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Multiple thermohaline states due to variable diffusivity in a hierarchy of simple models

Multiple thermohaline states due to variable diffusivity in a hierarchy of simple models
Multiple thermohaline states due to variable diffusivity in a hierarchy of simple models
The effect of variable vertical diffusivity is investigated in dynamically reduced models of the thermohaline circulation (THC) in a rectangular basin. In a simple box model, sufficiently strong variation of the diffusivity v with stability G can lead to the existence of two stable equilibria. Related behaviour is found in well-resolved frictional geostrophic (FG) models. A hierarchy of under-resolved FG models is constructed, the simplest of which is an 8-cell cube, to connect the two extremes of resolution. Multiple solutions in low-order models are found to correspond to the formation of high-gradient layers which are unlikely to be resolved by current ocean models. Physical arguments show that layering and multiple solutions require v to decrease more rapidly than 1/G and sensitivity experiments suggest that, in addition, v must vary by a factor of 10–100. In two-hemisphere runs with salinity forcing included, the dependence of diffusivity on stratification is found to marginally favour equatorially symmetric states. Finally, such variation is shown to have a profound effect on the periodic, flush-collapse cycle under strong saline forcing; specifically, if diffusivity is taken to be a function of stratification rather than depth, regime transitions can occur much more easily. It will therefore be important for climate modelling to determine which is more realistic.
woce, thermohaline circulation, vertical diffusivity
1463-5003
67-94
Edwards, Neil R.
9b7ab7c6-6229-4522-9780-1c24470ee7b4
Shepherd, John G.
f38de3ac-eb3b-403f-8767-c76be68d8bf2
Edwards, Neil R.
9b7ab7c6-6229-4522-9780-1c24470ee7b4
Shepherd, John G.
f38de3ac-eb3b-403f-8767-c76be68d8bf2

Edwards, Neil R. and Shepherd, John G. (2001) Multiple thermohaline states due to variable diffusivity in a hierarchy of simple models. Ocean Modelling, 3 (1-2), 67-94. (doi:10.1016/S1463-5003(01)00002-6).

Record type: Article

Abstract

The effect of variable vertical diffusivity is investigated in dynamically reduced models of the thermohaline circulation (THC) in a rectangular basin. In a simple box model, sufficiently strong variation of the diffusivity v with stability G can lead to the existence of two stable equilibria. Related behaviour is found in well-resolved frictional geostrophic (FG) models. A hierarchy of under-resolved FG models is constructed, the simplest of which is an 8-cell cube, to connect the two extremes of resolution. Multiple solutions in low-order models are found to correspond to the formation of high-gradient layers which are unlikely to be resolved by current ocean models. Physical arguments show that layering and multiple solutions require v to decrease more rapidly than 1/G and sensitivity experiments suggest that, in addition, v must vary by a factor of 10–100. In two-hemisphere runs with salinity forcing included, the dependence of diffusivity on stratification is found to marginally favour equatorially symmetric states. Finally, such variation is shown to have a profound effect on the periodic, flush-collapse cycle under strong saline forcing; specifically, if diffusivity is taken to be a function of stratification rather than depth, regime transitions can occur much more easily. It will therefore be important for climate modelling to determine which is more realistic.

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More information

Published date: 2001
Keywords: woce, thermohaline circulation, vertical diffusivity

Identifiers

Local EPrints ID: 7945
URI: http://eprints.soton.ac.uk/id/eprint/7945
ISSN: 1463-5003
PURE UUID: 6c872f2d-9282-4a60-a06f-9e01951cf822
ORCID for John G. Shepherd: ORCID iD orcid.org/0000-0002-5230-4781

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Date deposited: 10 Aug 2004
Last modified: 16 Mar 2024 02:47

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Author: Neil R. Edwards

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