The Pacific-North American (PNA) Pattern
The PNA pattern has important implications for Northern Hemisphere precipitation and temperatures. The PNA has been independently linked to tropical variability like El Nino-Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO), as well as extratropical internal variability. In Henderson et al. (2020), we do a comprehensive examination of the development and amplification of the PNA pattern by considering all of these relevant processes.
Using linear inverse modeling (LIM), we find that the PNA develops optimally from the interference between the modes strongly coupled to sea surface temperature (SST), such as ENSO, and the modes more internal to the atmosphere, such as the MJO. By decomposing the optimal initial condition leading to the PNA pattern (figure below, upper left) into both of these subspaces, we find that the greatest PNA amplification occurs when the extratropical circulation in these two subspaces go from destructive interference (left column) to constructive interference (right column).
Using linear inverse modeling (LIM), we find that the PNA develops optimally from the interference between the modes strongly coupled to sea surface temperature (SST), such as ENSO, and the modes more internal to the atmosphere, such as the MJO. By decomposing the optimal initial condition leading to the PNA pattern (figure below, upper left) into both of these subspaces, we find that the greatest PNA amplification occurs when the extratropical circulation in these two subspaces go from destructive interference (left column) to constructive interference (right column).
Figure: All contours show 200mb streamfunction anomalies. Shown are the coupled (top row) and internal atmospheric (bottom row) components of the extratropical initial condition (left column) that optimally leads to a PNA pattern after 15 days (right column). The full initial condition is the sum of the left column panels, shown in the upper left.
Adapted from Henderson et al. (2020)
Developing a linear inverse model from only the internal atmospheric modes allowed us to examine PNA growth in the absence of this interference. This is particularly important for ENSO neutral years. We find that in the absence of ENSO, PNA development is largely driven by the MJO in conjunction with a retrograding northeast Pacific streamfunction anomaly. In the tropics, Indian Ocean MJO heating optimally leads to PNA pattern development. This suggests that for the negative PNA phase, the early MJO phases are particularly important.