Marine heatwaves (MHWs) are usually defined as anomalously warm events in the surface ocean that can extend over thousands of kilometers and last for weeks to months. The frequency and duration of global MHWs have increased substantially over the past few decades and are projected to increase further under continued global warming. Over the past decades, MHWs of record-breaking intensity or/and duration have been observed in the open ocean, marginal seas, and coastal regions (Figure 1a), with widespread and profound environmental, ecological and socio-economic impacts. The effects of MHWs are not limited to the ocean surface, but extend into the atmosphere and the deep ocean.
MHWs with an expression at the ocean surface have been widely studied, partly due to the availability of satellite-observed sea surface temperature (SST) data since 1981. Recent studies illustrated that the ocean warming associated with surface MHWs can penetrate into the deep ocean. Moreover, the physical drivers and ecological responses to MHWs with different penetration depths are quite different. Still, little is known about the vertical structure of MHWs in the global ocean, and the typical characteristics of the vertical structure of global MHWs remain to be explored. The T/S vertical profiles of Argo floats with a depth ranging from 0-2,000 dbar provide an opportunity to explore the vertical structure of MHWs in the global ocean.
We identify four main types of MHWs with different vertical structures using Argo profiles (Figure 1b-e): 1) shallow MHWs, where warming is confined to the surface layer and decreases rapidly with depth; 2) subsurface-reversed MHWs, which have warming at the surface and anomalous cooling beneath this surface warming; 3) subsurface-intensified MHWs, which exhibit maximum warming anomalies in the subsurface layers; 4) deep MHWs, which display surface warming anomalies that decay slowly with depth. These MHW types are characterized by different spatial distributions, with hotspots of subsurface-reversed and subsurface-intensified MHWs at low latitudes and shallow and deep MHWs at middle-high latitudes. Both shallow MHWs and subsurface-reversed MHWs tend to have shallow impact depths, while both subsurface-intensified MHWs and deep MHWs tend to have deep impact depths.
These four types of MHW have different spatial distributions and vertical depths, which may depend on the regionally dominant physical processes not only in the mixed layer but also in the thermocline. The multiscale ocean dynamical processes play an important role in shaping the vertical structure of MHWs, involving oceanic planetary waves, large-scale currents, eddies, and mixing.
The occurrence area of MHWs has increased significantly during the Argo era (2001-2020), with the greatest increase in the subsurface-intensified MHWs, and followed by subsurface-reversed MHWs. The long-term warming of the global ocean dominates the increasing trend of MHWs, whereas the changes in internal temperature variability play a secondary role. The occurrence area of MHWs has also shown clear interannual variability related to climate mode variabilities.
MHWs have now emerged as one of the major challenges to marine ecosystems and the sustainability of marine resources due to their negative impacts on many marine organisms and ecosystems. As the MHWs extend warming to the depths, the impacts on marine organisms and ecosystems are not limited to the surface ocean. This study provides a better understanding of the physical processes and climate drivers of MHWs, contributes to improved predictions of MHWs in a warmer ocean, and provides managers of fisheries, aquaculture, and conservation with forecasts to support mitigation strategies.