Stratification

Why is it important?

Stratification is a term used to describe when two distinct layers occupy the vertical water column in the sea: the near-surface one is less dense than the near-bed one (see also Stratification / potential energy anomaly assessment). This can be due to differences in temperature (warm layer overlying a cooler layer), salinity (fresh water overlying saltier water), or both. The balance between inputs of fresh water and/or heat from the sun and mixing from tidal currents, wind, and waves determines whether or not the water column is stratified. The interface between the two layers is very efficient at limiting the exchange of water and its properties (such as nutrients).

What is already happening?

In large parts of Scottish shelf sea waters, stratification occurs on a seasonal basis: once the heat input from the sun is sufficiently strong, stratification develops in regions where the water depth is sufficiently deep or the tidal currents sufficiently weak (see also Stratification / potential energy anomaly assessment). In these regions, the strength of mixing is insufficient to keep the water column vertically homogenous.

In areas with significant river inputs (Clyde, Forth, Inverness, Beauly, Cromarty and Dornoch Firth), stratification due to river run-off (input of fresh water) can occur year-round. There is currently no evidence that the strength of stratification (the density difference between the two layers) in these regions is systematically changing, although it can change due to particular rainfall events.

Simulations using computer models suggest that there is already a trend towards earlier onset of seasonal stratification (Sharples, Holt & Wakelin, 2020). Research found this onset occurred 8 days earlier in the western Irish Sea in 1999 compared with 1960. In the north-western North Sea, Sharples, Ross, Scott, Greenstreet & Fraser (2006) found a weak trend in the earlier onset of stratification between 1991 and 2003 (1 day per year earlier). However, these trends are weak and natural variability in the timing of stratification is relatively large (standard deviation around the mean is ±7 days; Sharples et al., 2006). There are no clear trends in the strength of the stratification (both in seasonal warming or freshwater inputs; Sharples et al., 2020).

What is likely to happen in future?

Climate models predict that the onset of stratification in spring will occur about one week earlier by the end of the century (Sharples et al., 2020). Similarly, projections suggest that, by 2100, breakdown of stratification in autumn will occur 5 – 10 days later than at present. This pattern is based on comparing the present day situation (1961 - 1990) with the projected conditions in future (2070 - 2098) in a business as usual scenario (SRES A1B; Sharples et al., 2020). This pattern is due to the warmer air temperatures, although there is uncertainty as changes in wind patterns (especially the strength) may change stratification. Model projections suggest that the strength of stratification (the density difference between the two layers) will also intensify across the shelf sea regions (Sharples et al., 2020), which will reduce upward mixing of nutrients and therefore will have an impact on primary productivity of the marine ecosystem.

For the northern North Sea, changes in the strength of the exchange with the North Atlantic Ocean are also likely to occur, leading to possibly permanent stratification: due to salinity differences in winter and due to temperature differences in summer (Holt et al., 2018; Sharples et al., 2020).

Changes in stratification could have an impact on other properties, such as dissolved oxygen concentration due to longer periods of reduced air-sea exchange (see Climate change - Dissolved oxygen) and productivity from phytoplankton and lower trophic levels due to reduced upward mixing of nutrients from the deeper layer.

Uncertainties in future regional patterns of rainfall and winds cause uncertainty in the ability to predict future changes in stratification (Sharples et al., 2020). For regions where stratification is driven by freshwater inputs (such as many coastal areas), the lack of accurate rainfall predictions means no predictions of stratification change exist (Sharples et al., 2020). As rainfall events are likely to increase in number and intensity, future changes in the extent and frequency of stratification events are also likely.