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Flow and mixing in the rift valley of the Mid-Atlantic Ridge

Flow and mixing in the rift valley of the Mid-Atlantic Ridge
Flow and mixing in the rift valley of the Mid-Atlantic Ridge
High levels of diapycnal mixing and geothermal heating near midocean ridges contribute to the buoyancy fluxes that are required to close the global circulation. In topographically confined areas, such as the deep median valleys of slow-spreading ridges, these fluxes strongly influence the local hydrography and dynamics. Data from a segment-scale hydrographic survey of the rift valley of the Mid-Atlantic Ridge and from an array of current meters deployed there during an entire year are analyzed in order to characterize the dominant hydrographic patterns and dynamical processes. Comparison with historic hydrographic data indicates that the temporal variability during the last few decades has been small compared to the observed segment-scale gradients. The rift valley circulation is characterized by inflow from the eastern ridge flank and persistent unidirectional along-segment flow into a cul-de-sac. Therefore, most of the water flowing along the rift valley upwells within the segment with a mean vertical velocity >10?5 m s?1. The observed streamwise hydrographic gradients indicate that diapycnal mixing dominates the rift valley buoyancy fluxes by more than an order of magnitude, in spite of the presence of a large hydrothermal vent field supplying several gigawatts of heat to the water column. Hydrographic budgets in the rift valley yield diffusivity values of order 5 × 10?3 m2 s?1, consistent with estimates derived from statically unstable overturns, the largest of which were observed downstream of topographic obstacles in the path of the along-segment flow. This suggests vertical shear associated with cross-sill flows as the dominant contributor to the mechanical mixing in the rift valley.
WOCE, MID ATLANTIC RIDGE, DIAPYCNAL MIXING, HYDROGRAPHY, HEAT EXCHANGE
0022-3670
1763-1778
Thurnherr, A.M.
7b1feb8a-78a2-48b5-9b68-152ada222636
Richards, K.J.
fbeeea4f-7891-417a-a30d-182dd90916f9
German, C.R.
cd0eedd5-1377-4182-9c8a-b06aef8c1069
Lane-Serff, G.F.
129c1906-92f5-4c21-b039-f5d4790248f9
Speer, K.G.
4708eb98-c14c-414f-acab-9f10a364f3bd
Thurnherr, A.M.
7b1feb8a-78a2-48b5-9b68-152ada222636
Richards, K.J.
fbeeea4f-7891-417a-a30d-182dd90916f9
German, C.R.
cd0eedd5-1377-4182-9c8a-b06aef8c1069
Lane-Serff, G.F.
129c1906-92f5-4c21-b039-f5d4790248f9
Speer, K.G.
4708eb98-c14c-414f-acab-9f10a364f3bd

Thurnherr, A.M., Richards, K.J., German, C.R., Lane-Serff, G.F. and Speer, K.G. (2002) Flow and mixing in the rift valley of the Mid-Atlantic Ridge. Journal of Physical Oceanography, 32 (6), 1763-1778. (doi:10.1175/1520-0485(2002)032<1763:FAMITR>2.0.CO;2).

Record type: Article

Abstract

High levels of diapycnal mixing and geothermal heating near midocean ridges contribute to the buoyancy fluxes that are required to close the global circulation. In topographically confined areas, such as the deep median valleys of slow-spreading ridges, these fluxes strongly influence the local hydrography and dynamics. Data from a segment-scale hydrographic survey of the rift valley of the Mid-Atlantic Ridge and from an array of current meters deployed there during an entire year are analyzed in order to characterize the dominant hydrographic patterns and dynamical processes. Comparison with historic hydrographic data indicates that the temporal variability during the last few decades has been small compared to the observed segment-scale gradients. The rift valley circulation is characterized by inflow from the eastern ridge flank and persistent unidirectional along-segment flow into a cul-de-sac. Therefore, most of the water flowing along the rift valley upwells within the segment with a mean vertical velocity >10?5 m s?1. The observed streamwise hydrographic gradients indicate that diapycnal mixing dominates the rift valley buoyancy fluxes by more than an order of magnitude, in spite of the presence of a large hydrothermal vent field supplying several gigawatts of heat to the water column. Hydrographic budgets in the rift valley yield diffusivity values of order 5 × 10?3 m2 s?1, consistent with estimates derived from statically unstable overturns, the largest of which were observed downstream of topographic obstacles in the path of the along-segment flow. This suggests vertical shear associated with cross-sill flows as the dominant contributor to the mechanical mixing in the rift valley.

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More information

Published date: 2002
Keywords: WOCE, MID ATLANTIC RIDGE, DIAPYCNAL MIXING, HYDROGRAPHY, HEAT EXCHANGE

Identifiers

Local EPrints ID: 5989
URI: http://eprints.soton.ac.uk/id/eprint/5989
ISSN: 0022-3670
PURE UUID: 6996022c-9d05-4796-ad5f-272914b1175f

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Date deposited: 07 Jun 2004
Last modified: 09 Nov 2021 06:06

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Contributors

Author: A.M. Thurnherr
Author: K.J. Richards
Author: C.R. German
Author: G.F. Lane-Serff
Author: K.G. Speer

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