Online Marine & coastal projections 6.5 Changes in circulation
The mean surface circulation (Figure 6.11) shows substantial variations in the open ocean regions of the model domain between the recent past (RCM-P) and future scenario (RCM-F) model experiments. The circulation on the shelf is, in comparison, much less changed between these two model experiments. The largest effect is in the slope-shelf region where the slope-current to the west of Scotland (Souza et al. 2001) reduces substantially, whereas the portion of this current north of Scotland accelerates. There is also a marked reduction (by ~20%) in flow of water from East Anglia, along the continental coast to the German Bight. Across the majority of the shelf, however, there is little qualitative change in the circulation pattern. This is primarily dictated by the shape of the basin/topography, the locations of tidal mixing fronts and coastal currents.
To quantify the seasonal changes in the circulation we examine the transports across the sections shown in Figure 6.1. Figure 6.12 shows the substantial reduction in the slope-current west of Scotland (noted above) and the shelf-edge current west of Ireland. This is accompanied by an increase in the slope current in the Faeroe-Shetland Channel. Comparing the fluxes produced by RCM-F and RCM-P on the shelf, the Fair Isle Channel current closely matches the Hebrides shelf-edge current and both show little variation, whereas the flows into the North Sea east of Shetland do show a reduction in spring and summer. These currents flow into the Dooley current and on into the Skaggerrak, but there is little variation in either of these currents. This suggests that there is little large scale change in the transport of water into the North Sea, despite the strong variations in the slope current, and that the effects of changes in across-shelf-edge transport are limited to the shelf-edge regions. This supports the supposition that the northwest European shelf is isolated from the details of the circulation occurring in the northeast Atlantic. The changes in the horizontal transport component of the heat flux (Figure 6.7) seen in the central North Sea would therefore relate to local changes in circulation.
There is some evidence that the increased summer stratification causes increased density driven circulation at tidal mixing fronts. This circulation is in the form of sub-surface jets located above the bottom front. The effect here is seen as a weak increase in the transport during the summer north of Dogger Bank and into the St. George’s Channel.