The continuum of North Pacific sea level pressure patterns: Intraseasonal, interannual, and interdecadal variability

This study combines k-means cluster analysis with linear unidimensional scaling to illustrate the spatial and temporal variability of the wintertime North Pacific sea level pressure (SLP) field. The k-means clustering algorithm analyzes daily, wintertime SLP data derived from the NCEP/NCAR reanalysis to produce 24 SLP anomaly patterns that represent a discretized approximation of the continuum of North Pacific SLP patterns. These findings show that the continuum perspective for teleconnection patterns provides a much simpler framework for understanding North Pacific variability than the more commonly-used discrete modal approach.

The primary focus of this research is to show that variability in the North Pacific, on intraseasonal, interannual, and interdecadal time scales, can all be understood in terms of changes in the frequency distribution of the cluster patterns that comprise the continuum, each of which has a time scale of about 10 days. This analysis reveals 5-7 Pacific/North American-like (PNA-like) patterns for each phase, as well as dipoles and wave trains. A self-organizing map (SOM) analysis of coupled SLP and outgoing longwave radiation data shows that many of these patterns are associated with convection in the tropical Indo-Pacific region. As a result, on intraseasonal timescales, the frequency distribution of these patterns, in particular the PNA-like patterns, is strongly influenced by the Madden-Julian Oscillation (MJO). The link between the MJO and the continuum of North Pacific SLP patterns suggests the potential for improved forecasting of the PNA and other patterns in in the midlatitude North Pacific for lead times of 1-3 weeks. On interannual timescales, the El Niño-Southern Oscillation (ENSO) impacts the North Pacific continuum, with warm ENSO episodes resulting in the increased frequency of easterly displaced Aleutian low pressure anomaly patterns, and cold ENSO episodes resulting in the increased frequency of southerly displaced Aleutian high pressure anomaly patterns. In addition, the results of this analysis suggest that the interannual and interdecadal variability of the North Pacific SLP field, including the well known “regime shift” of 1976/77, also results from changes in the frequency distribution of SLP patterns with short, intraseasonal timescales. This implies that the dynamical processes that drive interannual and interdecadal variability are closely linked to processes that time place on much shorter intraseasonal time scales.