Interpretation of mean vertical velocity measured by isobaric floats during deep convective events

To understand isobaric float measurements of vertical velocity during oceanic deep convective events, we compare experimental results obtained in the Greenland Sea with time series from an ensemble of modeled isobaric drifters embedded in a Large Eddy Simulation (LES), carried out with realistic initial conditions and time-dependent surface forcing. Both real and simulated drifter time series show that in general the drifters observe a mean vertical velocity that may be upwards or downwards, depending on the vertical position of the float within the mixed layer. Over the course of the mixing phase, the net apparent vertical displacement of water may be on the order of several kilometers. The significant mean vertical velocity measured with these drifters is due to their tendency to preferentially sample zones of horizontal convergence. In the model, these zones are found to be associated with sinking plumes in the upper region of the mixed layer, or with rising warm return flow in the lower region.