Online Marine & coastal projections 6.1.1 Mechanisms for climatic influence
Atmospheric, oceanic and riverine forcing all have a role in controlling the temperature, salinity and circulation of shelf seas. Atmospheric forcing is in the form of surface fluxes of heat, momentum and freshwater, and horizontal gradients in the atmospheric pressure. Since about 90% of the extra heat trapped in the climate system (by anthropogenic greenhouse gas emissions) is contained in the ocean (IPCC, 2007, Chapter 5), it is natural to expect significant temperature changes in seas over the next 100 years.
The spatial distribution of temperature on the northwest European continental shelf is to a large extent determined by the effects of vertical mixing processes on the water column, with horizontal heat transport playing a more minor role (this is quantified below). Features of particular importance in tidally active seas are the seasonal thermal stratification and the formation of tidal mixing fronts. Thermal stratification arises where tidal and wind generated turbulence is insufficiently energetic to vertically mix an otherwise stable column. Tidal mixing fronts occur at the boundaries of areas where this vertical mixing can occur. Where the water column remains stratified, the deeper part of the water column remains close to the winter temperature while the upper part of the column warms rapidly under summer heating conditions (Figure 6.2 shows an example of this). The stratification of the column breaks down in the late autumn with increased wind and convective mixing. Given that the whole water column is in rapid communication with the atmosphere for a large fraction of the year, simply increasing the air temperature will not necessarily lead to an increase in the summer time stratification (as might be expected for the open ocean where temperatures at large depth change on centennial rather than seasonal time scales). In the case of shallow seas the variation of stratification is dependent on changes in the relative surface heat flux between winter and summer. In addition, the non-linear dependence of the water density on temperature plays a role.
The salinity distribution of shelf seas is primarily determined by the balance between inputs of fresh river water and saline oceanic water. The surface forcing (precipitation and evaporation) plays a role in setting the overall salinity budget and large scale gradients. Changes in precipitation not only affect the salinity budget but also the salinity component of the vertical density structure. This can play a significant role in pre-conditioning the vertical density structure and influence the onset of thermal stratification. In contrast, river discharges can have a dramatic effect on near-coastal regions (up to ~20 km from the coast), introducing large variations of salinity over short time scales (hours to weeks). Hence, changes in runoff (from precipitation over land) would be expected to have a large effect on near coastal salinity, stratification and currents. On a larger scale the combined effect of river discharges is to generate coastal currents (e.g. the Norwegian Coastal Current), which can form a substantial part of the circulation of shelf seas.
The basin-scale oceans influence the coastal seas through the impingement of larger scale currents onto the shelf and a number of smaller scale processes that mediate the ocean-shelf exchange (Huthnance, 1995). Because of the tendency for large scale ocean currents to flow around steep topography, rather than crossing onto the continental shelf, the northwest European shelf can be seen as a quasi-isolated system on time scales of ~1 yr (Wakelin et al. 2009). This is particularly applicable to properties that are strongly constrained by surface forcing, most notably the temperature field. In this case, where the transit time across the shelf is slow compared with the seasonal cycle, temperature fluctuations arising from variability in the open ocean temperature are generally lost. In contrast, there is no direct feedback between the surface salinity and the atmosphere, so the salinity field is highly dependent on the exchange of water with the open ocean (Huthnance, 1997).
In many shelf seas, tides provide the most energetic process for transport and mixing. These are determined by well established astronomical forces and by the bathymetry and coast line (changes in these are not considered here). In open shelf seas, tidal conditions might be expected to change only very slowly and hence tidal mixing has the potential to limit some of the effects of varying atmospheric forcing. For example, the locations of tidal mixing fronts are largely insensitive to the details of the atmospheric forcing (Young and Holt, 2007) and are not expected to change greatly under future climate conditions.