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On the mechanism of centennial thermohaline oscillations

On the mechanism of centennial thermohaline oscillations
On the mechanism of centennial thermohaline oscillations
Centennial oscillations of the ocean thermohaline circulation are studied in a 2-D latitude-depth model under mixed boundary conditions (i.e. restoring surface temperature and prescribed freshwater flux). The oscillations are revealed through linear stability analysis of a steady state obtained in a single hemisphere configuration. A density variance budget is performed and helps determine the physical processes sustaining these oscillations: the restoring surface temperature appears as a source of density variance – this is a consequence of positively-correlated temperature and salinity anomalies. A minimal model, the Howard-Malkus loop oscillator, enables us to understand physically the oscillatory and growth mechanisms. The centennial oscillation is connected to the advection of salinity anomaly around the loop; it is also related to the salinity feedback on the overturning which reinforces anomalies through a change of residence time in the freshwater flux regions. Analytical solutions of this loop model show that these centennial oscillations exist in a specific parameter regime in terms of the freshwater flux amplitude F0: oscillations are damped if F0 is too weak, but if F0 is too large, the instability grows exponentially without oscillating – the latter regime is known as the positive salinity feedback. The robustness of these oscillations is then analyzed in more realistic bihemispheric configurations, some including a highly idealized Antarctic Circumpolar Current: oscillations are then always damped. These results are rationalized with the loop model, and compared to the oscillations found in general circulation models.
0022-2402
355-392
Sévellec, Florian
01569d6c-65b0-4270-af2a-35b0a77c9140
Huck, Thierry
3de8511b-db7b-4ed1-bc27-92d2bbc7c6b9
Ben Jelloul, Mahdi
bb143e60-6a9a-4949-8a97-a0e52b79ec38
Sévellec, Florian
01569d6c-65b0-4270-af2a-35b0a77c9140
Huck, Thierry
3de8511b-db7b-4ed1-bc27-92d2bbc7c6b9
Ben Jelloul, Mahdi
bb143e60-6a9a-4949-8a97-a0e52b79ec38

Sévellec, Florian, Huck, Thierry and Ben Jelloul, Mahdi (2006) On the mechanism of centennial thermohaline oscillations. Journal of Marine Research, 64 (3), 355-392. (doi:10.1357/002224006778189608).

Record type: Article

Abstract

Centennial oscillations of the ocean thermohaline circulation are studied in a 2-D latitude-depth model under mixed boundary conditions (i.e. restoring surface temperature and prescribed freshwater flux). The oscillations are revealed through linear stability analysis of a steady state obtained in a single hemisphere configuration. A density variance budget is performed and helps determine the physical processes sustaining these oscillations: the restoring surface temperature appears as a source of density variance – this is a consequence of positively-correlated temperature and salinity anomalies. A minimal model, the Howard-Malkus loop oscillator, enables us to understand physically the oscillatory and growth mechanisms. The centennial oscillation is connected to the advection of salinity anomaly around the loop; it is also related to the salinity feedback on the overturning which reinforces anomalies through a change of residence time in the freshwater flux regions. Analytical solutions of this loop model show that these centennial oscillations exist in a specific parameter regime in terms of the freshwater flux amplitude F0: oscillations are damped if F0 is too weak, but if F0 is too large, the instability grows exponentially without oscillating – the latter regime is known as the positive salinity feedback. The robustness of these oscillations is then analyzed in more realistic bihemispheric configurations, some including a highly idealized Antarctic Circumpolar Current: oscillations are then always damped. These results are rationalized with the loop model, and compared to the oscillations found in general circulation models.

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Published date: May 2006
Organisations: Physical Oceanography

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Local EPrints ID: 339728
URI: http://eprints.soton.ac.uk/id/eprint/339728
ISSN: 0022-2402
PURE UUID: 07cc6d84-ef37-4700-84f2-870c3ca80fb0

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Date deposited: 29 May 2012 16:16
Last modified: 14 Mar 2024 11:14

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Contributors

Author: Thierry Huck
Author: Mahdi Ben Jelloul

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