Low-frequency air-sea interactions in the Southern Ocean

• Main Author: Maze, Guillaume
• Other Authors: 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), Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL), Université Pierre et Marie Curie - Paris VI, Alain Colin de Verdiere
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2006
• Subjects: Southern Ocean; annular mode; low frequency; ocean-atmosphere interactions; variability; climat; océan Austral; mode annulaire; basse fréquence; interactions océan atmosphère; variabilité; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Antarc*; Antarctic
• Online Access: https://theses.hal.science/tel-00515553; https://theses.hal.science/tel-00515553/document; https://theses.hal.science/tel-00515553/file/Maze.PhD2006.pdf

Patterns of interannual variability of the ocean-atmosphere coupled system in the Southern Hemisphere extratropics are studied with a simple dynamical model, in order to determine the basic physical processes of interaction independently of tropical forcing. The model used is an atmospheric quasi-geostrophic model coupled to a “slab” oceanic mixed layer, which includes mean geostrophic advection by the Antarctic Circumpolar Current (ACC). The ocean-atmosphere coupling occurs through surface heat fluxes and Ekman current heat advection. In a fully coupled simulation, the atmospheric part of the model, which includes high frequency transient eddies at midlatitudes, exhibits a strong Southern Annular Mode (SAM) as the first mode of variability at interannual time-scales. The SAM-related wind anomalies induce Ekman currents in the mixed layer which produce sea surface temperature anomalies. These are then advected along by the ACC. A forced mechanism where the ocean role is reduced to advect the SST appears sufficient to reproduce the main features of the variability. Nevertheless, a positive feedback of the ocean was also found. It operates through anomalous Ekman currents heat advection and contributes to the maintenance of the SST anomaly. The stationary atmospheric response to an idealised SST anomaly (SSTa) is next studied in the same simple model of the Southern Hemisphere. Sensitivity of the stationary atmospheric response to the SSTa location is determined. Differentiating the barotropic from the baroclinic responses, it was found that for extratropical SSTa, the baroclinic atmospheric response is independent of the SSTa longitude whereas the barotropic response can take two different patterns. The SSTa induces a warm air anomaly through the thermal wind balance, which gives a baroclinic response that creates a trough 45 degrees eastward. This response is simply due to the advection of the SSTa induced anomalous vortex stretching by quasi-stationary westerlies. Baroclinic transients eddies were found to have ...