Marine Heatwaves
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Annual Review of Marine Science Marine Heatwaves Eric C.J. Oliver,1 Jessica A. Benthuysen,2 Sofia Darmaraki,1 Markus G. Donat,3 Alistair J. Hobday,4 Neil J. Holbrook,5,6 Robert W. Schlegel,1,7 and Alex Sen Gupta8,9 1Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada; email: [email protected] 2Australian Institute of Marine Science, Crawley, Western Australia 6009, Australia 3Barcelona Supercomputing Center, 08034 Barcelona, Spain 4CSIRO Oceans and Atmosphere, Hobart, Tasmania 7000, Australia 5Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania 7001, Australia 6Australian Research Council Centre of Excellence for Climate Extremes, University of Tasmania, Hobart, Tasmania 7001, Australia 7Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 8Climate Change Research Centre, The University of New South Wales, Sydney, New South Wales 2052, Australia 9Australian Research Council Centre of Excellence for Climate Extremes, The University of New South Wales, Sydney, New South Wales 2052, Australia Annu. Rev. Mar. Sci. 2021. 13:313–42 Keywords First published as a Review in Advance on climate extremes, sea surface temperature, climate change, mixed-layer September 25, 2020 dynamics, ocean circulation, air–sea heat flux, climate variability The Annual Review of Marine Science is online at marine.annualreviews.org Abstract Annu. Rev. Mar. Sci. 2021.13:313-342. Downloaded from www.annualreviews.org https://doi.org/10.1146/annurev-marine-032720- Ocean temperature variability is a fundamental component of the Earth’s Access provided by Dartmouth College - Main Library on 01/11/21. For personal use only. 095144 climate system, and extremes in this variability affect the health of marine Copyright © 2021 by Annual Reviews. ecosystems around the world. The study of marine heatwaves has emerged All rights reserved as a rapidly growing field of research, given notable extreme warm-water events that have occurred against a background trend of global ocean warm- ing. This review summarizes the latest physical and statistical understand- ing of marine heatwaves based on how they are identified, defined, charac- terized, and monitored through remotely sensed and in situ data sets. We describe the physical mechanisms that cause marine heatwaves, along with their global distribution, variability,and trends. Finally,we discuss current is- sues in this developing research area, including considerations related to the 313 choice of climatological baseline periods in defining extremes and how to communicate findings in the context of societal needs. 1. INTRODUCTION Marine heatwaves (MHWs)—a term first used by Pearce et al. (2011)—are anomalous warm sea- water events that can substantially affect marine ecosystems. An MHW is qualitatively defined as a discrete period of prolonged anomalously warm water at a particular location, and quantitative definitions are based on ocean temperatures exceeding a fixed (Frölicher et al. 2018), seasonally Marine heatwave (MHW): a discrete varying (Hobday et al. 2016), or cumulative (Eakin et al. 2010) threshold. MHWs are caused by period of prolonged a combination of local oceanic and atmospheric processes, including air–sea heat flux and hori- anomalously warm zontal temperature advection, and may be modulated by large-scale climate variability, including water at a particular remote sources via teleconnections (Holbrook et al. 2019). MHWs have been identified primar- location ily using ocean surface temperatures, although they may also extend below the surface (Schaeffer Threshold: & Roughan 2017). The global count of MHW days per year has risen over the historical record a temperature value, due to increases in MHW duration and frequency (Oliver et al. 2018a). This trend is projected to which may be fixed or increase further under climate change (Frölicher et al. 2018, Oliver et al. 2019) as a consequence vary over time and/or space, above which of long-term ocean warming. temperatures are The study of MHWs as discrete events is motivated largely by their ecological and socioe- considered conomic impacts. The study of sea surface temperature (SST) variability has a long history in anomalously warm in physical oceanography and climate science (Philander 1983, Deser et al. 2010), and understand- defining MHWs ing the effects of warm ocean temperature extremes on marine ecosystems is vitally important Air–sea heat flux: (Garrabou et al. 2009, Mills et al. 2013, Wernberg et al. 2013, Smale et al. 2019, Benthuysen et al. turbulent (sensible and 2020). Specifically, studies of MHWs are set apart from studies of SST variability more broadly latent) and radiative in that they examine the aspects of SST variability that affect marine ecosystems, rather than just (longwave and shortwave) exchanges the physical and climatic properties of ocean temperature variability. of heat between the Here, we critically review our current understanding of MHWs from a physical and climato- ocean and atmosphere logical perspective. We review the definitions of MHWs, data sets useful for monitoring them, Temperature the physical processes causing MHWs, and their statistical properties. We examine the global advection: a local distribution of MHWs, including characteristics of their variability and trends, and the role of an- temperature change thropogenic change. The field of MHWs has grown as events continue to occur that have broad due to the action of impacts across marine ecosystems and socioeconomic systems, raising issues of how best to com- oceancurrentsinthe municate about MHWs and the most appropriate way to define MHWs in a warming ocean. presence of a spatial gradient in Annu. Rev. Mar. Sci. 2021.13:313-342. Downloaded from www.annualreviews.org temperature, which 2. A BRIEF HISTORY OF RECENT HIGH-IMPACT EVENTS Access provided by Dartmouth College - Main Library on 01/11/21. For personal use only. may act in the horizontal (e.g., While anomalously warm seawater events are not a new phenomenon, they have occurred with in- geostrophic boundary creasing frequency and duration over the past century (Oliver et al. 2018a) and have had significant currents, wind-driven Ekman flows) or the impacts on marine ecosystems. The global-scale coral bleaching events of 1998, 2010, and most vertical (e.g., up- recently 2014–2017 occurred with El Niño conditions and globally elevated ocean temperatures welling/downwelling) (Eakin et al. 2019). The increased occurrence of extreme warm-water events across the globe has MHW duration: focused efforts toward examining events within a common framework. Over the past two decades, the time from the start analyses of prominent MHW events (Figure 1) have revealed that they are often caused by com- date to the end date of pounding influences of atmospheric forcing and/or oceanic processes. Furthermore, the phaseof an MHW climate modes of variability can act to increase or decrease the likelihood of MHW occurrence regionally (Holbrook et al. 2019). Here, we highlight notable events that have been attributed to local changes in air–sea heat fluxes, ocean heat transport, or remote forcing via teleconnections. 314 Oliver et al. a 5.0 MMed.ed. AL 22003003 2016 NWA 2012 4.0 ECS NEP SST anomaly (°C) 2016 2015 3.0 2.5 NNAA 22016016 Peru 2017 2.0 Beng. WA 2001 2011 TTas.as. 22017017 WSA 1.5 2014 1.0 4.5 b Med. 2003 200 km2 WA 2011 1,000 km2 Extreme 4.0 Med. 2006 5,000 km2 ECS 2016 NWA 2012 3.5 Peru 2017 Severe 3.0 Tas. 2017 ECS WA 2012 2018 Tas. 2015 AL 2016 2.5 WSA 2014 Beng. 2001 Strong 2.0 WA 2013 Maximum MHW intensity (°C) 1.5 NEP 2015 Moderate NA 2016 1.0 0 100 200 300 400 500 MHW duration (days) Figure 1 Annu. Rev. Mar. Sci. 2021.13:313-342. Downloaded from www.annualreviews.org Key historical MHWs. (a) SST anomalies on the day of peak MHW intensity. MHW intensity was defined based on the time series of Access provided by Dartmouth College - Main Library on 01/11/21. For personal use only. SST averaged over the regions indicated by the black boxes. Light gray indicates areas of sea ice influence. (b) MHW properties for key historical events. The MHW intensity (y axis), MHW duration (x axis), and category (color; see Hobday et al. 2018a) were determined from the spatially averaged time series, as in panel a.TheMHWarea(circle size) is the total contiguous area reaching at least category 2 (strong). All events shown in panel b are referenced in Section 2. Abbreviations: AL, Gulf of Alaska and Bering Sea; Beng., Benguela; ECS, East China Sea; Med., Mediterranean; NA, northern Australia; MHW, marine heatwave; NEP, northeast Pacific; NWA, northwest Atlantic; SST, sea surface temperature; Tas., Tasman Sea; WA, Western Australia; WSA, western South Atlantic. Panel a inspired by a schematic from Frölicher & Laufkötter (2018). Record-breaking MHWs have been documented where the atmospheric state played a central role in their development and maintenance. Examples of such events include those in the Mediter- ranean Sea during the summers of 2003 (Olita et al. 2007) and 2006 (Bensoussan et al. 2010), off the northeast coast of the United States in 2012 (Chen et al. 2014, 2015), from the southeast tropi- cal Indian Ocean to the Coral Sea in 2015–2016 (Benthuysen et al. 2018), in the East China Sea in www.annualreviews.org • Marine Heatwaves 315 2016 (Tan & Cai 2018), along the southwest Atlantic shelf in 2017 (Manta et al. 2018), off coastal Peru in 2017 (Echevin et al. 2018), and in the TasmanSea in 2017–2018 (Perkins-Kirkpatrick et al. 2019, Salinger et al. 2019). In these cases, the anomalously warm ocean temperatures were related Climate modes of variability: recurrent to abnormally high air–sea heat fluxes into the ocean. spatial patterns of Other MHWs have been found to be caused primarily by anomalous ocean heat transport, specific climate such as the MHWs in the Angola Benguela Upwelling System in 2001 (Rouault et al.