The growth and triggering mechanisms of the PNA: A MJO-PNA coherence

A prominent mode of low-frequency variability in the northern extratropical winter known as the Pacific/North American (PNA) teleconnection pattern prevails not only on seasonal but also on intraseasonal timescales. In this study, processes governing the intraseasonal PNA are investigated using daily fields during 1957-2002. The results of the vorticity budget analysis illustrate that the positive phase of the PNA tends to grow by linear processes such as the barotropic energy conversion from the zonally asymmetric climatological flow. For the negative phase of the PNA, nonlinear low-frequency vorticity advection is as important as the linear processes. Composite life cycle of the PNA shows that at 9 days before the peak a pronounced wave train was observed along the Asian jet stream and it eventually developed to the PNA near the jet exit region. This wave train is found to be excited by divergent winds primarily associated with anomalous convection of the Madden-Julian Oscillation (MJO). Probability density functions of the MJO calculated separately following the polarity of the PNA reveal a phase locking between the PNA and the MJO. When the active (inactive) convection associated with the MJO reaches the Bay of Bengal to the western Pacific, occurrence frequency of the negative (positive) phase of the PNA is the highest. This MJO triggering explains roughly 30% of the total PNA events, suggesting that, even though the PNA may be inherent to the extratropical atmosphere, a specific tropical forcing is of importance to realize the PNA as dominant mode.