A new look at the midlatitude-MJO teleconnection in the Northern Hemisphere winter

The midlatitude-MJO teleconnection in the Northern Hemisphere wintertime was examined using the OLR and the NCEP/NCAR reanalysis data of twenty years. It is revealed through the EOF and regression analyses that the tropical upper-level divergence (convergence) associated with enhanced (reduced) tropical convection has its subtropical counterpart of upper-level convergence (divergence), which is manifested as a modulation of the local Hadley circulation. As the convective region of MJO moves eastward from the Indian Ocean to the western Pacific, the corresponding divergent circulation moves eastward. We show that this divergent circulation and the vertical wind shear of the climatological wintertime jet collaborate to produce a vorticity source for the midlatitude-MJO teleconnection. It is hypothesized and verified that the vertical vorticity advection and the tilting terms in vorticity equation are important sources of the midlatitude-MJO teleconnection; baroclinic jet source was defined by combining the two terms. By vertical modal decomposition technique, we showed that the importance of baroclinic jet source as an effective source of the external normal mode of the quasi-geostrophic equation. We further showed that, at least in the quasi-geostrophic framework, midlatitude Rossby wave source does not give a proper source for the midlatitude-MJO teleconnection when it is interpreted as a stretching term in vorticity equation.

A numerical experiment is carried out, in which a linear barotropic model is forced by the BJS only. The model clearly captured the main feature of the midlatitude-MJO teleconnection in the extratropics. This suggests that the interplay of the vertical wind shear of the wintertime Pacific-jet and the divergent (convergent) circulation accompanied by the enhanced (reduced) convection of the MJO is one of key ingredients for the midlatitude-MJO teleconnection.