The influence of subpolar North Atlantic hydrographic conditions on the North Sea is well recognized, yet the precise pathways taken by Atlantic Water to reach its gateway remain uncertain. Using satellite-derived velocity fields, we map the open-ocean routes leading to the North Sea. Our Lagrangian analysis shows that the Rockall Trough serves as the primary route in a time-mean sense, with its dominance becoming particularly evident under anomalously cold subpolar conditions. During anomalously warm periods, however, Atlantic Water is preferentially routed through the Iceland Basin. Empirical orthogonal function analysis of the Lagrangian trajectories reveals a dipole mode of variability between the Rockall Trough and the Iceland Basin, with its first principal component explaining 74% (R = 0.86) of the variance in multi-year ocean heat content variability. These trajectories further demonstrate that this variability is closely linked to the north-south shifts of the North Atlantic Current. Such spatial shifts are likely driven by variations in northward ocean heat transport at the intergyre boundary, with the strength of the subpolar overturning circulation in preceding years potentially playing a critical role. This connection suggests that the conditions in the North Sea as well as the pathways Atlantic Water is advected along to reach it could be predictable several years in advance.

Marine heatwaves are autocorrelation events exceeding a given  threshold for a given minimum duration. Marine heatwaves are driven locally by heat fluxes through the surface of a given volume, for example incoming solar radiation at the ocean surface, and horizontal advection of warm water within. Local drivers are, however, the consequence of weather systems and super-regional phenomena like teleconnections. Marine heatwaves can further be closely tied to extreme temperatures, with far-reaching consequences for ecosystems as for example coral reefs. This thesis is concerned with drivers of marine heatwaves, from local to super-regional, and furthermore with the attribution of extreme temperatures as cause for the decline of specific ecosystems.

Marine extremes are recognized to cause severe ecosystem and socioeconomic impacts. However, in polar regions, such as the Barents Sea, the driving mechanisms of these extremes remain poorly understood and require careful consideration of the observed long-term ocean warming. Here we show that on short time scales of a few days, marine heatwaves and marine cold spells are dynamically driven by a dipole atmospheric circulation pattern between the Nordic Seas and the Barents Sea. Importantly, the dipole’s eastern component determines anomalies in shortwave radiation and latent heat fluxes. On interannual time scales, both changes in ocean heat supply and persistent atmospheric patterns can support severe marine extremes. We apply conventional marine heatwave detection methodology to OISSTv2 data, for the period of 1982–2021, and combine the analysis with ERA5 data to identify drivers. The ocean-atmosphere interplay across scales provides valuable information that can be integrated into fisheries and ecosystem management frameworks.

Diapycnal mixing impacts vertical transport rates of salt, heat, and other dissolved substances, essential for the overturning circulation and ecosystem functioning in marine systems. While most studies have focused on mixing induced by individual obstacles in tidal flows, we investigate the net effect of non-tidal flow over multiple small-scale (<1 km) bathymetric features penetrating a strongly-stratified density interface in a coastal region. We combine high-resolution broadband acoustic observations of turbulence microstructure with traditional shear microstructure profiling, to resolve the variability and intermittency of stratified turbulence related to the rough bathymetry. Scale analysis and acoustic imaging suggest that underlying mixing mechanisms are related to topographic wake eddies and breaking internal waves. Depth averaged dissipation rates (1.1 × 10⁻⁷ Wkg⁻¹) and turbulent vertical diffusivities (7 × 10⁻⁴ m²s⁻¹) in the halocline exceed reference values by two orders of magnitude. Our study emphasizes the importance of rough small-scale bathymetric features for the vertical transport of salt in coastal areas.