Evolution of the Ocean Surface Boundary Layer during 2012-2013 as observed by the OSMOSIS Glider Array; a Challenge for Model Simulations

As part of the OSMOSIS project, a time series of profiling gliders was maintained at the Porcupine Abyssal Plain site in the northeast Atlantic from September 2012 to September 2013. Vertical resolution of 1 m and profiles every 2 hours allow us to characterize temporal variability throughout the upper 1000 m in temperature, salinity and dissolved oxygen concentration. Mixed/mixing layer depth is determined using a variety of thresholds and techniques; dissolved oxygen concentration identifies the active mixing layer well and tends to be a few meters shallower than the mixed layer depth determined from thresholds in physical parameters. During fall 2012, the mixed layer deepens gradually, but its depth does not vary much on time scales of hours to days. January-April 2013 is characterized by a deep but highly variable mixed layer depth, reaching 390 m in early February. In spring the mixed layer shoals rapidly, followed by periods of intermittent deepening and restratification in May and June. In summer the mixed layer is at its shallowest and least variable. The seasonality in the mixed layer temperature is consistent with the net surface heat flux, whereas the variability in mixed layer salinity is not consistent with the surface freshwater fluxes from the ERA-Interim data set.

The glider data set provides a challenge for verification of mixing and stratification processes in numerical models. We compare the observed variability and depth of the mixed layer with that simulated by various one-dimensional mixed layer models, including the new prognostic OSBL model developed during OSMOSIS that includes Langmuir turbulence. This demonstrates remarkable agreement in mixed layer depths between the simulations forced by the ERA-Interim fluxes and the glider observations.