Stratification

Low evidence, medium consensus

There is good agreement between the observation and modelling studies and good understanding of the basic controls of stratification caused by surface heating. However, long-term observational data is limited in scope and less is known about the roles of salinity and freshwater in coastal and shelf stratification. Models have less skill in describing salinity fields. There is a need for better long-term data on stratification, to capture the onset/breakdown, the strength, and the relative contributions of heating and salt/freshwater. Observations require both long-term sustained moored instrumentation and regular ship-based and autonomous vehicle sampling of key sites. Stratification caused by freshwater inputs (from estuaries and via rainfall) is particularly difficult to assess because of a lack of consistent long-term salinity observations.

Low evidence, low agreement

The UKCP09 predictions were the first attempt at regional-scale assessment of changes in the marine climate over the next century, and were extended using a model ensemble by Tinker et al (2016). There is broad confidence of the ability of the model to predict 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. The overall lack of confidence in the predictions arise from (1) these are the first predictions available, and (2) predictions of both the timing and strength of stratification are determined by changes in regional meteorology, which is a challenging aspect of future climate projections, and (3) stratification caused by salinity changes is much more difficult to model than that caused by surface heating/cooling.