Deep Convection in the Labrador Sea as Observed by Lagrangian Floats

Abstract

During the winters of 1997 and 1998, a total of 24 Lagrangian floats were deployed in the Labrador Sea. These floats were designed to match the buoyancy and compressibility of seawater. They measured temperature and three-dimensional position (pressure for vertical position and RAFOS acoustic tracking for latitude and longitude) as they followed water motions three-dimensionally. This data provides direct observation of mixed layer depth and excellent estimates of vertical velocity. Floats were repeatedly carried across the convecting layer by vertical velocities averaging several centimeters per second with vertical excursions of up to one kilometer. In the horizontal, several scales of eddy motion were resolved, as was a possible float predilection toward remaining in water preconditioned for convection. Heat flux estimates from this data reveal entrainment and surface heat 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 allows an increase in density without a net change in temperature. Comparison with NCEP reanalysis meteorological heat flux and wind magnitude data shows 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 nonrotating scaling of vertical velocity variance, (BoH)1/3, provides a marginally better fit than a rotating scaling, (Bo/f)1/2. Horizontal effects appear to play only a weak role during strong convection but result in rapid restratification when convective forcing weakens.