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Stability of the thermohaline circulation examined with a one-dimensional fluid loop

Stability of the thermohaline circulation examined with a one-dimensional fluid loop
Stability of the thermohaline circulation examined with a one-dimensional fluid loop
The Stommel box model elegantly demonstrates that the oceanic response to mixed boundary conditions, combining a temperature relaxation with a fixed salt flux forcing, is non-linear owing to the so-called salt advection feedback. This non-linearity produces a parameter range of bi-stability associated with hysteresis effects characterised by a fast thermally driven mode and a slow salinity-driven mode. Here, we investigate whether a similar dynamical behaviour can be found in the thermohaline loop model, a one-dimensional analogue of the box model. A semi-analytical method to compute possible steady states of the loop model is presented, followed by a linear stability analysis carried out for a large range of loop configurations. While the salt advection feedback is found as in the box model, a major difference is obtained for the fast mode: an oscillatory instability is observed near the turning point of the fast mode branch, such that the range of bi-stability is systematically reduced, or even removed, in some cases. The oscillatory instability originates from a salinity anomaly that grows exponentially as it turns around the loop, a situation that may occur only when the salinity torque is directed against the loop flow. Factors such as mixing intensity, the relative strength of thermal and haline forcings, the non-linearity of the equation of state or the loop geometry can strongly affect the stability properties of the loop.
0280-6495
Roquet, Fabien
df1aac99-2c3a-4497-a84d-63efec104bb3
Lindqvist, Rickard
683c4781-9d39-48e4-b3aa-579f4a003875
Pollmann, Friederike
ed1b7ab3-5f9e-4c78-a57b-426aca3b72ad
Ferreira, David
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Madec, Gurvan
ffb28deb-4bbd-4a4c-914f-492f813e4864
Roquet, Fabien
df1aac99-2c3a-4497-a84d-63efec104bb3
Lindqvist, Rickard
683c4781-9d39-48e4-b3aa-579f4a003875
Pollmann, Friederike
ed1b7ab3-5f9e-4c78-a57b-426aca3b72ad
Ferreira, David
47a79187-501a-4689-af4e-fc67ddc40c9a
Madec, Gurvan
ffb28deb-4bbd-4a4c-914f-492f813e4864

Roquet, Fabien, Lindqvist, Rickard, Pollmann, Friederike, Ferreira, David and Madec, Gurvan (2017) Stability of the thermohaline circulation examined with a one-dimensional fluid loop. Tellus A, 69 (1), [1380490]. (doi:10.1080/16000870.2017.1380490).

Record type: Article

Abstract

The Stommel box model elegantly demonstrates that the oceanic response to mixed boundary conditions, combining a temperature relaxation with a fixed salt flux forcing, is non-linear owing to the so-called salt advection feedback. This non-linearity produces a parameter range of bi-stability associated with hysteresis effects characterised by a fast thermally driven mode and a slow salinity-driven mode. Here, we investigate whether a similar dynamical behaviour can be found in the thermohaline loop model, a one-dimensional analogue of the box model. A semi-analytical method to compute possible steady states of the loop model is presented, followed by a linear stability analysis carried out for a large range of loop configurations. While the salt advection feedback is found as in the box model, a major difference is obtained for the fast mode: an oscillatory instability is observed near the turning point of the fast mode branch, such that the range of bi-stability is systematically reduced, or even removed, in some cases. The oscillatory instability originates from a salinity anomaly that grows exponentially as it turns around the loop, a situation that may occur only when the salinity torque is directed against the loop flow. Factors such as mixing intensity, the relative strength of thermal and haline forcings, the non-linearity of the equation of state or the loop geometry can strongly affect the stability properties of the loop.

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Accepted/In Press date: 11 September 2017
e-pub ahead of print date: 28 September 2017

Identifiers

Local EPrints ID: 414702
URI: http://eprints.soton.ac.uk/id/eprint/414702
ISSN: 0280-6495
PURE UUID: 83bb14bc-7bdf-4718-b6b2-a21a13ba5e6b

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Date deposited: 06 Oct 2017 16:31
Last modified: 15 Mar 2024 16:18

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Contributors

Author: Fabien Roquet
Author: Rickard Lindqvist
Author: Friederike Pollmann
Author: David Ferreira
Author: Gurvan Madec

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