Natural variability in the physical conditions of the North Sea

On decadal to seasonal scales, many atmosphere–ocean studies suggest complex modes of variability based on internal ocean dynamics or atmosphere–ocean interactions. For example, the large-scale atmospheric North Atlantic Oscillation (NAO) has a strong influence on European weather conditions in winter and hence on the atmospheric forcing of the Northwest European Shelf (NWES). However, there are clear indications that NAO variations do not fully explain the observed variability in the strength of North Atlantic inflow into the North Sea and related water temperatures and salinities. 

In this study, leading seasonal anomaly patterns in the North Sea general circulation and water temperature are identified by means of Empirical Orthogonal Functions (EOF) analysis. The main objective behind this approach is to find an explanation for regional variabilities in the North Sea that are not induced by the NAO (e.g. in the English Channel and Southern Bight). The analysis is applied to model output from the global ocean model MPIOM forced with ERA40 reanalysis data. For this regional application, MPIOM has been run on a stretched grid configuration with non-diametrical poles located over Central Europe and North America (Fig. 1). This setup circumvents the need for prescribed open boundary conditions in the ocean and it provides a high horizontal grid resolution in the North Atlantic and NWES.

Fig. 1: Stretched grid configuration of the global ocean model MPIOM, providing a higher horizontal grid resolution in the North Atlantic and NWES. Similar grids have been used e.g. by Gröger et al. (2013) and Su et al. (2014)

The leading variability mode of the North Sea circulation in winter (Fig. 2) well reflects the NAO influence of westerly wind anomalies (Fig. 3) on vertically averaged current speed anomalies in the entire North Sea. The explained variance of the first EOF is 44% and the correlation of the first principal component (PC) and the NAO-Index amounts to 0.74. Along the Fair-Isle and Jutland Currents, maximum anomalies in mean vertically averaged current speed of up to ±10 cm/s give rise to a variation of the overall North Sea volume throughflow by about ±0.3 Sv. Nevertheless, the inflow of Atlantic water masses through the English Channel is hardly affected by the leading variability mode of the general circulation.

Fig. 2: First EOF of the North Sea general circulation in winter

Fig. 3: Anomalies in the wind field associated with the first EOF of the North Sea general circulation shown in Fig. 2

The second EOF of the Noth Sea circulation in winter (Fig. 4) is characterized by vertically averaged current speed anomalies along the Norwegian Trench and inversed anomalies in the southern North Sea and English Channel. The explained variance of the second EOF is 22%. The associated anomalies in the wind field (Fig. 5) indicate that such circulation anomalies are induced by variations in large-scale high and low pressure areas centered over the British Isles. The resulting north-south oriented wind anomalies in the Southern Bight and English Channel cause maximum local residual flow anomalies that can not be explained by NAO-induced westerly wind anomalies, giving rise to substantial temperature and salinity variations of up to 40% of the local variabilities.

Fig. 4: Second EOF of the North Sea general circulation in winter

Fig. 5: Anomalies in the wind field associated with the second EOF of the North Sea general circulation shown in Fig. 4

The second PC of the general circulation is also (weakly but) positively correlated with the NAO index, suggesting that positive and negative NAO phases may favor the formation of large-scale high and low pressure systems over the British Islands, respectively. At some years, pronounced NAO phases thus coincide with pronounced North Sea circulation anomalies of the second type (Fig. 4) rather than of the first type (Fig. 2) or of both types. The integrated effect of the different anomaly directions in the southern and northern North Sea finally lowers the correlation of the NAO with the circulation in the southern North Sea.

Similar analyses of the North Sea water temperature reveal that temperature anomalies in Atlantic water masses propagating onto the NWES locally weaken the NAO correlations of both depth-averaged water temperature and SST in the inflow regions of the northern and southern North Sea. Our model results suggest that the weaker NAO correlations of observational winter SST in these regions originate from (i) the circulation anomalies driven by large-scale atmospheric pressure areas centered over the British Isles (Fig. 4 and 5) and (ii) anomalies in air temperature and wind speed over the northeastern North Atlantic in the preceding autumn and their impact on the temperature of the upper ocean.

 

More details can be found in:

Mathis, M., A. Elizalde, U. Mikolajewicz and T. Pohlmann (2015). Variability patterns of the general circulation and sea water temperature in the North Sea. Progress in Oceanography 135, 91-112 (link)

 

References:

Gröger, M., E. Maier-Reimer, U. Mikolajewicz, A. Moll and D. Sein (2013). NW European shelf under climate warming: implications for open ocean-shelf exchange, primary production, and carbon absorption. Biogeosciences 10, 3767-3792 (link)

Su, J., D.V. Sein, M. Mathis, B. Mayer, K. O’Driscoll, X. Chen, U. Mikolajewicz and T. Pohlmann (2014). Assessment of a zoomed global model for the North Sea by comparison with a conventional nested regional model. Tellus A, 66 (23927) (link)