Following publication has been announced by our division System Analysis and Modelling. This publication is part of a series of papers dealing with the causes of the variability of the North Sea system and their possible predictability on longer time scales. For further information please contact Prof Dr Corinna Schrum, co-author of the publication:
Koul, V., Schrum, C., Düsterhus, A., & Baehr, J. (2019): Atlantic Inflow to the North Sea Modulated by the Subpolar Gyre in a Historical Simulation with MPI‐ESM. J. Geophys. Res. Oceans, 124, doi:10.1029/2018JC014738
Plain Language Summary:
Earlier investigations on the causes of variability in the North Sea water properties have mostly focused on the role of the atmosphere. This follows from the general idea that shallowness of the North Sea makes it more responsive to wind speed and direction than the deeper ocean. In the present contribution, we identify variability in North Sea water properties other than that induced by the atmosphere. For our analysis, we use a historical simulation with a global coupled model for two reasons: (a) in the model, the connections between North Atlantic and North Sea are represented, which would not be the case in a regional model, and (b) the model provides a long continuous time series of water properties, which is not available from spatially and temporally scarce observations. We find that the strength of Subpolar Gyre (SPG) has an impact on the salinity of the North Sea. When the SPG is weak (strong), water from the subtropical (subpolar) North Atlantic dominates the inflow into the North Sea and thus increasing (decreasing) the salinity. This modulation of inflow properties happens at decadal periods and beyond, and is largely independent of the amount of Atlantic water entering the North Sea.
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, we investigate circulation and water mass variability in key regions, namely Rockall Trough and Faroe‐Scotland‐Channel (FSC) which link the North Atlantic to the North Sea. We find that salinity co‐varies with advective lags in these three regions, and that the northern North Sea salinity follows the Rockall Trough with a lag of one 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.