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Modulation of the semidiurnal cycle of turbulent dissipation by wind-driven upwelling in a coastal embayment

Modulation of the semidiurnal cycle of turbulent dissipation by wind-driven upwelling in a coastal embayment
Modulation of the semidiurnal cycle of turbulent dissipation by wind-driven upwelling in a coastal embayment
With two 25‐hour series of turbulent microstructure and currents observations carried out in August 2013, during spring (CHAOS 1) and neap tides (CHAOS 2), we investigated the semidiurnal cycle of turbulent dissipation in an embayment affected by coastal upwelling (Ría de Vigo, NW Iberia). At the time of sampling, the bay hosted a net, wind‐driven bi‐directional positive exchange flow and thermal stratification. Turbulent kinetic energy (TKE) dissipation (ε) at the interface between upwelled and surface waters was enhanced by two orders of magnitude during the ebbs (∼ 10−6 W kg−1) with respect to the floods (∼ 10−8 W kg−1). This pattern was caused by the constructive interference of the shear associated with the upwelling and tidal currents. The vertical structure of the tidal currents was consistent with a deformation of tidal ellipses by stratification, which was tightly coupled to the intensity of upwelling. This two‐pronged interaction resulted in a modulation of the semidiurnal cycle of turbulent dissipation by coastal upwelling. Thus, as a result of the upwelling relaxation conditions experienced during CHAOS 1, depth‐integrated interior TKE dissipation rates were higher, by a factor of∼ 2, compared to CHAOS 2. By using a simple model, we determined that observed variations in turbulent mixing had a limited influence on the tidal variations of stratification, which were dominated by straining and advection. The mixing mechanism described here is potentially relevant for the ecology of upwelling bays, as it can stimulate the transport of nutrients from deep‐upwelled waters to the sun‐lit surface layers where primary production takes place.
2169-9275
4034-4054
Fernández-castro, Bieito
f1dbe0ea-e2d2-4317-87fc-51cb586d12bd
Gilcoto, Miguel
431de478-8e3a-4e3a-83eb-bd6f27c9a82b
Naveira-Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Villamaña, Marina
9504a28b-9fee-457d-b71e-690cb5c7a60d
Graña, Rocío
1bfbf101-18c1-49e7-a08d-5e3e3d4299aa
Mouriño-carballido, Beatriz
1bfd941d-9ec6-473f-94bd-bb6faac56fa5
Fernández-castro, Bieito
f1dbe0ea-e2d2-4317-87fc-51cb586d12bd
Gilcoto, Miguel
431de478-8e3a-4e3a-83eb-bd6f27c9a82b
Naveira-Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Villamaña, Marina
9504a28b-9fee-457d-b71e-690cb5c7a60d
Graña, Rocío
1bfbf101-18c1-49e7-a08d-5e3e3d4299aa
Mouriño-carballido, Beatriz
1bfd941d-9ec6-473f-94bd-bb6faac56fa5

Fernández-castro, Bieito, Gilcoto, Miguel, Naveira-Garabato, Alberto C., Villamaña, Marina, Graña, Rocío and Mouriño-carballido, Beatriz (2018) Modulation of the semidiurnal cycle of turbulent dissipation by wind-driven upwelling in a coastal embayment. Journal of Geophysical Research: Oceans, 123 (6), 4034-4054. (doi:10.1002/2017JC013582).

Record type: Article

Abstract

With two 25‐hour series of turbulent microstructure and currents observations carried out in August 2013, during spring (CHAOS 1) and neap tides (CHAOS 2), we investigated the semidiurnal cycle of turbulent dissipation in an embayment affected by coastal upwelling (Ría de Vigo, NW Iberia). At the time of sampling, the bay hosted a net, wind‐driven bi‐directional positive exchange flow and thermal stratification. Turbulent kinetic energy (TKE) dissipation (ε) at the interface between upwelled and surface waters was enhanced by two orders of magnitude during the ebbs (∼ 10−6 W kg−1) with respect to the floods (∼ 10−8 W kg−1). This pattern was caused by the constructive interference of the shear associated with the upwelling and tidal currents. The vertical structure of the tidal currents was consistent with a deformation of tidal ellipses by stratification, which was tightly coupled to the intensity of upwelling. This two‐pronged interaction resulted in a modulation of the semidiurnal cycle of turbulent dissipation by coastal upwelling. Thus, as a result of the upwelling relaxation conditions experienced during CHAOS 1, depth‐integrated interior TKE dissipation rates were higher, by a factor of∼ 2, compared to CHAOS 2. By using a simple model, we determined that observed variations in turbulent mixing had a limited influence on the tidal variations of stratification, which were dominated by straining and advection. The mixing mechanism described here is potentially relevant for the ecology of upwelling bays, as it can stimulate the transport of nutrients from deep‐upwelled waters to the sun‐lit surface layers where primary production takes place.

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Fern-ndez-Castro_et_al-2017-Journal_of_Geophysical_Research__Oceans - Accepted Manuscript
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Accepted/In Press date: 28 April 2018
e-pub ahead of print date: 6 May 2018
Published date: June 2018

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Local EPrints ID: 420639
URI: http://eprints.soton.ac.uk/id/eprint/420639
ISSN: 2169-9275
PURE UUID: a273e617-e39c-440a-b488-17e13af460fa

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Date deposited: 11 May 2018 16:30
Last modified: 07 Oct 2020 05:05

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