An Annual Cycle of Vertical Flow and Restratification in the Upper Ocean

Recent numerical simulations show that submesoscale flows, with horizontal scales of 1 – 10 km, can induce strong vertical flows and contribute importantly to vertical transports in the upper ocean. Here, we present observational evidence of the enhancement and surface intensification of vertical flows and vertical fluxes of buoyancy at submesoscales from two nested clusters of mesoscale- and submesoscale- resolving moorings deployed in a mid-gyre area of the Northeast Atlantic in September 2012 – September 2013. Mesoscale and submesoscale vertical velocities, inferred from conservation of mass, are both found to be highly variable in time and to exhibit a small negative mean. Vertical flows are substantially stronger at submesoscales than at mesoscales. Differences between submesoscale and mesoscale vertical velocities are modest at times of weak mesoscale eddy activity, but are often large (on the order of 100 m/day) when energetic mesoscale features propagate through the mooring array. Vertical buoyancy fluxes associated with strong submesoscale ageostrophic motions are found to be more surface-intensified, thereby, achieving restratification in the ocean surface boundary layer. The physical phenomena underpinning these findings will be discussed.