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An annual cycle of submesoscale vertical flow and restratification in the upper ocean

An annual cycle of submesoscale vertical flow and restratification in the upper ocean
An annual cycle of submesoscale vertical flow and restratification in the upper ocean
Numerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification, and thus restratify the upper ocean via vertical flow. However, the observational evidence for this restratifying process has been lacking due to the difficulty in measuring such ephemeral phenomena, particularly over periods of months to years. This study presents an annual cycle of the vertical velocity and associated restratification estimated from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical mid-ocean area of the Northeast Atlantic. Vertical velocities inferred using the non-diffusive density equation are substantially stronger at submesoscales (horizontal scales of 1-10 km) than at mesoscales (horizontal scales of 10-100 km), with respective root mean square values of 38.0 ± 6.9 m/day and 22.5 ± 3.3 m/day. The largest submesoscale vertical velocities and rates of restratification occur in events of a few days’ duration in winter and spring, and extend down to at least 200 m below the mixed layer base. These events commonly coincide with the enhancement of submesoscale lateral buoyancy gradients, which is itself associated with persistent mesoscale frontogenesis. This suggests that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence that restratifies the upper ocean. The upper-ocean restratification induced by submesoscale motions integrated over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling, indicating that submesoscale flows play a significant role in determining the climatological upper-ocean stratification in the study area.
0022-3670
1439-1461
Yu, Xiaolong
3ffa551d-f57e-4f49-a130-475e43039c75
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Martin, Adrian P.
9d0d480d-9b3c-44c2-aafe-bb980ed98a6d
Buckingham, Christian E.
dba7c776-a8c6-4617-ad40-7919dd4ab3dc
Brannigan, Liam
55fbac37-1760-4491-b619-61d490092b64
Su, Zhan
5e72ecf8-2480-4239-88bd-9badeaceaf32
Yu, Xiaolong
3ffa551d-f57e-4f49-a130-475e43039c75
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Martin, Adrian P.
9d0d480d-9b3c-44c2-aafe-bb980ed98a6d
Buckingham, Christian E.
dba7c776-a8c6-4617-ad40-7919dd4ab3dc
Brannigan, Liam
55fbac37-1760-4491-b619-61d490092b64
Su, Zhan
5e72ecf8-2480-4239-88bd-9badeaceaf32

Yu, Xiaolong, Naveira Garabato, Alberto C., Martin, Adrian P., Buckingham, Christian E., Brannigan, Liam and Su, Zhan (2019) An annual cycle of submesoscale vertical flow and restratification in the upper ocean. Journal of Physical Oceanography, 1439-1461. (doi:10.1175/JPO-D-18-0253.1).

Record type: Article

Abstract

Numerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification, and thus restratify the upper ocean via vertical flow. However, the observational evidence for this restratifying process has been lacking due to the difficulty in measuring such ephemeral phenomena, particularly over periods of months to years. This study presents an annual cycle of the vertical velocity and associated restratification estimated from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical mid-ocean area of the Northeast Atlantic. Vertical velocities inferred using the non-diffusive density equation are substantially stronger at submesoscales (horizontal scales of 1-10 km) than at mesoscales (horizontal scales of 10-100 km), with respective root mean square values of 38.0 ± 6.9 m/day and 22.5 ± 3.3 m/day. The largest submesoscale vertical velocities and rates of restratification occur in events of a few days’ duration in winter and spring, and extend down to at least 200 m below the mixed layer base. These events commonly coincide with the enhancement of submesoscale lateral buoyancy gradients, which is itself associated with persistent mesoscale frontogenesis. This suggests that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence that restratifies the upper ocean. The upper-ocean restratification induced by submesoscale motions integrated over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling, indicating that submesoscale flows play a significant role in determining the climatological upper-ocean stratification in the study area.

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jpo-d-18-0253.1 - Accepted Manuscript
Restricted to Repository staff only until 8 April 2020.
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Accepted/In Press date: 28 March 2019
e-pub ahead of print date: 8 April 2019
Published date: June 2019

Identifiers

Local EPrints ID: 430970
URI: https://eprints.soton.ac.uk/id/eprint/430970
ISSN: 0022-3670
PURE UUID: 5442b1d5-651f-43b0-9e52-e19ae8c6b0b4

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Date deposited: 20 May 2019 16:30
Last modified: 09 Dec 2019 17:37

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Contributors

Author: Xiaolong Yu
Author: Adrian P. Martin
Author: Christian E. Buckingham
Author: Liam Brannigan
Author: Zhan Su

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