Seasonality in submesoscale turbulence

Creators: Callies, Jörn; Ferrari, Raffaele; Klymak, Jody M.; Gula, Jonathan

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.

Additional Information

© 2015 The Authors. This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ Received: 17 October 2014. Accepted: 06 March 2015. Published online: 21 April 2015. We thank the Office of Naval Research for funding and the entire LatMix group for conducting the experiment. We thank the Oleander group for collecting and making available the long-term ADCP data. In particular, we thank Kathleen A. Donohue for help with understanding the imprint of processing on the spectra derived from the first set of Oleander data (1994–2004). J.C. and R.F. acknowledge financial support under grants ONR-N00014-09-1-0458 and NSF-OCE-1233832; J.M.K. under grants ONR-N00014-11-1-0165 and NSERC-327920-2006; J.G. under grant ONR-N00014-12-1-0105. Author Contributions: J.C. and R.F. planned the research and wrote the manuscript. J.C. analysed the data. J.M. K. planned, collected and processed the Moving Vessel Profiler observations. J.G. performed the model simulations shown in Fig. 1. The authors declare no competing financial interests.

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