Modelling shallow mixed layers in the northeast Atlantic

A case study in winter at 40°N in the northeast Atlantic, using output from the Met Officeʹs Forecasting Ocean Assimilation Model (FOAM) system and a one-dimensional model initialised with Argo data, shows that the mixed layer (ML) can shallow from 100 to 10 m in a day because of the buoyancy input from rain. Rain induced shallow mixed layers have previously only been observed further south, and in the heating season. Their existence could be verified using the satellite surface salinity data that may soon be available. Z-coordinate models using bulk mixing schemes, such as FOAM, mix positive air–sea buoyancy fluxes over the upper grid box even when the wind stress is too small to achieve this. A second case study in spring demonstrates that having an upper box 10 m deep, as in FOAM, instead of a 2 m vertical resolution, causes an underestimate of the spring sea surface temperature (SST) by over 1 K. This problem can be alleviated by increasing the vertical resolution, or, as we suggest here, by occasionally adding a sublayer within the top model layer. When a sublayer was added to the 10 m model for the spring case study, the underestimate of the SST, mentioned above, was typically halved.