Lifting the lid on Marine Heatwaves

Life is ubiquitous throughout the ocean, with species abundance and richness often greatest below the surface. As a result, ocean extremes throughout the water column may impact resident marine organisms and ecosystems. However, ocean extremes, such as marine heatwaves, have been commonly described based on surface observations. Given the importance of subsurface ocean processes, such as nutrient recycling, (de)oxygenation, and carbon transport, there has been an increasing focus on subsurface marine heatwaves (MHWs). Subsurface MHWs are prolonged warm ocean temperature extremes, and have a diversity of vertical structures linked with different driving mechanisms. Warming may be confined to the surface mixed layer; it may extend much deeper, potentially affecting the entire water column; it may appear only below the surface, with no surface signature, or it may be isolated near to or connected with the seafloor. Based on existing literature and a new analysis of subsurface MHW structure, we propose a comprehensive naming convention, differentiating between mixed layer, deep, thermocline, submerged and benthic marine heatwaves. Most surface-confined MHWs are associated with surface heat fluxes or shallow ocean advection or mixing. Conversely, many subsurface events are likely related to the vertical or horizontal displacement of temperature gradients/fronts, deep advection, and/or subduction of warm waters below the mixed layer. Different MHW vertical structures also have varying impacts on ocean biogeochemistry. However, due to the sparsity of physical, biogeochemical and biological observations, as well as the complexity of identifying and describing subsurface MHWs, there is limited understanding of the impact of subsurface MHW extremes. The nomenclature proposed in this paper seeks to provide a common language for understanding subsurface MHWs, thus enabling inter-disciplinary studies to quantify their impact. • Proposed a classification scheme for subsurface marine heatwaves. • Reviewed emerging studies on subsurface ocean temperature extremes. • Identified unique drivers differing from surface heatwave mechanisms. • Explored biological impacts and observational challenges of subsurface events.