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Online Marine & coastal projections 6.4 Changes in water column stratification

The density of shelf seas is well characterised by a quantity called the . This is here defined as the energy (per unit depth) required to completely mix the top 200 m of the water column. For convenience the potential energy anomaly is defined to be positive for stable stratification (essentially where denser cooler or saltier water lie beneath lighter fresher or warmer water). Hence, this represents the potential energy that must be added (usually from a loss of kinetic energy) to completely mix the water column. Comparing the seasonal mean potential energy anomaly (Figure 6.9) between the future time slice (RCM-F) and the recent past (RCM-P) shows the future forcing produces a substantial increase in stratification across the whole region, except in those areas that are permanently well mixed; and that this increase is largest in the autumn. The extent of the stratification (indicated on the shelf by the 10 Jm-3 contour) does not greatly change, but where stratification does occur, its strength increases under the future forcing. The increase in stratification is substantially larger (both in absolute terms and proportionately) in the deep ocean than the shelf regions. The general conclusion is, therefore, that tidally active shelf seas are unlikely to experience the strong increase in stratification expected in the deep ocean (e.g. the mean profiles from the global scale coupled PPE model used to adjusted the boundary conditions show a ~50% increase in potential energy anomaly from the past to the future time slice). Instead a weaker increase is likely, resulting from the increased expansivity as the water warms, increased precipitation, and changes in the horizontal heat transport. Figure 6.7 demonstrates that changes in the seasonal heat flux are unlikely to contribute greatly to this.

While by this measure the increase in the strength of stratification is projected to be weaker in shelf seas than the open ocean, these smaller changes may still have important implications; particularly when/where the stratification is weak. For example spring phytoplankton blooms can be triggered by very small increases in water column stability. To explore this further, we examine the changes in the timing of the onset of seasonal stratification* in the spring and of the breakdown in the autumn. Figure 6.10 shows the timing (in days from 1 January) of the onset of persistent seasonal stratification and its breakdown for RCM-P and RCM-F, and the total number of stratified days. It demonstrates that seasonal stratification occurs ~5 days earlier in the future scenario (typically the 5 April) than the recent past (typically 10 April) across the whole shelf, with larger changes in the shelf break region west of the Celtic Sea. This is analogous to spring coming earlier in terrestrial systems. An exception is the salinity stratified regions off the coast of continental Europe. Here, the onset of stratification is delayed by ~10 days. The breakdown of stratification in the autumn occurs about 10 days later (in the future time slice compared with the recent past) across much of the region (accounting for the stronger SST warming trend in autumn noted above). However, the pattern is patchier than the onset and there are a number of regions that show little variation. These include the central North Sea, the Celtic Sea and the sea east of Scotland. Hence, the overall increase in the duration of the stratified period is typically 15 days except in these regions where it is closer to 5 days.

* Here defined as a sustained surface to bottom density difference equivalent to -0.5˚C and a mixed layer of shallower than 50 m.

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