Low-frequency variability in the coupled ocean-atmosphere system at midlatitudes
Notice bibliographique
Résumé
The winter-mean climate over the North Atlantic ocean shows a low pressure center near Iceland and a large high pressure center ranging from Florida to Spain. Between these centers, an eastward jet prevails, causing cold and dry winter conditions in Northeast America and moderate conditions in Western Europe. On this mean situation, variability shows up on various time scales. The dominant pattern of variability expresses as a weakening and strengthening of the mean pressure centers and is called the North Atlantic Oscillation (NAO). The impact of the NAO on the American and European winter climate is large and the understanding of a possible mechanism which drives the NAO from one phase into another is therefore of great importance. In the North Atlantic climate system, multiple subsystems appear to be highly related to the NAO. For instance, high correlations have been found between specific North Atlantic sea-surface temperature (SST) patterns of variability and the NAO. Therefore, it has often been suggested that the NAO is both an atmospheric and an oceanic phenomenon with possibly a crucial role for ocean-atmosphere coupling. Although the NAO fluctuates on all time scales, there seems to be a preference for low-frequency variability, ranging from a few years to a few decades. In this thesis, we focus on the NAO at the low-frequency time scale. The central question is: How do oceanic and/or atmospheric processes cause low-frequency fluctuations at midlatitudes? In order to explore this question, first, intermediate models are analyzed using a dynamical systems approach, showing the fundamental processes which lead to low-frequency variability at midlatitudes. Then, output of a complex coupled General Circulation Model (GCM) is analyzed, in order to explore whether these fundamental processes can be identified and understood in the more complex climate system. The results of chapters 2 and 3 show two fundamental mechanisms, which might generate low-frequency variability at midlatitudes: nonlinearly induced variability due to rectification processes and/or low-frequency variability arising through an internal ocean mode, the socalled gyre mode. In chapter 4, a robust decadal oscillatory signal is found, both in oceanic and atmospheric fields. In the ocean surface and subsurface fields, anomalies propagate from the northwestern Labrador Sea south-eastward along the coast and bend around Newfoundland before they enter the subtropical gyre. The pressure patterns in the atmosphere show a strong resemblance with those of the North Atlantic Oscillation. A new framework, in which the gyre mode found in chapter 3 is suggested to set the decadal time scale, is presented to explain the decadal variability. The results of chapters 2- 4 are discussed in chapter 5, leading to a general (personal) view on the NAO: the low-frequency variations of the NAO may very well be caused by low-frequency variability in the ocean.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,001 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,001 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,001 | 0,001 |
| Communication savante | 0,002 | 0,001 |
| Science ouverte | 0,001 | 0,001 |
| Intégrité de la recherche | 0,001 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,002 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».