Observations of vertical and horizontal aspects of deep convection in the Labrador Sea by fully Lagrangian floats.

摘要

This study focuses on data collected by twenty-four Lagrangian floats deployed during the Labrador Sea Deep Convection Experiment. These floats match the buoyancy and compressibility of seawater and hence follow water motions three dimensionally. These data provide direct observation of mixed layer depth, excellent estimates of vertical velocity, and an opportunity to observe the interaction of convection with horizontal variability.; Vertical aspects of convection are explored. Floats were repeatedly carried across the convecting layer (up to 1 km deep) by vertical velocities averaging several centimeters per second. Heat flux estimates reveal entrainment and surface fluxes similar in magnitude. The mixed layer acts as a vertical conveyor belt of temperature, transporting heat from depth to the surface without requiring a net change in mixed layer temperature since incorporation of salt from below increases density without requiring a net change in temperature. Comparison with NCEP re-analysis meteorological heat flux and wind magnitude data finds that the vertical velocity variance can be modeled with 80% skill as a linear function of lagged buoyancy flux (with the atmosphere leading the ocean by ∼1/2 day) without using the wind estimates. Mixed layer motions are clearly driven by the surface buoyancy flux, Bo. A non-rotating scaling of vertical velocity variance, (B_oH)1/3, provides a marginally better fit than a rotating scaling, (B_o/ f)1/2.; The meso- and submesoscale structure during active convection is explored. On the mesoscale (O 20 km), horizontal variability was nearly 1°C and 0.1 psu. An eddy of 40 km diameter was found. On a smaller scale (O 5 km), variability of .04 psu and .3°C was found. This smaller scale field was organized into both cyclonic and anticyclonic eddies of 1–12 km radius. These features were unsteady, with an anticyclone doubling in size in less than a week. There was exchange between eddies, producing horizontal diffusivities on the order of basin scale values. The influence of these structures on convection is explored: the deepest mixed layers were within an anticyclone and while the predicted slanting of plumes was small, several convective trajectories were found to be significantly non-vertical.