Journal Article

Turbulent Vertical Velocities in Labrador Sea Convection

Merckelbach, L., Clément, L., Frajka‐Williams, E.

2024

Overview

Turbulent vertical velocity measurements are scarce in regions prone to convection such as the Labrador Sea, which hinders our understanding of deep convection dynamics. Vertical velocity, w$w$ , is retrieved from wintertime glider deployments in the convective region. From w$w$ , downward convective plumes of dense waters are identified. These plumes only cover a small fraction of the convective area. Throughout the convective area, the standard deviation of w$w$agrees with scaling relations for the atmospheric surface and boundary layers. It initially depends on surface buoyancy loss in winter, and later, on wind stress after mid‐March. Both periods are characterized by positive turbulent vertical buoyancy flux. During convective periods in winter, the positive buoyancy flux is mostly forced by surface heat loss. After mid‐March, when buoyancy loss to the atmosphere is reduced, the positive buoyancy flux results from a restratifying upward freshwater flux, potentially of lateral origins and without much atmospheric influence. Plain Language Summary Deep convection is an essential component of our climate system as it uptakes and redistributes atmospheric properties, such as anthropogenic carbon and oxygen, into the abyssal ocean. Intense ocean heat loss to the atmosphere in winter triggers convection, resulting in kilometer‐sized plumes with high downward vertical velocities and deep mixed layer depth. These plumes remain challenging to observe and parameterize in climate models. Here we show that autonomous vehicles (gliders) can sample dense downwelling plumes in the Labrador Sea. Gliders sampled a positive vertical buoyancy flux that depicts downwelling of dense water parcels and upwelling of light water parcels during convection, and that compensates a buoyancy loss from the ocean to the atmosphere. At the end of convection, an observed freshwater import produces a similar buoyancy flux unmatched by the surface flux. This flux adds buoyancy and shoals the mixed layer. Additional measurements from sufficiently long glider deployments like these ones could potentially allow us to establish a crucial link between deep water formation and an expected increase in freshwater fluxes from Arctic and Greenland sources. Key Points Vertical velocity during deep ocean convection follows scalings from the atmospheric boundary layer under wind and buoyancy forcing Vertical velocity help identify convective plumes with a horizontal scale of 620 m and a downward velocity magnitude up to 4.6 cm s−1${\\mathrm{s}}^{-1}$Positive vertical buoyancy flux occurs during convection, caused by atmospheric cooling and then by freshwater flux during restratification

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Abyssal zone / Anthropogenic factors / Atmosphere / Atmospheric convection / boundary layer scalings / Boundary layers / Buoyancy / Buoyancy flux / buoyancy fluxes / Climate / Climate models / Climate system / Convection / Convection dynamics / Deep water / Deep water formation / Dense water / Downwelling / Fluctuations / Freshwater / Heat loss / Human influences / Inland water environment / Light water / Mixed layer / Mixed layer depth / Ocean circulation / Oceanic convection / Oceans / Plumes / Scaling / Shoals / Upwelling / Velocity / Vertical velocities / vertical velocity / Wind stress / Winter