Stratification

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Stratification
What is already happening?
  • Thermal stratification of the UK shelf seas continues to start slightly earlier in the year, but there are no long-term trends in the strengthening of stratification. 
     
  • Stratification in coastal regions influenced by freshwater inputs still show no discernible long-term trends against the background of natural variability. 
CONFIDENCE LEVEL
MEDIUM

Medium evidence, medium agreement

There is good understanding of the basic controls on stratification caused by surface heating, but there is a lack of long-term data to capture the onset/breakdown, strength, and the relative contributions of heating and salinity. Coastal stratification caused by freshwater inputs is particularly difficult to assess because of a lack of consistent long-term observations.

What could happen in the future?
  • Projections suggest that thermal stratification in UK shelf seas by 2100 will start earlier, typically by one week, and end 5–10 days later, due to changes in air temperature. 
     
  • Changes in the seasonal heating cycle are projected to increase the strength of stratification in UK shelf seas. 
     
  • Projected changes to shelf-sea stratification may lead to less upward mixing of nutrients and possible reductions in primary productivity and potential increases in eutrophication. 
CONFIDENCE LEVEL
LOW

Low evidence, low agreement

There is broad confidence in predicting changes over the open-shelf seas where surface heating/cooling is the dominant control. There are some uncertainties locally close to the shelf edge and in regions influenced by estuaries; more confidence is also required in how changes in salinity, both in the shelf interior and in the open ocean, will contribute to future stratification.

Key Challenges and Emerging Issues
  • Increasing coastal observing data to assess stratification trends. 
     
  • Addressing model limitations in simulating shelf edge processes and salinity. 
     
  • Addressing model limitations in simulating river inflows and intermittent thermal stratification near the coast. 
     
  • Conducting more research into the key role stratification strength and duration plays in bottom water dissolved oxygen concentration. 
     
  • Addressing the fact that shelf sea biology and physics are very sensitive to water mixing across the pycnocline, but this mixing is poorly resolved in models. 
     
  • Exploring the role of rainfall and horizontal changes in salinity across the shelf sea in triggering spring stratification, which has major implications for the timing of stratification.
     
  • Improving modelling of regional changes in rainfall and winds this century, and their impacts on stratification.