Classification, Seasonality and Persistence of Low-Frequency Atmospheric Circulation Patterns

Orthogonally rotated principle component analysis (RPCA) of Northern Hemisphere 1-month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability. The analysis is detailed and comprehensive, in that 1) a high resolution, approximately equal-area 358-point grid is used for the virtually maximum possible 35-year period of record, 2) a positive bias in the NMC data base in the early 1950s in the subtropics is largely eliminated for the first time, and 3) homogeneous, separate analyses of each month of the year are carried out, detailing the mouth-to-month changes in the dominant circulation patterns. Winter results are similar to those of other recent RPCA and teleconnection studies except that some less obvious patterns are identified and further detail of the better-known patterns is provided. Two north-south dipole patterns are found over the Pacific Ocean (West Pacific Oscillation and East Pacific pattern) and over the Atlantic Ocean (North Atlantic Oscillation and East Atlantic pattern); two uncorrelated phases of 3-center, approximately east–west wave trains are found over the Eurasian continent (Eurasian Type 1 and Eurasian Type 2 patterns) and North American continent (Pacific/North American and Tropical/Northern Hemisphere patterns) and a Siberian north–south dipole emerges (Northern Asian pattern). The strongest summer pattern is also the strongest winter pattern—the North Atlantic Oscillation, which systematically contracts northward in summer and expands southward in winter, being the only pattern found for every month of the year. Another strong summer pattern, named Subtropical Zonal, is a north–south dipole of great zonal extent at low latitudes. A single-center Asian summer pattern is also found. Two other regular patterns are found during transition seasons. An evaluation of the intermonthly and interseasonal persistence of the patterns shows that many of the strong winter patterns have statistically significant persistence in the middle of their active periods, and the Subtropical Zonal summer pattern shows considerable interannual, as well as intermonthly and interseasonal persistence. The robustness of the RPCA results is examined through consistency with results of other studies and of adjacent month solutions within this study, and by replicating the results using 3-month and 10-day means of 700 mb height. (Results using 10-day means point the way to use of a larger sample without noticeably obscuring the low-frequency signal.) Moreover, the analyses are repeatedly rerun withholding different sets of years from the record, and results are objectively compared with those using the full 35-year record. The conclusion from all considerations is that the RPCA method provides a physically meaningful, as well as statistically stable product with the simplicity of teleconnection patterns but with pattern choice and depiction superior to those of the teleconnection method.