Diagnosing the optimal growth of the PNA pattern using linear inverse modeling
The Pacific-North American (PNA) pattern is a large-scale wave-like pressure pattern that forms over the North Pacific and across North America with important implications for precipitation and temperatures. In this work, we examine PNA development by considering both the role of tropical variability and the mid-latitude circulation. Using linear inverse modeling (LIM), we find that the PNA pattern optimally grows when there is an El Niño-Southern Oscillation (ENSO) event, which persists through a winter season, and concurrently, a Madden-Julian Oscillation (MJO) event, which evolves on weekly timescales. These two tropical phenomena independently force Rossby waves, or teleconnection patterns, that significantly alter the extratropical circulation. ENSO teleconnections and variability internal to the atmosphere (including MJO teleconnections) interfere destructively, with opposite-sign anomalies largely cancelling each other. As the MJO evolves, these extratropical anomalies progress towards constructive interference, leading to the rapid growth of a PNA pattern. The results of this study suggest there is enhanced predictability of the PNA pattern in subseasonal to seasonal timescales during ENSO events.
