An Atlantic wide assessment of marine heatwaves beyond the surface in an eddy-rich ocean model

Marine heatwaves (MHWs) were shown to have devastating impacts on marine ecosystems and to influence the atmospheric circulation changing inland temperature and precipitation. While various studies utilise model and observation based datasets to detect MHWs at the surface, little is known about the characteristics and drivers of MHWs at depth. Detecting MHWs requires continuous daily temperature records over a multi-year time period, which are only scarcely available from observations, in particular in the deep ocean. Although models provide such a temporally and spatially coherent dataset, a basin-wide detection of MHWs remains challenging due to the large number of grid points, at least in realistic high-resolution models. Additionally, model biases and unrealistic model trends (‘drift’) need to be taken into consideration.

In order to fill this knowledge gap, we investigate the impact of horizontal model resolution, the choice of the temperature baseline and the impact of spurious model trends on the characteristics of MHWs at various depths. We detect MHWs over the course of more than 40 years at all three-dimensional grid points of an eddy-rich (1/20°) ocean model, covering the entire Atlantic Ocean from 34.5°S to approximately 65°N.

Our results highlight the importance of horizontal and vertical heat transport variations within the ocean on sub-surface, but also on near-surface, MHWs. The surface heat flux is important in the mixed layer, but does not affect MHWs beyond approximately the top 100 m of the ocean. As a consequence, the temporal evolution of MHWs at depth is dominated by spurious temperature trends, if this is not adequately considered by using an extensive model spin-up, or by applying a linear temperature baseline. Independent of the baseline used, we find that ocean dynamics lead to different characteristics of MHWs along the western boundary, interior and eastern boundary of the Atlantic. Furthermore, we find sub-surface MHWs to be coherent over layers of a few 100 to 1000 m thickness. These layers are closely related to the vertical structure of the temperature field.