What was done
The authors used "wavelet analyses to investigate multi-decadal oscillations in precipitation across tree-ring records from Douglas-fir and ponderosa, limber, and piņon pines that extend back to at least A.D. 1400 from the central (Montana, Wyoming) and southern (Utah, Colorado, New Mexico) Rocky Mountains."  In doing so, they examined 15 ring-width series, grouping sites "with similar low-frequency behavior into five regions by averaging chronologies with similar wavelet characteristics."

What was learned
The authors report that "strong multidecadal phasing of moisture variation was present in all regions during the late 16th century megadrought" and that "oscillatory modes in the 30-70 yr domain persisted until the mid-19th century in two regions, and wet-dry cycles were apparently synchronous at some sites until the 1950s drought."  In the 20th century, they say "the 'cool' or negative PDO [Pacific Decadal Oscillation] (La Niņa-like) regime prevailed from 1890-1924 and 1947-1976, while the 'warm' or positive PDO (El Niņo-like) regime prevailed from 1925-1946 and from 1977-1998," noting that "another such 'regime shift' may have started in 1998 when the PDO index turned sharply negative ('cool' mode)."  Combining these two sets of observations, the authors conclude that "severe drought conditions (e.g., the 1950s) across consecutive seasons and years in the central and southern Rockies may ensue from coupling of the cold phase PDO with the warm phase AMO [Atlantic Multidecadal Oscillation]."

What it means
Climate alarmists tend to become especially outspoken whenever there are significant flood or drought conditions somewhere in the world, almost demanding that everyone accept their contention that CO2-induced global warming is clearly to blame for the aberrant weather conditions.  As this paper demonstrates with even greater clarity, however, significant periods of relatively greater wetness or dryness are simply the result of various climate regime shifts that are part and parcel of the natural behavior of the atmosphere.  They come and go in cycles of various frequencies and amplitudes that sometimes find themselves in phase with each other and end up creating both positive and negative "mega-anomalies."  It's just the way the world's climate system happens to work.