Horizontal Patchiness in Mixing at the Thermocline

Oceans 2025 Theme 3, work package 3.2

Computer models of vertical turbulent mixing in shelf seas generally perform well in the bottom boundary layer and within mixed water columns. However, the models often perform poorly in response to surface wind and wave forcing, and when the water column is stratified. The example below shows the type of problems we are addressing, by comparing output of the POLCOMS numerical model with a Scanfish high-resolution section across the Celtic Sea:

POLCOMS numerical model with a Scanfish high-resolution section across the Celtic Sea:

Getting the position and structure of the thermocline correct in our models is important. Overall, poor thermocline simulations are symptomatic of general model failings that need to be addressed. In detail, getting the thermocline vertical position wrong has serious implications for modelling the sub-surface growth of phytoplankton (by affecting the light experienced). Also, inaccurate thermocline thicknesses will lead to poor simulations of fluxes between the thermocline and the surface and bottom mixed layers (e.g. resulting in incorrect nutrient, sediment, and carbon fluxes). The figure above suggests that processes that mix downward from the sea surface, homogenising the surface layer and producing a tighter thermocline, are missing from the models. We also have evidence that there is significant horizontal patchiness in the structure of the thermocline related to changes in the seabed. The figure below highlights this by showing a section of temperature and chlorophyll along a bank in the Celtic Sea. At the edges of the bank there is a clear increase in chlorophyll inside the thermocline, and in the bottom mixed layer. The pattern is not correctly simulated by our models, but it could be locally very important to the biological production at these seabed banks.

A section of temperature and chlorophyll along a bank in the Celtic Sea

Our work aims to address the issues described above with three components:

1. Impacts of surface waves and Langmuir circulation on the surface mixed layer. [Contact: Judith Wolf]
2. Seabed bathymetry and mixing at the thermocline. [Contact: Jonathan Sharples]
3. High resolution modelling of tidal flows over seabed banks. [Contact: Jiuxing Xing]