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

To understand isobaric float measurements of vertical velocity during ocean ic deep convective events, we compare experimental results obtained in the Greenland Sea with time series from an ensemble of modeled isobaric drifter s embedded in a Large Eddy Simulation (LES), carried our with realistic ini tial conditions and time-dependent surface forcing. Both real and simulated drifter time series show that in general the drifters observe a mean verti cal velocity that may be upwards or downwards, depending on the vertical po sition of the float within the mixed layer. Over the course of the mixing p hase, the net apparent vertical displacement of water may be on the order o f several kilometers. The significant mean vertical velocity measured with these drifters is due to their tendency to preferentially sample zones of h orizontal 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. (C) 2001 Elsevier Science B.V. All r ights reserved.