Theoretical Investigation of the Atlantic Multidecadal Oscillation
International audience A weakly damped mode of variability, corresponding to the oceanic signature of the Atlantic multidecadal oscillation (AMO) was found through the linear stability analysis of a realistic ocean general circulation model. A simple two-level model was proposed to rationalize both...
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ftccsdartic:oai:HAL:hal-01575291v1 2023-05-15T17:32:05+02:00 Theoretical Investigation of the Atlantic Multidecadal Oscillation Sévellec, Florian Huck, Thierry Laboratoire de physique des océans (LPO) Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS) 2015-09 https://hal.archives-ouvertes.fr/hal-01575291 https://doi.org/10.1175/JPO-D-14-0094.1 en eng HAL CCSD American Meteorological Society info:eu-repo/semantics/altIdentifier/doi/10.1175/JPO-D-14-0094.1 hal-01575291 https://hal.archives-ouvertes.fr/hal-01575291 doi:10.1175/JPO-D-14-0094.1 ISSN: 0022-3670 EISSN: 1520-0485 Journal of Physical Oceanography https://hal.archives-ouvertes.fr/hal-01575291 Journal of Physical Oceanography, American Meteorological Society, 2015, 45, pp.2189-2208. ⟨10.1175/JPO-D-14-0094.1⟩ Geographic location/entity North Atlantic Ocean Circulation/ Dynamics Dynamics Meridional overturning circulation Atm/Ocean Structure/ Phenomena Thermohaline circulation Physical Meteorology and Climatology Climate variability Variability Multidecadal variability [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography info:eu-repo/semantics/article Journal articles 2015 ftccsdartic https://doi.org/10.1175/JPO-D-14-0094.1 2021-11-21T02:23:11Z International audience A weakly damped mode of variability, corresponding to the oceanic signature of the Atlantic multidecadal oscillation (AMO) was found through the linear stability analysis of a realistic ocean general circulation model. A simple two-level model was proposed to rationalize both its period and damping rate. This model is extended here to three levels to investigate how the mode can draw energy from the mean flow, as found in various ocean and coupled models. A linear stability analysis in this three-level model shows that the positive growth rate of the oscillatory mode depends on the zonally averaged isopycnal slope. This mode corresponds to a westward propagation of density anomalies in the pycnocline, typical of large-scale baroclinic Rossby waves. The most unstable mode corresponds to the largest scale one (at least for low isopycnal slope). The mode can be described in four phases composing a full oscillation cycle: 1) basin-scale warming of the North Atlantic (AMO positive phase), 2) decrease in upper-ocean poleward transport [hence a reduction of the Atlantic meridional overturning circulation (AMOC)], 3) basin-scale cooling (negative AMO), and 4) AMOC intensification. A criterion is developed to test, in oceanic datasets or numerical models, whether this multidecadal oscillation is an unstable oceanic internal mode of variability or if it is stable and externally forced. Consistent with the classical theory of baroclinic instability, this criterion depends on the vertical structure of the mode. If the upper pycnocline signature is in advance of the deeper pycnocline signature with respect to the westward propagation, the mode is unstable and could be described as an oceanic internal mode of variability. Article in Journal/Newspaper North Atlantic Archive ouverte HAL (Hyper Article en Ligne, CCSD - Centre pour la Communication Scientifique Directe) Journal of Physical Oceanography 45 9 2189 2208 |
institution |
Open Polar |
collection |
Archive ouverte HAL (Hyper Article en Ligne, CCSD - Centre pour la Communication Scientifique Directe) |
op_collection_id |
ftccsdartic |
language |
English |
topic |
Geographic location/entity North Atlantic Ocean Circulation/ Dynamics Dynamics Meridional overturning circulation Atm/Ocean Structure/ Phenomena Thermohaline circulation Physical Meteorology and Climatology Climate variability Variability Multidecadal variability [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography |
spellingShingle |
Geographic location/entity North Atlantic Ocean Circulation/ Dynamics Dynamics Meridional overturning circulation Atm/Ocean Structure/ Phenomena Thermohaline circulation Physical Meteorology and Climatology Climate variability Variability Multidecadal variability [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography Sévellec, Florian Huck, Thierry Theoretical Investigation of the Atlantic Multidecadal Oscillation |
topic_facet |
Geographic location/entity North Atlantic Ocean Circulation/ Dynamics Dynamics Meridional overturning circulation Atm/Ocean Structure/ Phenomena Thermohaline circulation Physical Meteorology and Climatology Climate variability Variability Multidecadal variability [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography |
description |
International audience A weakly damped mode of variability, corresponding to the oceanic signature of the Atlantic multidecadal oscillation (AMO) was found through the linear stability analysis of a realistic ocean general circulation model. A simple two-level model was proposed to rationalize both its period and damping rate. This model is extended here to three levels to investigate how the mode can draw energy from the mean flow, as found in various ocean and coupled models. A linear stability analysis in this three-level model shows that the positive growth rate of the oscillatory mode depends on the zonally averaged isopycnal slope. This mode corresponds to a westward propagation of density anomalies in the pycnocline, typical of large-scale baroclinic Rossby waves. The most unstable mode corresponds to the largest scale one (at least for low isopycnal slope). The mode can be described in four phases composing a full oscillation cycle: 1) basin-scale warming of the North Atlantic (AMO positive phase), 2) decrease in upper-ocean poleward transport [hence a reduction of the Atlantic meridional overturning circulation (AMOC)], 3) basin-scale cooling (negative AMO), and 4) AMOC intensification. A criterion is developed to test, in oceanic datasets or numerical models, whether this multidecadal oscillation is an unstable oceanic internal mode of variability or if it is stable and externally forced. Consistent with the classical theory of baroclinic instability, this criterion depends on the vertical structure of the mode. If the upper pycnocline signature is in advance of the deeper pycnocline signature with respect to the westward propagation, the mode is unstable and could be described as an oceanic internal mode of variability. |
author2 |
Laboratoire de physique des océans (LPO) Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS) |
format |
Article in Journal/Newspaper |
author |
Sévellec, Florian Huck, Thierry |
author_facet |
Sévellec, Florian Huck, Thierry |
author_sort |
Sévellec, Florian |
title |
Theoretical Investigation of the Atlantic Multidecadal Oscillation |
title_short |
Theoretical Investigation of the Atlantic Multidecadal Oscillation |
title_full |
Theoretical Investigation of the Atlantic Multidecadal Oscillation |
title_fullStr |
Theoretical Investigation of the Atlantic Multidecadal Oscillation |
title_full_unstemmed |
Theoretical Investigation of the Atlantic Multidecadal Oscillation |
title_sort |
theoretical investigation of the atlantic multidecadal oscillation |
publisher |
HAL CCSD |
publishDate |
2015 |
url |
https://hal.archives-ouvertes.fr/hal-01575291 https://doi.org/10.1175/JPO-D-14-0094.1 |
genre |
North Atlantic |
genre_facet |
North Atlantic |
op_source |
ISSN: 0022-3670 EISSN: 1520-0485 Journal of Physical Oceanography https://hal.archives-ouvertes.fr/hal-01575291 Journal of Physical Oceanography, American Meteorological Society, 2015, 45, pp.2189-2208. ⟨10.1175/JPO-D-14-0094.1⟩ |
op_relation |
info:eu-repo/semantics/altIdentifier/doi/10.1175/JPO-D-14-0094.1 hal-01575291 https://hal.archives-ouvertes.fr/hal-01575291 doi:10.1175/JPO-D-14-0094.1 |
op_doi |
https://doi.org/10.1175/JPO-D-14-0094.1 |
container_title |
Journal of Physical Oceanography |
container_volume |
45 |
container_issue |
9 |
container_start_page |
2189 |
op_container_end_page |
2208 |
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1766130022550077440 |