggering processes and impacts on ocean productivity. The most intense events preferentially occur in summer, when climatological oceanic mixed layers are shallow and winds are weak, but at a time preceding climatological maximum sea surface temperatures. Most subtropical extreme marine heatwaves were triggered by persistent atmospheric high-pressure systems and anomalously weak wind speeds, associated with increased insolation, and reduced ocean heat losses. Furthermore, the most extreme events tended to coincide with reduced chlorophyll-a concentration at low and mid-latitudes. Understanding the importance of the oceanic background state, local and remote drivers and the ocean productivity response from past events are critical steps toward improving predictions of future marine heatwaves and their impacts.

Description: Concepts and analyses were developed during three workshops organized by an international working group on marine heatwaves (https://www.marineheatwaves.org) funded by a University of Western Australia Research Collaboration Award and a Natural Environment Research Council (UK) International Opportunity Fund (NE/N00678X/1). D.A.S. is supported by a UKRI Future Leaders Fellowship (MR/S032827/1). The Australian Research Council supported T.W. (FT110100174 and DP170100023) and A.S.T. (FT160100495). N.J.H. and L.V.A. are supported by the ARC Centre of Excellence for Climate Extremes (CE170100023). M.S.T was supported by the Brian Mason Trust. P.J.M. is supported by a Marie Curie Career Integration Grant (PCIG10-GA-2011–303685) and a Natural Environment Research Council (UK) Grant (NE/J024082/1). E.C.J.O. was supported by National Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant RGPIN-2018-05255 and Marine Environmental Observation, Prediction and Response Network (MEOPAR) project 1-02-02-059.1. C.C.U. acknowledges financial support through the Early Career Scientist Endowed Fund, George E. Thibault Early Career Scientist Fund, and The Joint Initiative Awards Fund from the Andrew W. Mellon Foundation at WHOI. M.G.D. received funding by the Spanish Ministry for the Economy, Industry and Competitiveness Ramón y Cajal 2017 grant reference RYC-2017-22964. NOAA High Resolution SST data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at https://www.esrl.noaa.gov/psd/.

Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Schlegel, R. W., Oliver, E. C. J., & Chen, K. Drivers of marine heatwaves in the Northwest Atlantic: the role of air-sea interaction during onset and decline. Frontiers in Marine Science, 8, (2021): 627970, https://doi.org/10.3389/fmars.2021.627970

Description: Marine heatwaves (MHWs) are increasing in duration and intensity at a global scale and are projected to continue to increase due to the anthropogenic warming of the climate. Because MHWs may have drastic impacts on fisheries and other marine goods and services, there is a growing interest in understanding the predictability and developing practical predictions of these events. A necessary step toward prediction is to develop a better understanding of the drivers and processes responsible for the development of MHWs. Prior research has shown that air–sea heat flux and ocean advection across sharp thermal gradients are common physical processes governing these anomalous events. In this study we apply various statistical analyses and employ the self-organizing map (SOM) technique to determine specifically which of the many candidate physical processes, informed by a theoretical mixed-layer heat budget, have the most pronounced effect on the onset and/or decline of MHWs on the Northwest Atlantic continental shelf. It was found that latent heat flux is the most common driver of the onset of MHWs. Mixed layer depth (MLD) also strongly modulates the onset of MHWs. During the decay of MHWs, atmospheric forcing does not explain the evolution of the MHWs well, suggesting that oceanic processes are important in the decay of MHWs. The SOM analysis revealed three primary synoptic scale patterns during MHWs: low-pressure cyclonic Autumn-Winter systems, high-pressure anti-cyclonic Spring-Summer blocking, and mild but long-lasting Summer blocking. Our results show that nearly half of past MHWs on the Northwest Atlantic shelf are initiated by positive heat flux anomaly into the ocean, but less than one fifth of MHWs decay due to this process, suggesting that oceanic processes, e.g., advection and mixing are the primary driver for the decay of most MHWs.

Description: RS was supported by the Ocean Frontier Institute International Postdoctoral Fellowship hosted jointly by Dalhousie University and Woods Hole Oceanographic Institution, through an award from the Canada First Research Excellence Fund. EO was funded through the National Sciences and Engineering Research Council of Canada Discovery Grant RGPIN-2018-05255 and Marine Environmental Observation, Prediction, and Response Network Early Career Faculty Grant 1-02-02-059.1. KC was supported by National Oceanic and Atmospheric Administration Climate Program Office Modeling, Analysis, Predictions, and Projections (MAPP) program under grant NA19OAR4320074 and Climate Variability and Predictability (CVP) program under grant NA20OAR4310398.

Description: Deep convection and associated deep water formation are key processes for climate variability, since they impact the oceanic uptake of heat and trace gases and alter the structure and strength of the global overturning circulation. For long, deep convection in the subpolar North Atlantic was thought to be confined to the central Labrador Sea in the western subpolar gyre (SPG). However, there is increasing observational evidence that deep convection also has occurred in the eastern SPG south of Cape Farewell and in the Irminger Sea, in particular, in 2015–2018. Here we assess this recent event in the context of the temporal evolution of spatial deep convection patterns in the SPG since the mid-twentieth century, using realistic eddy-rich ocean model simulations. These reveal a large interannual variability with changing contributions of the eastern SPG to the total deep convection volume. Notably, in the late 1980s to early 1990s, the period with highest deep convection intensity in the Labrador Sea related to a persistent positive phase of the North Atlantic Oscillation, the relative contribution of the eastern SPG was small. In contrast, in 2015–2018, deep convection occurred with an unprecedented large relative contribution of the eastern SPG. This is partly linked to a smaller north-westward extent of deep convection in the Labrador Sea compared to previous periods of intensified deep convection, and may be a first fingerprint of freshening trends in the Labrador Sea potentially associated with enhanced Greenland melting and the oceanic advection of the 2012–2016 eastern North Atlantic fresh anomaly.

Description: Highlights • A large body of literature exists on extreme cold ocean temperature events. • These events have well documented negative impacts but no consistent definition. • We propose here a definition for use in the detection of marine cold-spells (MCSs). • MCSs are decreasing in count, duration, and intensity over most ocean surfaces. • MCSs are increasing in count and duration in the Southern Ocean. Characterising ocean temperature variability and extremes is fundamental for understanding the thermal bounds in which marine ecosystems have adapted. While there is growing evidence of how marine heatwaves threaten marine ecosystems, prolonged periods of extremely cold ocean temperatures, marine cold-spells, have received less global attention. We synthesize the literature on cold ocean temperature extremes and their ecological impacts and physical mechanisms. Ecological impacts of these events were observed across a range of species and biophysical processes, including mass mortalities, range shifts, marine habitat loss, and altered phenology. The development of marine cold-spells is often due to wind-induced ocean processes, but a range of physical mechanisms are documented in the literature. Given the need for consistent comparison of marine cold-spells, we develop a definition for detecting these events from temperature time series and for classifying them into four categories. This definition is used to consistently detect marine cold-spells globally over the satellite record and to compare the characteristics of notable cold events. Globally, marine cold-spells’ occurrence, duration, and intensity are decreasing, with some areas, such as the Southern Ocean, showing signs of increase over the past 15 years. All marine cold-spell categories are affected by these decreases, with the exception of “IV Extreme” events, which were so rare that there has been little decrease. While decreasing occurrences of marine cold-spells could be viewed as providing a beneficial reduction in cold stress for marine ecosystems, fewer cold spells will alter the temperature regime that marine ecosystems experience and could have important consequences on ecological structure and function.

Description: Distinct epigenetic landscapes underlie the pathobiology of pancreatic cancer subtypes Distinct epigenetic landscapes underlie the pathobiology of pancreatic cancer subtypes, Published online: 17 May 2018; doi:10.1038/s41467-018-04383-6 Pancreatic ductal adenocarcinoma (PDAC) is a complex disease and its underlying epigenomic heterogeneity is not fully understood. Here, the authors utilize patient-derived PDAC xenografts to define the epigenomic landscape of PDAC, highlighting chromatin states linked to differing disease aggressiveness and survival.