Presenter: Hendrik Großelindemann (GEOMAR Helmholtz Center for Ocean Research Kiel)

Description:
Marine Heatwaves (MHWs) are ocean extreme events, characterized by anomalously high temperatures, which can have drastic ecological impacts. The Northeast U.S. continental shelf is of great economical importance being home to a highly productive ecosystem. Local warming rates exceed the global average and the region experienced multiple MHWs in the last decade with severe consequences for regional fisheries. Due to the lack of subsurface observations, the depth-extent of MHWs is not well known, which however hampers assessing impacts on pelagic and benthic ecosystems. This study utilizes a global ocean circulation model with a high-resolution (1/20°) nest in the Atlantic to investigate the depth structure of MHWs and associated drivers on the Northeast U.S. continental shelf. It is shown that MHWs exhibit varying spatial extents, with some only appearing at depth. Highest intensities are found around 100m depth with temperatures exceeding the climatological mean by up to 7°C, while surface intensities are typically smaller around 3°C. Distinct vertical structures are associated with different spatial patterns and drivers. Investigation of the co-variability of temperature and salinity revealed that over 80% of MHWs at depth (>50m) coincide with extreme salinity anomalies. Two case studies provide insight into opposing MHW patterns at the surface and at depth, being forced by anomalous air-sea heatfluxes and Gulf Stream warm core ring interaction, respectively. The results highlight the relevance of subsurface MHWs, underlining the need of continuous subsurface measurements. It is shown that MHW formation can depend on local ocean dynamics. Better understanding of these is necessary to assess potential future changes in a changing climate. This study demonstrates the model's excellent skills to realistically simulate this highly dynamical region, though underlining the need of sufficient high-resolution.