Mechanisms Determining the Winter Atmospheric Response to the Atlantic Overturning Circulation

• Published in: Journal of Climate
• Main Authors: Gastineau, Guillaume, L’hévéder, Blandine, Codron, Francis, Frankignoul, Claude
• Other Authors: Processus de la variabilité climatique tropicale et impacts (PARVATI), Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN), Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2016
• Subjects: Variability; Models and modeling; Climate models; Geographic location/entity; Sea ice; Circulation/ Dynamics; Meridional overturning circulation; Atm/Ocean Structure/ Phenomena; Stratosphere-troposphere coupling; Multidecadal variability; North Atlantic Oscillation; [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology; Labrador Sea; Nordic Seas; North Atlantic; North Atlantic oscillation
• Online Access: https://hal.sorbonne-universite.fr/hal-01331257; https://hal.sorbonne-universite.fr/hal-01331257/document; https://hal.sorbonne-universite.fr/hal-01331257/file/Gastineau_2016_Mechanisms.pdf; https://doi.org/10.1175/JCLI-D-15-0326.1

International audience In climate models, an intensification of the Atlantic meridional overturning circulation (AMOC) precedes a warming in the North Atlantic subpolar basin by a few years. In the IPSL-CM5A-LR model, this warming may explain the atmospheric response to the AMOC observed in winter, which resembles a negative phase of the North Atlantic Oscillation (NAO). To firmly establish the causality links between the ocean and the atmosphere and illustrate the underlying mechanisms in this model, ensembles of atmosphere-only simulations are conducted, prescribing the SST and sea ice anomalies that follow an AMOC intensification. In late winter, the North Atlantic SST and sea ice anomalies drive atmospheric circulation anomalies similar to those found in the coupled model. Simulations only driven by the SST anomalies related to the AMOC show that the largest oceanic influence is due to the warm subpolar SST anomaly, which enhances the oceanic heat release and decreases the lower-tropospheric baroclinicity in the region of maximum eddy growth, resulting in a weaker meridional eddy heat flux in the atmosphere. The transient eddy feedback leads to a negative NAO-like response. An AMOC intensification is also followed by less sea ice over the Labrador Sea and more sea ice over the Nordic seas. The simulations with full boundary forcing suggest that such anomalies act to strengthen both the poleward momentum flux and the upward heat flux into the polar stratosphere and lead to a stratospheric warming, which then reinforces the negative NAO signal in late winter.