title: Seasonality in submesoscale turbulence.
abstract: Although the strongest ocean surface currents occur at horizontal scales of order 100 km, recent numerical simulations suggest that flows smaller than these mesoscale eddies can achieve important vertical transports in the upper ocean. These submesoscale flows, 1-100 km in horizontal extent, take heat and atmospheric gases down into the interior ocean, accelerating air-sea fluxes, and bring deep nutrients up into the sunlit surface layer, fueling primary production. Here we present observational evidence that submesoscale flows undergo a seasonal cycle in the surface mixed layer: they are much stronger in winter than in summer. Submesoscale flows are energized by baroclinic instabilities that develop around geostrophic eddies in the deep winter mixed layer at a horizontal scale of order 1-10 km. Flows larger than this instability scale are energized by turbulent scale interactions. Enhanced submesoscale activity in the winter mixed layer is expected to achieve efficient exchanges with the permanent thermocline below.
authors.affiliation: MIT/WHOI Joint Program in Oceanography, Massachusetts Institute of Technology, Building 54-1615, 77 Massachusetts Avenue, Cambridge/Woods Hole, Massachusetts 02139, USA.
authors.forename: Jörn
authors.initials: J
authors.lastname: Callies
authors.affiliation: Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Raffaele
authors.initials: R
authors.lastname: Ferrari
authors.affiliation: University of Victoria, Victoria, British Columbia V8W 3P6, Canada.
authors.forename: Jody M
authors.initials: JM
authors.lastname: Klymak
authors.affiliation: University of California, Los Angeles, California 90095, USA.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Gula