A new look at the midlatitude–MJO teleconnection in the northern hemisphere winter

Abstract

AbstractThe midlatitude to Madden–Julian (MJO) teleconnection in the northern hemisphere wintertime was investigated using twenty years of outgoing long‐wave radiation and National Center for Atmospheric Research/National Centers for Environmental Prediction re‐analysis data. It is revealed through empirical orthogonal functions and regression analyses that the tropical upper‐level divergence (convergence) associated with enhanced (reduced) tropical convection has a subtropical counterpart of upper‐level convergence (divergence). As the convective region of MJO moves eastward from the Indian Ocean to the western Pacific, the divergent circulation connecting the Tropics and subtropics moves eastward. From an analysis of vertical motion diagnosed by the generalized omega equation, it was revealed that the position of the subtropical Rossby gyres of the MJO with respect to the Asian‐Pacific jet was the most important factor affecting the existence of the localized divergence (convergence) and associated lower‐level vertical motion in the subtropics. As the subtropical Rossby gyres pass by the longitudinal position of the Asian‐Pacific jet, vertical motion occurs to meet the balance required by quasi‐geostrophic theory. Since this anomalous vertical motion is large enough to cause convection at that region, the diabatic heating process reinforces the vertical motion. Furthermore, other dynamical processes neglected in quasi‐geostrophic theory are activated and contribute to the vertical motion balancing the diabatic heating. As a result of the vertical motion near the jet entrance region in the subtropics, the vorticity advection by vertical wind and the tilting of vorticity become important. Since these two terms are similar in shape and magnitude, they reinforce each other during the development phase of the midlatitude–MJO teleconnection. Combining these two terms, we define the baroclinic jet source (BJS). In order to evaluate the BJS as a source for the midlatitude–MJO teleconnection, a numerical experiment is carried out. The linear barotropic model is forced by only two terms, yet the model captured clearly the main feature of the midlatitude–MJO teleconnection. Copyright © 2006 Royal Meteorological Society