Characteristics and Seasonality of the Submesoscale Regime in the Gulf Stream Region

Abstract:

Frontogenesis and frontal instabilities in the mixed layer are known to be important processes in the formation of submesoscale features. We study the seasonality of such processes in the Gulf Stream (GS) region.

To approach this problem, a realistic simulation with the Hybrid Coordinate Ocean Model (HYCOM) is integrated for 18 months at two horizontal resolutions: a high resolution (1/48th deg) simulation able to resolve part of the submesoscale regime and the full range of mesoscale dynamics, and a coarser resolution (1/12th deg) case, in which submesoscales are not resolved.

Results provide an insight into submesoscale dynamics in the complex GS region. A clear seasonal cycle is observed, with submesoscale features mostly present during the winter season in the mixed layer. The submesoscale field is characterized in terms of deviation from geostrophy and 2D dynamics.

The limiting and controlling factor in the formation of submesoscales appears to be the depth of the mixed layer, that controls the release of APE stored in the mesoscale fronts present most of the year. Atmospheric forcings are the main energy source behind submesoscale formation, but mostly indirectly through mixed layer deepening. The mixed layer instability scaling proposed by Fox-Kemper et al. 2008 appears to hold, indicating that the parametrization is appropriate even in this complex and mesoscale dominated area.