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

Abstract. Periods of prolonged anomalously high temperatures in the ocean, known as marine heatwaves (MHWs), can have devastating effects on ecosystems. Although MHWs are extensively studied in the near-surface ocean, little is known about MHWs at depth. As continuous observations in space and time are very sparse away from the surface, basin-wide studies on MHWs at depth have to rely on models. This introduces additional challenges due to the long adjustment timescale of the deep ocean, resulting in long-term drift following the model's initialization. This unrealistic model drift dominates the MHW statistics below approximately 100 m when a fixed baseline is used. As a result, MHW studies at depth require a long model spin-up or have to apply a detrended baseline removing temperature trends. Based on a comparison of two model configurations with eddy-permitting and eddy-rich horizontal resolution, we show that the representation of mesoscale dynamics leads to pronounced differences in the characteristics of MHWs, in particular along the boundaries and along pathways of highly variable currents. Although a high horizontal resolution of the model is important, MHW statistics can be calculated on a coarser grid, which largely decreases the amount of data that needs to be processed. Our results highlight the importance of horizontal and vertical heat transport within the ocean to sub-surface, but also near-surface, MHWs. By investigating the vertical coherence of MHWs in an example region – here, the Cabo Verde archipelago – we show that MHWs are coherent over layers of a few hundred to 1000 m thickness, independent of the baseline used. These ranges are closely related to the vertical structure of the temperature field.