Damped multidecadal stratosphere/troposphere/Ocean-coupled oscillation as framework for northern hemisphere climate variability

Nour-Eddine Omrani, University of Bergen and Bjerknes Centre, Geophysical Institute, Bergen, Norway, Noel S Keenlyside, Geophysical Institute, Juneau, Norway, Katja Matthes, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, Davide Zanchettin, Università Ca'Foscari, Venice, Italy and Johann Jungclaus, Max Planck Institute for Meteorology, Hamburg, Germany
Using long-term forced simulation with Max-Planck Institute Earth System Model (MPI-ESM), we show that the framework of damped coupled stratosphere/troposphere/Ocean Oscillation can help understanding several observed coherent changes in the Northern Hemispheric climate system. In this Oscillation, the large-scale Atlantic cooling associated with cold phase of the Atlantic Multidecadal Variability (AMV) enhances the divergence of resolved atmospheric waves in the stratosphere/troposphere-coupled system leading to positive winter North Atlantic Oscillation (NAO) and strengthening the stratospheric polar vortex. The net result is an increase in upward heat fluxes and salinity over Labrador-seas, which results in enhanced convection, delayed strengthening of the Atlantic Overturning circulation (AMOC) and sub-polar gyre and enhanced poleward oceanic heat transport. The net result is a large-scale Atlantic warming that shift the AMV into its warm phase and the NAO into its negative phase. The negative NAO acts in turn to weaken the AMOC and subpolar gyre, which shifts the oscillation into warm AMV-phase and wave-induced negative NAO. This Oscillation can explain large part of the coherent changes between sea-ice and NAO through changes in the ocean circulation and associated poleward heat transport.