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Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

Thompson, Andrew F. and Lazar, Ayah and Buckingham, Christian and Naveira Garabato, Alberto C. and Damerell, Gillian M. and Heywood, Karen J. (2016) Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders. Journal of Physical Oceanography, 46 (4). pp. 1285-1307. ISSN 0022-3670.

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The importance of submesoscale instabilities, particularly mixed layer baroclinic instability and symmetric instability, on upper-ocean mixing and energetics is well documented in regions of strong, persistent fronts such as the Kuroshio and the Gulf Stream. Less attention has been devoted to studying submesoscale flows in the open ocean, far from long-term, mean geostrophic fronts, characteristic of a large proportion of the global ocean. This study presents a year-long, submesoscale-resolving time series of near-surface buoyancy gradients, potential vorticity, and instability characteristics, collected by ocean gliders, that provides insight into open-ocean submesoscale dynamics over a full annual cycle. The gliders continuously sampled a 225 km^2 region in the subtropical northeast Atlantic, measuring temperature, salinity, and pressure along 292 short (~20 km) hydrographic sections. Glider observations show a seasonal cycle in near-surface stratification. Throughout the fall (September–November), the mixed layer deepens, predominantly through gravitational instability, indicating that surface cooling dominates submesoscale restratification processes. During winter (December–March), mixed layer depths are more variable, and estimates of the balanced Richardson number, which measures the relative importance of lateral and vertical buoyancy gradients, depict conditions favorable to symmetric instability. The importance of mixed layer instabilities on the restratification of the mixed layer, as compared with surface heating and cooling, shows that submesoscale processes can reverse the sign of an equivalent heat flux up to 25% of the time during winter. These results demonstrate that the open-ocean mixed layer hosts various forced and unforced instabilities, which become more prevalent during winter, and emphasize that accurate parameterizations of submesoscale processes are needed throughout the ocean.

Item Type:Article
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URLURL TypeDescription DOIArticle
Thompson, Andrew F.0000-0003-0322-4811
Buckingham, Christian0000-0001-9355-9038
Heywood, Karen J.0000-0001-9859-0026
Additional Information:© 2016 American Meteorological Society. Manuscript received 29 August 2015, in final form 21 January 2016. We are grateful for the efforts of the OSMOSIS team and the crews of the RRS Discovery, the R/V Celtic Explorer, and the RRS James Cook. The data presented in the study are the result of five research cruises and a full year of glider piloting and involved many contributions not represented in the author list. The authors acknowledge constructive reviews from John Taylor and Jörn Callies as well as helpful conversations with Liam Brannigan, Baylor Fox Kemper, Amala Mahadevan, Leif Thomas, Patrice Klein, and Eric D’Asaro. AFT was supported by NSF OCE1155676, AL was supported by NSF-OCE 1235488, CB and ACNG were supported by OSMOSIS NERC Grant NE/I019999/1, and GMD and KJH were supported by OSMOSIS NERC Grant NE/I019905/1.
Funding AgencyGrant Number
Natural Environment Research Council (NERC)NE/I019999/1
Natural Environment Research Council (NERC)NE/I019905/1
Subject Keywords:Geographic location/entity; North Atlantic Ocean; Circulation/ Dynamics; Instability; Turbulence; Atm/Ocean Structure/ Phenomena; Boundary layer; Observational techniques and algorithms; Profilers, oceanic; Variability; Seasonal variability
Issue or Number:4
Record Number:CaltechAUTHORS:20160427-101133146
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:66510
Deposited By: Ruth Sustaita
Deposited On:02 May 2016 20:47
Last Modified:09 Mar 2020 13:18

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