Keeping pace with marine heatwaves

Abstract

Marine heatwaves (MHWs) are prolonged extreme oceanic warm water events. They can have devastating impacts on marine ecosystems -for example, causing mass coral bleaching and substantial declines in kelp forests and seagrass meadows -with implications for the provision of ecological goods and services. Effective adaptation and mitigation efforts by marine managers can benefit from improved MHW predictions, which at present are inadequate. In this Perspective, we explore MHW predictability on shortterm, interannual to decadal, and centennial timescales, focusing on the physical processes that offer prediction. While there may be potential predictability of MHWs days to years in advance, accuracy will vary dramatically depending on the regions and drivers. Skilful MHW prediction has the potential to provide critical information and guidance for marine conservation, fisheries and aquaculture management. However, to develop effective prediction systems, better understanding is needed of the physical drivers, subsurface MHWs, and predictability limits. [H1] IntroductionProlonged extreme ocean warming events -also known as marine heatwaves (MHWs) -can severely impact marine ecosystems and the services they provide 1-6 . Yet despite their significance, dedicated and coordinated research only became prominent following the extreme event off Western Australia in 2011 7,8 . Indeed, it was during this event that the term 'marine heatwave' was first used to characterise an extensive, persistent and extreme ocean temperature event 9 (Box 1), spurring a new wave of research into their physical processes and corresponding impacts. Since 2011, MHWs have been observed and analysed both retrospectively and contemporaneously, and are now recognised to occur over various spatio-temporal scales. For example, given the ocean's heat capacity and dynamical scales, MHW events can persist for weeks to years [10][11][12][13][14][15][16] . They further vary in spatial extent and depth depending on the processes that cause and maintain them, as well as the geometry of the regions in which they occur. For instance, MHWs can be locally confined to individual bays 17 , around small islands or along short sections of coastline, or be broadly distributed over regional seas 10,18 , ocean basins 15,19 , or even spanning multiple oceans 20,21 (for a map of major MHW events, see Fig. 1). As well as the physical drivers, the ecological impacts of MHWs have also been studied in considerable depth. The effects include biodiversity loss and changes in species behaviour or performance 3,7 , loss of genetic diversity and adaptive capacity 22 , economic impacts from changes in fishery catch rates 1,[23][24][25] , and mortality or altered performance of farmed aquaculture species 13 .The impacts of MHWs are particularly evident on coral reefs (promoting widespread bleaching, including pan-tropical events 26 ), kelp forests (driving significant loss of kelp forest habitats off the coast of Western Australia, New Zealand, Mexico and the North Atlantic 7,27-30 ...

Drivers and impacts of the most extreme marine heatwave events

“…Moreover, we have only characterized the surface, not sub-surface, signature of these MHWs. Examination of subsurface processes remain a challenge 22 given the lack of observations available to constrain mixed-layer depths, heat advection and mixing processes. While ARGO data, for example, are becoming increasingly useful, the record is short (e.g.…”

“…trigger or modulate their likelihood 3,16,[19][20][21] , historical changes and trends in future projections e.g. 11,13 , and their predictability 19,22 . Quantifying MHW characteristics enables relationships to be developed between events and their impacts on marine species and ecosystems 1,7 .…”

A Global, Multiproduct Analysis of Coastal Marine Heatwaves: Distribution, Characteristics, and Long‐Term Trends

“…Subsurface MHWs are likely to occur in most places with or without surface corresponding surface expressions (Elzahaby & Schaeffer, 2019; Schaeffer & Roughan, 2017). However, the ability to assess subsurface MHWs globally is limited by the sparsity and lack of continuity of such observations (Holbrook et al., 2020). Global ocean circulation models can be an alternative in providing continuous subsurface observations.…”

Compound high temperature and low chlorophyll extremes in the ocean over the satellite period

“…CC BY 4.0 License. critical information for fisheries management and adaptation interventions to reduce risks and impacts on marine organisms and ecosystems during such events (Holbrook et al, 2020).…”