The north Atlantic subpolar gyre (SPG) has been widely implicated as the source of large-scale changes in the subpolar marine environment. However, inconsistencies between indices of SPG-strength have raised questions about the active role SPG-strength and size play in determining water properties in the eastern subpolar North Atlantic (ENA). Here, by analyzing various SPG indices derived from observations and a global coupled model, we show that the choice of the SPG index dictates the interpretation of SPG strength-salinity relationship in the ENA. Variability in geostrophic currents derived from observed hydrography and model based Lagrangian trajectories reveal zonal shifts of advective pathways in the enA and meridional shifts in the western intergyre region. Such shifts in advective pathways are manifestations of variability in the size and strength of the SPG, and they impact salinity by modulating the proportion of subpolar and subtropical waters reaching the enA. SpG indices based on subsurface density and principal component analysis of sea surface height variability capture these shifts in advective pathways, and are therefore best suited to describe SPG-salinity relationship in the enA. our results establish the dynamical constraints on the choice of the SpG index and emphasize that SPG indices should be cautiously interpreted.

While the influence of the subpolar gyre (SPG) on thermohaline variability in the eastern North Atlantic is well documented, the extent and timescale of the influence of the SPG on North Sea is not well understood. This is primarily because earlier investigations on the causes of variability in the North Sea water properties mostly focused on the role of atmosphere and deployed regional models. Here using a historical simulation with the Max Planck Institute Earth System Model (MPI‐ESM), we investigate circulation and water mass variability in key regions, namely, the Rockall Trough and the Faroe‐Scotland Channel, which link the North Atlantic to the North Sea. We find that salinity covaries with advective lags in these three regions and that the northern North Sea salinity follows the Rockall Trough with a lag of 1 year. We show that recurring and persistent excursions of salinity anomalies into the northern North Sea are related to the SPG strength and not to the local acceleration of the inflow. Furthermore, we illustrate that the SPG signal is more pronounced in salinity than in temperature and that this simulated SPG signal has a period of 30–40 years. Overall, our study suggests that, at low frequency, water mass variability originating in the North Atlantic dominates changes in the North Sea water properties over those due to local wind‐driven volume transport.

Reliable information about the future state of the ocean and fish stocks is necessary for informed decision-making by fisheries scientists, managers and the industry. However, decadal regional ocean climate and fish stock predictions have until now had low forecast skill. Here, we provide skilful forecasts of the biomass of cod stocks in the North and Barents Seas a decade in advance. We develop a unified dynamical-statistical prediction system wherein statistical models link future stock biomass to dynamical predictions of sea surface temperature, while also considering different fishing mortalities. Our retrospective forecasts provide estimates of past performance of our models and they suggest differences in the source of prediction skill between the two cod stocks. We forecast the continuation of unfavorable oceanic conditions for the North Sea cod in the coming decade, which would inhibit its recovery at present fishing levels, and a decrease in Northeast Arctic cod stock compared to the recent high levels.